Neurology 9 - Neuromuscular and Spinal Cord

Question 1

Describe the transmission across synapses

Answer 1

• Can make membrane potential less negatve (excitatory post synaptic potential)
• Can make membrane potential more negative (inhibitory post synaptic potential)
• Added together this is summation

Question 2

Where are the alpha motor neurons found?

Answer 2

• Brainstem and ventral horn of the spinal cord
• Innervate the extrafusal muscle fibres of skeletal muscles
• Activation causes voluntary muscle contraction
• Motor neuron pool contains all alpha motor neurons innervating a single muscle

Question 3

What is a motor unit?

Answer 3

• Smallest functional unit with which to produce force
• The name given to a single motor neuron together with all the muscle fibres that it innervates.
• Stimulation of one motor unit causes contraction of all of the muscle fibres innervated

Question 4

List the types of motor unit

Answer 4

• Slow (S type 1, smallest cell bodies, small dendritic trees, thin axons)
• Fast, fatigue resistant (FR type IIA, large diameter and dendritic trees, thicker axons)
• Fast, fatiguable (FF type IIB, large diameter and dendritic trees and thicker axons).

Question 5

How are the motor unit types classified?

Answer 5

• Amount of tension generated
• Speed of contraction
• Fatiguability

Question 6

What are the two mechanisms by which the brain regulates the force produced by a muscle?

Answer 6

Recruitment and rate coding

Question 7

What is recruitment?

Answer 7

• Motor units are not randomly recruited.
• Governed by the “Size Principle”. Smaller units are recruited first (these are 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 8

What is rate coding?

Answer 8

• 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 9

What are neurotrophic factors?

Answer 9

• A type of growth factor
• Prevent neuronal death and promote growth of neurons after injury
• Activity of the motor unit can be modified by these factors, to increase speed of contraction of a muscle usually innervated by slow neurones for example.

Question 10

Which fibre types change?

Answer 10

• Type IIB to type IIA is the most common
• Type I to II is possible in severe spinal cord injury (and during spaceflight)
• Ageing is associated with loss of type II preferentially as well as some type I, resulting in slower contraction times
• Governed by neurotrophic factors

Question 11

List the extrapyramidal tracts

Answer 11

• Extrapyramidal tracts are outside of the pyramids
• Rubrospinal causes automatic movements of the arm in response to posture change
• Reticulospinal tract coordinates automated movements of locomotion and posture
• Vestibulospinal tract maintains balance by regulating posute, and facilitates alpha motorneurones of the postural anti–gravity muscles

Question 12

What is a reflex?

Answer 12

• 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 (as a muscle or gland) without reaching the level of consciousness.
• An involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli.

Question 13

What are the components of a reflex arc?

Answer 13

• Sensory receptor
• Sensory neuron
• Integrating center
• Motor neuron
• Effector

Question 14

Describe the pathway of the monosynaptic (stretch) reflex

Answer 14

• Sensory receptor stimulated by stretch which triggers contraction of the same muscle via a single synapse
• At the same time, via an interneuron, antagonistic muscles are inhibited so they relax. This takes longer.

Question 15

What is the Hoffman reflex?

Answer 15

• Electrical stimulation causing a monosynaptic reflex
• Eg. A precise way of measuring the patellar reflex
• M wave is triggered at higher intensity stimulus, causing a faster response than hitting the patellar
• Used to determine conduction times

Question 16

Describe the pathway of flexion withdrawal (polysnaptic reflex).

Answer 16

• Sensory receptor to neuron following stepping on a pin
• To integrating centre which activates interneurons in other segments and motor neurones- therefore, polysynaptic
• Results in flexor muscle contraction to withdraw leg
• Inhibition across the midline and stimulation of the other leg in order to maintain the balance

Question 17

What is the Jendrassik manoevre?

Answer 17

• Higher centres of the CNS exert inhibitory and excitatory regulation upon the stretch reflex.
• Inhibitory control dominates in normal conditions (N).
• Decerebration reveals the excitatory control from supraspinal areas (D).
• Therefore, you can increase reflexes. (The patient clenches the teeth and interlocks the fingers, and the patellar reflex will increase)

Question 18

List the ways higher centres influence reflexes.

Answer 18

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

Question 19

List the higher centres and pathways involved in supraspinal control of reflexes

Answer 19

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 20

What is the gamma reflex loop?

Answer 20

• Extrafusal voluntary muscle outside (alpha motor neurones)
• Intrafusal supplied by the gamma motor neurones, reacting to contraction of the extrafusal muscle. They are like springs.

Question 21

What is the babinski sign?

Answer 21

• Looks for upper motor neuron dysfunction
• Stimulus to the lateral aspect of the foot and balls of the toes
• Great toe will extend and the small toes will fan out when there is nerve damage, while normally the toes will flex

Question 22

What is a neuromuscular junction?

Answer 22

A specialised synapse between the motor neuron and the motor neuron end plate (muscle fibre cell membrane)

Question 23

How are neuromusclar junctions activated?

Answer 23

• Action potential reaches terminal bouton increasing calcium influx
• ACh vesicles fuse with membrane
• ACh binds to receptors on postsynaptic membrane, resulting in an action potential due to sodium influx
• Acetylcholine can be released randomly to release miniature end-plate potentials

Question 24

What can cause hyperreflexia?

Answer 24

• Stroke affecting the descending control of movement

- Upper motor neuron issues

Question 25

What can cause hyporeflexia?

Answer 25

- Caused by damage to the lower motor neurones

Question 26

Describe the arrangement of alpha motor neurones in the spinal cord

Answer 26

- In the ventral root there is functional mapping - Extensor muscles more anterior - Flexor muscles more posterior - Proximal neurones are medial - Distal musculature motor neuron pool more lateral

Question 27

Describe the functional differences between the 3 types of motor unit

Answer 27

- Type IIb motor units are fast twitch, high tension and high fatigue - Type IIa motor units are fast twitch, moderate tension and fatigue resistant - Type 1 motor units are slow twitch, low tension and fatigue resistant