A Level Biology - Nervous System

A Level Biology - Nervous System
Covers:
Important definitions,
Neurone structure,
Action potential,
Refractory period,
Factors affecting nervous impulse,
Synapses,
Summation,
Drugs affecting synapses,
Cholinergic synapses

Transcript

1. The Nervous System
2. Important definitions • Absolute refractory period – 1st part of the period at which the membrane of an axon cannot produce an action potential • Acetylcholine – neurotransmitter at cholinergic synapses, common in vertebrate central nervous systems and at neurotransmitter junctions • Action potential – change which occurs in the electrical charge across the membrane of an axon when it is at stimulated and a nerve impulse occurs • Presynaptic neurone – neurone that releases neurotransmitters • Postsynaptic neurone – neurone that has receptors for neurotransmitters • Relative refractory period – 2nd part of the period at which it takes a stronger than usual stimulus to trigger an action potential • Resting potential – when a cell is at rest, a difference in electrical charge is maintained across the membrane of the axon • Saltatory conduction – occurs in myelinated axons, action potential jumps from one node of Ranvier to the next
3. Important definitions - continued • Summation – process of the build up of neurotransmitters in the synapse which triggers a new action potential in the postsynaptic neurone • Synapse – a point where the action of one neurone connects with the dendrite of another or with an effector • Unidirectional – movement in only one direction
4. Neurone structure (motor neurone) • Cell body – contains nucleus and large endoplasmic reticulum for protein and neurotransmitter production • Dendrons – extensions of cell body which branch into dendrites, carry nerve impulses towards the cell body • Axon – single, long fibre which carries impulses away from the cell body • Schwann cells – surrounds and protects axon, providing electrical insulation may carry out phagocytosis during nerve regeneration • Myelin sheath – lipid membrane around Schwann cells • Nodes of Ranvier – gaps between Schwann cells where there is no myelin sheath
5. Myelinated neurones • Quicker at carrying impulses • Saltatory conduction can occur • Increases speed of conduction Non-myelinated neurones • Slower at carrying impulses • No saltatory conduction • Slower conduction velocity
6. Types of neurone Neurone Image Features/function Motor neurone Long axons and short dendrites with cell body at one end, transmits impulses from an intermediate or sensory neurone to an effector Sensory neurone Short axon and long dendrites with cell body along the neurone, transmits impulses from a receptor to an intermediate or motor neurone Intermediate or relay neurone Transmit impulses between neurones and have numerous processes
7. All or nothing response • Action potentials have the same potential difference for the full distance, no matter how long the nerve fibre is • Strength of stimuli does not effect the impulse • It either reaches the threshold and triggers an impulse or it doesn’t
8. Action potential Time (s) Potential difference across axon membrane (mV) -70 0 +40 Stimulus Resting potential Hyperpolarisation Threshold Absolute refractory period Relative refractory period 1 2 3 4 5 6 7
9. Action potential - continued 1. Sodium-potassium pump establishes resting potential (3x Na+ out and 2x K+ in), Na+ voltage-gated channels closed, K+ voltage-gated channels open, K+ diffuses out 2. Energy from stimulus opens some Na+ voltage-gated channels, small amount of Na+ diffuses in 3. Remaining voltage-gated Na+ channels open and Na+ rapidly diffuses in (depolarisation) 4. Voltage-gated Na+ channels close 5. Voltage-gated K+ channels open and K+ rapidly diffuses out and down the electrochemical gradient (repolarisation) 6. More K+ diffuse out than Na+ diffuse in (hyperpolarisation) 7. Resting potential re-established by the sodium-potassium pump
10. Refractory period • After an action potential a neurone has a refractory period during which the neurone can resist another action potential • Absolute refractory period – the 1st part of the period in which the axon membrane can not produce another action potential • Relative refractory period – the 2nd part of the period in which it would take a stronger than usual stimulus to trigger another action potential
11. Factors affecting the speed of the nervous impulse • Myelin sheath – myelinated axons carry impulses much quicker than unmyelinated axons (from approx. 30m/s in unmyelinated to 90m/s in myelinated) • Diameter of axon – greater the diameter the faster the speed of conduction and less ion leakage, greater the volume relative to the membrane so more sodium ions can diffuse across resulting in quicker conduction • Temperature – higher the temperature, the quicker ions diffuse across the membrane and so the quicker the rate of conduction
12. Synapses and their functions • Where an axon of one neurone connects with the dendrite of another or an effector • Link neurones together and are important for coordinating activities • Neurotransmitters are chemicals which synapses use to transmit impulses from one neurone to another (presynaptic and postsynaptic) • Allows a number of simultaneous responses - one impulse along one neurone can be transmitted to a number of different neurones at one synapse • Produce a single response - a number of impulses can be combined at a synapse so allows stimuli from different receptors to interact • Unidirectional
13. Transmission across a synapse Depolarisation of presynaptic neurone Voltage-gated calcium ion channels open and calcium ions diffuse in Vesicles containing neurotransmitters bind to presynaptic membrane, exocytosis Neurotransmitters diffuse across the membrane Neurotransmitters bind to receptor before enzymes digest them in the synapse Depolarisation of postsynaptic neurone Synaptic bulb Synaptic cleft
14. Summation • Process of the build up of neurotransmitters in the synapse • Allows low frequency action potentials (produce insufficient amount of neurotransmitter) to trigger a new action potential in postsynaptic neurone • Spatial summation – number of different presynaptic neurones together release enough neurotransmitter to exceed the threshold of the postsynaptic neurone and trigger an action potential • Temporal summation – single presynaptic neurone releases neurotransmitter many times in a short period, total amount may exceed the threshold of the postsynaptic neurone and trigger an action potential
15. Drug effects on synapses Stimulate the nervous system > • Mimic the neurotransmitter • Stimulate the release of more neurotransmitters • Inhibit the enzyme that breaks down the neurotransmitter • Blocks reuptake of the neurotransmitter Inhibit the nervous system > • Inhibit the release of neurotransmitter • Blocks the receptors on sodium/potassium ion channels on postsynaptic neurone • Some neurotransmitters can inhibit themselves
16. Cholinergic synapses • Neurotransmitter is acetylcholine • Common in vertebrate central nervous systems and at neuromuscular junctions • Can be affected by: • Dart frog poison (blocks acetylcholine receptors and inhibits muscle contraction) • Anaesthesia (allows intubation and relaxes muscles) • Organophosphate insecticides (inhibits action of acetylcholinesterase, military nerve gas)
17. Hydra nervous system • Nerve net • 1 cell type • Rapid regeneration • No myelin sheath • Slow conduction speed • 5ms-1 approx. • Can regenerate neurones • Action potential can travel in any direction • Respond to limited number of stimuli • Small number of effectors Human nervous system • CNS • Many cell types • Very slow or no regeneration • Myelin sheath • Fast conduction speed • Up to 120ms-1 • Can not regenerate neurones • Action potential is unidirectional • Respond to many different stimuli • Large number of effectors