Somatic Motor Control

Question 1

Explain the difference between the somatic and autonomic parts of the motor division in PNS

Answer 1

The somatic nervous system
innervates skeletal muscle, this type of muscle is voluntary and so somatic nerves control
body movements.

The autonomic nervous system innervates blood vessels and viscera, which are composed of smooth muscle, and the heart which is composed of cardiac muscle. These types of muscle are involuntary and so the autonomic nervous system controls unconscious muscle activity, it also controls glandular secretion.

Question 2

Motor areas of Frontal Lobe (explain)

Answer 2

Motor association (premotor) area:
generates a plan for the degree and sequence of muscle
contraction and relays this information to the primary motor area

Primary motor area (precentral gyrus):
from here the ‘program’ is sent
out along the relevant pathway to eventually reach the appropriate muscles

Question 3

Motor Homunculus (describe and explain)

Answer 3

This term refers to the map of body parts which can be superimposed over the surface of the cerebral cortex.

The amount of cortex in the primary motor area for a given body region is proportional to the number of muscles and motor units in that region

Areas that require fine motor control, such as the hand, possess a higher number of individual muscles, and those muscles express higher numbers of motor units than muscles found elsewhere. The greater the degree of control over muscle activity required to carry out movements of a given body part, the larger the area of cortex devoted to that control

Question 4

Corticospinal tract (describe and explain)

Answer 4

The pathway which motor neurones take to reach skeletal muscle

Most fibres in this tract cross in the medulla oblongata on
their way to the spinal cord (80%). The remaining 20% cross at the level of the innervated
muscle. This means that the motor areas of the left side of the brain control the skeletal
muscle of the right side of the body and vice versa.

The corticospinal tracts innervate the distal muscles of the body, i.e. the furthest from the trunk, these are the muscles which mediate
voluntary skeletal muscle activity.

Question 5

Describe the other two descending tracts involved in motor control

Answer 5

Reticulospinal tract (from the reticular formation)

Vestibulospinal tract (from the vestibular nucleus)

neither of these tracts cross

These tracts are thought to be important in balance and posture, innervating proximal muscles i.e. the muscles closest to the trunk to do this, with the latter particularly associated with the control of head position. Regulating balance
and posture is important since it creates a stable background for voluntary body movement.

Question 6

Contribution of cerebellum (explain)

Answer 6

Located just above the brainstem and beneath the occipital lobes at the base of the skull. It receives signals from the primary motor cortex about intended movements, as
well as signals from sensory receptors in skeletal muscle (the proprioceptors) and the ear about actual movements occurring. This helps to smooth movements and is important in
balance and posture.

Question 7

Contribution of Basal Ganglia (explain)

Answer 7

Our knowledge of the precise function of the basal ganglia is very limited; however
abnormalities associated with diseases of the basal ganglia have conferred two functions.
These areas are believed to be important in initiating movements and inhibiting resting muscle tone.

Question 8

Contribution of Muscle Spindles (explain)

Answer 8

Important in posture control

This type of proprioceptor consists of a number of highly specialised muscle fibres within a collagenous capsule. These muscle fibres are known as intrafusal fibres and have a central portion which is devoid of actin and myosin. Because the central portion doesn’t contract it is able to act as a sensory receptor. Since the intrafusal fibres run in parallel with the normal contractile fibres (extrafusal fibres) they detect changes in muscle length. So when the muscle
is stretched the spindles are also stretched.

As well as being important in proprioception, activation of muscle spindles also results in the activation of a reflex arc, this involves activation of motor neurones (via the spinal cord) to the stretched muscle itself so that the muscle contracts more forcefully. This response is called the stretch reflex and it helps to maintain posture. For example, if your head starts to tip forward, it stretches muscles at the back of your neck, stimulating the muscle spindles which in turn relay signals to the spinal cord. This results in the activation of motor nerves innervating those muscles, causing them to contract more forcefully, raising your head,

Question 9

Contribution of Golgi Tendon Organs

Answer 9

These mechanoreceptors are nerve endings which innervate tendons and detect differences
in muscle tension rather than length. They discharge impulses under two conditions:
* In response to excessive tension created in the muscle when it shortens itself
* In response to excessive tension created in the muscle when it is passively stretched

As well as sending signals to the cerebellum they also bring about reflex inhibition of muscles that they supply, protecting muscles from being overstretched.

Question 10

Posterior and Anterior Spinocerebellar tracts (describe and explain)

Answer 10

The tracts carrying signals from proprioceptors to the cerebellum

These tracts therefore allow the cerebellum to gather
information about the degree of stretch in the muscle, the tension generated by individual
muscles and the relative positions of parts of the body at any one time. This can also be referred to as unconscious proprioception.