Somatic Motor Control (Physiology)

Question 1

Explain the difference betweent the somatic and autonomic parts of the motor division in the 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, as well as glandular secretion.

Question 2

Describe how the motor areas of the cerebral cortex control skeletal muscle activity

Answer 2

• Two areas of the frontal lobe are involved in initiating and creating muscle movements.
• The motor association (premotor) area generates a plan for the degree and sequence of muscle contraction and relays this information to the primary motor area.
• The primary motor area (precentral gyrus) sends the plan along the relevant pathway to eventually reach the appropriate muscles.

Question 3

Describe the motor homunculus and explain the functional significance of its organisation

Answer 3

• The motor homunculus shows that the amount of cortex in the primary motor area for a given body part 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 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.
• The pathway which motor neurones take to reach skeletal muscle is called the corticospinal tract.
• Most fibres in the corticospinal tract cross in the medulla oblongata on their way to the spinal cord while the remaining 20% cross at the level of the innervated muscle. This means that the motor areas on the left side of the brain control the skeletal muscle on the right side of the body and vice versa.
• The corticospinal tracts innervate the distal muscles in the body.
• Two other descending tracts, the reticulospinal tract and the vestibulospinal tract, are thought to be important in balance and posture, and innervating proximal muscles (the vestibulospinal tract is particularly associated with the control of head position).
• Unlike the corticospinal tract, neither the reticulospinal or vestibulospinal tracts cross.

Question 4

Explain how the cerebellum and basal ganglia contribute to skeletal muscle mediated movement.

Answer 4

• The cerebellum receives signals from the primary motor cortex about intended movements, as well as signals from proprioceptors in skeletal muscle and the ear about actual movements occurring.
• The cerebellum smoothes movements and is important in maintaining posture.
• Basal ganglia play a role in initiating movement. Individuals suffering from lesions of the basal ganglia tend to exhibit akinesia and bradykinesia.
• Basal ganglia also maintain resting muscle tone. Ischemic damage to the basal ganglia during a stroke causes spastic paralysis. Parkinsons patients exhibit rigidity and a tremor at rest.

Question 5

Describe how proprioception contributes to somatic motor control

Answer 5

• There are two types of proprioceptor important in the detection of mechanical stimuli: muscle spindles and golgi tendon organs.

Muscle spindles

• These muscle fibres are known as intrafusal fibres and have a central portion which is devoid of actin and myosin, meaning the central portion doesn’t contract and can therefore act as a sensory receptor.
• Since the intrafusal fibres run in parallel with the extrafusal fibres they detect changes in muscle length because when the muscle is stretched the spindles are also stretched.
• Activation of muscle spindles also results in the activation of a reflex arc which involves the activation of motor neurones to the stretched muscle itself so that the muscle contracts more forcefully. This response is called the stretch reflex and helps maintain posture.

Golgi tendon organs

• These are nerve endings which innervate tendons and detect differences in muscle tension.
• They discharge impulses in response to excessive tension created in the muscle when it shortens itself, and in response to excessive tension created in the muscle when it is passively stretched.
• The anterior and posterior spinocerebellar tracts carry signals from proprioceptors to the cerebellum. These tracts allow the cerebellum to gather information about the degree of stretch in the muscle, the tension generated by individual muscles, and the relative position of body parts at any one time. This can be referred to as unconscious proprioception.
• The posterior column (dorsal column) is composed of two tracts, the gracile fasciculus and the cuneate fasciculus, which cross in the medulla oblongata.
• The gracile fasciculus carries sensations of limb and trunk position and movement from vertebrae T6 and below.
• The cuneate fasciculus carries sensations from above T6.
• Signals carried via these two tracts eventually reach the somatosensory cortex of the parietal lobe to give us a conscious awareness of body position and movement. This is referred to as conscious proprioception.