The sensorimotor system

Question 1

Muscle innervation by motor neurons of the spinal cord

Answer 1

• Movements are controlled by muscles that contract in response to neural signals from efferent motor neurons projecting from the spinal cord.
• Motor neurons exit the spinal cord via the ventral root and terminate on individual muscle fibres.
• An action potential in a motor neuron triggers the release of a neurotransmitter, acetylcholine, which stimulates muscle fibres to change their length accordingly.

Question 2

Motor unit

Answer 2

The group of fibres innervated by a single motor neuron is called a motor unit. Motor units with the fewest muscle fibres, such as those in the face and hands, permit the greatest degree of selective motor control.

Question 3

Flexors

Answer 3

act to bend or flex a joint

flexors and extensors work antagonistically

Question 4

Extensors

Answer 4

act to straighten a joint

flexors and extensors work antagonistically

Question 5

Muscle spindles

Answer 5

monitor the activity of the skeletal muscles and provide information to the CNS regarding muscle length

Question 6

Hierarchical control in the sensorimotor system

Answer 6

1. association cortex (prefrontal, parietal)
2. secondary motor cortex (premotor, supplementary motor)
3. primary motor cortex
4. brainstem motor nuclei
5. spinal motor circuits

Question 7

Somatopic map of the primary motor cortex

Answer 7

• each region along the strip of the precentral gyrus represents muscles of a particular body part
• the majority of the primary motor cortex is devoted to those body parts that are capable of making fine movements, such as the mouth, lips, tongue and hand
• the primary motor cortex receives sensory feedback, via somatosensory cortex in the post central gyrus, from the muscle spindles

Question 8

Basal ganglia

Answer 8

transmits neural signals back to the cortex via the thalamus. Therefore they play a modularity role in control

1. sequencing of movement
2. initiation of voluntary movement ( select the most appropriate action from a given set of possible responses)

Huntington’s chorea and Parkinson’s disease are characterised by disfunctions of the basal ganglia

Question 9

cerebellum

Answer 9

It is thought that the cerebellum uses this motor and sensory information to monitor and correct ongoing movements that may depart from their intended course. The cerebellum is also thought to play a crucial role in aspects of timing of movements.

Lesions of the cerebellum disrupt a person’s ability to control the direction, velocity, force and amplitude (extent) of voluntary movements; damage to the cerebellum also impairs a person’s ability to adjust to changing conditions (e.g., changes in the surface or slope of the ground will cause problems with walking).

Question 10

Huntington’s disease

Answer 10

Huntington’s disease (HD) is a progressive disorder that begins with degeneration of the caudate nucleus and rapidly spreads to other nuclei of the basal ganglia, and finally to the cortex.

the patient shows a variety of motor abnormalities: clumsiness, problems with balance, and restlessness of the limbs.

Question 11

Parkinson’s disease

Answer 11

Parkinson’s disease (PD) is characterised by degeneration of dopamine-producing neurons in the substantia nigra (‘black substance’), which is located in the brainstem.

PD has several symptoms:

• Motor tremor (shaking), which is present when the limb is at rest but disappears once a movement is initiated.
• Rigidity of the body and limbs, which occurs due to simultaneous contraction of agonist and antagonist muscles.
• Festinating gait (small, shuffling steps with a stooped posture).
• Hypokinesia (difficulty initiating movements).
• Problems in sequencing and set shifting.

Question 12

single finger flexion

Answer 12

When participants performed flexion and extension movements with the index finger of their right hand, areas in the primary motor and somatosensory cortex of the left hemisphere were selectively active.

Question 13

finger sequencing

Answer 13

When the same participants were required to perform a sequence of finger movements that followed a specified pattern (index, ring, middle, index, ring, middle, etc.), increased activity was observed not only in the primary motor and somatosensory cortex of the left hemisphere, but also in the prefrontal cortex and supplementary motor areas in both hemispheres. Also active (but not shown in the figure) were the basal ganglia and cerebellum. Thus, the more complicated finger sequencing task recruited many more components of the motor hierarchy than the simple finger flexion task.

Question 14

imagined finger sequencing

Answer 14

In a final experiment, Roland and his colleagues had participants simply imagine they were performing the finger sequencing task, without actually moving their fingers at all. Only the supplementary motor area (SMA), in both hemispheres, was selectively active during this imagined movement task. This pattern of brain activity is consistent with the notion that the SMA is involved in the programming of patterns of movements, rather than in their actual execution (which is carried out by the primary motor cortex and other structures lower down in the motor hierarchy).

Question 15

Apraxia

Answer 15

• Disorder of skilled actions
• Cannot be attributed to motor weakness or sensory loss
• More frequent after left hemisphere damage than right hemisphere damage
• Degree of impairment greatest for pantomimed actions (less for imitation and real object use)

It has been suggested that the left parietal cortex plays a central role in representing learned actions, particularly those involving gesture and tool use.