Sensory and Motor Innervation of Muscle

Question 1

Where are voluntary movements planned, initiated and directed?

Answer 1

Motor Cortex

Question 2

What is the role of the Basal Ganglia in the descending motor pathway?

Answer 2

The Basl Ganglia helps to process and modulate proper initiation of movement.

It helps to dampen down movement. E.g. Parkinson’s disease is loss of neurons in the BG, and so there is a loss of control (tremors).

Question 3

What is the role of the cerebellum in the descending motor pathway?

Answer 3

The cerebellum provides feedback to adjust and smooth movement.

It helps with sensory and motor co-ordination.

It makes sure the correct movement is carried out.

Question 4

Describe the descending motor pathway.

Answer 4

1) Motor Cortes
(plans / initiate voluntary movement) (*Basal Ganglia modulates)

2) Brainstem Centres
(basic movement and postural control) (*Cerebellum smooths and co-ordinates)

3) Local Circuit Neurons / Interneuron / relay neuron
(most neurons go via )
OR
Motor Neuron Pools (LMN)

4) Skeletal muscles

Question 5

Which Brodmann’s Areas are involved in voluntary movement?

Answer 5

Brodmann’s area 4 and 6

Question 6

What are Betz and Non-betz cells?

Answer 6

Betz Cells (pyramidal cells) are upper motor neurons that make up the indirect pathway to the LMN (e.i. Via local circuit neurones)

Betz Cells are the largest neurones.

Non-Betz cells are the upper motor neurones that make up the direct pathway to the LMN (e.i. go directly to the motor neuron pools)

Question 7

Where is the primary motor cortex located?

Answer 7

Precentral Gyrus

Question 8

In what cortical layer are the upper motor neurones?

Answer 8

Pyramidal Cells (UMN) are in cortical layer 5.

Question 9

Describe the pathway of the corticospinal (pyramidal) tract.

Answer 9

1) Primary motor centre (precentral gyrus) gives rise to neurone.
2) via posterior limb of interal capsule to brainstem.
3) Cerebral peduncles (midbrain) through pons.
4) 95% of axons decussate at medulla and descend contralaterally as the posterior/laterl corticospinal tract

5) 5% descend ipsilaterally (as anterior/ventral corticospinal tract) then cross at level of spinal segment.
6) at level of exit, synapse in ventral grey horn. LMN exits to innervate muscle.

Question 10

what is the role of the “red nucleus”

Answer 10

Motor coordination

Question 11

How is muscle innervated by Lower Motor Neurones?

Answer 11

• The efferent axon exits the spinal cord via the ventral root and spinal nerve.
• Alpha motor neurons generate force of muscle.
• Gamma motor neurons sense proprioception.
• Each muscle is comprised of many fibers.
• Each muscle fiber is innervated by a single axon.

Question 12

In which segments is Lamina IX (grey matter in ventral horn) largest?

Answer 12

Cervical and Lumber enlargements.

Question 13

What are motor neuron pools?

Answer 13

Each muscle is innervated by its own group of alpha motor neurones. This group is known as a motor neuron pool.

Question 14

Where does the motor neuron pool lie?

Answer 14

Rostro-caudally along the spinal cord axis.

Question 15

What is the difference between the motor neuron pool of Gastrocnemius and soleus?

Answer 15

Gastrocnemius is a muscle that has a lot more power and force than soleus. So has large motor neuron pool as it is controlled by more LMN.

Question 16

What is the distribution of Lower motor neurons in the spinal cord in relation to the muscle it innervates?

Answer 16

Motor neurones that control axial muscles lie medial to those controlling distal muscles.

Question 17

What is a motor unit?

Answer 17

Is the alpha motor neuron and all the muscle fibers that it innervates.

*1 muscle fiber can only be innervated by one alpha axon. However 1 alpha axon provide innervation to more than 1 muscle fiber.

Question 18

Why and how do motor units vary in size?

Answer 18

Small motor unit provide little force e.g.extraocular muscle of the eye may have 3 muscle fibres : 1 alpha MN.

Large motor neurone provide more force e.g. Gastrocnemius has 1000 muscle fibres : 1 alpha MN.

This allows variability in the degree of force produced and the level of control needed. (gastrocnemius needs a lot of force but eye muscles need precision)

More precision needs more neurons.

Question 19

What is a slow motor unit?

Answer 19

Slow muscle fibers are composed of small motor units. They generate low force bu are resistant to fatigue.

Slow fibres work at maximum force throughout activity.

It is Red muscle: high in myoglobin, mitochondria and have a good blood supply. This allows aerobic respiration.

Question 20

What are fast fatigable motor units?

Answer 20

They are composed of large motor units and larger pale muscle fibers that generate large forces but are easily fatigues.

The have fewer myoglobin, so less O2. Also less mitochondria so quickly start to generate ATP via anaerobic respiration and so generate lactic acid quicker.

Question 21

What are fast fatigue resistant motor units?

Answer 21

Moderate force and some fatigue.

between slow and fast units

Question 22

How can motor unit activity be measured?

Answer 22

Electromyograph - needle used to examin electrical activity of motor unit.

Question 23

Describe how muscle acitivity can go from a twitch to a smooth contraction.

Answer 23

• At low frequency Motor neuron activated&raquo_space;>produces muscle twitch.
• Increasing frequency of MN firing leads to summation of twitch
• Higher frequency of MN firing leads to smooth muscle contraction.

(muscle fibers are activated by the next action potential before they have time to relax)

Question 24

What is a muscle spindle?

Answer 24

• DETECTS CHANGE IN MUSCLE LENGTH.
• contributes to proprioception.
• MAINTIANS MUSCLE TONE - interaction between muscle spindle and alpha motor neurons ensure muscles are always under some degree of stretch

Question 25

What is the structure and innervation of the muscle spindle?

Answer 25

Spindle lies parallel to muscle fibres.

2 types of Spindle:

Chain:

• innervated by Ia fibres and II afferent fibres.
• static change

Bag:

• innervated by only Ia afferent fibres.
• dynamic response

Question 26

What is the role of gamma motor neurons?

Answer 26

Gamma MN regulate the length of the muscle spindle.

Intrafusal fibres (spindles) have a sensory function and do not generate tension.

Intrafusal fibres recieve their motor innervation from gamma motor neurons.

Question 27

What is the role of the golgi tendon organ.

Answer 27

Golgi tendon organ is a mechanoreceptor.

Detects change in MUSCLE TENSION.

Innervated by Ib afferents.

Question 28

What is the difference between muscle spindles vs. golgi tendon organs.

Answer 28

Both are sensory .

Spindles detect : MUSCLE LENGTH.

Golgi tendon organ detects :
MUSCLE TENSION.

Question 29

What is an isometric contraction?

Answer 29

Tension without change in muscle length.

Golgi tendon organ and Ib afferent fibres detect

Question 30

What is an isotonic contraction?

Answer 30

Change in length but not tension.

spindle and Ia afferent fibres detect

Question 31

What is upper motor neurone syndrome?

Answer 31

Damage to descending motor pathways gives rise to upper motor neurone syndrome.

Normally if a pin is run from heel to toe, the normal plantar response will be seen where the toes curl down.

In UPNS the toes fan - this is Babinski sign.
also there is a loss of fine movements.

Question 32

What is Motor Neuron Disease?

Answer 32

Presents as muscle weakness and atrophy at an early age. (steven hawkings)

no loss of cognitive function.