Neuromuscular and Spinal Cord

Question 1

What inputs can alter the membrane potential of a neuronal body?

Answer 1

1. Excitatory post synaptic potential (EPSPs) - bring the potential closer t the threshold for firing
2. Inhibitory post synaptic potential (IPSPs) - move the potential further away from. The threshold for firing
   * EPSPs and IPSPs can also summary and the degree to which this occurs determines how readily a neuron will reach its threshold potential

Question 2

What happens at the NMJ?

Answer 2

AP -> Ca2+ influx -> ACh release-> ACh binds to motor end-plate receptors -> ion channels open -> Na+ influx -> AP in muscle fibre

At. Rest individual vesicles release ACh at a very slow rate causing miniature end-plate potentials (mEPPs)

Question 3

What are alpha motor neurone?

Answer 3

Lower motor neuronsof the brain stem and spinal cord

Innervate (extrafusal) muscle fibres of th skeletal muscle
*extrafusal is muscles that actually do the contracting

Activation causes muscle contraction

Motor neuron pool =all alpha neurones innervating one muscle fibre

**almost somatotropin arrangement of neurons in spinal cord - flexor axons tend to be located in posterior portion of ventral horn

Question 4

What is the motor unit?

Answer 4

Single motor neurone together with the muscle fibre it innervates

Each muscle fibre only innervated by one distinct neuron

Competition for innervation of muscle fibres usually happens at birth

Motor unit is smallest functional unit that creates force

Question 5

What are the different types of motor units?

Answer 5

Slow (S), type I - smallest soma, small dendritic trees, thinnest axons (slow conduction velocity)

Fast, fatigue resistant (FR), type IIA - larger Oma, larger dendritic tree, thicker axon (fast conduction)

Fats, fatigable (FF) type IIB - larger soma, larger dendritic tree, thicker axon (fast conduction)

*types not discretely located - spread out

Question 6

What is the innervation ratio?

Answer 6

Number of muscle fibres innervated by a motor neuron

Higher number = more precise movement

Question 7

How are the 3 types of muscle or units classified?

Answer 7

3 types are classified by amount of tension generated, speed and conduction and fatigability of motor unit

Question 8

What mechanisms are used to regulate force produces?

Answer 8

1. Recruitment - governered by ”size principle”
   - smaller units ar recruited first (generally slow twitch)
   - as more force is required, more units are required
   - this allows for fine control when low force is required
2. Rate coding - rate of firing
   - slow units fire at a slower frequency
   - as the firing rate increases,force by the unit increases
   - summation occurs when units fire at a rate too fast to allow the muscle to relax in between contractions

Question 9

What are neurotrophic factors?

Answer 9

Form of growth factors - prevent neuronal cell death and promote growth of neurons after injury

Question 10

What are the characteristics of motor units dependent on?

Answer 10

Nerve that innervated them

Question 11

What can change the properties of muscle types?

Answer 11

training - type IIB -> IIA

deconditioning of spinal cord (microgravity) - type I -> type II

ageing - loss of type I and II (preferential loss of the II so larger proportion of type I)

Question 12

How are the motor tracts organised in the spinal cord?

Answer 12

**Diagram from lecture

Pyramidal tracts 1a nd 1b are the lateral and anterior corticospinal tracts - responsible for majority of motor function

Some semblance of somatotropin representation in 1a - sacral, lumba, thoracic and cervical

Question 13

What is a reflex?

Answer 13

An autonomic and often inborn response to a stimulus that involves a nerve passing inward from receptor to a nerve and then outward to an effector without reaching the level of consciousness

*once initiated, cannot be stopped

Reflex arc :
Sensory receptor -> sensory neuron -> integration in inter neuron -> motor neuron -> effector muscle

Question 14

How can you see how many synapses there are in a reflex arc?

Answer 14

Difference win time from afferent volley to intracellular efferent recording gives experimenters an idea on the number of synapses in the reflex arc pathway - in increments of about 0.7ms

*momosynaptic = 0.7ms
disynaptic  = 1.6 ms

**inhibitory tends to be polysynaptic

Question 15

What is the monosynaptic stretch reflex?

Answer 15

when patella ligament is hit

sensory stretch receptors activated -> integrating centre -> both inhibitory neuron and an excitatory neuron to two different muscle groups (one muscle stretched and one relaxed)

Question 16

What is the Hoffman (H-) reflex?

Answer 16

done electronically ( not mechanically with patella hammer) -> differences in response due to person and not human error

initial M- wave is direct response of muscle to stretch with no synapses (or very few) - M- wave activates higher stimulus than H- wave

H- reflex is response of muscle a few ms later as response has travelled through a few synapses (multiple fibres

• H- wave activates at lower stimulus strength but is slower to fire
• H- wave decreases at higher stimulus because at higher stimuli, motor cells get activated that fire against normal flow and cancel out AP

Question 17

What is the flexion withdrawal response?

Answer 17

withdrawal of one leg in reflex action and (crossed) extensor of the other leg in same reflex arc to ensure you continue to balance

Question 18

Describe the supraspinal control of reflexes

Answer 18

e.g Jendrassik manoeuvre - clench teeth while knee jerk

higher centres of CNS exert inhibitory an excitatory regulation on stretch reflex whilst in normal conditions - dominant inhibitory control - decerebration/ reduced inhibition from CNS leads to hyperreflexia and spasticity

higher centres can influence reflexes by activating :

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

higher centres and pathways involved are:

• cortext : corticospinal - fine control
• red nucleus: rubrospinal - autonomic movements of arm in response to posture/ balance changes
• vestibular nucleus : vestibulospinal - altering posture to maintain balance
• tectum: tectospinal - head movements in response to visual information

Question 19

What is the gamma reflex loop?

Answer 19

ensures the muscle never remains slack and so can remain sensitive to stretch movements

gamma neurons shorten the spindle to maintain sensitivity

gamma and alpha motor neurons are found synonymously in the nervous system (they are found together and fire together when needs be)

Question 20

What happens to reflexes in stroke patients?

Answer 20

loss of descending inhibition -> hyperreflexia

clonus due to continuous stretch and contraction until voluntary stopping

Babinski’s sign is another classic stroke sign

Question 21

What is hyporeflexia commonly associated with?

Answer 21

lower motor neuron disease