What is a stimulus? A2 Unit 5: Survival and response Organisms increase their chances of survival by responding to changes in their environment.

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1 A2 Unit 5: Survival and response Organisms increase their chances of survival by responding to changes in their environment. What is a stimulus? A stimulus is a change in the internal or external environment of an organism that produces a response. Since survival often requires rapid action, responses are often under nervous control. The pathway for a typical nervous response is. Stimulus Receptor Coordinator Effector Response Taxes and kinesis: Both are simple responses that maintain a mobile organism in a favourable environment. Taxes A taxis is a simple response whose direction is determined by the direction of the stimulus. (A directional response to a directional stimulus). A positive taxis involves moving towards the stimulus. A negative taxis involves movement away from the stimulus. Examples Algae moving towards light (positive phototaxis). This aids survival, by increasing glucose production by photosynthesis. Earthworms moving away from light (negative phototaxis). This aids survival by avoiding predators, reducing water loss etc. Bacteria moving towards food/glucose (positive chemotaxis). This aids survival by finding a food source. Kineses A kinesis is a random movement in response to a non-directional stimulus (eg humidity), which helps an organism return to a more favourable environment. Often the more intense the stimulus, the greater the rate of random movement becomes. Example: woodlice move rapidly in any direction, turning frequently when in a dry environment. This helps increase their chances of finding a moist habitat, helping reduce water loss and increase their survival chances.

2 Label the motor neurone and add an arrow to show the direction a nerve impulse travels Cell body/nucleus Myelin sheath axon Reflex arc A simple reflex arc typically involves just 3 neurones. A person is hit below the knee (P). On the reflex arc diagram identify A, B, C, + label the relay/intermediate neurone and effector. Relay neurone Dendrites Node of ranvier C= Spinal cord (grey matter) Simple Reflexes Reflexes are rapid, involuntary actions and often help the body to avoid damage. Suggest two reasons why a typical reflex response is able to be rapid. The slowest part of any nervous pathway involves transmission of the message across a synapse. With just 3 neurones there are only 2 synapses involved making the pathway short and fast. Simple reflex responses are involuntary, so no time is wasted waiting for decisions/responses from the brain. B= Motor neurone A = sensory neurone effector

3 Resting potential At rest, there is a difference in charge (potential difference) between the inside and outside of a neurone. In most cases the size of the resting potential is about -70 mv. The resting potential is established by the use of a sodium-potassium ion pump and a combination of ion channels. The sodium-potassium ion pump uses active transport to move sodium ions out of the axon, whilst potassium ions are pumped in the opposite direction. However, the axon membrane is more permeable to potassium ions than sodium ions because open potassium ion channels allow the diffusion of more potassium ions out of the axon. With fewer positive ions inside the axon than outside, there is an electrochemical gradient across the membrane.

4 Action potential When a nerve impulse arrives changes in membrane permeability lead to depolarisation and the generation of an action potential. The main events are as follows.. a. Sodium ion channels open allowing sodium ions to diffuse into the axon. b. This causes depolarisation. c. When the sodium ion concentration reaches a threshold level more sodium ion channels open, resulting in a positive feedback effect. d. Once the action potential is reached (+40 mv), sodium ion channels close and potassium ion channels open. e. The outflow of potassium ions from the axon causes repolarisation g. The axon briefly becomes hyperpolarised during the refractory period. h. The resting potential is then restored using the Na+ and K+ ion pump together with facilitated diffusion. Identify the stages in the action potential shown in the diagrams below. Which number on the graph might each represent? Repolarisation (5) Depolarisation (2 +3)

5 Movement of an action potential along a neurone Non-myelinated neurones (continuous conductance) When an action potential occurs, some sodium ions that enter the neurone diffuse sideways. This causes gated sodium ion channels in the adjacent region of the neurone to open. Sodium ions diffuse into that part of the neurone and in this way the wave of depolarisation spreads along the neurone. After the impulse has passed a section of neurone, repolarisation occurs and a resting potential is quickly established so that another nerve impulse can follow if required. Myelinated neurones In myelinated neurones ion movement and hence depolarisation/action potentials/repolarisation can only occur at the nodes of Ranvier (as the myelin sheath insulates against ion movement). Impulses effectively jump from one node to the next in a process called saltatory conduction. This speeds up nerve impulses.

6 All-or-nothing principle All action potentials are the same size and duration, providing the stimulus exceeds a minimum threshold level. Hyperpolarisation During repolarisation gated potassium ion channels are open, whilst gated sodium ion channels are closed. This allows the flow of potassium ions out of the neurone. However, for a brief moment the potential difference drops too low (below -70 mv). This is called hyperpolarisation. Label on the graph where hyperpolarisation has occurred. The refractory period There is always a gap between nerve impulses. The minimum time between action potentials and hence nerve impulses is the refractory period. During this time the gated sodium ion channels stay closed allowing time for repolarisation to occur. This makes sure that impulses do not merge and remain discrete.

7 Factors affecting speed of conductance Axon diameter Nerve impulses travel faster along wider neurones. This is because they are able to maintain their resting potential more efficiently as a lower proportion of their ions leak in or out by diffusion. Temperature In mammals temperature rarely has any effect on the speed of nerve impulses because they are endotherms and have a fairly constant body temperature. In animals with myelinated neurones this is less of a problem and so they can have quite narrow neurones which are still able to conduct rapid impulses. Invertebrates have only non-myelinated neurones which are normally slow at conducting nerve impulses (see below). BUT, some invertebrates have adapted to have wide neurones to allow rapid reactions. eg giant axons in squid. However, in animals which are ectotherms, and whose body temperature varies with their environment, cold temperatures can significantly slow nerve impulses and hence increase their reaction times. Myelination The presence of a myelin sheath insulates neurones preventing ion movement except at gaps called nodes of Ranvier. This forces action potentials to jump from node to node in a process called salutatory conduction. This uses less energy than continuous conductance along a nonmyelinated neurone. In non-myelinated neurones nerve impulses have to travel the whole length of the axon membrane making it much slower. Suggest why saltatory conduction uses less energy than continuous conductance. In saltatory conduction ATP is only needed to restore the resting potential using the Na+/K+ ion pump at the nodes of Ranvier. In contrast ATP is required to restore the resting potential along the entire length of a non-myelinated neurone.

8 Synapses A synapse is a junction between two neurones. Electrical nerve impulses are unable to pass directly across a synaptic gap or cleft and neurotransmitters are released instead. Typical events at a synapse 1. Action potential reaches synaptic knob/presynaptic membrane 2. (gated) Calcium ion channels open. 3. Calcium ions diffuse into the synaptic knob. 3. Calcium ions trigger movement of vesicles to pre-synaptic membrane. 4. Neurotransmitter is released. 5. Neurotransmitter diffuses across synaptic cleft. 6. Neurotransmitter binds to receptors (proteins) on postsynaptic membrane. 7. Sodium ion channels open allowing these ions to diffuse into the axon, causing depolarisation and triggering an action potential (if the threshold is exceeded). Events after transmission across a synapse. Calcium ions are removed from the synaptic knob using active transport. The neurotransmitter is removed from receptors and broken down by enzymes (eg acetylcholinesterase breaks down acetylcholine). The breakdown products diffuse back into the synaptic knob and are the neurotransmitter is re-synthesised and then enters the vesicles ready for re-use. What is a cholinergic synapse? A synapse that uses acetylcholine as a neurotransmitter. (Most synapses outside the CNS are of this type). What is a neuromuscular junction? A synapse that occurs between a motor neurone and a muscle. (they use acetylcholine as a neurotransmitter).

9 Key aspects of synapses part 1 Unidirectionality Synapses make sure that nerve impulses are only able to travel in one direction along a neurone. Explain how a synapse is able to do this. Only the synaptic knob contains vesicles of neurotransmitter. Only the post synaptic membrane has the receptors to bind to neurotransmitters. Summation Often many neurones and their synapses meet at the same cell body of a neurone. Impulses arriving at a synapse do not always result in a impulse in the next neurone. Temporal summation: occurs when several impulses arrive within a short-time at a synapse. Together they release enough neurotransmitter to cause sufficient gated sodium ion channels to open on the postsynaptic membrane. Enough sodium ions enter the axon to exceed the threshold and trigger an action potential. Spatial summation: occurs when several nerves impulses arrive at the same time, causing sufficient release of neurotransmitter and depolarisation, to trigger an action potential. Label to show examples of the two types of summation Temporal. 3 impulses along P in quick succession trigger impulse in B Q spatial. 1 impulse from P and 1 from Q at the same time trigger impulse in B

10 Key aspects of synapses part 2 Inhibition All the synapses considered so far have been excitatory ones which are able to generate an action potential in a post synaptic neurone. But, some synapses are inhibitory synapses. These are able to stop nerve impulses from continuing along post synaptic neurones. Inhibition via gated potassium ion channels In some cases inhibition occurs when a neurotransmitter stimulates gated potassium ion channels to open. As potassium ions move out of the cell body of the post synaptic neurone the chances of the threshold for an action potential being reached is reduced. Excitatory and inhibitory synapses can both occur at the same cell body. If a nerve impulses arrives at the same time from both synapses the effect of sodium movement into the cell body is cancelled out by movement of potassium ions out, so no action potential is produced and inhibition occurs. Inhibition due to chloride ion movement Some neurotransmitters can cause chloride ion channels to open in the post synaptic membrane. This leads to diffusion of chloride ions into the axon making the resting potential more negative (hyperpolarisation). This makes an action potential less likely to occur. Neural circuits A neural circuit is a group of neurones which work together eg sensory, relay and motor neurones in a simple reflex arc. Inhibition is important in neural circuits. It enables specific pathways to be stimulated, while preventing random stimulation all over the body. For example in reflex actions neural pathways produce a response only where its needed eg only your arm muscle contracts when touching a hot object. What evidence is there that the membrane potential in neurone B was affected by an inhibitory synapse? The resting potential of the neurone got more negative (hyperpolarisation occurred) as more chloride ions entered the axon or potassium ions left.

11 Key aspects of synapses part 3 Drugs Some drugs have an effect on the body by interfering with the normal functions of synapses. Interfering with neurotransmitter receptors Some drugs are able to bind to neurotransmitter receptors on post synaptic membranes. Curare Curare is extracted from the bark of some Amazon forest trees and was traditionally used in poison darts. Curare binds to receptors on membranes of neuromuscular junctions at the ends of motor neurones. Acetylcholine is unable to bind to the blocked receptors, so impulses that pass down motor neurones fail to stimulate muscle contraction, paralysing prey. Inhibition of acetylcholinesterase Organophosphate insecticides and similar nerve gas agents (eg sarin) interfere with synapses by inhibiting the enzyme acetylcholinesterase. As a result acetylcholine remains attached to receptors and cannot be broken down. Continued stimulation of the postsynaptic membranes causes contraction of the muscles in uncontrollable spasms and eventually death. (Atropine is used as an antidote because it blocks ACh receptors). Painkillers Endorphins are neurotransmitters which naturally block the sensation of pain by binding to pain receptors, preventing action potentials along pain pathways. Morphine and heroin bind to the same receptors and act as painkillers. Other drugs bind to receptors and mimic the neurotransmitter stimulating the nervous system by creating more action potentials. There is lots of further useful information on the nervous system at websites such as biology mad.com p://biologymad.com/nervoussystem/ner voussystem.htm

12 Other neurotransmitters - The heart Acetylcholine is the neurotransmitter used at all neuromuscular junctions in skeletal muscles. It is also used by some neurones in the autonomic nervous system (ANS) that controls unconscious activities eg heartbeat. The rate of your heartbeat is regulated by the sino atrial node (SAN). It is connected to the nervous system by nerves from the parasympathetic and sympathetic nervous systems. Other neurotransmitters - The brain The brain has more than 50 known types of neurotransmitter! Some are excitatory, whilst others are inhibitory. Each functions by binding to specific receptors. Dopamine Dopamine is a neurotransmitter used in parts of the brain required with control of muscle actions. In people with Parkinson s disease neurones that produce dopamine degenerate and muscles become too tense resulting in trembling. Neurones from the parasympathetic nervous system produce acetylcholine, whilst neurones from the sympathetic nervous system produce a different neurotransmitter, noradrenaline. During exercise the sympathetic neurones produce noradrenaline in the SAN. This stimulates the SAN to increase the rate at which it sends electrical nerve impulses to the atria/ventricles, increasing cardiac output. After exercise parasympathetic neurones produce acetylcholine which lowers the rate at which the SAN produces nerve impulses, returning heart rate to normal. One Parkinson s drug acts as a dopamine mimic. It is able to bind to dopamine receptors and have the same effect as the natural neurotransmitter. Dopamine is also involved in schizophrenia. In some suffers there are an excess of dopamine receptors in the brain. How might a drug to prevent this type of schizophrenia work? What possible side effects might result? Bind to and block dopamine receptors, preventing dopamine from binding. Side effect may be loss of muscle control as found in Parkinson s