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Flashcards in The sensorimotor system Deck (15):

Muscle innervation by motor neurons of the spinal cord

- 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.


Motor unit

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.



act to bend or flex a joint

flexors and extensors work antagonistically



act to straighten a joint

flexors and extensors work antagonistically


Muscle spindles

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


Hierarchical control in the sensorimotor system

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


Somatopic map of the primary motor cortex

- 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


Basal ganglia

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



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).


Huntington's disease

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.


Parkinson's disease

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.


single finger flexion

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.


finger sequencing

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.


imagined finger sequencing

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).



- 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.