9.5 - Nervous transmission

What is a neurone?

A nerve cell that is specially adapted to rapidly carry electrical nerve impulses from one part of the body to another

What are the functions & structures of the three types of neurones?

Sensory neurone:

• carry impulses from receptors to the brain & spinal cord in the CNS

• have a cell body that branches in the middle of the axon & dendrites connecting to receptor cells

Relay neurone:

• carry impulses within the CNS & connect sensory neurones to motor neurones

• have short neurones with axons & highly branched dendrites

Motor neurone:

• carry impulses from the CNS to effector muscles or glands

• have a large cell body at one end that lies within the spinal cord or brain & many highly-branched dendrites extending from the cell body

Label this diagram

What is a cell body?

Contains the nucleus & large amounts of rough endoplasmic reticulum → needed to make proteins (i.e. neurotransmitters)

What are dendrons?

A long extension of a neurone that carries impulses towards the cell body & branches into dendrites

What is an axon?

A single long fibre that carries nerve impulses away from the cell body

What are Schwann cells?

• Surround the axon, protecting it & providing electrical insulation → also carry out phagocytosis & play a part in nerve regeneration

• Wrap themselves around the axon many times, so layer of the membranes build up around it

What is the myelin sheath?

A fatty insulating layer that surrounds the axon of a neurone, made of the membranes of the Schwann cells

What are nodes of Ranvier?

Gaps between adjacent Schwann cells where there is no myelin sheath

What is the name of the plasma (cell surface) membranes of a neurone?

Plasmalemma

What is the definition of resting membrane potential?

The potential difference (difference in electrical charge/voltage) across the axon of a neurone when the neurone is not transmitting an impulse:

• the cytoplasm inside the axon of the neurone is negatively charged relative to the tissue fluid outside of the neurone

• the inside of the axon has a voltage of -70mV relative to the outside

Outline how resting membrane potential is maintained

1. The Na⁺/K⁺ ATPase pump moves 3 Na⁺ ions out of the axon for every 2 K⁺ ions that it pumps back into the axon → this process requires energy from ATP from respiration

2. The axon membrane is far more permeable to K⁺ ions than Na⁺ ions as the membrane has more intrinsic leakage protein channels for K⁺ than for Na⁺

3. This means that K⁺ ions can diffuse out of the neurone faster than Na⁺ ions can diffuse back in

4. The intrinsic leakage protein channels ensure, that in the absence of a stimulus, there are always more positively charged ions outside the axon than inside → in the resting state, the axon membrane is said to be polarised

What is the definition of an action potential?

A brief reversal in the electrical charge across the membrane of a neurone axon when it is stimulated & a nerve impulse passes

What is the definition of depolarisation?

A temporary reversal of the charges on the cell surface membrane of a neurone that occurs when a nerve impulse is being transmitted (i.e. positive on the inside relative to the outside)

What is the definition of repolarisation?

The return of the resting membrane potential of the axon of a neurone after an action potential

Outline the production of an action potential

1. At resting potential, Na⁺ ion concentration is higher outside the axon & K⁺ ion concentration is higher inside. The overall concentration of cations is greater outside, making the outside positive relative to the inside (i.e. the membrane is polarised)

2. When a stimulus occurs, some voltage-gated Na⁺ ion channels open & Na⁺ ion diffuse into the axon down its concentration gradient → reduces the negativity inside

3. If the threshold value (~ -55 mV) is reached, more Na⁺ ion channels open via positive feedback, causing rapid Na⁺ ion influx & depolarisation to ~ +40 mV → reversal of charge creates an action potential

4. After the action potential is established, Na⁺ ion channels close & voltage-gated K⁺ ion channels open. K⁺ diffuses out down its concentration gradient, repolarising the membrane → continued K⁺ ion influx causes hyperpolarisation (i.e. below -70 mV)

5. Finally, K⁺ ion channels close & the resting membrane potential is restored by the Na⁺/K⁺ ATPase pump & diffusion through permanently open channels, fully repolarising the axon.

How do impulses pass along myelinated & unmyelinated axons?

Once an action potential has been created, it continues along the length of an axon:

• nothing physically moves → it is simply that the reversal of electrical charge at depolarisation is reproduced at successive points along the axon membrane

• as one region depolarises & produces an action potential, it acts as a stimulus for the depolarisation of the next region of the axon

• the previous region of the membrane returns to its resting potential (i.e. is repolarised)

Outline the passage of an action potential along an unmyelinated neurone

Stage 1:

• At resting potential, the axon membrane is polarised

• High concentration of Na⁺ ions on outside; high concentration of K⁺ ions on inside

• But, more positive on outside than inside overall

Stage 2:

• A stimulus causes sudden influx of Na⁺ ions

• Charge on axon membrane reversed

• Membrane depolarises, leading to an action potential

Stage 3:

• Voltage-gated Na⁺ ions channels are now triggered to open a little further along the axon

• Na⁺ ions enter & depolarisation occurs here

• Behind this, the voltage-gated Na⁺ ions channels close & the K⁺ ones open → K⁺ ions leave the axon

Stage 4:

• The outward movement of K⁺ ions causes the initial region to repolarise

• The next region has become depolarised & this action potential is propagated (i.e. passed along) in the same way further along the neurone

Stage 5:

• Repolarisation means the neurone returns to its resting potential, ready for a new stimulus

Why do myelinated axons propagate impulses faster than unmyelinated axons?

Saltatory conduction:

• the impulse ‘jumps’ from one node of Ranvier to another

• depolarisation cannot occur where the myelin sheath acts as an electrical insulator → so, the impulse does not travel along the whole length of the axon

Outline the process of synaptic transmission

1. An action potential arrives at the presynaptic terminal & causes depolarisation of the axon membrane, which triggers the opening of Ca²⁺ gated ion channels, causing Ca²⁺ ions to diffuse into the presynaptic neurone

2. Vesicles of acetylcholine (i.e. neurotransmitter) move towards & fuse with the presynaptic membrane

3. Acetylcholine is hydrolysed by acetylcholinesterase (i.e. enzyme) & is broken down into acetic acid & choline, which are actively reabsorbed into the pre-synaptic neurone to be re-assembled into acetylcholine

4. Acetylcholine is released by exocytosis into the synaptic cleft & diffuses across down its concentration gradient. It then binds to specific complementary receptors on the postsynaptic membrane

5. A change occurs in the receptor which opens Na⁺ gated ion channels in the postsynaptic membrane, allowing acetylcholine to be released. Na⁺ ions diffuse into the postsynaptic neurone, down their concentration gradient, resulting in depolarisation of the membrane

6. If the threshold value is reached, an action potential is generated & propagated along the postsynaptic neurone

What is the all-or-nothing principle?

If a stimulus is too weak, then threshold potential will not be reached & there will be no action potential, while a stimulus that is strong enough for threshold potential to be reached will always result in an action potential:

• action potentials are always the same size (around +30mV)

• a strong or long-lasting stimulus will result in the generation of multiple action potentials in quick succession

  • stronger stimulus → high frequency of action potentials

  • weaker stimulus → lower frequency of action potentials

What are excitatory ion-channel synapses?

These synapses have neuroreceptors that are sodium (Na⁺) channels:

• when the channels open, positive Na⁺ ions diffuse in, causing a local depolarisation → excitatory postsynaptic potential (EPSP) & make an action potential more likely

• typical neurotransmitters in these synapses are acetylcholine & glutamate

What are inhibitory ion-channel synapses?

These synapses have neuroreceptors that are chloride (Cl^-) channels:

• when the channels open, negative Cl^- ions diffuse in, causing a local hyperpolarisation → inhibitory postsynaptic potential (IPSP) & make an action potential less likely

• with these synapses, an impulse in one neurone can inhibit an impulse in the next

• typical neurotransmitters in these synapses are glycine or GABA

What are non-channel synapses?

These synapses have neuroreceptors that are not channels at all, but instead are membrane-bound enzymes:

• when activated by the neurotransmitter, they catalyse the production of a ‘messenger chemical’ (e.g. Ca²⁺) inside the cell, which in turn, can affect many aspects of the cell’s metabolism

• they can alter the number & sensitivity of the ion channel receptors in the same cell → these synapses are involved in slow & long-lasting responses (e.g. learning & memory)

• typical neurotransmitters are adrenaline, noradrenaline, dopamine, serotonin & acetylcholine