NERVOUS COORDINATION- nerve impulses & synaptic transmission

🔹 1. The structure of a myelinated motor neurone

What is the function of the cell body in a motor neurone?

- contains organelles (e.g nucleus and ribosomes)

- responsible for producing proteins and neurotransmitters.

What do dendrites do?

- carry action potentials from other neurons toward the cell body.

What is the axon and what does it do?

- long fiber

- conducts nerve impulses

- away from the cell body

- toward the effector or next neuron.

What are Schwann cells and what do they form?

- wrap around the axon

- form the myelin sheath (fatty lipid layer) that insulates the axon.

What is the function of the myelin sheath?

- insulates the axon

- preventing ion movement across the membrane in those areas

- speeding up conduction.

What are the gaps between Schwann cells called?

- Nodes of Ranvier

- gaps in the myelin sheath

- ion movement and action potential regeneration occur.

🔹 2. The establishment of a resting potential

What is a resting potential?

- difference in electrical charge across the neurone membrane when not transmitting an impulse (approx. -70mV inside relative to outside).

What maintains the resting potential?

- sodium-potassium pump (active transport using ATP)

- pumps 3 Na⁺ ions out and 2 K⁺ ions in

- 3 NAH IM OUT, 2 KOOL IM IN

Why is the inside of the axon more negative than the outside?

- More positive ions (Na⁺) pumped out than K⁺ pumped in

- K⁺ diffuses out more easily through permanently open K⁺ channels

- creating an electrochemical gradient.

What is the role of membrane permeability in maintaining the resting potential?

- membrane is more permeable to K⁺ than Na⁺

- more K⁺ leaves the axon

- makes the inside relatively negative.

🔹 3. Changes in membrane permeability: Depolarisation and the all-or-nothing principle

What happens when a stimulus reaches a neurone?

- opens voltage-gated sodium channels

- allowing Na⁺ to enter

- causing depolarisation (membrane becomes more positive).

What is the threshold potential?

- minus 55mV

- if the membrane potential reaches this value, an action potential is triggered.

What is depolarisation?

- inside of the membrane becomes less negative

- when Na⁺ floods in.

What happens during repolarisation?

- Sodium channels close

- potassium channels open

- K⁺ exits

- restoring the negative potential inside.

What is hyperpolarisation?

- membrane becomes even more negative than resting potentia

- minus 80mV

- due to excess K⁺ leaving.

What is the all-or-nothing principle?

- If the threshold is reached

- an action potential always fires

- of same size and intensity

- If not, no action potential occurs.

What determines the strength of a stimulus in the nervous system?

- frequency of action potentials

- NOT THEIR SIZE

🔹 4. Passage of an action potential along non-myelinated and myelinated axons

How is an action potential propagated in a myelinated neurone?

- By saltatory conduction

- impulse jumps from one Node of Ranvier to the next.

How is conduction different in non-myelinated neurones?

- action potential must be regenerated at every point along the axon

- makes conduction slower.

Why does saltatory conduction increase speed?

- fewer action potentials need to be generated

- impulses jump over myelinated sections.

🔹 5. Refractory period: nature and importance

What is the refractory period?

- short time after an action potential

- during this another cannot be generated

- (due to sodium channels being inactive)

What are the three main roles of the refractory period?

- Ensures unidirectional transmission (action potentials only move forward_

- Produces discrete impulses (each AP is separate)

- Limits frequency of action potentials (prevents overstimulation)

🔹 6. Factors affecting speed of conductance

How does myelination affect conduction speed?

- Increases speed via saltatory conduction

- impulse jumps between nodes

- not across entire axon.

How does axon diameter affect speed?

- Wider axons have less ion leakage

- leading to faster impulse conduction.

How does temperature affect speed?

- Higher temperature = faster diffusion of ions (more kinetic energy).

- Increased enzyme activity for ATP production (sodium-potassium pump).

- Too high temperature can denature enzymes → decreased speed.

What is a synapse?

- gap between two neurons

- allows the transmission of signals via neurotransmitters.

What happens when an action potential reaches the synaptic knob?

- Depolarisation of the synaptic knob opens calcium ion channels

- causing calcium ions to diffuse into the synaptic knob.

What do calcium ions do in the synaptic knob?

- :cause vesicles containing neurotransmitters to move to the presynaptic membrane

- fuse with it

- release neurotransmitters into the synaptic cleft.

How do neurotransmitters cross the synapse?

- diffuse across the synaptic cleft

- from high to low concentration

- to the postsynaptic membrane.

What happens when neurotransmitters bind to receptors on the postsynaptic membrane?

- cause sodium ion channels to open

- allows sodium ions to diffuse in

- triggering an action potential.

What ensures the unidirectionality of a synapse?

- Neurotransmitter vesicles are only in the presynaptic neuron

- receptors are only on the postsynaptic membrane

- transmission only occurs in one direction.

What is a cholinergic synapse?

- a synapse

- uses acetylcholine as its neurotransmitter.

What happens to acetylcholine after it binds to receptors?

- broken down by the enzyme acetylcholinesterase

- into choline and acetate

- prevent continuous stimulation.

What is summation in synapses?

-buildup of neurotransmitter

- to reach threshold

- to trigger an action potential.

What is spatial summation?

- Multiple presynaptic neurons

- release neurotransmitters

- to the same postsynaptic neuron

- to trigger an action potential.

What is temporal summation?

- :One presynaptic neuron releases neurotransmitters repeatedly

- over a short time

- cause enough buildup to trigger an action potential.

What is an inhibitory synapse?

- synapse

- prevents action potentials

- causing chloride ions to enter the postsynaptic neuron

- leading to hyperpolarisation.

What happens in hyperpolarisation?

- Chloride ions enter the postsynaptic membrane

- reducing membrane potential to around -80 mV

- making it less likely to fire an action potential.

What is a neuromuscular junction?

- synapse between a motor neuron and a muscle fibre

- allows the muscle to contract as a response.

What are similarities between cholinergic synapses and neuromuscular junctions?

-Both are unidirectional

- both involve acetylcholine binding to complementary receptors to initiate a response.

What are differences between cholinergic synapses and neuromuscular junctions?

- Neuromuscular junctions are always excitatory

- connect neurons to muscles

- cholinergic synapses can be excitatory or inhibitory

- connect neurons to neurons.

What receptors are involved in neuromuscular junctions?

- Acetylcholine binds to receptors on the muscle fibre membrane

- triggering contraction.

Why is it important for acetylcholine to be broken down quickly?

- prevent continuous stimulation of the postsynaptic membrane

- prevent ongoing muscle contraction/nerve impulse.

How could a drug affect synaptic transmission?

- drugs can mimic, block, or inhibit neurotransmitters or their enzymes

- reducing the likelihood of action potential generation.

Why do inhibitory synapses prevent an action potential?

- cause hyperpolarisation

- allowing negative ions (e.g., chloride) in

- making it harder to reach the threshold for firing.