Neuromuscular and Spinal cord

Question 1

Describe a synapse and what is the contact ratio of a synapse?

Answer 1

See notes - ranges from 1:1 for muscle to 10^3:1 in the CNS.
Size of the synaptic cleft 10-50nm
Pre-synaptic terminal - bouton

Question 2

Describe the transmission across synapses

Answer 2

The membrane potential of the post synaptic neurone can be altered in two directions by 2 inputs: NOTE: standard membrane potential is -70mv, threshold potential about -55mV

Made less negative - brought closer to the threshold potential; EPSP

Made more negative - brought further away from threshold potential; IPSP

The graded effect is called summation - EPSP and IPSP can be summmated to determine whether the postsynaptic neurone will fire or not

Question 3

What is the nueromuscular junction?

Answer 3

A specialised synapse between the motor neurone and the motor end plate, the muscle fibre cell membrane.
See diagram - the post synaptic membrane is highly folded.

Question 4

Describe the activation of the neuromuscular junction?

Answer 4

When an action potential arrives at the NMJ, Ca2+ influx causes ACh release. ACh binds to receptors on the motor end plate.
Activation of ACh receptors on the postsynaptic membrane (Na+ influx) propagates the action potential, which travels down into the muscle fibres via T-tubules

At rest, individual vesicles release ACh at a very low rate causing miniature end-plate potentials (mEPP)

Question 5

What is the alpha motor neurone?

Answer 5

This is the lower motor neurone which runs from the spinal cord/brain stem to the muscle.
They innervate the extrafusal muscle fibres of the skeletal musucle - activation of a-motor neurone causes contraction.

Question 6

What are extrafusal muscle fibres and intrafusal muscle fibres?

Answer 6

Extrafusal - standard skeletal muscle fibres that cause contraction
Intrafusal - contain sensory organs that tell the CNS how much the muscle is being stretched

Question 7

What is the motor neurone pool?

Answer 7

The motor neurone pool contains all alpha motor neurones innervating a single muscle

Question 8

Where is the alpha motor neurone located?

Answer 8

The ventral horn of the spinal cord. They are organised into flexors, proximal, distal and extensors - see diagram

Question 9

What is the motor unit?

Answer 9

Is the smallest functional unit that describes a nerve and all the muscle fibres it innervates, and can produce a contraction.
Each muscle fibre is only innervated by one motor unit. Stimulation of the motor neurone will activate all the muscle fibres that it innervates
See lecture

Question 10

What are the types of motor units?

Answer 10

Slow (Type I) - smallest diameter cell bodies, small dendritic trees, thinnest axons, slowest conduction velocity. Slow twitch, low tension (force), fatigue resistant

Fast, fatigue resistant (Type IIa) - larger diameter cell bodies, larger dendritic trees, thicker axons, faster conducting velocity. Fast twitch, moderate tension, fatigue resistant

Fast, fatiguable (Type IIb) - same as above. Fast twitch, high tension, high fatigue

The motor unit types are classified by the amount of tension generated, speed of contraction and fatigueability of the motor unit.

Question 11

How is muscle force regulated?

Answer 11

Recruitment and rate coding

Recruitment - of motor units. Smaller units are recruited first (slow twitch) then as more force is required more units are recruited; allows for fine control (when writing), under which low force levels are required. Motor units are derecruited in the same order. See diagram

Rate coding - Slow units fire a lower frequency. As firing rate increases the force produced by the unit increases. Summation occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials

Question 12

What are neurotropic factors?

Answer 12

Type of growth factor that prevents neuronal death (regeneration) and promotes growth of neurones after injury.
Motor unit and fibre characteristics are dependent on the nerve that innervates them. Muscle wasting can be seen if there is nerve damage. See diagram

Question 13

Describe the plasticity of motor units/muscle fibres

Answer 13

Fibre types can change properties:
Type IIB to type IIA is the most common following training
Type I to II only occurs in cases of severe deconditioning or spinal cord injury. Microgravity causes shift from slow to fast twitch muscle fibres
Ageing = loss of type I and II but preferential loss of II

Question 14

Draw and describe where the motor tracts are located in the spinal cord

Answer 14

See diagram/kenhub

Question 15

What do the pyramidal tracts do?

Answer 15

Voluntary control of movement.

For the limbs the tracts cross whereas for the trunk the tracts are uncrossed.

Question 16

What are the three main extrapyramidal tracts and what do they do?

Answer 16

Extrapyramidal tracts are involved in posture control

Vestibulospinal tract - Regulates posture to maintain balance, and facilitates mainly a-motorneurones of the postural, anti-gravity (extensor) muscles

Reticulospinal tract - Coordinated automated movements of locomotion and posture (e.g to painful stimuli)

Rubrospinal tract - Automatic movements of arm in response to posture/balance changes

Question 17

What is a reflex?

Answer 17

An automatic and often inborn response to a stimulus that involves a nerve impulse passing inward from a receptor to a nerve centre and then outwards to an effector (as a muscle or a gland) without reaching the level of consciousness.

An involuntary coordinated pattern of muscle contraction and relaxation by peripheral stimuli

Question 18

What are the components of a reflex arc?

Answer 18

sensory receptor + neurone, (interneurone), motor neurone + effector

Question 19

How many synapses are there in a reflex arc?

Answer 19

See diagram (although its bad) - in a monosynaptic pathway the sensory neurone synapses to a motor neurone which when stimulated causes a contraction. However in a disynaptic pathway the sensory neurone synpases with an interneurone which inturn inhibits the motor neurone. Think about this in terms of a reflex where one muscle has to relax while the other contracts for movement - e.g tapping tendon below the patella.

Question 20

Describe the Hoffman reflex

Answer 20

see diagram
electrical stimulation of a nerve will give you two responses. One from the direct activation of the motor nerve and the second one being up along the sensory neurone to the spinal cord and then back down the same motor neurone.

You get two waves. M wave (motor) and the H wave (hoffman reflex). You then work out how long it takes for nerve transmission and work out if it is peripheral or central damage.

Question 21

What are flexion withdrawal reflexes?

Answer 21

See diagram
Tread on a pin = flexion withdrawal of the leg and then crossed extensor (the other leg extends to take the weight of your other leg coming up)

Question 22

Describe supraspinal control of reflexes

Answer 22

Reflexes can be influenced - Jendrassik manoeuvre (larger reflex response occurs)

Higher centres of the CNS exert inhibitory and excitatory regulation upon the stretch reflex. Inhibitory control dominates in normal conditions. The cerebellum inhibits reflexes, revealed by decerebration. Decerebration also revealed that excitatory control comes from supraspinal areas (brain stem).

See diagrams

Question 23

What are gamma motor neurones?

Answer 23

Cause sensory organs to change their sensitivity

Question 24

What is a classic symptom of a upper motor neurone disorder caused by stroke?

Answer 24

Hyperreflexia - links back to supraspinal control of reflexes

Question 25

What is hyporeflexia?

Answer 25

Below normal of absent reflexes - mostly associated with lower motor neurone disease