U8: THE SENSORIMOTOR SYSTEM
1. Principles of Sensorimotor Hierarchically Organized System
function - Has a hierarchical arrangement allowing higher levels to focus on
o Ex. Compared to a company – association cortex
(president) general commands muscle + motor
neurons (lower levels)
- After much practice, lower levels perform well-learned tasks with
little higher involvements.
- This system is also characterized by functional segregation, each
level of the senorimotor and company hierarchies tends to be
composed of different units, each of which performs a different
Motor Output is Guided by Sensory Input
- Sensory feedback is also necessary for monitoring the body’s
responses and feeding their information back into sensorimotor
- The only responses that are not normally influenced by sensory
feedback are ballistic movements – brief, all-or-none movements
Learning changes the Nature and Locus of Sensorimotor control
- When a company is starting up, decisions are made by company
president after careful consideration but as the company develops,
many individual actions are coordinated into sequences of
- Likewise, after much practice, individual responses become
organized into continuous motor programs.
Posterior Parietal Association Cortex - Receives input from visual, auditory, and somatosensory systems
and output goes to secondary motor cortices.
- Plays a role in directing behavior by providing spatial information
and in directing attention. Helps localize body and external objects
o Ex. Mountcastle et al: posterior parietal cortex uses
tactile, visual and positional information to act as a
command centre for limb, and eyes operation.
o Electrical stimulation applied to inferior portions of
posterior parietal cortex caused patients to perform a
particular action at high levels of stimulation.
- Lesions in this region produces apraxia (unable to perform
movements) and contralateral neglect (inability to respond to
signals on the opposite side of body)
o Ex. Patients with contralateral neglect often beave as if
the left side of their world doesn’t exist and they often fail
to appreciate that they have a problem.
o Associated with large lesions of right posterior parietal
o The deficit occurs for their egocentric left or the left of
their own bodies. Also applies to the left side of objects.
- Two main types of apraxia:
1. Ideomotor Apraxia
- Patients can’t copy movements or make gestures (i.e. wave hello)
and results from damage to left posterior parietal lobe
- Can`t perform movements when requested to do so, especially when
movements are out of context but they can perform the same
movements under natural conditions, when they`re not thinking
about doing so.
2. Constructional Apraxia
- Spatial organization that is disrupted. - Patients can’t assemble puzzles, build tree houses, draw a picture or
copy a series of facial movements.
- Develop after injury to either the left or right posterior parietal lobe.
- Both these can be thought of as resulting from disruption of
parietofrontal connections that control movement.
Dorsolateral Prefrontal Association
Cortex - Receives projections from the posterior parietal cortex and sends
projections to areas of secondary motor cortex to primary motor
cortex and the frontal eye field.
- Plays a role in the evaluation of external stimuli and the initiation of
voluntary reactions to them
- The activity of some neurons depends on the charactristics of
objects; the activity of others depends on the locations of objects or
the combination of both.
Primary Motor Cortex - Used electrical brain stimulation techniques to map the motor
Wilder Penfield homunculus in the human brain
- Found that stimulation of each particular cortical site activated a
particular contralateral muscle and produced a simple movement.
- Found the primary motor cortex is in the precentral gyrus of the
frontal lobe and is somatopically organized.
- Primary motor cortex is dedicated to controlling parts of the body
that are capable of intricate movements, such as the hands and
- Each site in the primary cortex receives sensory feedback from
receptors in the muscles and joints that the site influences.
o Monkeys are an exception, with 2 different hand areas in
the primary motor cortex of each hemisphere, one
receives input from skin receptors instead of from
o Helps with stereognosis – identifying objects by touch.
- The motor homunculus has a disproportionate representation of
hands and mouth
o Ex. 2 areas of each primary motor cortex control the
contralateral/ opposite hand
- Neurons in primary motor cortex code for a preferred direction of
movement, they fire most just before and during the movement and
when the movement is in the preferred direction and less as the
direction deviates from the preferred one.
- Lesions produce contralateral astereognosia, they reduce the speed
and force of contralateral movements and make it difficult to move
one body part (e.g. a finger) independently of others – do not
Current View of Primary Motor Cortex Function
- Using longer bursts of current at slightly higher intensities, this
produced seemingly natural response sequences.
o Ex.Stimulation at one site reliably produced a feeding
- Sites that moved a particular body part overlapped greatly with sites
that moved other body parts.
- The conventional view that many primary motor cortex neurons are
tuned to movement in a particular direction has also been
challenged – as monkeys moved about freely the firing of many
primary motor cortex neurons were most closely related to the end
point of a movement, not to the direction of the movement.
- If a monkey reached toward a particular location, primary motor
neurons sensitive to that target location activated regardless of the
direction of the movement that was needed to get to the target.
Remote Control of Mind - Developed technology needed for a primate to use the activity of its Nicolelis and Chapin primary motor cortex to control the movements of a robotics arm
hundreds of miles away.
- Other work involves developing a brain machine interface (BMI)
for seizure control that would function somewhat like a heart
pacemaker. It would monitor the brain’s electrical activity for
patterns that indicate an imminent attack.
- It would then deliver an electrical stimulus to quench the storm or
release antiepileptic medication.
Cerebellum and Basal Ganglia - Doesn’t participate directly in transmission of signals to spinal cord.
They integrate and coordinate the activity of structures at various
levels of the sensorimotor system
- The interconnections between sensory and motor areas via the
cerebellum and basal ganglia are the reason why damage to cortical
connections between visual cortex and frontal motor areas doesn’t
abolish visually guided responses.
- Constitutes only 10% of brain mass, it contains more than half of
the brain’s neurons
- Receives information from primary and secondary motor cortex,
information about descending motor signals from brain stem motor
nuclei and feedback from motor responses via the somatosensory
and vestibular systems.
- Cerebellum compares these 3 sources of input and correct ongoing
movements deviating from intended course.
- Cerebellum functions in fine-tuning and learning of cognitive
Aside: Witch trials in Salem had women responses as well as motor responses.
who had spastic movements were witches. Basal Ganglia
- Basal ganglia receives information from various parts of the cortex
and transmits it back to motor cortices via thalamus.
- Perform a modulatory function, contributing few fibres to
descending motor pathways, instead they are part of neural loops
that receive cortical input from various cortical areas and transmit it
back to the cortex via the thalamus.
- Many of these loops carry signals to and from the motor areas of the
- In addition to their role in modulating motor output, they also have
- Have been shown to participate in learning to respond correctly in
order to obtain reward and avoid punishment.
- Attacks 0.5% of the population and develops in people in their 50s
- Characterized by involuntary movements, tremor at rest, muscular
rigidity, disorders of posture, slowness of movement, disturbances
in speech and a masklike face but no intellectual deterioration.
- Unknown cause, but associated with the degeneration of dopamine
in cells in some of the nuclei in basal ganglia as well as neurons in
the substantia nigra.
- Treatment involves using L-Dopa, the precursor to dopamine to be
taken up by dopaminergic neurons and is converted to dopamine
- Characterized by ceaseless, involuntary, jerky movements and
progressive dementia caused by a loss of cells in the basal ganglia
- Unlike Parkinsons, it is associated with dementia.
- Disease is caused by a single gene so 50% of all offspring of a
Huntington’s disease is caused by a single dominant gene so 50% of
all offspring of a Huntington’s parent will get it.
- Symptoms don’t appear until after reproduction, so this disease is
passed on. 2. Secondary Motor Cortex - Areas of the secondary motor cortex are those that receive much
of their input from the association cortex and send much of their
output to primary motor cortex
- Only 2 areas of secondary motor cortex were known: the
supplementary motor area and the premotor cortex
1. Supplementary motor area:
- Wraps over the top of the frontal lobe and extends down its medial
surface into the longitudinal fissure
2. Premotor cortex
- Runs in a strip from the supplementary motor area to the lateral
- There are at least 8 areas of secondary motor cortex in each
hemisphere, each with its own subdivisions.
o Ex. Three different supplementary motor areas, 2
premotor areas (dorsal and ventral), 3 small areas
(cingulate motor areas)
- Areas of secondary motor cortex are involved in the programming
of specific patterns of movements after taking general instructions
from dorsolateral prefrontal cortex.
o Ex. Brain imaging studies in which the patterns of activity
in the brain have been measured while the subject is either
imagining his or her own performance of a series of
- Mirror neurons are neurons that fire when an individual performs
a particular goal-directed hand movement or when they observe the
same goal-directed hand movement performed/copied by another
- This discovery is significant because they provide a mechanism for
social cognition – knowledge of the perceptions, ideas, and
intentions of others
- Mapping the actions of others onto one’s own action repertoire
would facilitate social understanding, cooperation and imitation.
- Mirror neurons reacted to the sight of an action that made a sound
were found to respond just as robustly to the sound alone (e.g.
cracking a peanut)
- Mirror neurons have also been found in the inferior portion of the
posterior parietal lobe, which respond to the purpose of an action
rather than to the action itself.
- Although not confirmed in humans, fMRI studies have found areas
of human motor cortex that are active when a person performs,
watches or imagines a particular action.
3. Descending Motor Pathways - Neural signals conduct from primary motor cortex to the motor
neurons of the spinal cord over 4 different pathways:
1. 2 pathways descend in the dorsolateral region of spinal cord
2. 2 descend in the ventromedial region of the spinal cord Dorsolateral Corticospinal Tract and Dorsolateral corticospinal tract
Dorsolateral Corticorubrospinal Tract - Group of axons descending from primary motor cortex through the
medullary pyramids (2 bulges on the ventral surface of the medulla)
decussates descend in the contralateral dorsolateral spinal
- Betz cells are extremely large pyramidal neurons of the primary
motor cortex are most notable neurons of these axons.
- Axons synapse on small interneurons of spinal grey matter
synapse onto motor neurons of distal muscles of wrist, hands,
fingers and toes.
- Second group of axons that descends from primary motor cortex
synapses in the red nucleus of the midbrain decussate
descend through the medulla where some of them terminate in the
nuclei of the cranial nerves to control muscles of the face.
Dorsolateral Corticorubrospinal Tract
- The rest of the axons continue to d