6B Nervous Co-ordination

Nerve Impulses and Synaptic Transmission

What is the role of sensory neurones?

Transmit impulses from sensory receptors to the central nervous system

What is the role of motor neurones?

Transmit impulses from the central nervous system to effectors (muscles or glands)

What is the role of relay neurones?

Connect sensory and motor neurones within the central nervous system

What is the myelin sheath?

A fatty insulating layer surrounding the axon, formed by Schwann cells

What is the function of the myelin sheath?

Provides electrical insulation and speeds up impulse conduction by saltatory conduction

What are the nodes of Ranvier?

Gaps between Schwann cells where the axon membrane is exposed

What is saltatory conduction?

Impulse jumps from node to node, speeding up transmission

What is a nerve impulse?

An action potential transmitted along the axon of a neurone

What is resting potential?

The difference in electrical charge across the axon membrane when a neurone is not conducting an impulse, typically -70mV

How is resting potential maintained?

By the sodium-potassium pump and open potassium ion channels

Describe the role of the sodium-potassium pump.

Actively transports 3Na⁺ out and 2K⁺ into neurone cell, maintaining concentration gradients

Describe the role of potassium ion channels during resting potential

Allow K⁺ to diffuse out of the axon down its concentration gradient, making the inside more negative after it[‘s been pushed inside by the pump

What two things cause the inside of the axon to be negative at resting potential?

Sodium-potassium pump and potassium ion channels being kept open

What happens in axon when a stimulus is received?

Voltage-gated sodium channels open, allowing Na⁺ to diffuse rapidly into the axon

What is threshold potential?

Approximately -55mV; once reached, an action potential is triggered

What happens during depolarisation?

Voltage-gated sodium channels open, Na⁺ floods in, making the inside positive (+40mV) and reach threshold value (-55mV)

What happens during repolarisation?

Voltage-gated sodium channels close; voltage-gated potassium channels open, K⁺ diffuses out, making the inside negative again

Why is there a refractory period?

Voltage-gated sodium and potassium channels are temporarily inactivated and recoverig, preventing another action potential immediately

What is hyperpolarisation?

Membrane potential becomes more negative than resting potential (e.g. -80mV) due to slow closing of potassium channels

How is resting potential restored after hyperpolarisation?

The sodium-potassium pump gets back to work and restores ion concentrations

What is the all-or-nothing principle?

Once threshold is reached, an action potential of fixed size is generated; below threshold, no action potential occurs

How does stimulus intensity affect response?

Stronger stimuli generate action potentials more frequently, not larger action potentials

What factors affect the speed of conduction down an axon?

Myelination (saltatory conduction), axon diameter (wider = faster), temperature (higher = ions diffuse more faster)

What is the structure of a synapse?

Gap (synaptic cleft) between pre-synaptic and post-synaptic membranes; synaptic knob contains vesicles of neurotransmitter

What is the role of neurotransmitters?

Chemical messengers that transmit signals across the synaptic cleft and bind to receptors on the post synaptic membrane to trigger some form of response

What neurotransmitter is used at neuromuscular junctions?

Acetylcholine (ACh)

What type of neurotransmitter is acetylcholine?

Excitatory

Describe the sequence of events at a cholinergic synapse.

Action potential arrives, Ca²⁺ channels in the presynaptic membrane open, cause vesicles to move to the presynaptic membrane and fuse, ACh released into synaptic cleft, ACh binds to receptors on postsynaptic membrane, Na⁺ channels open, depolarisation occurs

Why is calcium important in synaptic transmission?

Calcium ions cause synaptic vesicles to fuse with the pre-synaptic membrane, releasing neurotransmitter

What happens to ACh after it binds to receptors and has done it’s job?

Acetylcholinesterase breaks it down into acetate and choline, which are reabsorbed by the presynaptic cleft and remade into ACh

Why is ACh broken down rapidly?

To prevent continuous stimulation and allow the post-synaptic membrane to repolarise

What is an excitatory neurotransmitter?

Causes depolarisation of the post-synaptic membrane, making it more likely to generate an action potential

What is an inhibitory neurotransmitter?

Causes hyperpolarisation of the post-synaptic membrane, making it less likely to generate an action potential

Give an example of an inhibitory neurotransmitter.

GABA (gamma-aminobutyric acid)

What is spatial summation?

Multiple pre-synaptic neurones release neurotransmitter at the same time, together reaching threshold

What is temporal summation?

A single pre-synaptic neurone releases neurotransmitter repeatedly in rapid succession, summing to reach threshold

What is a neuromuscular junction?

A specialised synapse between a motor neurone and a muscle fibre

How does the neuromuscular junction differ from a typical synapse?

Neurotransmitter is always excitatory, muscle fibres have specialised motor end plates, ACh is always used, postsynaptic membrane has lots of folds that form clefts that store acetylcholinesterase AChE)

What are the effects of drugs on synaptic transmission?

Can act as agonists (mimic neurotransmitters) or antagonists (block receptors), inhibit or enhance breakdown, or affect release

What are the advantages of reflex arcs?

Rapid, protective, do not require conscious thought

How’s ATP used in muscle contraction, specifically focusing on myosin etc.

1. energy released when ATP is broken down causes the myosin head to bend which pulls the actin filament closer to the m line in a power stroke

2. Another ATP provides energy to break the actin-myosin bridge so the myosin head detached from the actin binding site

What’s phosphocreatine stores good for?

Phosphocreatine binds with ADP to phosphorylate it and make ATP and creatine. Phosphocreatine is stored inside cells for short bursts of energy.

Describe and explain different properties of fast twitch muscle fibres

1. Muscle fibres that contract very quickly- fast movement

2. Short bursts of power and energy

3. Get tired quickly

4. Energy released from anaerobic respiration- few mitochondria

5. Whitish in colour because it doesn’t store much oxygen

Describe and explain different properties of slow twitch muscle fibres

1. Contract slowly- eg muscles used for posture in back

2. Good for endurance and long activities, eg long distance running

3. Long time before tired

4. Energy released slowly through aerobic respiration- lots of mitochondria and blood vessels

5. Reddish because rich in oxygen stores