A-Level Biology - 3.6.2 Nervous coordination

Draw the structure of a myelinated motor neurone

Name the parts of a myelinated motor neurone

cell body

dendrons

dendrites

axon

myelin sheath made of Schwann cells

nodes of Ranvier

What is the role of dendrons in a myelinated motor neurone?

transmit action potential towards cell body

divide into *dendrites*

What is the role of the axon in a myelinated motor neurone?

transmits action potential away from cell body

What is the structure and role of the myelin sheath in a myelinated motor neurone?

electrically insulated axon to speed up transmission of impulses

made up of *Schwann cells* wrapped around axon

Describe how a resting potential is established

sodium-potassium pump actively transports *3* Na⁺ ions *out* of the axon for every *2* K⁺ that are actively transported *in*

Higher concentration of K⁺ inside and higher concentration of Na⁺ ions outside produces an electrochemical gradient

Membrane more permeable to K⁺ ions leaving than Na⁺ ions entering (Na⁺ channels closed, K⁺ channels open)

K⁺ diffuses out down concentration gradient, generating resting potential of -70mV

Describe how an action potential is generated

when a stimulus reaches the threshold value, *voltage-gated sodium ion channels* open

Na⁺ ions diffuse into axon down electrochemical gradient, causing more Na⁺ V-G channels to open by positive feedback

increase in Na⁺ ions inside axon produces charge of +40mV

axon is depolarised

Describe how repolarisation and hyperpolarisation happens in the axon

once action potential of +40mV is established, Na⁺ V-G close and K⁺ V-G open

K⁺ diffuse out, repolarising axon

temporary overshoot of electrical gradient (too many K⁺ diffuse out) causes inside of axon to be relatively more negative

V-G K⁺ channels close and Na⁺/K⁺ pump restores resting potential

What is the all or nothing principle?

An action potential is only generated when threshold value is reached

*size of stimulus* does not affect *size of action potential*

but stronger stimuli more likely to reach threshold value

How do organisms perceive larger stimuli?

larger stimulus increases *frequency* of action potentials

does NOT cause larger action potentials

Give two roles of the refractory period

allows discrete impulses to be produced

limits the frequency of impulse transmission

What is the refractory period?

after an action potential, another action potential can't be produced immediately

due to temporary inactivation of the Na+ channels.

Name three factors affecting speed of conductance

myelination and saltatory conduction

axon diameter

temperature

How does temperature affect speed of conductance?

Higher temperature = faster speed

Faster rate of diffusion of ions

optimum temperatures required for enzymes involved in active transport (sodium-potassium pump)

Temperature too high = membrane proteins denature.

How does axon diameter affect speed of conductance?

greater diameter = faster speed

less leakage of ions makes it easier to maintain membrane potential

How does myelination and saltatory conduction affect speed of conductance?

Myelination increases speed of conductance

Myelination provides electrical insulation

In myelinated saltatory conduction (depolarisation at nodes of Ranvier)

In non-myelinated, depolarisation occurs along whole length of axon

What is saltatory conduction?

Myelin sheath around axon act as an electrical insulator preventing action potentials from forming

at regular intervals, there are nodes of Ranvier (breaks in myelin sheath) where action potentials can occur

action potentials jump from node to node

action potentials pass faster along myelinated neurons

because depolarisation occurs along the whole axon in unmyelinated neurons

Draw the structure of a synapse

Describe the structure of a synapse

Presynaptic neuron:

-ends in synaptic knob

-has pre-synaptic membrane

has mitochondria, ER and vesicles containing neurotransmitter

synaptic cleft (gap between neurons)

Postsynaptic neuron:

-has complementary receptors to neurotransmitter

-has post-synaptic membrane

Name the neurotransmitter used in a cholinergic synapse

acetylcholine

Describe the sequence of events involved in transmission across a cholinergic synapse

1) Depolarisation of presynaptic membrane

2) Calcium channels open and calcium ions enter synaptic knob

3) Calcium ions cause synaptic vesicles to fuse with presynaptic membrane and release acetylcholine

4) Acetylcholine diffuses across synaptic cleft

5) Acetylcholine attaches to receptors on the postsynaptic membrane;

6) Sodium channels open and sodium ions enter postsynaptic neurone, leading to depolarisation and action potential

Describe how a nerve impulse causes the release of neurotransmitter at a synapse

1) Nerve impulse causes Ca²⁺ channel proteins to open

2) Ca²⁺ enter by facilitated diffusion

3) Causes synaptic vesicles to fuse with presynaptic membrane

What is spatial summation at a synapse?

many different presynaptic neurones release enough neurotransmitter

to exceed threshold value of post-synaptic neurone

creates action potential

What is temporal summation at a synapse?

A single presynaptic neurone releases a high enough concentration of neurotransmitter in a short period

exceeds threshold value of post-synaptic neurone

creates action potential

Describe the unidirectionality of synapses

Neurotransmitter is only made in and released from the pre-synaptic neurone

Receptors are only present on the post-synaptic membrane

(so transmission can only happen in one direction across the synapse)

How do inhibitory synapses work?

cause hyperpolarisation in postsynaptic neurones (Cl- channels open and Cl- diffuse in, K+ channels open and K+ diffuse out)

1) Inside postsynaptic neurone is more negative - potential is below resting potential

2) More Na⁺ required to enter for depolarisation to reach threshold and produce action potential

Describe how the release of acetylcholine into a neuromuscular junction causes muscle contraction

1) Acetylcholine diffuses

across synaptic cleft

2) Acetylcholine binds to receptors on sarcolemma (post-synaptic membrane)

3) Na⁺ channels open and Na⁺ enter, leading to depolarisation and action potential

4) Ca²⁺ released by endoplasmic reticulum

Describe the structure of neuromuscular junctions

Presynaptic neuron:

-ends in synaptic knob

-has pre-synaptic membrane

has mitochondria, ER and vesicles containing neurotransmitter

synaptic cleft (gap between neurone and muscle)

muscle:

-has complementary receptors to neurotransmitter

-has sarcolemma (membrane)

Compare transmission across a cholinergic synapse and a neuromuscular junction

*cholinergic synapse:*

-excitatory or inhibitory

-links neurones to neurones or other effector organs

-new action potential may be produced along another neurone

-acetylcholine binds to receptors on membrane of postsynaptic neruone

*NM junction:*

-excitatory

-links neurones to muscles

-action potential ends here

- acetylcholine bind to receptors on membrane of muscle fibre

Drugs that stimulate the nervous system are called:

agonists

Drugs that inhibit the nervous system are called:

antagonists