The Sensorimotor System (i.e.
, How You Move)
A Sensory System: The sensory nervous system is a part of the nervous system responsible for
processing sensory information. A sensory system consists of sensory neurons, neural pathways, and
parts of the brain involved in sensory perception and interoception.
VS
The Sensorimotor System: The sensorimotor system is a network that includes the sensory organs
(eyes, ears, skin), parts of the nervous system, and the body's motor controls. It governs the human
body's ability to perceive and respond to the external environment, as well as its ability to move and
other functional activities.
The main difference between the sensory systems and the sensorimotor system is the primary
direction of information flow.
- In sensory systems, information mainly flows up through the hierarchy
- In the sensorimotor system, information mainly flows down.
3 Principles of Sensorimotor Control/Function:
1. The sensorimotor system is hierarchically organized.
2. Motor output is guided by sensory input.
3. Learning can change the nature and the locus of sensorimotor control.
Principle 1: The Sensorimotor System Is Hierarchically Organized
- The sensorimotor system is a parallel hierarchical system.
- Parallel hierarchical systems are hierarchical systems in which signals flow between levels
over multiple paths.
- This parallel structure enables the association cortex to exert control over the lower levels of
the hier- archy in more than one way.
- The sensorimotor system is functionally segregated.
- Each level of the sensorimotor hierarchy tends to be composed of different units (neural
structures), each of which performs a different function.
Principle 2: Motor Output Is Guided By Sensory Input
- The eyes, the organs of balance, and the receptors in skin, muscles, and joints all monitor the
body’s responses, and they feed their information back into sensorimotor circuits.
- In most instances, this sensory feedback plays an important role in directing the continuation
of the responses that produced it.
- The only responses that are not normally influenced by sensory feedback are ballistic
movements—brief, all-or-none, high-speed movements, such as swatting a fly.
- Many adjustments in motor output that occur in response to sensory feedback are controlled
unconsciously by the lower levels of the sensorimotor hierarchy without the involvement of
the higher levels.
Principle 3: Learning Changes the Nature and Locus of Sensorimotor Control
- During the initial stages of motor learning each individual response is performed under
conscious control
- Then, after much practice, individual responses become organized into continuous integrated
sequences of action that flow smoothly and are adjusted by sensory feedback without
conscious regulation.
- The organization of individual responses into continuous motor programs and the transfer of
their control to lower levels of the CNS characterize most sensorimotor learning.
Sensorimotor Association Cortex
There are two major areas:
1. The posterior parietal association cortex
2. The dorsolateral prefrontal association cortex
Posterior Parietal (Association) Cortex
- Before an effective movement can be initiated, certain information is required.
- The nervous system must know the original positions of the parts of the body that are to be
moved, and it must know the positions of any external objects with which the body is going
to interact.
- The posterior parietal association cortex (the portion of parietal neocortex posterior to the
primary somatosensory cortex) plays an important role in integrating these two kinds of
information, in directing behavior by providing spatial information, and in directing
attention.
- The posterior parietal cortex is classified as association cortex because it receives input from
more than one sensory system.
- It receives information from the three sensory systems that play roles in the localization of the
body and external objects in space: the visual system, the auditory system, and the
somatosensory system.
- Much of the output of the posterior parietal cortex goes to areas of motor cortex, which are
located in the frontal cortex: to the dorsolateral prefrontal association cortex, to the various areas
of secondary motor cortex, and to the frontal eye field—a small area of prefrontal cortex that
controls eye movements.
- Damage to the posterior parietal cortex can produce a variety of deficits, including deficits in
the perception and memory of spatial relationships, in accurate reaching and grasping, in the
control of eye movement, and in attention.
- Apraxia is a disorder of voluntary movement that is not attributable to a simple motor deficit
(e.g., not to paralysis or weakness) or to any deficit in comprehension or motivation.
- Apraxic patients have difficulty making specific movements when they are requested to do
so, particularly when the movements are out of context.
- However, apraxic patients can often readily perform the very same movements under natural
conditions when they are not thinking about what they are doing.
- Apraxia is caused by signalling problems between the brain and the muscles.
- Although its symptoms are bilateral, apraxia is often caused by unilateral damage to the left
posterior parietal cortex or its connections.
- Contralateral neglect is a disturbance of a patient’s ability to respond to stimuli on the side of the
body opposite (contralateral) to the side of a brain lesion in the absence of simple sensory or motor
deficits.
- Patients with this condition behave as if the left side of their words doesn’t exist, and this
disturbance is often associated with large lesions of the right posterior parietal cortex.
- Egocentric left OR object-based contralateral neglect.
