The nervous system

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  • Created by: Bethfr
  • Created on: 01-06-15 21:30

The axon

The fatty myelin sheath electrically insulates axons, and is formed by Schwann cells by myelination: the membrane of the Scwann cell becomes extended and wraps around the axon. The combination of the membrane phospholipids and the myelin insulates the neurone by preventing ion movement in and out.

There are small gaps between the Schwann cells where the axon is exposed and ion movement can occur - the Nodes of Ranvier. Only at these points can local circuits be set up. This effectively increases the disctance over which local currents can bring about depolarisation --- it causes the action potential to jump from node to node by saltatory conduction, increasing speed of transmission.

- Temperature increases until 40C, as it increases rate of respiration so more ATP - generation of nerve impulses is an active process

- In non myelinated axons, transmission speed faster if axon diamter larger, as then their is less longitudinal resistance of the cytoplasm/axoplasm, increasing local electrical circuit length.

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The resting neurone

At rest, the axon membrane is more permeable to K+ than Na+ as K+ channel proteins are more abundant.

The sodium-potassium pump actively transports 3 Na+ ions out of the neruone for every 2 K+ pumped in. This results in concentration gradients across the membrane.

Due to this concentration gradient, K+ ions try to diffuse out and Na+ try to diffuse in. As the membrane is more permable to K+, the K+ ions rapidly diffuse out of the neruone, but Na+ only slowly diffuse in.

Overall outward movement of ions creates potential difference across the membrane: inside of axon is negatively charged relative to the outisde: membrane is polarised. Resting potential is -70mV.

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The active neurone

When the neurone is stimulated, voltage-gated Na+ channels open, increasing membrane permeability to Na+, Na+ rapidly diffuse in, depolarising the membrane.

Potential difference across the membrane briefly reversed to +40mV on the inside.

This change in polarity is the action potential.

At the end of depolarisation, voltage gated Na+ channels close, decreasing membrane permeability to Na+. The repolarisation of the membrane opens the voltage gated Na+ channels, increasing permability of membrane to K+, increasing their rate of outward diffusion.

This causes the potential difference of the membrane to overshoot to -80mV - the neurone is hyperpolarised. This closes the voltage-gated K+ channels, and the sodium-potassium pump restores the membrane's resting potential.

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The refractory period

Brief period after an action potential.

Voltage gated Na+ channels inactivated, preventing inward movement of Na+ ions.

Therefore, another action potential cannot be generated - the refractory period.

Ensures that an action potential can only be propogated in one direction.

Ensures action potentials are seperate, limiting frequency of action potentials along a neurone

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The threshold

For an action potential to be generated, depolarisation must exceed the threshold level which is 10-15mV above resting potential, so about -60mV. 

Threshold stimulus=stimulus sufficent to generate an action potential

Sub-threshold stimuluis= stimulus that fails to generate an action potential

Above the threshold, size or strength of action potential is the same irrespective of the size or strength of the stimulus. This is the all-or-nothing law.

The greater the intensity of the stimulus, the more frequent the action potentials.

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Transmission of a nerve impulse along an axon

1. When the neurone is stimulated, Na+ ions rush into the axon, depolarising the membrane.

2. The action potential causes local electrical circuits to be established, causing voltage gated Na+ channels to open in the adjacent part of the membrane - Na+ ions rush in, depolarising that section of the membrane.

3. Behind the impulse, K+ ions begin to leave, repolarising the neurone behind the impulse. Also, the voltage gated Na+ ions are inactivated behind the impulse. By the time the refractory period is over, the action potential is too far down the axon for its local electrical circuit to affect the first section. This ensures impulses travel only in one direction.

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Synapse structure

20nm junction between two neruones.

Impulse transmitted between two neurones by chemical transmitters e.g. noradrenaline

At the end of a neruone, the axon swells to form a synaptic knob.

Contains synpatic vesicles, which contain neurotransmitter

Contains many mitochondria to produce ATP for resynthesis of neurotransmitter and also for the sodium-potassium pump.

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Transmission across a synapse

1. Nerve impulse depolarises the presynaptic membrane, causing voltage gated Ca+ channels to open. Calcium ions diffuse into the presynaptic neurone, causing synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synpatic cleft by exocytosis.

2. Aceylcholine diffuses across synaptic cleft, where it binds with specific receptor proteins on the post synaptic membrane. The receptor proteins are attached to gated Na+ channels: binding of acteylcholine causes protein to change shape, opening the Na+ channels.

3. Na+ ions diffuse into postsynpatic neruone. If threshold is reached, action potential generated in postsynaptic neuone,

4. Acetylcholinesterase on the postsynaptic neurone hydrolyses acetylcholine into choline and ethanoic acid, preventing successive impulses merging at the synapse. The products diffuse back across the synaptic cleft and are actively transported back into the synpatic knob of the presynpatic neuone. 

5. Energy from ATP hydrolysis is used to reform acetylcholine, which is then stored in synpatic vesicles.

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Synapse function

Transmit impulses in one direction only (neurotransmitter only released from presynaptic, recpetors only on postsynaptic, neurotransmitter diffuses from pre to postsynaptic)

Filter out low level stimuli - if low intensity threshold stimulus, then low frequency of impulses, so little neurotransmitter released, opens few Na+ channels on postsynaptic neurone - may be insufficient to exceed threshold value, so no action potential generated in postsynaptic neurone.

Acts as junctions/allow summatiom to occur - high frequency of impulses arrive at synapse, lots of neurotransmitter released, so many Na+ channels open on postsynpatic, generating action potential in post synaptic.

Temporal summation - high frequency impulses from same presynaptic neurone

Spatial summation - several impulses arrive at once from several different presynaptics

Prevent passage of impulses between neurones e.g. inhibitory synapses cause hyperpolarisation in the postsynaptic neruone, so no action potential can be generated e.g dont drop the hot glass dish, more harm than a burn, place it down.

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Drugs and synaptic transmission

Excitatory drugs: similar shape and effect as neurotransmitter, or may inhibit enzyme that breaks down neurotransmitter e.g organophosphorus insecticides inhibit cholinesterase so acetylcholine remains bound to postsynaptic neurone... agonists bring about continous stimulation of postsynaptic neurone/muscle. Paralysis or death due to continous contraction of cardiac/intercostal muscles.

Inhibitory drugs bind to and block the receptors on postsynaptic e,g, beta blockers, preventing action of neurotransmitter as cant bind. Paralysis or death due to inability of muscles to contract

Psychoactive drugs alter brain function, temporarily effect perception, mood, consciousness and behaviour, e.g. cocain - excitatory drug that prevents normal reuptake of dopamine neurotransmitter, so that it accumulates in synapses, causing repeated action potentials in postsynaptic neruone - dopamine stimulates pleasure centres in the brain, giving happiness, but cocain - overstimulation - euphoria.

Active ingredient of marijuana, THC is an inhibitory drug that binds to receptors in the presynatpic neuone, preventing the release of neurotransmitter, preventing stimulation of postysnatpic, making users feel relaxed and calm.

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Reflexes

A reflex is a rapid, automatic, involuntary response to a particular stimulus. Not under conscious control of the brain. Usually have a protective function.

Reflex arc is the neurones forming the pathway taken by the nerve impulses in a reflex action....

1. Heat from hot place detected by heat receptor in finger. Action potential generated, impulse transmitted along sensory neurone to spinal cord via the dorsal root (cell body in ganglion).

3. Sensory neurone synapses with relay neurone in grey matter of spinal cord. Neurotransmitter rleased to generate action potential. This is repeated at the relay neurone:motor neuone synapse still within the grey matter.

4. Action potnetial transmitted along motor neurone via ventral root to the bicep muslces. Release of acetylcholine causes bicep to contract, resulting in automatic withdrawal of hand.

However, in the grey matter, the sensory neurone may snapses with another neurone transmitting impulses to the brain. Info recieved can be stored, to modify the response. Brain may relay this info the other receptors e.g. the eyes - if its a glass dish, the brain makes a conscious decision not to drop it, as it would cause furthe harm, so transmits impulses down the spinal cord via neuones that terminate in inhibitory synapses, so not immediately dropped.

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Nerve nets

e.g. Hydra - no recognisable brain or true muscles

Nerve net is a simple system composed of photoreceptor and touch-sensitive nerve cells in the body wall and tentacles - hence only respond to 2 stimuli - light intensity and touch. Hence, small number of effectors.

The nerve net is simple nerve cells with short extensions joined to each other and branching in many directions... hence, transmission is low. 

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