Sensory and Motor Innervations of Muscles

Question 1

Upper motor neurons originate from which two areas of the brain?

Answer 1

Motor - planning, initiating and directing voluntary movements. Corticospinal
Brainstem - basic movements and posteral control. Cranial nerves

Question 2

What two parts of the brain assist the upper motor neurons in proper movement and coordination/recognising errors?

Answer 2

Basal ganglia

Cerebellum

Question 3

What is the local circuit neurons?

Answer 3

most impulses travel down this route
via interferons
involved in reflex action

Question 4

What are motor neuron pools?

Answer 4

Route involved in innervating spinal muscles

Pattern generation - hard to regain function when this goes wrong

Question 5

What Brodmann’s areas are involved in the motor cortex?

Answer 5

4 & 6

Question 6

What are the two tpes of pyramidal cells?

Answer 6

BETZ - part of local circuit

Non BETZ - lower motor neuron circuit

Question 7

Why do around 5% of fibres in the corticospinal tract descend ipsilaterally and don’t cross till the spinal cord?

Answer 7

essential for postural muscles that have to have both sides working at the same time

Question 8

How many axons innervate each fibre in a muscle?

Answer 8

One!

Not enough for all muscles therefore fibres diverge

Question 9

What do alpha neurons do?

Answer 9

Generation of force by muscles

Question 10

What do gamma neurons do?

Answer 10

Proprioception

Question 11

What is the segmental distribution of LMN in the spinal cord

Answer 11

Ventral horn
Flexors and Axial muscles = further dorsal
Extensors and distil muscles = further lateral
See slide 12 for image of flexed arm fitting into the ventral horn

Question 12

Explain where in the body small, medium and large motor units would be used and what for?

Answer 12

Small: extraoccular muscles of the eye - fine movement
Medium: Soleus muscle - postural
Large: Gastrocnemius - power

Question 13

In what order do motor units get recruited?

Answer 13

Small to large

Gradual increase in force

Question 14

What are the adaptive features of red muscle fibres?

Answer 14

Resistant to fatigue
Low force
high in:
Myoglobin
Blood supply
Mitochondria

Question 15

Where are red and pale muscle fibres located?

Answer 15

Red = small motor units
Pale = large motor units

Question 16

What is a electromyograph used for?

Answer 16

recording motor unit activity

uses a needle

Question 17

At what frequency of motor neuron firing does initiation of movement start and continue to cause smooth movement?

Answer 17

10-20Hz

Question 18

What is muscle tone?

Answer 18

The muscles always being under some degree of stretch, caused by muscle spindles and alpha motor neurons

Question 19

What are muscle spindles?

Answer 19

Intrafusal muscle fibres
Sensory receptors within the belly of a muscle
Detect changes in the length of this muscle - important in proprioception
Lies parallel to muscle fibre

Question 20

What are the two main classes of spindles and what do they do?

Answer 20

Chain - Ia and II fibres, continue firing as long as muscles are stretched
Bag - Ia fibres only, involved in rapid stretching

Question 21

What are the differences between alpha and gamma fibres?

Answer 21

Alpha: innervate extrafusal fibres = detecting stretch
Gamma: innervate intrafusal (spindle) fibres = keep spindles taunt to stop the muscle fibres around it collapsing

Question 22

How do alpha and gamma fibres respond to each other?

Answer 22

If intrafusal fibre is shortened by alpha fibres then extrafusal fibre collapses = loss of sensitivity
Gamma fibres causes extrafusal fibres to contract the same amount to compensate

Question 23

What is the Golgi Tendon Organ?

Answer 23

Mechanoreceptor
Detects changes in muscle tension (not length) = proprioception
Innervated by Ib fibres
Made of collegen and axons intertwined

Question 24

What is the Babinski sign and when is it seen?

Answer 24

tickling bottom of foot = toes to point up and fan out
Seen in:
Upper motor neuron syndrome
Newborns - immature corticospinal tract