Chapter 9: The Sensorimotor System
9.1 Three Principles of Sensorimotor Function:
Pinel introduced these principles with a description of Rhonda the cashier, and also gave
examples of how the principles would govern the operation of a large, efficient company.
1. Hierarchical organization
- Association cortex represents the highest level of processing (general goals for
behavior), whereas lower levels are concerned with the specific plans of action.
- Association cortex represents the president of the company and the workers
represent the muscles. This type of organization leaves the higher levels free to
perform more complex functions.
- Functional segregation refers to the fact that at each hierarchical level there are
different units (neural structures or departments) that perform different functions
in parallel.
2. Motor Output is Guided by Sensory Input
- Sensory systems continually monitor the progress of responses and feed their
information back into sensorimotor circuits.
- 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 (e.g., swatting a
fly).
- Many of the motor adjustments that occur in response to sensory feedback are
controlled by the lower levels of the sensorimotor hierarchy. For example, the
workers at the large company wouldn’t want to disturb the presendent every time
a minor problem occurred.
- Case G.O. - an infection selectively destroyed the somatosensory nerves of the
arms and impaired performance of intricate responses (doing buttons, picking up
coins) even under visual guidance.
- He could not adjust his motor output to an unexpected external disturbance (e.g.
he couldn’t avoid spilling a cup of coffee if someone brushed against him)
- G.O.’s greatest problem was not being able to maintain a constant level of muscle
contraction.
- How did the destruction of his somatosensory nerves affect his dart game? Pinel
doesn’t tell us, but throwing darts may be considered a ballistic movement.
- However, since G.O. had difficulty picking up coins, even under visual guidance, it
seems unlikely that his dart game was still championship quality.
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3. Learning changes the nature and locus of sensorimotor control
- During the early stages of motor learning each response is performed under
conscious control (representing the highest levels of processing).
- However, after sufficient practice, the skills become organized into continuous
integrated sequences of action that are adjusted through sensory feedback,
without conscious regulation.
- In the business example, when a company is just starting out the president makes
all the decisions; however, as the company grows, junior executives make the
more routine decisions and carry out prescribed procedures.
For example, Hassler's secondary automatisms:
“...like walking, running, ascending and descending stairs, bicycling, brushing one’s
teeth, changing gears and using the clutch in automobile driving, can all be
performed without intentional or conscious control once such skills have been
thoroughly acquired. To acquire such secondary automatisms, however,
intentional activity and voluntary concentration of attention and repeated conscious
efforts are necessary. Before they become secondary automatisms, they belong to
such intentional actions as lighting a match, shaving, doing one’s hair, reading,
writing, driving, crossing a street, and all activities which need constant visual,
auditory, or somatosensory control. One must realize that most of our motor
actions, the reactive and mainly the spontaneous ones, are originally initiated by
conscious intentions and do not take place without conscious representation and
interference.” (Hassler, 1978, p. 189)
Hassler postulated that the formation of secondary automatisms is mediated by the
basal ganglia.
9.2 Sensorimotor Association Cortex
1. Posterior Parietal Cortex
- is important for integrating sensory information (e.g., the spatial positions of
external objects and parts of the body) that is necessary for initiating voluntary
responses.
OVERHEAD T38
- This area receives input from 3 sensory systems important for defining the spatial
locations of objects and body parts; somatosensory, visual and auditory
(vestibular) systems.
- Output of this area is directed to dorsolateral prefrontal association cortex, the
secondary motor cortex and to the frontal eye field - which is a small area of
the prefrontal cortex that controls eye movements.
Damage to the posterior parietal cortex can result in two movement disorders:
Apraxia and Contralateral Neglect.
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- Apraxia is a disorder of voluntary movement that is not attributable to motor,
comprehension, or a motivational deficit.
- Apraxic patients have difficulty making specific movements upon request, but are
able to perform the same movements when they are not thinking about what they
are doing.
- Apraxia is associated with left posterior parietal damage (i.e., unilateral damage)
even though the movement disruption is bilateral.
- Right posterior parietal damage is associated with constructional apraxia - a
bilateral disruption of movements designed to assemble components of an object
to form a whole.
- There are two tests sensitive to constructional apraxia:
(1) WAIS block design subtest
(2) WAIS object assembly (basically a jigsaw puzzle)
Contralateral neglect
- is a disturbance in the ability to respond to visual, auditory, and somatosensory
stimuli on the side of the body contralateral to the side of brain damage.
- it is associated with large lesions of the right posterior parietal lobe (e.g., Mrs. S.
who only eats from the right half of the plate and makes up one side of her face).
2. Dorsolateral Prefrontal Association Cortex *
- receives projections from the posterior parietal cortex
- sends projections to the secondary motor cortex, primary motor cortex and the
frontal eye field.
- Based on the discovery that many dorsolateral prefrontal neurons have memory
fields (oculomotor delayed-response task) it has been suggested that one of the
functions of this association cortex is to provide a mental representation of stimuli
to which the subject will respond.
- Since neurons in the dorsolateral prefrontal cortex seem to be the first to respond
in anticipation of a motor response the decision to make a voluntary response
may be made by this association cortex.
9.3 Secondary Motor Cortex
- Secondary motor cortex includes 4 areas: the supplementary motor area, the
premotor cortex and two cingulate motor areas.
- Receives most of its input from association cortex and sends most of its output to
the primary motor cortex.
- In addition, areas of secondary motor cortex are reciprocally connected with each
other and with the primary motor cortex, and send direct output to the motor
circuits of the brainstem.
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- Secondary motor cortex is somatotopically organized - i.e., organized
according to a map of the body.
- Input to the supplementary motor area is primarily somatosensory, whereas input
to the premotor area is primarily visual.
- Electrical stimulation of each secondary motor cortical region results in complex
movements of corresponding body parts and neurons in each region responds
prior to and during voluntary motor responses.
OVERHEAD T39
1. Supplementary Motor Area (SMA)
- located within and adjacent to the lateral fissure, anterior to the primary motor
cortex
- Lesions of SMA impair the ability to perform movements in proper sequence. For
example, monkeys with unilateral SMA lesions can perform all the individual
movements involved in picking up a peanut with the contralateral hand, but
cannot coodinate the movements properly.
- The SMA is particularly important for learning new motor sequences: neurons that
were active during learning an arm movement are not active after learning. Also,
neurons are activated by internal representations in memory rather than by
external stimuli.
2. Premotor Cortex
- located between the SMA and the lateral fissure, anterior to the primary motor
cortex
- The premotor cortex is involved in voluntary movements that are carried out
under the guidance of external stimuli rather than internal representations.
- In a PET study, the human premotor cortex was activated when subjects tapped
in time to a metronome but not when they tapped in the absence of an external
sensory influence.
9.4 Primary Motor Cortex
- is located in the precentral gyrus and is the point of convergence and departure of
cortical sensorimotor signals
- The motor homunculus is the somatotopic map of the human primary motor
cortex. Areas such as the hand and face are allocated larger areas of
representation in the primary motor cortex due to the complex nature of
movements associated with these body areas.
- Recent evidence suggests that there are two areas in each hemisphere that
control the contralateral hand.
- Recordings of the hand area during movements of individual fingers show that
finger neurons are involved in the movement of more than one finger and that
there is overlap in the locations of neurons that control the various fingers.
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- Each area of the primary motor cortex controls movements of a particular group
of muscles and receives sensory feedback from receptors in the muscles and
joints via the somatosensory cortex.
- One exception to this organization involves direct sensory input from receptors in
the skin to one of the hand areas in the primary motor cortex of each hemisphere.
- Presumably, this direct input is responsibile for stereognosis - the process of
identifying objects by touch.
- Although the primary motor cortex is the major departure site of cortical motor
fibers, lesions of this area do not cause paralysis. Rather, primary motor damage
disrupts the ability to move one body part independently of others, produces
astereognosia, and reduces the speed, accuracy and force of body movements.
9.5 Cortical Blood Flow Studies: A Recap of the Sensorimotor
Areas of the Human Cerebral Cortex
- Changes of regional blood flow associated with 4 activities:
(1) a sequence of finger movements of the right hand increased blood flow in
prefrontal cortex, SMA, and the hand area of the primary motor and
somatosensory cortices
(2) just thinking of the finger movements activated the SMA only
(3) forceful flexion of one finger increased blood flow to the hand areas of the
primary motor and somatosensory cortices
(4) doing a fingermaze test blindfolded while receiving verbal instructions
increased activity in posterior parietal cortex, prefrontal cortex, SMA, premotor
area and the hand areas of primary motor and somatosensory cortices.
- Basically these findings support what was previously said about sensorimotor
cortex; to see the specific conclusions (in one of the longest sentences I’ve ever
seen in a textbook) refer to page 219.
9.6 Cerebellum and Basal Ganglia
- Pinel says that the cerebellum and basal ganglia are not part of the descending
sensorimotor hierarchy, but do modulate and coordinate the activities at different levels.
1. Cerebellum
- constitutes 10% of the mass of the brain but contains more than half of the brain's
neurons.
- The 3 main sources of input to the cerebellum are from the primary and
secondary motor cortex, brainstem motor nuclei, and somatosensory and
vestibular systems.
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- The cerebellum is thought to correct ongoing movements through its output
connections with brainstem motor nuclei, and may play a role in learning and
cognition, although this suggestion remains controversial.
- Lesions of the cerebellum have devastating effects; influencing the direction,
force, velocity and amplitude of movements and the ability to adapt patterns of
motor output to changing conditions. Cerebellar lesions also influence balance,
gait, speech and the control of eye movements.
2. Basal ganglia
- like the cerebellum the basal ganglia are important for modulating sensorimotor
function.
-They receive information from various areas of cortex and transmits information
back to cortex via the thalamus (cortico-striatal-thalamocortical loops).
- Support for the view that the basal ganglia are involved in learning comes from a
study of tonically active neurons (TANs) in the striatum; clicks that had been
paired with juice acquired the ability to inhibit firing and the ability was lost with
extinction (Graybiel et al., 1994) (this is not strong evidence - e.g. there are
neurons in the hippocampus that fire after a CS is learned, however lesions of the
hippocampus do not impair classical conditioning).
- What about converging operations? There are a lot of studies that show that the
striatum is involved in learning. Pinel could have made a much stronger
arguement!
- Although, Pinel’s discussion is an improvement from the last edition where
learning wasn’t even mentioned for the basal ganglia but was stressed for the
cerebellum (note that now the cerebellum’s role in learning is described as
“controversial”). Maybe it would be better to just describe some actual data, so
that the readers could make their own INFORMED assessment.
9.7 Descending Motor Pathways
Neural signals are conducted from the primary motor cortex to the motor neurons
of the spinal cord over 4 different pathways: 2 dorsolateral and 2 ventromedial
pathways.
OVERHEAD T40
1. Dorsolateral corticospinal tract - Betz cells (primary motor cortex)  axons
cross at the medullary pyramids and continue to descend in the contralateral
dorsolateral white matter of the spinal cord  terminate on motor neurons in the
lower spinal cord that project to leg muscles; other fibers terminate on
interneurons of the spinal grey matter which project to distal muscles of the wrist,
hands, fingers and toes. This tract is important for voluntary movement of the
legs and independent movements of the digits.
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2. Dorsolateral corticorubrospinal tract - axons from the primary motor cortex
synapse in the red nucleus of the midbrain  fibers of red nucleus neurons
decussate and descend through the medulla where some terminate on nuclei that
send fibers through the cranial nerves and innervate muscles of the face; the rest
of the fibers synapse on interneurons, that in turn synapse on motor neurons that
control distal muscles of the arms and legs.
OVERHEAD T41
3. Ventromedial corticospinal tract - long axons descend ipsilaterally from the
primary motor cortex into the ventromedial white matter of the spinal cord. These
axons branch diffusely and innervate interneurons of several spinal segments on
both sides of the spinal grey matter.
4. Ventromedial cortico-brainstem-spinal tract - motor cortex axons feed into a
complex network of brainstem structures  brainstem axons then desend
bilaterally, carrying signals from both hemispheres, in the ventromedial white
matter of the spinal cord and innervate interneurons at multiple spinal segments.
Four brainstem structures:
tectum - receives auditory and visual information about
spatial location.
vestibular nucleus - receives balance information from
receptors in the semicircular canals.
reticular formation - contains motor programs for complex
species common movements.
motor nuclei - of cranial nerves that control muscles of the face.
The dorsolateral tracts differ from the ventromedial tracts in two major ways:
1) Ventromedial tracts are much more diffuse; their axons innervate interneurons
on both sides of the spinal gray matter and at multiple segments. Dorsolateral
tracts terminate in the contralateral half of one segment, sometimes directly on a
motor neuron.
2) Ventromedial tracts innervate motor neurons that project to proximal muscles of
the trunk and limbs; Dorsolateral innervate motor neurons that project to distal
muscles.
Experiments by Lawrence and Kuypers (1968):
- Bilateral lesions of the dorsolateral corticospinal tract in monkeys caused a
transient impairment in reaching (weak and poorly directed); also, the monkeys
could not move their fingers independently of one another and could not release
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objects from their grasp (however, they could release the bars of the cage when
climbing).
- Additional lesions of dorsolateral corticorubrospinal tract resulted in an impairment
in reaching (they could only throw their arms out and rake the object toward
them).
- Combined dorsolateral corticospinal and complete ventromedial transection
caused severe postural abnormalities; difficulty standing, walking and sitting.
They could not control their shoulders.
Conclusions:
1) Both dorsolateral tracts control reaching movements of the limbs, so there is a
certain degree of redundancy that was presumably responsible for the recovery
after the initial dorsolateral corticospinal tract lesion; however, since these animals
never regained the ability to move their digits independently of one another,
independent digit movement must be a specific function of the dorsolateral
corticospinal tract.
2) The two ventromedial tracts are involved in the control of posture and wholebody movements such as walking and climbing (and can exert control over the limb
movements involved in such activities).
9.7 Sensorimotor Spinal Circuits
Muscles
motor unit - a single motor neuron and all the muscle fibers it innervates.
motor pool - all motor neurons that innervate the fibers of a single muscle.
neuromuscular junction (acetylcholine) - the synapse between the terminals of
motor neurons and the membrane of muscle fibers.
motor end plate - the postsynaptic area of muscle fibers.
fast and slow muscle fibers - Fast fibers contract and relax quickly, generate great
force and fatigue quickly because of poor vascularization. Slow muscle fibers are
slower and weaker but capable of more sustained contraction because of rich
vascularization.
flexors and extensors - flexor muscles bend or flex a joint while extensors
straighten or extend them.
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synergistic and antagonistic muscles - two muscles are synergistic if their
contraction produces the same movement whereas those that work in opposition
are antagonistic.
isometric and dynamic contraction - isometric contraction does not shorten or pull
bones together whereas dynamic contraction does.
The Receptor Organs of Muscles
Golgi tendon organs - are receptors embedded in the tendons that respond to
increased muscle tension. When contraction of a muscle is extreme these
receptors excite inhibitory interneurons in the spinal cord that cause the muscle to
relax.
Muscle spindles - are receptors embedded in the muscle that respond to changes
in muscle length. Muscle spindles have their own threadlike intrafusal muscles
which are innervated by intrafusal motor neurons. The intrafuscal muscle and
motor neuron enable the muscle spindle to shorten during skeletal muscle
contraction so that the middle stretch sensitive portion remains sensitive to
changes in muscle length.
The Stretch Reflex is a monosynaptic reflex that is elicited by a sudden external stretching force on a
muscle.
Patellar tendon reflex - is a stretch reflex elicited by an external force that stretches
the patellar tendon. This causes the extensor muscle in the thigh to lengthen and
stretch the muscle spindle receptors. Afferent input from the muscle spindle is sent
through the dorsal root to the spinal cord where it stimulates motor neurons, which
in turn, stimulates contraction of the thigh extensors. The stretch reflex helps keep
external forces from altering the intended position of the body. For example, if you
are holding a cup of coffee (or a bottle of beer) and someone brushes up against
you, the lengthening of the biceps muscle will cause stimulation of the muscle
spindle which will result in a compensatory contraction of the muscle. Of course
this reflex may not be as effective if you are drinking beer rather than coffee. What
two principles of sensorimotor function are illustrated by this example? 1) how
sensory feedback regulates motor output and 2) the ability of lower circuits in the
motor hierarchy to deal with problems without the involvement of higher levels.
Withdrawal reflex is not monosynaptic like the stretch reflex. The withdrawal reflex involves
stimulation of both excitatory and inhibitory interneurons which in turn stimulate the
appropriate motor neurons that innervate flexor and extensor muscles (flexors are
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excited while extensors are inhibited). Thus the withdrawal reflex takes advantage
of reciprocal innervation - the fact that antagonistic muscles are innervated in such
a way that when one is contracted, the other relaxes to permit a smooth motor
response. During more voluntary movements, both agonist and antagonist
muscles are contracted to some degree, and movement is a product of adjustment
in the level of relative cocontraction. Cocontraction affords smooth movements
that can be stopped with precision by a slight increase in the contraction of the
antogonistic muscles. It also insulates us from the effects of unexpected external
forces.
Recurrent collateral inhibition muscle fibers need occasional breaks which are provided through recurrent
collateral inhibition - stimulation of a motor neuron not only excites muscle fibers
but also stimulate interneurons (called Renshaw cells) that feedback onto the motor
neuron and inhibit its activity for a brief period. This momentary inhibition shifts the
responsibility for maintained contraction of the muscle to other members of its
motor pool.
9.8 Central Sensorimotor Programs
Chapter 9 is based on the theory that the sensorimotor system is hierarchically
organized. Central sensorimotor programs are a critical part of the theory.
These programs do not produce behavior by controlling individual muscles but
rather produce functional patterns of coordinated muscle activity; all but the highest
levels of the sensorimotor system have certain patterns of activity programmed into
them, and complex movements are produced by activating the appropriate
combinations of these programs. Have you ever decided to look for something and
then after getting up and walking to a different place forgotten what it was you
wanted. Although this example illustrates how our memory can sometimes fail us,
it also illustrates the independence of certain central motor programs from the
higher levels that innitiated an idea or general plan of action.