thebiotutor.com A2 Biology Unit 5 Responses, Nervous System & Muscles

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1 thebiotutor.com A2 Biology Unit 5 Responses, Nervous System & Muscles 1

2 Response Mechanism tropism Definition A growth movement of part of plant in response to a directional stimulus examples Positive: Negative: taxis A simple response whose direction is determined by the direction of the stimulus Positive: Negative: kinesis A response by random movement to an unpleasant stimulus. The more unpleasant, the more rapid Reflex arc An involuntary response to a See below Organisms must sense and respond to changes in the external environment in order to survive. 2

3 sensory stimulus brought about by nerves Annotate the diagram below to explain the following terms: stimulus, receptor, sensory neurone, intermediate neurone, motor neurone, effector, response Distinguish between 1. central and peripheral nervous systems 2. voluntary and autonomic nervous systems 3. sympathetic and parasympathetic nervous systems Control of the heart rate Annotate the diagram (from Toole and Toole) to explain how a) increased CO 2 levels and b) increased blood pressure result in a lower heart rate. 3

4 Receptors 1. Pacinian corpuscle It is specific: It responds to only one kind of stimulus, mechanical pressure 4

5 It is a transducer: It converts the energy of the stimulus into a generator potential Found deep in skin particularly fingers, soles of feet and genitalia, also joints, ligaments and tendons. Explain how it works (use the terms: stretch-mediated sodium channels, resting potential, generator potential, action potential.) 2. Rods and Cones in the retina. A generator potential is created by the break down of the visual pigment rhodopsin (in rods) or iodopsin (in cones) into two component parts. In the case of rhodopsin these are opsin (a protein) and retinal (a derivative of vitamin A). The pigment reforms in the dark. Label the diagram here then use the table which follows to compare the two in terms of visual acuity and sensitivity to light feature rods cones Colour Vision Sensitivity to light Visual Acuity Distribution in the retina 5

6 Principles of coordination Nervous system: Hormonal system nerve cells transmit electrical impulses Stimulate target cells with neurotransmitter substances Response is short lived Use chemical messengers (hormones) Travel in blood system Stimulate specific target cells Longer term response Chemical mediators Histamine and prostaglandins released by specific cells in response to injury affect cells in local area cause increase in permeability of capillaries Plant growth factors IAA and phototropism diffuse from growing areas to tissues enable response to light, gravity and water control seasonal events, e.g. flowering and leaf fall IAA (indoleacetic acid) causes plant cells to elongate by loosening the bonds between cellulose microfibrils. It has many effects. Phototropism is only one. It is the growth of a stem towards a light source (positive photropism) 6

7 Nerve impulses Label the following structures: cell body, dendrons, axon, Schwann cell, myelin sheath, node of Ranvier The nerve impulse is Resting potential a self-propagating wave of electrical disturbance that moves across the surface of the nerve membrane The inside of the nerve membrane is negatively charged relative to the outside (about -7mv) Explain how it is created by gated channels and a sodium potassium exchange pump. 7

8 Action potential: a temporary reversal in charge (depolarisation) across the membrane (about +30 mv) Depolarisation results from a stimulus which changes the shape of the gated channels. They are therefore called voltage gated channels. Summary of events of action potential stimulus causes Na + voltage gates to open Na + ions stream into axon along their electrochemical gradient Depolarisation occurs as axon is more positive inside than out Na+ gates close and K+ gates open K+ ions flood out causing an overshoot Resting potential is re-established 8

9 Movement of action potential Why does it move? (localised currents) What s the difference between myelinated and non-myelinated nerve? (saltatory conduction) What other factors might influence the speed of conduction? Why does it only move in one direction? (refractory period) What s meant by all or nothing principle? The synapse 9

10 Summarise the events described in the diagram above (see p179 Toole and Toole) Explain the following features of synapses: Unidirectionality Spacial summation Temporal summation Inhibitory synapse Transmitters and drugs 10

11 The most common transmitter is acetylcholine. This type of synapse is called cholinergic and the post-synaptic membrane will be fitted with acetylcholine receptors. The enzyme breaking it down will be cholinesterase. There are other transmitters, some are excitatory (inducing an action potential), and some are inhibitory. In all cases, the action depends on the presence of specific receptor molecules and enzymes. Exam questions are likely to focus on these issues, possibly taking the example of a drug. e.g. morphine blocks receptors responsible for the transmission of pain impulses; some snake venom and natural insecticides contain a competitive inhibitor of cholinesterase. The effect of constant synaptic transmission is muscle spasm and paralysis. The neuromuscular junction Motor neurones terminate at a neuromuscular junction. The structure is very similar to the synapse and it works the same way with a transmitter substance in this case binding to receptor sites on the membranes of muscle fibres, triggering muscle contraction (see next topic notes). Muscle structure 11

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13 Sliding filament theory In a resting muscle, the actin / myosin binding sites are blocked by another protein called When is released at the.junction, it binds to on the muscle fibre membrane and causes ions to be released from the endoplasmic reticulum deep in the muscle cytoplasm (sarcoplasm). This unblocks the binding sites. The myosin heads with ADP attached bind with actin sites. The myosin heads change their shape, ADP is released and the cross bridge contracts to pull one filament over the other, shortening the muscle fibre. A molecule of now attaches to each myosin head and is to ADP. This process is catalysed by. Which is itself activated by..ions. The energy released is used to return the.. head to its original position. When nervous stimulation ceases, ions are actively transported back into the. And the binding sites are blocked again by. Energy supply for muscle contraction ATP is used for the movement of the myosin heads and for the active transport of Ca+ ions. When oxygen is in short supply (e.g. in energetic exercise) the aerobic supply of ATP is not sufficient. Muscle cells store a substance called phosphocreatine In anaerobic conditions it splits to phosphate and creatine it releasing energy and phosphate for ATP manufacture. NB it is not directly used as an energy resource. Fast or slow twitch muscles? Feature Fast twitch Slow twitch Speed of muscle contraction Used mainly for Found in 13