Limbs and Back Physiology

Question 1

Describe why neuronal cells have a negative resting potential and the ion flux basis of this

Answer 1

A negative membrane potential is created due to the unequal distribution of positive ions. The Na⁺/K⁺ ATPase transport 2 K⁺ into cells and 3 Na⁺ out of cells (this is an active process). This creates an electrochemical gradient.

Leaky ion channels allow diffusion of ions.

Question 2

Describe how action potentials arise

Answer 2

Actions potentials arise due to the activation of voltage gated channels, allowing the movement of ions and depolarisation to occur.

Question 3

Describe the mechanism of action potential propagation in myelinated and unmyelinated axons

Answer 3

Myelinated axons: The action potential undergoes saltatory conduction in which the action potential is observed at the Nodes of Ranvier, which occur between the myelinated areas. These are areas with concentrated levels of sodium channels.

Unmyelinated axons: The action potential is propagated along the full length of the axon. Continuous conduction using local circuits

Question 4

Describe why the conduction velocity is proportional to axon diameter

Answer 4

A greater axon diameter allows for less resistance to ion flow, leading to increased conduction velocity

Question 5

Describe absolute and relative refractory periods

Answer 5

Absolute: The activation and inactivation gate of voltage gated sodium channels are closed. No APs can be initiated

Relative: The activation gate is closed, the channel is in its resting state, BUT if a large stimulus occurs an action potential can be initiated

Question 6

List different types of stimuli (sensory modularities) conducted by different classes of nerve fibres in peripheral nerves

Answer 6

Question 7

Describe the cell types that make up the nervous system

Answer 7

Neurons

Support cells: Astrocytes, Schwann cells, microglia

Question 8

Describe the basic structure of a neurone, using appropriate terminology

Answer 8

Soma, axon hillock, dendrites, axon, axon terminal

Question 9

Describe the main cell types that make up peripheral nerves

Answer 9

Neurones

Schwann Cells: Layers of Schwann cell membranes form the myelin sheath of neurons

Question 10

Recognise and define the terms endoneurium, perineurium and epineurium

Answer 10

Endoneurium: A thin internal covering of a nerve fasicle

Perineurium: A fibrous layer covering a bundle of nerve fasicles

Endoneurium: An outer covering of the multiple bundles of nerves

Question 11

Describe the major distinguishing features of smooth, cardiac and skeletal muscle

Answer 11

x

Question 12

Recognise and define the terms endomysium, perimysium, epimysium, fascicle, muscle fibre, myofibril

Answer 12

Fascicle > muscle fibres > myofibril

Exterior to interior: Epimysium, perimysium, endomysium

Question 13

Recognise and define the term motor unit

Answer 13

Motor unit: A single alpha motor neuron and the all muscle fibres which it innervates

☆ A muscle fibre may only be innervated by one motor neuron BUT a motor neuron may innervate multiple muscle fibres

Question 14

Describe the morphological, biochemical and functional characteristics of the various types of muscle fibres that can make-up skeletal muscle in different parts of the body

Answer 14

Slow twitch: Type I

Fast twitch: Glycolytic (Type IIa) and Oxidative (Type IIOx)

Question 15

Describe the structure of the neuromuscular junction (NMJ)

Answer 15

Formed from the axon terminal of a neuron, a synapse and the motor end plate of a muscle.

T-tubules seen along the sarcolemma of the muscle. The T-tubules allow for the action potential to be conducted into the muscle, intro close proximity of the SR - with these structures occuring at the junction between the A and I bands. This allows for the calcium influx/release required for muscle contraction.

Key term: Excitation-contraction coupling

Question 16

Use a diagram to illustrate the structure of a NMJ

Answer 16

3 outcomes of neurotransmitter release at the NMJ: Metabolism (acetylcholine esterase), action or diffusion (MAD)

Question 17

Describe how acetylcholine release at the NMJ results in a muscle action potential

Answer 17

Acetylcholine binds to nictinic receptors on the surface of the neuromuscular motor end plate. 2 ACh molecules must bind to one receptor in order to activate the receptor. Binding of multiple receptors, allowing threshold to be reached, allows for the generation of a motor end plate potential.

• A miniature end plate potential may be generate through the release of a single ACh vesicle

This causes depolarisation and generates an action potential, through opening Na⁺ channels, which propagates along the muscle sarcolemma. The action potential travels deeper into the muscle via the T tubules.

The T tubules are seen at the junction between the A and I bands. Calcium is released from the SR. Calcium binds to troponin C allowing for a conformational change and movement of tropomyosin, exposing the myosin binding site on actin.

Question 18

Describe the structure of a muscular sarcomere

Answer 18