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.

Question 5

What does the degree of summation determine

Answer 5

how readily a neurone can reach threshold to produce an AP

Question 6

How is the neuromuscular junction activated

Answer 6

1. AP arrives at the pre-synaptic terminal causing opening of VGCa2+ channels and Ca2+ moves in
2. Ca2+ causes the ACh vesicles to bind to the pre-synaptic membrane
3. Ach is released into the synaptic cleft
4. ACh binds to the receptors on the nicotinc ACh receptor (nAChR) and opening it
5. Na+ moves into the sarcolemma and causes an AP here often leading to muscle contraction

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 extrafusal muscle fibres of the skeletal muscles
Activation causes contraction

Question 9

What are the alpha motor neurons made up of and where is it

Answer 9

Cell bodies in grey matter of the spinal cord/brainstem
Cranial and spinal nerves included

In the spinal cord and brainstem

Question 10

What is a motor unit

Answer 10

Single motor neurone together with all the muscle fibres that it innervates
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 is recruitment

Answer 15

The acquisition of more motor units as more force is required
Allows fine control (e.g. writing), under which low force levels are required
(basically spatial summation)

Question 16

In what order does recruitment occur in

Answer 16

Governed by the size principle - smaller motor units are recruited first (generally slow twitch)
S to FR to FF

Question 17

What is Rate Coding

Answer 17

The increase in firing rate to generate more force by the motor unit
As firing rate increases, force produced increases
(basically temporal summation)

Question 18

Explain how rate coding is associated with summation

Answer 18

Summation/tetany occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials.

Question 19

Describe the plasticity of motor units/muscle fibres

Answer 19

Fibre types can change properties under conditions

Question 20

What are the changes in motor units during training

Answer 20

FF(IIB) to FR(IIA)

Question 21

What are the changes in motor units in ageing

Answer 21

Loss of Type I and II, but more II
S type (I) mainly

Question 22

Where is the vestibulospinal tract and what is it involved in

Answer 22

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

Question 23

Where is the anterior corticospinal tract and what is it involved in

Answer 23

Dorsal to vestibulospinal tract

Control voluntary movements

Question 24

Where is the rubrospinal tract and what is it involved in

Answer 24

Lateral

Automatic movements of arm in response to posture/balance changes

Question 25

Where is the reticulospinal tract and what is it involved in

Answer 25

Dorsal to the anterior corticospinal tract | Coordinate automated movements of locomotion and posture (e.g. to painful stimuli)

Question 26

What are the components of a reflex arc

Answer 26

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

Question 27

What are the types of spinal reflexes

Answer 27

Stretch reflex Flexion reflex Crossed extensor reflex

Question 28

Describe the general reflex pathway

Answer 28

1. Receptors sense the stimuli 2. Sensory neurones stimulated 3. Synapse with interneurons in posterior horn 4. Synapse with motor neurones in anterior horn 5. Effector activated

Question 29

Describe the stretch reflex pathway

Answer 29

Monosynaptic | Same as the normal reflex but there is an inhibitory signal to the antagonistic muscle causing relaxation

Question 30

What is the Hoffman reflex

Answer 30

If you stimulate a muscle fibre then it will be stimulated by: The direct stimulation to the motor neurone and cause contraction (SHORT ROUTE) also creating an M wave The sensory neurone going back through the spinal cord to the motor neurone and causing contraction (LONG ROUTE) also creating an H wave Hence you end up with 2 contractions of the muscle

Question 31

Describe the flexion reflex

Answer 31

Polysynaptic (as there is an interneurone stimulated in the spinal cord) Several spinal segments are stimulated from one sensory neurone Causes contraction of the flexor muscles and therefore withdraws the arm/leg

Question 32

Describe the crossed extensor reflex

Answer 32

Polysynaptic The sensory neurone synapses with several interneurons including those from the other side and so you get flexion of the stimulated side and extension of the non-stimulated side (in this case) So there is excitatory stimulation to both flexor and extensor muscles on both sides

Question 33

What are muscle spindles

Answer 33

Specialised receptors within the skeletal muscles to detect changes in length and proprioception avoid damage/overuse/pain

Question 34

What do gamma neurones do

Answer 34

They keep the muscle spindles sensitive by causing intrafusal muscle contraction to keep up with the changing muscle fibre length caused by alpha motor neurons

Question 35

What is the gamma reflex loop

Answer 35

If the knee is extended and the muscles fo slack, the spindle is shortened to maintain its sensitivity

Question 36

Give an example fo supra spinal control of reflexes

Answer 36

1 .If your bicep muscle is at normal length (N) then if a weight is suddenly dropped on your hand it will move to a stretched length(S) 2. When it moves to S then the spindles within the muscle fib§res are stretched and cause the a reflex to occur and contract your bicep (as there are sensory neurons wrapped around the spindle) 3. So your extrafusal muscle fibres have shortened but your spindles are still stretched 4. Supraspinal areas control the gamma motor neurones, causing stimulation of these to contract the spindle back to the correct now contracted length and keep the muscle sensitive to any other changes (Like if you dropped the weight suddenly)

Question 37

Where does UMN originate and terminate

Answer 37

originate in cerebral cortex and terminate in the spinal cord/brainstem

Question 38

Where does LMB ornate and terminate

Answer 38

LMN originate in the spinal cord/brainstem and terminate at the effector muscle

Question 39

What causes hyperreflexia

Answer 39

loss of the supraspinal reflex control when the UMNs are damaged and so the reflexes are present and very active as there is no suppression

Question 40

What causes hyporeflexia

Answer 40

LMNs are not functioning to effectively contract the muscles in the reflex loop

Question 41

What occurs in an upper motor neurone lesion

Answer 41

``` Hypertonia Hyperreflexia No atrophy Clonus prescient Lesion from motor cortex through to anterior horn ```

Question 42

What occurs in a lower motor neurone lesion

Answer 42

Hypotonia Hyporeflexia Severe atrophy Fasciculations and fibrillations