Neuromuscular and spinal cord

Question 1

How may the membrane potential of the postsynaptic neurone be altered by inputs?

Answer 1

It can be made less negative, i.e. be brought closer to threshold for firing- this is an excitatory postsynaptic potential (EPSP).

It can be made more negative, i.e. be brought further away from threshold for firing- this is an inhibitory postsynaptic potential (IPSP).

Question 2

What is an excitatory postsynaptic potential (EPSP)?

Answer 2

Membrane potential of the postsynaptic neurone can be made less negative, i.e. be brought closer to threshold for firing.

Question 3

What is an inhibitory postsynaptic potential (IPSP)?

Answer 3

Membrane potential of the postsynaptic neurone can be made more negative, i.e. be brought further away from threshold for firing.

Question 4

What does the degree of summation of EPSPs and IPSPs determine?

Answer 4

How readily a neuron can reach threshold to produce an action potential.

Question 5

What is a neuromuscular junction?

Answer 5

A specialised synapse between the lower motor neuron and the motor end plate (highly folded muscle fibre membrane).

Question 6

How is a neuromuscular junction activated?

Answer 6

When an action potential arrives at the NMJ, Ca2+ influx causes ACh release from vesicles.
ACh binds to receptors on the motor end plate.
Ion channel opens- Na+ influx causes action potential in muscle fibre.

Question 7

What is a miniature end plate potential (mEPP)?

Answer 7

At rest, individual vesicles release ACh at a very low rate.

Question 8

What is an alpha motor neurone?

Answer 8

Lower motor neurones of the brainstem and spinal cord.
Innervate the extrafusal muscle fibres of the skeletal muscles.
Their activation causes muscle contraction.
Motor neurone pool contains all alpha motor neurones innervating a single muscle.

Question 9

Where are alpha motor neurones found?

Answer 9

Ventral horn of spinal cord.

Question 10

What is a motor unit?

Answer 10

A single motor neurone and all of the muscle fibres it innervates.
The smallest functional unit that causes contraction/ produces force.
Stimulation of one motor unit causes contraction of all the muscle fibres in that unit.

Question 11

Approximately how many motor neurones do humans have?

Answer 11

420,000

Question 12

Approximately how many skeletal muscle fibres do humans have?

Answer 12

250 million

Question 13

On average how many muscle fibres does each motor neurone supply?

Answer 13

About 600

Question 14

What are the different types of motor unit?

Answer 14

Slow (S, type I).
Fast, fatigue resistant (FR, type IIA).
Fast, fatiguable (FF, type IIB).

Question 15

What are the features of a slow (S, type I) motor unit?

Answer 15

Smallest diameter cell bodies. 
Small dendritic trees. 
Thinnest axons. 
Slowest conduction velocity. 
Slow twitch. 
Low tension. 
Fatigue resistant.

Question 16

What are the features of a fast, fatigue resistant (FR, type IIA) motor unit?

Answer 16

Larger diameter cell bodies. 
Larger dendritic trees. 
Thicker axons. 
Faster conduction velocity. 
Fast twitch. 
Moderate tension. 
Fatigue resistant.

Question 17

What are the features of a fast, fatiguable (FF, type IIB) motor unit?

Answer 17

Larger diameter cell bodies. 
Larger dendritic trees. 
Thicker axons. 
Faster conduction velocity. 
Fast twitch. 
High tension. 
High fatigue.

Question 18

How are the 3 different motor unit types classified?

Answer 18

Amount of tension generated.
Speed of contraction.
Fatiguability of the motor unit.

Question 19

What are the 2 mechanisms by which the brain regulates the force that a single muscle can produce?

Answer 19

Recruitment.

Rate coding.

Question 20

What is recruitment?

Answer 20

Motor units are not randomly recruited- there is an order.
Governed by the ‘size principle’.
Smaller units are recruited first (generally the slow twitch units).
As more force is required, more units are recruited.
This allows fine control (e.g. when writing), under which low force levels are required.

Question 21

What is rate coding?

Answer 21

A motor unit can fire at a range of frequencies.
Slow units fire at a lower frequency.
As the 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 22

What are neurotrophic factors?

Answer 22

Growth factors.
Prevent neuronal death.
Promote growth of neurons after injury.

Question 23

What are the effects of neurotrophic factors?

Answer 23

Motor unit and fibre characteristics are dependent on the nerve which innervates them.
If a fast twitch muscle and a slow muscle are cross innervated, slow becomes fast and fast becomes slow.
The motor neuron has some effect on the properties of the muscle fibres it innervates.

Question 24

Discuss the plasticity of motor units/ muscle fibres.

Answer 24

Fibre types can change properties under many different conditions.
Type IIB to IIA most common following training.
Type I to II possible in cases of severe reconditioning or spinal cord injury. Microgravity during spaceflight results in shift from slow to fast muscle fibre types.
Ageing associated with loss of type I and II fibres but also preferential loss of type II. This results in a larger proportion of type I fibres in aged muscles (slower contraction times).

Question 25

What are the pyramidal tracts?

Answer 25

Lateral corticospinal tract.

Anterior corticospinal tract.

Question 26

What are the extrapyramidal tracts?

Answer 26

Rubrospinal tract.
Reticulospinal tract.
Vestibulospinal tract.
Olivospinal tract.

Question 27

What is a reflex?

Answer 27

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 outward to an effector without reaching the level of consciousness.
An involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli.

Question 28

What are the components of a reflex arc?

Answer 28

Sensory receptor- responds to a stimulus by producing a generator or receptor potential.
Sensory neuron- axon conducts impulses from receptor to integrating centre.
Integrating centre- one or more regions within the CNS that relay impulses from sensory to motor neurons.
Motor neuron- axon conducts impulses from integrating centre to effector.
Effector- muscle or gland that responds to motor nerve impulses.

Question 29

What is the Hoffman (H-) reflex?

Answer 29

Electrically stimulate the nerve instead of stretching muscle mechanically to produce reflex response.
More reliable than using a tendon hammer, exactly the same stimulus every time and in the same place, etc.
Used clinically to assess excitability of reflex pathways, and which nerves may be affected by a disorder.

Question 30

What is the Jendrassik maneouvre?

Answer 30

Traditionally we think of reflexes as being automatic (knee jerk) and stereotyped behaviours (sneeze, cough) in response to stimulation of peripheral receptors. Can they be influenced?
The Jendrassik manoeuvre- clench teeth or make a fist when having patellar tendon tapped, reflex is more pronounced due to voluntary command to other muscles.

Question 31

Describe the supraspinal control of reflexes.

Answer 31

Higher centres of the CNS exert inhibitory and excitatory regulation upon the stretch reflex.
Inhibitory control dominates in normal conditions.
Decerebration reveals the excitatory control from supraspinal areas.
Rigidity and spasticity can result from brain damage giving over-active or tonic stretch reflex.

Question 32

How do higher centres influence reflexes?

Answer 32

Activating alpha motor neurons. 
Activating inhibitory interneurons. 
Activating propriospinal neurons. 
Activating gamma motor neurons. 
Activating terminals of afferent fibres.

Question 33

What are the higher centres and pathways involved in the supraspinal control of reflexes?

Answer 33

Cortex- corticospinal (fine control of limb movements, body adjustments).
Red nucleus- rubrospinal (automatic movements of arm in response to posture/balance changes).
Vestibular nuclei- vestibulospinal (altering posture to maintain balance).
Tectum- tectospinal (head movements in response to visual information).

Question 34

What is the gamma reflex loop?

Answer 34

If the knee is extended and the muscle goes slack, the spindle is shortened to maintain its sensitivity.

Question 35

What can hyperreflexia result from?

Answer 35

e.g. a stroke.

Loss of descending inhibition.

Question 36

What is hyporeflexia?

Answer 36

Below normal or absent reflexes.

Mostly associated with lower motor neuron diseases.