The Sensorimotor System
Sunday, October 28, 2012
Module 5.1 - Sensorimotor System
Accurate movements depend on our ability to monitor the position and placement of our body
and its part, which relies on somatosensory feedback from our joints, tendons, muscles and skin.
Case Study of G.O - Role of Somatosensory Feedback
Former darts champion who experienced almost complete destruction of the somatosensory
nerves in both arms due to an infection.
Still had fine motor skills in fingers - could make shapes in the air, could perform complex motor
Hands however were useless to him - unable to perform intricate motor skills (e.g. picking up
spilled cereal, fastening buttons) , unable to use visual feedback to monitor his movements,
unable to use somatosensory feedback to monitor the position of his hands, unable to know
whether he was maintaining a constant level of muscle contraction (resulted in dropping objects).
He was also unable to use either tactile feedback to stop his movements or position/force
information from his muscles and joints to judge the speed and direction of his grasp (spilling
This case study indicates that many automatic adjustments occur unconsciously; they are made
without the involvement of higher cortical areas and are relatively resistant to interference from
higher cortical areas.
Types of receptors are functionally grouped into three types of somatic information:
(1) Nociception: sensation of pain and temperature.
(2) Hapsis: sensation of fine touch and pressure
(3) Proprioception - awareness of the body and its position in space
Most sensory receptors in the skin are mechanoreceptors (regardless of function) - they react to
distortion such as bending or stretching.
There are also mechanoreceptors wrapped around hairs on our body
Many different types of mechanoreceptors although most are axons that have mechanosensitive
ion channels in them
The axons that contain these ion channels are primary afferent axons that enter the spinal cord
through the dorsal roots.
Cell bodies (somas) of primary afferent axons reside in the dorsal root ganglia of the spinal cord.
Spinal cord is organized into dorsal and ventral root ganglia
Dorsal root ganglia are somatosensory
Ventral root ganglia are motor
30 pairs of spinal nerves, each of which is made up of dorsal and ventral roots that exit the spine
through a notch in the vertebrae
Spinal segments: - cervical (C) 1-8, thoracic (T) 1-12, lumbar (L) 1-5 and sacral (S) 1-5.
Each of the 30 dorsal roots of the spinal cord innervates different areas of skin known as
Dermatomes are innervated by specific spinal nerves. (see figure 5.1)
Cutting a dorsal root, results in the inability for the spinal cord to obtain information from that
But not all sensation from that dermatome is lost - extensive overlap of dermatomes To lose complete sensation in one dermatome, you must cut three dorsal roots, one serving the
dermatome and one above and below it.
THING IN A BOX (LOL) --> The Real World: Chicken Pox, Shingles & Dermatomes:
The virus of chicken pox livings in your cranial and spinal nerves. Normally dormant however
stress, fatigue and other events compromising the immune system can cause it to become active
resulting in the condition known as shingles. Typically virus becomes active in only one dorsal root
ganglion leading to hyperexcitability of that dorsal root ganglion. Leads to excessive firing,
perceived as a burning sensation or sharp stabbing pain. Shingles virus maps the dermatome of
the affected dorsal root by producing blisters on the skin along the nerve endings. Treatment of
shingles immediately may help prevent chromic neuralgia (pain that does not
result from any obviously lesion)
Somatosensory Pathways in the Brain:
Two main pathways: dorsal spinothalamic tract and ventral spinothalamic tract
Dorsal Spinothalamic Tract: responsible for transmitting information about proprioception and
Ventral spinothalamic tract: responsible for transmitting nociceptive information.
For both tracts, some of the neurons send projections to the somatosensory cortex.
Although somatosensory information for hapsis and nociception is transmitted separately,
because they send information through the same pathways to the same destinations, damage to
the brainstem or thalamus results in equal loss of both hapsis and nociception.
Damage to the spinal cord can result in different patterns of deficits.
Damage to the spinal cord results in a loss of sensorimotor function below the site of injury.
If the spinal cord is not completely transected (cut through), nociception is lost for the side of the
body contralateral to the injury, and hapsis is lost for the side of the body ipsilateral to the
Fig 5.3 - Hierarchical Organization of the Sensorimotor System
Two different areas of association cortex at the top of the hierarchy.
Secondary and Primary motor areas carry out commands fairly independently - sending
commands through descending motor pathways.
Basal Ganglia and Cerebellum modulate motor responses (indirect roles.)
Critical feedback, both somatosensory and motor, is achieved through ascending sensorimotor
Posterior Parietal Association Cortex:
Parietal lobes are active whenever the brain is interacting with space or with spatial information
Posterior parietal association cortex has an important role in determining body position and
position of objects around the body in space.
This knowledge is required to move effectively through the world.
Posterior parietal association cortex receives input from a variety of sensory systems - including
proprioception, hapsis and vision.
Helps PPAC to create a mental picture of the body in space.
In PPAC, is Brodmann's area 5 (BA5) which receives inputs from primary somaotosensory cortical
BA3, BA1, BA2 and BA7 receive higher order visual information. Individuals with damage to these areas have difficulty with spatial relations and have disturbances
in body image.
PPAC has extensive interconnections with the dorsolateral prefrontal association cortex - work
together to guide movement.
PPAC - has extensive reciprocal connections with lower areas of motor hierarchy e.g. secondary
and primary motor cortex.
Dorsolateral Prefrontal Cortex (BA8)
Involved with decision to make executive voluntary movements
Actively directs lower areas in the motor hierarchy (secondary (BA6) and primary (BA4))
Is activated even when we just think about performing a movement.
DPC decides what movement to make and lower levels specify how the movements will be made.
Secondary Motor Cortex
Consists of the supplementary motor cortex, premotor cortex and cingulate motor areas
All reciprocally connected to each other
Also sends direct projections to brainstem nuclei
Play a role in voluntary motor production
All areas appear to be bilaterally active before and during voluntary movements, suggesting that
they are involved in the planning and execution of motor movements.
It is assumed that each area has its own unique role.
E.g. activation of the supplementary motor area is associated with self-generated movements.
Self-generated movements are typically controlled by internal feedback, which is consistent with