Unit 5: Energy, Exercise and Coordination Topic 8: Grey Matter. 8.1. The nervous system and nerve impulses: 8.1.1.What 8.1.1.What are nerve cells like? The nervous system is system is a highly organised network of nerve cells and fibres that transmit nerve impulses around impulses around the body. The nervous system carries messages around the body using neurones. eurones.
Nerves A nerve is nerve is a complex structure containing containing a bundle of axons of axons of many neurones surrounded by a protective covering. There is usually a fatty insulating layer called the myelin sheath around sheath around the axon which is made up of Schwann cells – cells – the sheath affects how quickly never impulses pass along the axon. Not all organisms have myelinated axons. myelinated axons. Neurones Neurones are single cells (although cells (although there are different types) t ypes) which are highly specialised and adapted for the rapid transmission of electrical impulses ( action potentials) potentials) around the body. Most neurones have neurones have a similar basic structure: long – can can transmit the action potential over potential over a long distance - Most are long – body contains a nucleus and cell - The cell body contains organelles organelles within the cytoplasm types of of thin extensions from the - Contain two types cell body: Dendrites which Dendrites which conduct impulses towards o cell body Axons which Axons which transmits impulses away from o the cell body - The cell surface membrane has gated ion channels that control movement of Na, K or Ca ions pumps that use use ATP for active active - Have Na/K pumps transport - Maintain a potential difference across their cell surface membrane - Have a cell body containing the nucleus, mitochondria and ribosomes
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Motor Neurones (effector neurones) – neurones) – the the cell body is always situated within the CNS and the axon extends out, conducts impulses from the CNS to effectors (muscles or glands). The axons of some motor neurones can be very long (e.g. in the leg).
Sensory Neurones – carry impulses from sensory cells to the CNS.
Relay Neurones (connector neurones / interneurones) – found mostly in the CNS. They have a large number of connections with other nerve cells.
8.1.2. Reflex Arcs Nerve impulses follow routes/pathways through routes/pathways through the nervous system – system – these these are called reflex arcs (controlled arcs (controlled by the autonomic nervous system). Reflex arcs are responsible for reflexes (rapid, involuntary responses to stimuli). Reflexes are fast and help to t o avoid damage to the body. 1. Receptors first detect a stimulus (e.g. hot cup) and generate a nerve impulse. 2. Sensory neurones conduct nerve impulses to the CNS along the sensory pathway. 3. Sensory neurones enter the spinal cord through the dorsal route. 4. Sensory neurone synapses with a relay neurone. 5. Relay neurone synapses with motor neurone which leaves the spinal cord through the ventral route. 6. Motor neurone carries impulses to an effector (e.g. muscle) which produces a response (movement). If there is a relay neurone involved, involved, the reflex can be overridden by the brain.
1. Receptor
2. Sensory Neurone
3. Enter dorsal route of spine
4. Relay Neurone
5.1. Motor Neurone
5.2. Exit through ventral route of spine
6. Effector
2|Grey Matter The Pupil Reflex - The iris controls the size of the pupil and it contains a pair of antagonistic muscles: radial and circular muscles (controlled by autonomic nervous system). - Radial muscles (like the spokes of a wheel) are controlled by a sympathetic reflex (fight vs flight). - Circular muscles are controlled by a parasympathetic reflex (rest & digest). - When the pupil constricts the radial - When the pupil dilates the radial muscles relax while the circular muscles contract while the circular muscles contract (high light). muscles relax (low light).
- When
high light strikes the photoreceptors present in the retina (back of eye), nerve impulses pass along the optic nerve to sites within the CNS (including a group of coordinating cells in the midbrain). Photoreceptors are structures found in sensory cells or sense organs that respond to light.
- Impulses from these cells from are sent along parasympathetic motor neurones to the circular muscles of the iris, causing them to contract.
- At
the same time, the radial muscles relax which constricts the pupil and reduces the amount of light entering the eye.
1|Grey Matter 8.1.3. How nerve cells transmit impulses: All cells have a potential difference (electrical voltage) across their surface membrane. The inside of an axon is more negative than the outside (membrane is said to be polarised) – the value of this is -70 mV and is known as the resting potential. Neurones have sodium-potassium pumps in their surface membranes and these actively pump Na+ ions out of the cell by active transport. This uses ATP as ions are moved against their concentration gradients. When a neurone is not transmitting impulses, it is at rest. K + ions are pumped into the cell through channels. K + and Na+ ions diffuse back down their concentration gradient but K + diffuses back out faster than Na + can diffuse back in – net movement out of cell making the inside more negative. This is the resting potential and the membrane is polarised.
There is an uneven distribution of ions across the cell surface membrane of an axon – this is achieved by the action of sodium-potassium pumps in cell surface membrane. 1. Na+ pumped out of cell while K + is pumped into the cell – acts against concentration gradients and uses energy from the hydrolysis of ATP. 2. Once concentration gradients are set up: K+ diffuses out of cell (down the gradient) making the outside of the cell membrane positive and the inside negative. The membrane is permeable to K+ ions but virtually impermeable to Na + ions. 3. The more K+ that diffuses out the larger the potential difference across the membrane. The increased negative charge inside the cell attracts K + ions back across the membrane and into the cell. 4. At -70mV the electrical gradient balances the chemical gradient – no net movement of K+ which maintains the potential difference.
2|Grey Matter What happens when a nerve is stimulated? Neurones are electrically excitable cells; the potential difference across their cell surface membrane changes when they are conducting an impulse. What causes an action potential (change in voltage across a membrane) ? Once threshold stimulation occurs, an action potential (large change in voltage across the membrane) is caused by changes in the permeability of the cell surface membrane to Na + and K+, due to the opening and closing of voltage-dependent Na + and K+ channels. At the resting potential, these channels are blocked by gates preventing the flow of ions through them. Changes in the voltage across the membrane cause the gates to open, and so they are referred to as voltage-dependent gated channels. There are three stages in the generation of an action potential: 1. Depolarisation: occurs when a neurone is stimulated, changing the potential difference across the membrane. This causes a change in the shape of the Na + gate, opening some of the voltage dependent Na + channels. Na+ enters the axon, more depolarisation occurs, triggering more gates to open. This is an example of positive feedback (a change that encourages further change if the same sort). Action potentials are either there or they are not (all-or-nothing). The potential difference across the membrane is now +40mV and there is a higher concentration of Na + outside of the axon, so Na + ions flow rapidly inward causing a positive charge inside the cell membrane. 2. Repolarisation: after about 0.5ms, the voltage dependent Na + channels spontaneously close thus restoring permeability to a low level. K + channels open due to depolarisation of the membrane, thus K+ ions move out of axon causing a negative charge inside membrane, 3. Restoring Resting Potential: membrane is now highly permeable to K + ions which continue to move out of the cell making the potential difference more negative than normal resting potential – this is called hyperpolarisation of the membrane. Resting potential is reestablished by the closing of K + channels causing the ions to move back into the axon.
3|Grey Matter How is a nerve impulse passed along an axon (propagation of a nerve impulse)? Stimulation of a neurone causes a sequence of action potentials along the axon (domino effect). As part of the membrane becomes depolarised at the site of an action potential, a local electrical current is created as the charged Na + ions flow between the depolarised part of the membrane and the adjacent resting region. The depolarisation spreads to the adjacent region and the nearby Na+ gates will respond to this by opening as described earlier, triggering another action potential. These events are then repeated along the membrane. As a result, a wave of depolarisation will pass along the membrane. This is the nerve impulse. A new action potential cannot be generated in the same section of membrane for about five milliseconds - known as the refractory period. It lasts until all the voltage-dependent Na + and K+ channels have returned to their normal resting state (closed) and the resting potential is restored. The refractory period ensures that impulses only travel in one direction. 1. At resting potential there is a +ve charge on the outside of the membrane and -ve charge on the inside, with higher Na + ion concentration outside and higher K + ion concentration inside. 2. When stimulated, voltage-dependent Na+ ion channels open and Na + ions flow into the axon, depolarising the membrane. Localised electric currents are generated in the membrane. Na + ions move to the adjacent polarised (resting) region causing a change in electrical charge (potential difference) across this part of the membrane. 3. This change in potential difference in the membrane adjacent to the first action potential initiates a second action potential. At the site of the first action potential, the voltage-dependent Na + ion channels close and voltage-dependent K + ion channels open. K+ ions leave the axon, repolarising the membrane. The membrane is hyperpolarised 4. A third action potential is initiated by the second. In this way, local electric currents cause the nerve impulse to move along the axon. At the site of the first action potential, K+ ions diffuse back into the axon, restoring resting potential Are impulses different sizes? A stimulus must be above threshold level to generate an action potential. The all-or-nothing effect for action potentials means that the size of the stimulus has no effect on the size of the action potential. The size of a stimulus affects: - the frequency of impulses. - the number of neurones in a nerve that are conducting impulses. A high frequency of firing and the firing of many neurones are usually associated with a strong stimulus.
4|Grey Matter Speed of impulse conduction: The speed of nervous conduction is very fast, allowing fast responses to stimuli. They are affected by 3 factors: - Temperature – the higher temperature, the faster the speed. Homoeothermic (warm-blooded) animals have faster responses than poikilothermic (cold-blooded) ones. - Axon Diameter – the wider the diameter, the faster the impulse travels (less resistance to flow of ions so depolarisation travels faster). Marine invertebrates lining at low temps have developed thick axons (1000µm) to speed up their responses. Mammals axons are ± 1-20µm. - Myelin Sheath – only vertebrates have a myelin sheath which acts as an electrical insulator and prevents flow of ions across the membrane. Gaps known as nodes of Ranvier occur in the myelin sheath at regular intervals and are the only places where depolarisation can occur. The impulse jumps from one node to the next – increases speed of propagation (wave of depolarisation). The ‘jumping’ conduction is called salutatory conduction.
8.1.4. How does a nervous impulse pass between cells? Where two neurones meet is known as a synapse. The cells do not actually touch - there is a small gap called the synaptic cleft. Synapse structure:
How does the synapse transmit an impulse? There are three stages leading to the nerve impulse
5|Grey Matter passing along the post synaptic neurone. - Neurotransmitter release – when the presynaptic membrane is depolarised by an action potential, Ca 2+ channels open and Ca2+ diffuses across the membrane due to the increased Ca 2+ concentration outside the cell. This causes synaptic vesicles containing neurotransmitter (e.g. acetylcholine) to fuse with the presynaptic membrane and release their contents into the synaptic cleft by exocytosis. - Stimulation of the postsynaptic membrane – the neurotransmitter takes ±0.5ms to diffuse across the synaptic cleft and reach the postsynaptic membrane. Embedded in the postsynaptic membrane are specific receptor proteins that have a binding site with a complementary shape to the neurotransmitter. The neurotransmitter binds to the receptor changing the shape of the protein, and opening cation channels thus making the membrane permeable to Na + ions. The flow of the Na + ions across the postsynaptic membrane causes depolarisation, producing an action potential which is propagated along the postsynaptic neurone. The extent of the depolarisation depends on the volume of neurotransmitter. - Inactivation of neurotransmitter – some neurotransmitters are taken up by the presynaptic membrane and the molecules are used again. With others, the neurotransmitter rapidly diffuses away from the synaptic cleft or is taken up by other cells. With acetylcholine, acetylcholinesterase at the post synaptic membrane breaks it down so it can no longer bind to receptors. Some breakdown products are reabsorbed by the presynaptic cleft and reused. 8.1.5. What is the role of synapses in nerve pathways? Control and Coordination Synapses have two main roles: - Control of nerve pathways, allowing flexibility of response. - Integration of information from different neurones, allowing a co-ordinated response. The posynaptic cell can receive inputs from many synapses at the same time. The overall effect of all of these synapses determines
whether an action potential will be generated The two factors affecting the likelihood that the postsynaptic membrane will depolarise: - Type of synapse (excitatory or inhibitory) - Number of impulses received (the balance of excitatory and inhibitory synapses)
6|Grey Matter Types of synapse: 1. Excitatory synapses: Excitatory synapses make the postsynaptic membrane more permeable to Na + ions. - Summation is the idea that each impulse adds to the effect of the others – one alone does not have enough of an effect. There are two types of summation: Spatial Summation: here impulses are o from different synapses, from different neurones. The number of different sensory cells stimulated can be reflected in t he control of the response. Temporal Summation – here several o impulses arrive at a synapse after travelling along a single neurone, one after another. Their combined release of neurotransmitter generates an action potential in the postsynaptic membrane. 2. Inhibitory synapses These synapses make it less likely that an action potential will occur. The neurotransmitter from these synapses opens channels for Cl - and K+ ions in the postsynaptic membrane. Cl- ions (-ve charge) will enter the membrane and K + ions (+ve charge) will leave the membrane resulting in a greater potential difference across the membrane (inside becomes more negative than usual at ±-90mV) – hyperpolarisation. Subsequent depolarisation cannot occur and more excitatory synapses will be required to depolarise the membrane. Comparing nervous and hormonal coordination: The body is not only co-ordinated by the nervous system - hormones secreted by endocrine glands send chemical messages to cells and control long t erm changes e.g. growth/sexual development. Nervous Control Hormonal Control Electrical transmission by nerve impulse and Chemical transmission through the blood chemical transmission at synapses Fast acting Slower acting Associated with short term-changes e.g. muscle Can control long term changes e.g. contraction growth/sexual development Action potentials carried by neurones with Blood carries hormones to all cells, but only connections to specific cells specific cells can respond Response is very local, e.g. in a specific muscle Response may be widespread, e.g. in growth cell or gland and development C ontrasting nervous and hormonal cells in animals
7|Grey Matter Coordination in plants: Plants lack a nervous system so use chemicals to control growth, development and responses to the environment. - These chemicals (e.g. auxins), called plant growth regulators/plant growth substances, are produced in low concentrations before being transported to where they cause a response. - Experiments were conducted on phototropism (bending of plants towards a light source). These experiments showed that an oat coleotopile with the tip cut off stops bending towards the light. Replacing the tip starts regrowth towards the light again. - It was concluded that influence (a chemical) made in the tip was passed down to the rest of the plant causing it to bend. - The chemical is indoleacetic acid (auxin) and its main function is to stimulate grow as a result of cell elongation. - Experiments show that there is no difference in chemical production between sides of the plant in light/dark, however more auxin has passed down the shaded side – this increased concentration of auxin increases cell elongation and reduced concentration on the illuminated side inhibits cell elongation. As a result the shoot grows towards the light. Comparison of coordination in plants and animals:
Animals
Plants
Coordination in both plants and animals involves receptors, a communication system and effectors Plants do not have a nervous system or neurone, however some parts of plants do transmit action potentials, but this is done Animals have a nervous system, containing very much more slowly than animals, and the specialised neurones which transmit action potential differences involve are generally potentials very rapidly. less than those in mammals – e.g. Venus fly trap – action potential brings about it’s closure when fly lands on leaf Both animals and plants use chemical that are produced in one part of the organism and travel to other parts where they have their effects. In plants, there are no glands where these chemicals are made, but plant hormones are made in one area (e.g. auxin made in In animals, these substances are called hormones and they’re made in the endocrine meristems) and travel to another part of the glands, which secrete them directly to the plant where they have their effect. Unlike blood. They’re then carried in solution in the animal hormones, they often do not travel in blood plasma – they affect target organs vessels but instead move through cells, which have receptors for them either by facilitated diffusion through protein channels or by active transport through protein transporters Animal hormones are almost all small protein No protein or steroid plant hormones have molecules or steroids been found
8|Grey Matter 8.2. Reception of stimuli 8.2.1. How does light trigger nerve impulses? Receptors - Stimuli (changes that occur in an animal’s environment) are detected by receptor cells that send electrical impulses to the CNS. - Many receptors are spread throughout the body, but some types are grouped together into sense organs (eg. the eyes) which help protect the receptor cells and improve their efficiency. - In the eye: the lens and cornea refract (bend) light so that it is focused on the retina where the photoreceptor cells are located.
Type of receptor Chemoreceptors Mechanoreceptors Photoreceptors Thermoreceptors
Stimulated by Chemicals Forces that stretch, compress, or move the sensor Light
Examples of role in body Taste, smell and regulation of chemical concentrations in the blood Balance, touch and hearing
Sight Temperature control and awareness of Heat or cold changes in the surrounding temperature Four main types of receptors
Part of Eye Function Ciliary muscle Alters the thickness of the lens for focusing Choroid A black layer that prevents internal reflection of light Vitreous Humour Transparent jelly Retina Contains light sensitive cells Fovea Most sensitive part of retina located within the macula, central area of retina Optic nerve Sends impulses to the brain Blind Spot No light sensitive cells where optic nerve leaves the eye Sclera Protective layer, , allows attachment of external muscles Iris Controls the amount of light entering the eye by controlling pupil size Lens Focuses light on the retina Cornea Refracts (bends) light Pupil Circular opening for directing light to the lens Conjuctiva Protects the cornea
9|Grey Matter Photoreceptors: The retina contains two types of photoreceptor cells which are sensitive to light: rods and cones. - Rods only give black and white vision, but can work in dim and bright light. - Cones allow colour vision in bright light. The image below shows the arrangement of the 3 layers of cells which make up the retina: the rods and cones synapse with the bipolar neurone cells which synapse with ganglion neurone (whose axons make up the optic nerve). In both rods and cones, a photochemical pigment absorbs the light entering the eye resulting in a chemical changes. In rods the pigment is rhodopsin.
How does light stimulate photoreceptor cells? In the dark 1. Na+ diffuses through open cation channels into outer segment. 2. Na+ move down concentration gradient into inner segment. 3. Na+ is actively pumped out of cell using ATP and ion pumps. 4. Membrane slightly depolarised 40mV. 5. Inhibitory neurotransmitter (glutamate) is continuously released from rod cells and binds to the bipolar cell, preventing it from depolarising.
In the light 1. Light energy breaks down rhodopsin into opsin and retinal. 2. This breakdown causes Na+ channels to close, reducing influx of Na+ into the rod. 3. Na+ still actively pumped out but cannot diffuse into outer segment therefore inside of cell is more negative. 4. Membrane is hyperpolarised (-90mV) and release of neurotransmitter stops. 5. Lack of neurotransmitter results depolarisation of bipolar cell and neurones of optic nerve, resulting in an action potential.
10 | G r e y M a t t e r ‘Dark Adaptation’ - Once the rhodopsin molecule has been broken down it needs to be converted back so that more stimuli can be received (this takes a few minutes). - The higher the light intensity the more rhodopsin molecules are broken down and the longer it takes for all the rhodopsin to reform. - The reforming of rhodopsin – ‘dark adaptation.’
8.2.2. Plants and Environmental Cues - Plants can also detect and respond to stimuli, adapting their growth and development to make sure they survive and reproduce. - Plants detect the quantity (duration), intensity, direction, and wavelength (quality) of light using photo receptors and respond to changes in light conditions. They also respond to environmental cues such as gravity, touch, and mechanical stress. - All messages in a plant are chemical so responses are slower. - Plants contain several types of photoreceptor; the most extensively studied being phytochromes – 5 of which have been identified.
Phytochromes – plant photoreceptors - Phytochromes consist of a protein component bonded to a non-protein light absorbing pigment molecule. - The five phytochromes differ in their protein component while the non-protein component exist in the form of two different isomers which are photoreversible: Pr Pfr
Phytochrome Red (absorbs red light (660nm)) Phytochrome Far-red (absorbs far-red light (730nm))
- Plants synthesise phytochromes in the Pr form – absorption of red light converts P r into Pfr and absorption of far-red light converts P fr into Pr . - White light, including sunlight, contains both red and far-red light. - After daylight there is more Pfr . After darkness there is more Pr . - Phytochromes regulate: seed germination, stem elongation, leaf expansion, chlorophyll formation and flowering.
11 | G r e y M a t t e r Germination: - Red light triggers germination while far-red inhibits germination, therefore the effects are reversible. Flowering: - The photoperiod (relative length of the day and night) determines time of flowering. - The ratio of Pr and Pfr present enables the plant to determine the length of day and night. - Long day plants only flower when day length exceeds a critical value (period of uninterrupted darkness is less than 12 hours). Pfr stimulates flowering. - Short day plants only flower when the period of uninterrupted darkness is greater than 12 hours. Pr is required, and any P fr inhibits flowering. Greening: Changes in form and biochemistry due to exposure to sunlight.
How do phytochromes switch processes on or off? - Exposure to light causes the molecules to change form and shape. - Activated phytochromes then interact with other proteins, binding to them or disrupt the binding of a protein complex. - These signal proteins act as transcription factors, or activate transcription factors, which bind to DNA allowing transcription of genes. - The transcription and translation of proteins results in the plant’s response to light.
8.3. The Brain The cerebral hemispheres: - The outer layer of the brain is called grey matter . - The cortex at the top of the brain is made mostly of cell bodies, synapses and dendrites (grey matter ). It is divided into left and right cerebral hemispheres. Each hemisphere is composed of 4 regions: frontal lobe, pariental lobe, occipital lobe and temporal lobe. - Each lobe interprets and manages its own sensory inputs. The two cerebral hemispheres are connected by white matter (nerve axons) called corpus callosum.
12 | G r e y M a t t e r Each cerebral hemisphere is composed of 4 regions: 1. Frontal lobe 2. Parietal lobe 3. Occipital lobe 4. Temporal lobe Other structures lying directly below the corpus callosum include: - Cerebellum: Coordinates movement and balance. It receives information from the primary motor cortex, muscles and joints. - Medulla oblongata: Regulates involuntary body processes e.g. control of heart rate (cardiovascular centre), breathing movements (respiratory centre) and blood pressure. - Hypothalamus: Thermoregulation (maintaining core body temperature at ± 37.8°C). Controls sleep, hunger, thirst, and blood concentration. Connects directly to the pituitary gland which secretes other hormones. - Thalamus: Responsible for routing all incoming sensory information to the correct part of the brain (via axons of white matter). - Hippocampus: Involved in laying down long term memory. - Basal ganglia: A collection of neurones responsible for selecting and initiating stored programmes for movement. - Brain stem (reptilian brain): Top of the spinal column; it extends from midbrain to medulla. - Mid brain: Relays information to the
cerebral hemispheres, including auditory information to the temporal lobe and visual information to occipital lobe.
13 | G r e y M a t t e r The effects of strokes: - Brain damage caused by a stroke can cause problems with speaking, understanding speech, reading and writing. - Some patients can recover some abilities after a stroke, showing the potential of neurones to change in structure and function (neural plasticity). - Brain structure remains flexible even in later life & can respond to changes in the environment. Brain Imaging CAT scans (CT scans – Computerised Axial Tomography): - Use thousands of narrow beam X-rays rotated around the patient to pass through tissue from different angles. - Each narrow beam is attenuated (reduced in strength) according to the density of the tissue in its path. - The X-rays are detected and used to produce an image of the brain in which different soft tissues can be distinguished. - Are cheaper than MRI’s. - Don’t show which tissues are active; c an only show structure, not function of the brain. - Used to detect brain disease and monitor changes in tissue over the course of an illness.
MRI (Magnetic Resonance Imaging):
- Use a magnetic field and radio waves to detect soft tissues. - When placed in a magnetic field the nuclei of atoms line up with
the direction of the magnetic field. - Hydrogen atoms in water are monitored in MRI as there is a high water content in the tissues under investigation. - In a scanner: a magnetic field runs down the centre of the tube in which a patient lies, while another magnetic field is superimposed on this, which comes from the magnetic component of high frequency radio waves. The combined fields cause the direction (axis) and frequency of spin of the hydrogen nuclei to change, taking energy from the radio waves to do so. - When the radio waves are turned off, hydrogen nuclei return to their original alignment and release the absorbed energy – this energy is detected and sent to a computer for analysis. - Different tissues respond differently to the magnetic field and so produce contrasting signals and distinct regions in the image. - Used in: diagnosis of tumours, strokes, brain injuries and infection of the brain and spine. fMRI (Functional Magnetic Resonance Imaging): - Looks at the function of the different areas of the brain by following the uptake of oxygen in active brain areas. - Deoxyhaemoglobin absorbs the radio wave signal, whereas oxyhaemoglobin does not. - Increased neural activity in a brain area results in an increased demand for oxygen and hence an increase in blood flow. - The less radio signal there is absorbed, the higher the level of activity in that area, so different areas of the brain will ‘light up’ when they are active.
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8.3.1. From the eye to the brain The axons of the ganglion cells that make up the optic nerve pass out of the eye and extend to several areas of the brain areas, including the thalamus. Impulses are then sent along other neurones to the primary visual cortex where information is processed. Before reaching the thalamus, some of the neurones in each optic nerve branch off to the midbrain where they connect to motor neurones involved in controlling the pupil reflex and eye movement. Audio signals also arrive at the midbrain so we can quickly turn our eyes in the direction of a visual or auditory stimulus.
8.4. Visual Development 8.4.1. Axon growth: - Axons of the neurones from the retina grow to the thalamus where they form synapses with neurones in the thalamus in a very ordered arrangement. Axons from these thalamus neurones grow towards the visual cortex in the occipital lobe. - The visual cortex is made of columns of cells and axons from the thalamus synapse within these columns. - Columns are formed before the critical period for development of vision in which dendirites and synapses are stimulated by light - During the critical period, axons compete for target cells in the visual cortex. Every time a neurone fires onto a target cell, the synapses of another neurone sharing the target cell are weakened and release less neurotransmitter. Synapses not firing will be cut back. The human nervous system begins to develop at birth. There is no large increase in the number of brain cells but there is a large increase in brain size due to the elongation of axons, myelination and the development of synapses. We ‘see’ because our brain processes the image formed from the retina, using past experience and other sensory inputs. The capacity of the brain to process and interpret the action potentials that arrive along the optic nerve is acquired during early childhood. Newborn babies have little ability to interpret information, but this is highly developed by age of 5 or 6. The experiences the child has determines the way the brain cells are ‘wired up’ during development. 1960s – Hubel and Wiesel carried out important experiments using monkeys and kittens. They found that if they prevented light reaching the retina of one or both eyes as t he young animals matured, this stopped them developing normal visual abilities. Depriving them of vision at different stages of development affected different aspects of vision. They concluded that t here were specific ‘windows’ during development, known as critical periods/windows during which particular types of visual input were needed in order for particular visual capacities to develop normally.
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8.5. Making sense of what we see Depth perception: - Close objects (<30m away) – we depend on the presence of cells in the visual cortex that obtain information from both eyes at once – the visual field is seen from 2 angles which allows comparison of the view from one eye to the other. This is stereoscopic vision and it allows relative position of objects to be perceived. - Distant objects (>30m away) – the images on our retinas are similar, so visual cues and past experiences are used to interpret the images. Cross-cultural studies: People from different cultures may not share the same beliefs and behaviours. Carpentered world hypothesis: those who live in a world dominated by straight lines and right angles perceive depth cues differently to those who live in a ‘circular culture.’ Studies with newborn babies: The visual cliff – babies are encouraged to crawl across a transparent table, which is a visual cliff. Patterns placed below the glass create the appearance of a steep drop. If the perception of depth is innate the babies should be aware even if they have not previously experienced this stimulus. Young babies were reluctant to crawl over the ‘cliff’ even when the mothers encouraged them. 8.6. Learning and memory Learning is when organisms modify their behaviour as a result of experience. One of the simplest types of learning is habituation: a decrease in intensity of a response when the same stimulus is given repeatedly (e.g. humans show habituation when hearing a loud bang repeatedly). The nervous system changes when there are changes in the synapses that underpin learning and memory changes. Memory is in different parts of the cortex and short-and long term memory is controlled by different parts of the brain. 8.6.1. How memories are stored Memory can be created by altering: the pattern of connections, and the strength of synapses. The sea slug breathes through a gill in a cavity on the upper side of its body and water is expelled through a siphon tube. If the siphon is touched, the gill is withdrawn into the cavity – a protective reflex. Sea slugs are habituated to waves which stimulate the siphon. After repeated stimulation, the siphon no longer withdraws. Habituation allows animals to ignore unimportant stimuli so that limited sensory, attention and memory resources can be concentrated on more threatening or rewarding stimuli. How is habituation achieved? With repeated stimulation, calcium ion channels become less responsive: 1. Less Ca+ ions cross presynaptic membrane into presynaptic neurone. 2. Fewer synaptic vesicles fuse with presynaptic membrane. 3. Less neurotransmitter released into synaptic cleft. 4. Less Na+ ion channels open on posysynaptic membrane. 5. Less Na+ ions flow into postsynaptic membrane. 6. Less or no action potential is triggered in postsynaptic motor neurone.
16 | G r e y M a t t e r Practical – investigating habituation in pond snails: 1. Collect pond snails of the same species and place them in the same tank. Leave for a few days to acclimatise. 2. Place one snail in a dish and leave to rest for 5 minutes until active. 3. Gently touch snail between tentacles. Snail will withdraw and then slowly extend again. 4. Repeat stimulus several times with set intervals of < 1min. Record and time the time taken for the tentacles to be returned to fully extended position. 5. Plot a graph of time against number of stimuli given. More connection – longer memory: Long-term memory storage involves an increase in the number of synaptic connections. Repeated use of a synapse leads to creation of additional synapses between neurones. Sensitisation: the opposite of habituation. Happens when an animal develops an enhanced response to a stimulus. Humans can learn by sensitisation. E.g. if a predator attacks sea slugs, they become sensitised to other changes in their environment and respond strongly to them. There is a greater calcium ion uptake, more neurotransmitter released, greater depolarisation and a higher frequency of action potentials. Nature vs nurture There’s a long standing interest as to the extent to which the behaviour of animals is determined by genes or by the environment – that is, the experiences an animal has whilst it develops. The effect of genes is sometimes known as nature and the effect of the environment as nurture. We understand today that most patterns of behaviour are determined by the interactions of both. A behaviour pattern that is shown at the very beginning of an organism’s life is said to be innate, and is considered to be caused entirely by genes. However, in humans, its environment has influenced even a newborn baby, as it was developing in the uterus. Newborn babies show various reflex reactions, such as the startle reflex. This happens when a baby hears a sudden loud noise, or is dropped a short distance. The baby responds by flinging out arms and legs and contracting the neck muscles. This response is largely innate but it may also be influenced by the experiences of the baby while it was a foetus in the uterus. One way to investigate is using identical twins as they have identical genes. Brain development and behaviour of twins brought up in different families and environments are compared – these differences were caused by the environment. Animal studies confirm that innate behaviour patterns can be modified by experience. 8.6.2. Animal testing – ethical issues For May be the only way to fully test new drugs and substances, or find out about an aspect of physiology or behaviour which may lead to less suffering of humans and animals Only done when necessary – humans have a greater right to life Only way to study how a drug affects the whole body
Against We have no right to submit animals to procedures that may cause discomfort or make their lives unpleasant There is no need to use animals in research as there are other ways of conducting the same investigations Animals are different to humans – no certainty that drugs tested on animals will have the same effect on humans
17 | G r e y M a t t e r Institutions in the UK that test on animals follow the same codes of conduct: - Limit the use of animals to circumstances where there is no alternative method (e.g. using cells grown in tissue culture). - Only allow research after thorough scrutiny of the proposal, which must show no other method is possible and the animal welfare will be given high priority at all times. - All people involved, including scientists, are given fully training in ensuring the health and wellbeing of the animals. Utilitarianism: the belief that the right course of action is the one that maximises the overall happiness in the world. A utilitarian framework allows certain animals to be used in medical experiments provided the overall expected benefits are greater than the overall expected harms. 8.7. Problems with synapses The brain contains neurones which transmit nerve impulses to each other across synapses. Many different neurotransmitters are involved, including dopamine and serotonin. 8.7.1. Dopamine and Parkinson’s disease Dopamine is a neurotransmitter secreted by neurones, including many located in the midbrain. The axons of the neurones in this area extend throughout the frontal cortex, brain stem and spinal cord. Parkinson’s disease occurs when dopamine-secreting neurones in the basal ganglia die so dopamine is no longer produced, resulting in loss of control of muscular movement. Symptoms of the disease include stiffness of muscles, tremors, slowness of movement, poor balance, walking problems, depression and difficulties with speech and breathing. Treatment for Parkinson’s disease: - Slowing the loss of dopamine from the brain with the use of drugs like selegiline. This inhibits enzyme monoamine oxidase, which is responsible for breaking down dopamine in the brain. - Treating symptoms with drugs. Dopamine itself cannot treat Parkinson's because it cannot cross into the brain from the bloodstream, but L-dopa (a precursor in the manufacture of dopamine) can be given. Once in the brain, L-dopa is converted into dopamine, increasing the concentration of dopamine and controlling the symptoms of the disease. - Use of dopamine agonists. Dopamine agonists are drugs that activate dopamine receptors directly. These drugs mimic the role of dopamine in the brain, binding to dopamine receptors at synapses and triggering action potentials. They are useful in the treatment of Parkinson's disease since they avoid higher than normal levels of dopamine in the brain. Abnormally high dopamine levels can have unpleasant side effects. - Results of gene therapy trials in animals & phase I trials in humans show promise. Genes for proteins that increase dopamine production and promote growth and survival of nerve cells are inserted into brain. Cell therapy where proteins themselves are injected is also being trialled. - New surgical approaches are being trialled, some of which are generating encouraging results.
8.7.2. Serotonin and depression Serotonin is a neurotransmitter involved in functions including mood, appetite, temperature regulation, sensitivity to pain and sleep. Neurones that secrete serotonin are situated in the brain stem and their axons extend into the cortex, cerebellum and spinal cord. A lack of serotonin is linked to clinical depression. Symptoms include sadness, anxiety, hopelessness, loss of interest in pleasurable activities, insomnia, restlessness and thoughts of death. When someone is depressed, fewer nerve impulses than normal are transmitted around the brain, related to low levels of neurotransmitter production. The causes are not completely understood, but depression may be multifactorial: There may be a genetic element – it runs in families Could be environmental – trauma or stress related
18 | G r e y M a t t e r Treatment for depression: SSRI drugs (selective serotonin reuptake inhibitor) e.g. Prozac, inhibit the uptake of serotonin from the synaptic cleft unto the presynaptic cell. This increases the level of serotonin available to bind to the postsynaptic receptor. 8.7.3. How drugs affect synaptic transmission Clinical depression can be treated with drugs that increase serotonin in t he brain. The drug MDMA (ecstasy) increases the concentration of serotonin in the synaptic cleft by binding to molecules in the presynaptic membrane that are responsible for transporting serotonin back into the cytoplasm. This prevents its removal from the synaptic cleft. The drug may also cause the transporting molecules to work in reverse, further increasing the amount of serotonin outside the cell. These higher levels of serotonin bring about the mood changes seen in users of the drug. It is possible that ecstasy has a similar effect on molecules that transport dopamine as well. Effects of using ecstasy MDMA can produce feelings of euphoria, friendliness, well-being, enhanced senses and energy. However, it has many other side effects on the body, causing depression, clouded thinking, agitation, muscle spasms, hyperthermia, confusion and anxiety. Animal research shows the regular use of MDMA causes damage to several parts of the brain. - Short term effects – changes in behaviour and brain chemistry, sweating, dry mouth, increased heart rate, fatigue, muscle spasms and hypothermia - Long term effects – changes in behaviour and brain structure 8.7.3. Better treatments The Human Genome Project and drug development The Human Genome Project (HGP) has deciphered the base sequence of the human genome (all DNA of an organism). From this, we can work out the amino acid sequences of the proteins they produce, leading to understandings of how the proteins work. This enables researchers to develop new drugs which target specific proteins, enhancing or lessening their activity. 1977 – Fred Sanger – first DNA sequencing process. Not everyone responds in the same way to drugs, knowledge of differences in a person’s base sequence can help understand this. Knowledge of a particular DNA sequence will enable suitable drugs to be chosen on an individual basis. Issues surrounding the Human Genome Project: - Who should decide about the use of tests and on whom should they be used? - Making and keeping records of individual genotypes raises issues of confidentiality - Medical treatment through the development of genetic technologies will initially be very expensive - Restricted availability of many medical treatments will be a problem to health services in deciding who is eligible for treatment 8.7.4. Using genetically modified (GM) organisms to produce drugs Genetic modification (artificial introduction of genetic material from another organism) produces transgenic or genetically modified organisms (GMOs). GM is also known as genetic engineering, genetic manipulation, or recombinant DNA technology. Genes for the synthesis of particular proteins can be inserted into an organism’s DNA, so the organism expresses that gene and synthesises the protein. This involves: - Identifying and isolating the gene that is inserted by cutting it from DNA using restriction enzymes or by reverse engineering using a sequence of amino acids in the protein to be made and constructing a length of DNA with the appropriate base sequence to code for this protein - A bacterium that infects the species – genes from plasmid DNA are incorporated into the plant chromosome when they infect them - Inserting the vector into the organism using microprojectules: tiny pellets carrying the desired genes are shot into the plant cells using a particle gun
19 | G r e y M a t t e r First success in genetic engineering was with bacteria: - Cheap and easy to culture - Rapid reproduction – transferred gene copied rapidly Prokaryotic cells do not have the correct biochemistry to make some of the more complex human proteins – so much use eukaryotes e.g. yeast, plants and animals Bacteria contain simple DNA structures, plasmids, which can be transferred between cells. Using restriction enzymes, the circular plasmid can be cut and using other enzymes, a piece of DNA from another species can be inserted. E.g. bacteria to produce human insulin: 1. Isolated human gene, modified if necessary 2. Extracted plasmid is cut with restriction enzyme 3. Isolated human gene is spliced into plasmid 4. Modified plasmid placed back into bacterial cells 5. Cells multiply in fermenter 6. Bacterium produces human insulin 7. Insulin protein extracted and purified 8. Bacterial cells destroyed Genetically modified plants: Genetic engineers introduce new genes with alleles for desired characteristics into a plant’s DNA, resulting in genetically modified plants. 1. Plasmid carrying desired gene and an antibiotic resistance gene (marker gene) used 2. DNA insertion of new gene into a virus, used to incorporate genes into DNA of plant cells 3. Incubation in growth medium with antibiotic 4. Micropropagation: cells grow in sterile culture medium containing sucrose, amino acids, inorganic ions and plant growth substances 5. Plant growth substances stimulate root and shoot growth 6. Transgenic plant – all new cells contain the new genes 7. Plantlets separated and grown into full size plants
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Benefits of GMOs Pest resistant crop plants – reduces the use of pesticides – increases yield, reduces risk of harming beneficial insects Herbicide resistant crops allow the herbicide to kill weeds but not crops Crop plants can be modified to produce high quantities of nutrients Could benefit human health Could help to feed the developing world GM crops are more cost-effective
Risks of GMOs Genes inserted into a crop might spread to others – cause changes in the genotypes of plant populations – adversely affect other organisms in an ecosystem Pests might develop resistance through natural selection to the substances in GM crops, resulting in ‘super -pests’ or ‘superweeds’ Consuming foods containing GMOs can be considered harmful to health Environmental concerns – increased chemical use in crops Could damage organic farmers Raises ethical conflicts over the control of food production
Genetically modified animals: Several techniques can be used to artificially introduce genes into animal cells. For example: - Retroviruses: virus inflects cells by inserting their DNA/RNA into host’s genome - Liposome wrapping: gene wrapped in a lipid bilayer which can then fuse with the cell membrane and deliver the DNA into the cytoplasm - Microinjection: DNA injected directly into nucleus of a fertilised egg using a micropipett e (only successful in 1% of embryos) Concerns about genetic modification: The main health concerns that have been raised are: - Transfer of antibiotic-resistance genes to microbes - Formation of harmful products by new genes - Transfer of viruses from animals to humans. The main environmental concerns about GMOs are: - Transfer of genes to non-GM plants (cross-pollination) - Increased chemical use in crops.
21 | G r e y M a t t e r Nervous system keywords Nerve impulse: An electrical impulse sent along a nerve to allow information to travel to effectors to carry out a response to a stimulus Neurone: A single cell which has dendrites, an axon, a cell body and terminal branches, making up part of the nervous system Nerve: Contains a bundle of the axons of many neurones surrounded by a protective covering Cell body: Contains the organelles and is found in different locations for different neurones with extensions called dendrites and the axon Dendrites: Conduct impulses towards the cell body. Each neurone has many Axon: Conducts impulses away from the cell body, a single long process Motor neurone: Carry impulses from CNS to effectors Sensory neurone: Carry impulses from sensory cells to CNS Relay neurone: Have a large number of connections with other nerve cells Myelin sheath: Fatty insulating layer around the axon; affects speed of impulse, made of Schwann cells Effector: Produce a response when an impulse is received from motor neurones Reflex arc: Nerve impulses follow simple pathways Reflexes: Rapid, involuntary responses to stimuli Photoreceptors: Receptors in the retina that respond to light levels Resting potential: -70mV, when the inside of the axon membrane is more negative due to the movement of potassium ions Repolarisation: The charge is reversed, becoming positive on the inside (+40mV) because voltage-dependent Na+ channels open and Na+ flow in Repolarisation: The Na+ channels close and K+ channels open, allowing K+ ions to flow out making the inside more negative Action potential: The change in voltage across the axon membrane Positive feedback: The opening of Na+ ion gates makes more open All or nothing: There is either enough / not enough depolarisation to create an action potential Hyperpolarisation: The K+ ion gates open to re-polarise the membrane and too many are left out, the potential reaches -90mV Refractory period: A new action potential cannot be generated for a few milliseconds until all the gates are closed and resting potential is restored. This ensures the impulse only travels in one direction Nodes of Ranvier: Gaps at regular intervals in the myelin sheath on the axon. They are the sites of depolarisation Saltatory conduction: The action potential jumps across the axon to each node of Ranvier, making the impulse travel faster Synapse: Where two neurones meet Synaptic cleft: The gap between the two meeting neurones Presynaptic neurone: Has Ca+ ion channels and neurotransmitter vesicles. It is the stimulating neurone that passes on the impulse Postsynaptic membrane: Receives neurotransmitter and impulse, it fires off another impulse Synaptic vesicles: Contain the neurotransmitter to depolarise the membrane Neurotransmitter: When Ca+ ions enter the presynaptic membrane, vesicles fuse with the membrane & release neurotransmitter into synaptic cleft. They bind with channels on the postsynaptic membrane Acetylcholine: A neurotransmitter, the first to be discovered Summation: Each impulse adds to the effect of the others Spatial summation: Impulses from different neurones Temporal summation: Several impulses along one neurone Inhibitory synapses: Make it less likely that an impulse will be fired in the postsynaptic neurone by allowing it to hyperpolarise by letting Cl- ions in and allowing K+ ions out Excitatory synapses: Make the postsynaptic neurone more permeable to Na+ ions meaning it is more likely that depolarisation will occur and lead to an action potential
