Neuromuscular and spinal cord control of movements

Question 1

What is the function of the synapse

Answer 1

Allows for contact from neurone to muscle or from neurone to neurone

Question 2

Describe synapses

Answer 2

Structure is similar throughout the nervous system
May be simple or complex
Contact ratio ranges from 1:1 (muscle) to 10^3:1 (CNS)
synaptic cleft = 10-50nm

Question 3

What are the two directions in which a membrane potential of the post synaptic neurone can be altered

Answer 3

-ve = further away from threshold for firing (inhibitory post synaptic potentials)

+ve = brought closer to threshold for firing (excitatory post synaptic potential)

Question 4

What is summation

Answer 4

Graded effects of synaptic transmission
Occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials.
Excitatory post synaptic potentials and inhibitory post synaptic potentials can summate
The degree of summation will determine how readily a neurone can reach threshold to produce an AP

Question 5

Describe the neuromuscular junction

Answer 5

Specialised synapse between the motor neurone and the motor end plate, the muscle fibre cell membrane

Question 6

How is the neuromuscular junction activated§

Answer 6

1. AP arrives
2. Calcium influx
3. ACh release
4. ACh binds to receptor on motor end plate
5. Ion channels open to allow sodium to influx -> AP

Question 7

What are mature end-plate potentials

Answer 7

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

Question 8

Describe the alpha motor neurone

Answer 8

Lower motor neurones of the brainstem and spinal cord
Innervates the extramural muscle fibres of the skeletal muscles
Activation causes contraction

Question 9

What is the motor neurone pool

Answer 9

Contains all alpha motor neurons innervating a single muscle

Question 10

What is a motor unit

Answer 10

Single motor neurone together with all the muscle fibres that it innervates
Smallest functional unit to produce force
Stimulation of one motor unit causes contraction of all the muscle fibres in that unit

Question 11

What are the 3 types of motor unit

Answer 11

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

Question 12

Describe slow motor units

Answer 12

Smallest diameter cell bodies
Small dendritic trees
Thinnest axons
Slowest conduction velocity

Question 13

Describe fast, fatigue resistant motor units

Answer 13

Larger diameter cell bodies
Larger dendritic trees
Thicker axons
Faster conduction velocity

Question 14

Describe fast, fatiguable motor units

Answer 14

Larger diameter cell bodies
Larger dendritic trees
Thicker axons
Faster conduction velocity

Question 15

What are the 3 motor unit types classified according to

Answer 15

Amount of tension generated
Speed of contraction
Fatiguability

Question 16

What is recruitment

Answer 16

Mechanism by which the brain regulates the force a single muscle can produce

Governed by the size principle - smaller motor units are recruited first (generally slow twitch)
More force required -> more units required
Allows fine control (e.g. writing), under which low force levels are required

Question 17

What is rate coding

Answer 17

Mechanism by which the brain regulates the force a single muscle can produce

Motor units can fire at a range of frequencies
Slow units = lower frequency
As firing rate increases, force produced increases
Summation occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials.

Question 18

What are neurotrophic factors

Answer 18

Type of growth factor
Prevents neuronal death
Promotes growth of neurones after injury

Question 19

What are the effects of neurotrophic factors

Answer 19

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, the soleus becomes fast and the FDL becomes slow.

The motor neurone has some effect on the properties of the muscle fibres which it.

Question 20

Describe the plasticity of motor units/muscle fibres

Answer 20

Fibre types can change properties under conditions

Training = IIB -> IIA
Severe deconditioning/ spinal cord injury = =I -> II
Microgravity in spaceflight = slow -> fast
Ageing = loss of type I slower contraction

Question 21

What is the vestibulospinal tract involved in

Answer 21

Ventral
regulates posture to maintain balance, and facilitates mainly α motoneurones of the postural, anti-gravity (extensor) muscles

Question 22

What are the Sacral and anterior corticospinal tracts involved in

Answer 22

Control voluntary movements

Question 23

What is the role of the reticulospinal tract

Answer 23

Coordinates automated movements of locomotion and posture e.g. to painful stimuli

Question 24

What is the role of the rubrospinal tract

Answer 24

Automatic movements of arm in response to posture/balance changes

Question 25

What is a reflex

Answer 25

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.

Question 26

What are the components of a reflex arc

Answer 26

``` Sensory receptor Sensory neuron Integrating center Motor neuron Effector ```

Question 27

Describe the monosynaptic (stretch) reflex

Answer 27

1. Stretching stimulates the sensory receptor (muscle spindle) 2. Sensory neuron is excited 3. Sensory neuron activates motor neurones in the integrating centre (spinal cord) 4. Motor neuron excited 5. Effector contracts and relives the stretching 3. Inhibitory neurone activated 4. Motor neurone to antagonistic muscles inhibited 5. Antagonistic muscles relax

Question 28

What is the Hoffman reflex

Answer 28

-

Question 29

Describe a polysynaptic reflex

Answer 29

Flexion withdrawal 1. Stepping on a tack stimulates the sensory receptor 2. Excited sensory neurone 3. Sensory neurones activates interneurones in several spinal cord segments, ascending and descending 4. Motor neurones are excited 5. Effector contracts

Question 30

Describe the supra spinal control of reflexes

Answer 30

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 supra spinal nerves

Question 31

How may rigidity and spasticity occur via supra spinal reflex control

Answer 31

Brain damage giving over-active or tonic stretch reflex.

Question 32

What are the mechanisms by which higher centres influence reflexes

Answer 32

Activating: 1. Alpha neurones 2. Inhibitory interneurones 3. Propriospinal neurons 4. Gamma motor neurons 5. Terminals of afferent fibres

Question 33

Which higher centres and pathways are involved in supra spinal 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 muscles fo slack, the spindle is shortened to maintain its sensitivity

Question 35

What is hyper-reflexia

Answer 35

Below normal or absent reflexes Loss of descending inhibition Mostly associated with lower motor neuron diseases May be due to stroke

Question 36

Describe the structure of the alpha motor neuron

Answer 36

Extensors are more ventral, flexors dorsal | Grey matter is more proximal