Neuromuscular and spinal cord

Question 1

Give another name for a presynaptic terminal.

Answer 1

Bouton

Question 2

How long is a synaptic cleft?

Answer 2

10-50nm

Question 3

What is the role of a synapse?

Answer 3

Allow for communication/contact between neurones or neurone –> muscle

Question 4

How does the contact ratio of synapses vary?

Answer 4

1:1 muscle

10³:1 in the CNS

Question 5

What is summation?

Answer 5

The graded effects of combined EPSPs and IPSPs on the neurone membrane potential.

The degree of summation determines whether an AP will be generated.

Question 6

What is the difference between an IPSP and an EPSP.

Answer 6

An EPSP makes the neurones potential less negative, vice versa.

Question 7

What is a neuromuscular junction?

Answer 7

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

Question 8

Diagram of NMJ.

Answer 8

Question 9

How is the NMJ activated?

Answer 9

AP arrives at terminal bouton.

Ca2+ influx.

ACh vesicles fuse with pre-synaptic membrane + diffuses across cleft.

ACh binds to post-synaptic receptors.

Na+ influx stimulated.

AP in muscle fibre.

Question 10

what are miniature end-plate potentials?

Answer 10

At rest individual vesicles release ACh at a very low rate causing minimal stimulation.

Question 11

Where are alpha motor neurons found?

Answer 11

Brainstem and spinal cord.

Question 12

What do alpha motor neurons innervate?

Answer 12

The muscle fibres of skeletal muscle - activation causes contraction.

Question 13

What is a motor neuron pool?

Answer 13

all the alpha motor neurons innervating a single muscle.

Question 14

Cross section of vertebrae showing where in grey matter alpha motor neurons are located.

Answer 14

Question 15

What is a motor unit?

Answer 15

A motor neuron and all the muscle fibres innervated by that particular motor neuron.

Stimulation of one motor unit causes contraction of all muscle fibres in that unit.

Question 16

How many motor neurons and skeletal muscle fibres does a human have?

Answer 16

420,000 motor neurons

250 million skeletal muscle fibres.

Question 17

How many muscles fibres are supplies by each motor neuron?

Answer 17

Approx. 600

Question 18

Table comparing types of motor unit:

Slow (type 1)

Fast, fatigue resistant (type IIA)

Fast, fatiguable (type IIB).

Compare cell body diameter, no. dendritic trees, axon thickness, velocity of conduction,

Answer 18

Question 19

In what 3 ways can different motor units be classified?

Answer 19

Amount of tension generated.

Velocity of conduction.

Fatiguability of motor unit.

Question 20

Compare the tension, contraction speed, and fatiguability of Type I, Type IIA and Type IIB motor units.

Answer 20

Type IIB - fast twitch, high tension, high fatigue.

Type IIA - less fast twitch, moderate tension, somewhat fatigue resistant.

Type I - slow twitch, low tension, fatigue resistant.

Question 21

What two mechanisms does the brain use to regulate the force that a single muscle produces?

Answer 21

Recruitment and rate coding.

Question 22

Explain recruitment.

Answer 22

Recruit different motor units to build the force of contraction.

Smaller units recruited first - slow twitch.

Slow –> FR –> FF.

Question 23

Explain rate coding.

Answer 23

Different motor units fire at different frequencies.

Slow units fire at a lower frequency.

As firing rate increases, force produced by the unit increases.

Summation occurs when units fire too fast to allow muscle to relax between arriving APs.

Question 24

What are the roles of the motor cortex, brainstem, and cerebellum/basal ganglia in the motor system?

Answer 24

Motor cortex - receive info from other cortical areas and relays them to brainstem and thalamus.

Brainstem - passes command to spinal cord for body muscles and directly to muscles of face, head and neck.

Cerebellum/basal ganglia - adjust commands from other parts of motor system before outputting them to the thalamus and brainstem.

Question 25

What are neurotrophic factors?

Answer 25

growth factors that prevent neuronal death and promote growth after injury

Question 26

Explain normal relative to crossed innervation.

Answer 26

normal innervation - slow units have slow nerve fibres innervating them, while fast units have rapid ones. Crossed innervation - fast and flow innervation is swapped so slow units become faster and fasr units become slower (e.g. Soleus faster, FDL slower).

Question 27

Define plasticity and give some examples of it

Answer 27

The ability of fibre type to change. IIB --\> IIA after training I --\> II after severe deconditioning or spinal cord injury (or space microgravity) Ageing - preferential loss of type II.

Question 28

What is the role of pyramidal corticospinal tracts?

Answer 28

Control voluntary movements

Question 29

What is the role of reticulospinal tracts?

Answer 29

coordinate automated movements of locomotion and posture e.g. To painful stimuli

Question 30

What is the role of the vestibulospinal tract?

Answer 30

regulate posture to maintain balance and uses anti-gravity muscles.

Question 31

What is the role of the rubrospinal tract?

Answer 31

automatic movements of arm in response to posture/balance changes.

Question 32

Which tags correspond to the: lateral and antrerior corticospinal tracts (pyramidal) rubrospinal, reticulospinal, vestibulospinal, olivospinal tracts (extrapyramidal)?

Answer 32

1. a,b 2. a,b,c,d

Question 33

What is a reflex?

Answer 33

involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli.

Question 34

Diagram showing components of a reflex arc.

Answer 34

Question 35

Compare monosynaptic stretch reflexes with polysynaptic flexion withdrawal reflexes.

Answer 35

Polysynaptic involves activated interneurons at several spinal cord levels

Question 36

Diagram showing monosynaptic reflex.

Answer 36

Question 37

Give the steps of a polysynaptic flexion withdrawal reflex.

Answer 37

1. step on painful stimulus --\> sensory receptor stimulated --\> sensory neurone stimlated. 2. sensory neurone activates interneurons in several spinal cord segments within integrating centre 3. interneurons as/descend several levels and stimulate motor neurones from several levels to contract flexor muscles to move leg.

Question 38

Explain crossed extension.

Answer 38

interneurons cross the cord and stimulate effectors on opposite limb to contract and extens other limb.

Question 39

Define hyporeflexia.

Answer 39

below normal/absent reflexes. mostly associated with LMN disease.

Question 40

Define hyperflexia.

Answer 40

supra-normal reflexes.

Question 41

Explain supraspinal control of reflexes.

Answer 41

Higher CNS centres regulate stretch reflex. Under normal conditions, inhibitory control dominates. Decerebration: reveals excitatory control from supraspinal areas - leading to rigidity and spasticity (overactive and tonic stretch reflex)

Question 42

Via which neurons do higher centres of the CNS regulate reflexes?

Answer 42

1. Activating alpha motor neurons 2. Activating inhibitory interneurons 3. Activating propriospinal neurons 4. Activating gamma motor neurons 5. Activating terminals of afferent fibres

Question 43

What spinal pathways are involved in reflex regulation?

Answer 43

Rubrospinal: red nucleus for arms in balance Vestibulospinal: vestibular nuclei for balance Tectospinal: head movements due to visual information