Somatic motor control

Question 1

Somatic nervous system

Answer 1

• Innervates skeletal muscle
• Skeletal muscle is voluntary so somatic nerves control body movements

Question 2

Motor areas of the frontal lobe

Answer 2

Motor association area/pre-motor cortex:

• Generates a plan for the degree and sequence of muscle contraction
• Relays this info to the primary motor area

Primary motor area/pre-central gyrus/motor cortex:

• Sends signals down the relevant pathways to reach the relevant muscles

Question 3

Motor homunculus

Answer 3

• Each body part is allocated an area of cortex in the primary motor area

Size of cortex allocated to each body part is related to:
- Number of muscles in the region
- Number of motor units

• Individual motor unit is small if a high level of control is needed (ie. 1 motor neurone will innervate few muscle cells)
• These areas eg. hand will have more individual muscles, and these muscles have more individual motor units

Question 4

Descending tracts of spinal cord

Answer 4

Corticospinal tract:

• Pathway motor neurones take to reach skeletal muscle
• 80% of fibres cross in the medulla oblongata on way to SC and 20% cross at the level of innervated muscle; motor areas on the left side of brain control skeletal muscle of the right side of the body and vice versa
• Control the distal muscles of the body to produce fine, articulated movements

Reticulospinal tract (from reticular formation) and vestibulospinal tract (from vestibular nucleus):

• Neither of these tracts cross
• Control the proximal muscles ie. important for maintaining balance and posture

Question 5

Contribution of the cerebellum and basal ganglia

Answer 5

CEREBELLUM

• Receives signals from primary motor cortex about intended movements (pre-motor cortex)
• Also receives signals from sensory receptors in skeletal muscle (proprioceptors) and the ear about actual movements occurring
• If intended + actual do not match, cerebellum makes necessary changes by communicating with motor cortex (compares planned movement with reality of movement and makes necessary adjustments)
• Smoothes movements; is important in balance and posture
• Intention tremor only appears when you want to do something

BASAL GANGLIA

• Role in initiating movements
• If damaged, motor deficits are observed
• Lesions can cause akinesia and bradykinesia
• Inhibitory effect on resting muscle tone
• Ischemic damage to basal ganglia (stroke) causes:
  
  • Spastic paralysis: all muscles contract at the same
    time (muscle tone is far too great)
• Parkinson’s patient exhibit muscle rigidity and a resting tremor
• Normally, basal ganglia dampens down muscle tone at rest

Question 6

Proprioception + ascending tracts of SC

Answer 6

Proprioception:
Brain receives info about what muscles and limbs are doing at any particular point in time; info about body movements

SUBCONSCIOUS PROPRIOCEPTION:

• Info about muscle activity and body movements/positions is relayed to the cerebellum
• Information is carried by posterior and anterior spinocerebellar tract
• Allows cerebellum to carry out its function of maintaining balance and posture and smoothing movements
• Constant feedback from muscles to cerebellum, so allows cerebellum to make adjustments in order to refine movements if required

CONSCIOUS PROPRIOCEPTION:

• Info is carried by dorsal/posterior column of SC to the somatosensory cortex in parietal lobe
  
  • Gracile fasciculus: info about position and
    movement of lower chest and body parts
  • Cuneate fasciculus: info about position and
    movement of upper chest and limbs