Chapter Five

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21 Apr 2012
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PSYB65 Chapter Five: The Sensorimotor System
5.1: Somatosensory Feedback
Somatosensory feedback: information that’s obtained from the sensory network that monitors the
body’s surface and its movements; the system includes joints, muscles, tendons and skin
The somatosensory info that’s provided ensures that your brain makes accurate movements
Somatosensory Receptors
Variety of different sensory receptors present in the skin
Grouped into three types of somatic information:
o Nociception: sensations of pain and temperature
o Hapsis: sensations of fine touch and pressure
o Proprioception: awareness of the body and its position in space
Most of the sensory receptors in the skin are mechanoreceptors
o Mechanoreceptors: sensory receptors in the skin with many functions, including
reacting to distortion and wrapping around the hairs that cover our bodies
The spinal cord is organized into dorsal root ganglia (somatosensory) and ventral root ganglia
(motor)
o Dorsal root ganglia: somatosensory nodules on dorsal roots that contain afferent spinal
nerve neuron cell bodies; there’s one dorsal root ganglion for each of the thirty pairs of
spinal nerves
o Ventral root ganglia: motor nodules on ventral roots that contain efferent spinal nerve
neuron cell bodies; there’s one ventral root ganglion for each of the thirty pairs of spinal
nerves
Spinal segments can be divided into four groups on the basis of where the nerves originate:
o Cervical (C) 1-8; Thoracic (T) 1-12; Lumbar (L) 1-5; Sacral (S) 1-5
o Each of the thirty dorsal roots of the spinal cord innervates different areas of the skin
referred to as dermatomes
o When a dorsal root is cut, the spinal cord can no longer obtain info from that nerve.
However, in order to lose complete sensation in one dermatome, you must cut three
dorsal roots: the one serving the dermatome and the ones above and below it
Somatosensory Pathways in the Brain
Two main sensory pathways in the brain. Named for their position in the spinal cord and the
connections made: the dorsal spinothalamic tract and the ventral spinothalamic tract
o Dorsal spinothalamic tract: sensory pathway in the brain that’s responsible for
transmitting information about hapsis and proprioception; enters the spinal cord
through the dorsal root ganglion and synapses ipsilaterally in the dorsal column nuclei of
the spinal cord
o Ventral spinothalamic tract: sensory pathway in the brain that’s responsible for
transmitting nocioceptive information; enters the spinal cord contralaterally to the
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brainstem, where its axons join the medial lemniscus and ascend to the ventrolateral
thalamus
Although somatosensory information for hapsis and nociception is transmitted separately,
because they send info 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 results in a loss of sensorimotor function below the site of injury.
However, 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 injury
Association Cortex
In the case of the sensorimotor control, the two different areas of association cortex are at
the top of the hierarchy
The secondary and primary motor areas actually carry out the commands fairly
independently
The basal ganglia and cerebellum modulate motor responses
Critical feedback, both somatosensory and motor, is achieved through the ascending
sensorimotor pathways
Posterior Parietal Association Cortex
Deals with spatial information
Determines both the original position of the body and around the body in space
creates a mental picture; plays a critical role in the production of accurate movements
It’s an association cortex because it receives input from a variety of sensory systems,
including proprioception, hapsis, and vision
Within this cortex, Brodmann’s area 5 receives inputs from primary somatosensory
cortical areas
Dorsalateral Prefrontal Cortex
Involved with the decision to execute voluntary movements; sends extensive
projections to the secondary and primary motor cortex
Directs lower areas in the motor hierarchy (eg. secondary and primary motor cortex)
The sensory info that’s provided to the dorsolateral prefrontal association cortex by the
posterior parietal association cortex plays a large role in the decision to make the
movement
The dorsolateral prefrontal lobe also is assessing the likely outcome of planned
movements
Secondary Motor Cortex
Areas include the supplementary motor area, the premotor cortex, and the cingulate motor
areas all reciprocally connected to the each other
Sends direct projections to brainstem nuclei
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All areas play a role in voluntary motor production and are bilaterally active before/during it,
suggesting that these areas are involved in the planning and execution of motor movements
Activation of the supplementary motor area = self generated movements (typically controlled by
internal feedback eg. rhythmic tapping of the finger parietal lobe)
Activation of the premotor cortex = externally generated movements (typically externally
controlled or triggered eg. tapping the finger at the same speed as a metronome
visual/auditory cortical areas)
Primary Motor Cortex
Controls the movement of the muscles, and plans out the coordinated activity of the muscles
Directly in front of the central fissure (prefrontal gyrus), which is right next to the primary
somatosensory cortex (postcentral gyrus)
Heavy interconnections with the somatosensory cortex (ability to modify motor programs)
Connections between the primary motor cortex and somatosensory areas causes stereognosis
(ability to identify objects by touch)
Damage to the PMC doesn’t result in limb paralysis – it reduces speed, accuracy and force
Causes astereognosia
The primary somatosensory cortex and the PMC are organized somatopically
o When the somatotopic map of the sensorimotor or primary motor cortex is mapped,
the result is the homunculus
Shows how much cortical area is devoted to sensation from that area
The somatosensory and motor representations for the same body part are side
by side; representation of the sensory properties of the left hand is next to the
motor representation of the left hand
Rather than there being a specific area for each component of the hand, there
appears to be a network of neurons distributed throughout the premotor cortex
that become active when the index finger is moved representation of
individual fingers that overlap representations of adjacent fingers
Basal Ganglia
Subdivision of the telencephalon, and composed of three structures: the caudate nucles, the
putamen (collectively referred to as the striatum), and the globus pallidus
o Caudate Nucleus/Putamen: components of the basal ganglia; they’re both collectively
known as the striatum and receives the vast majority of cortical inputs to the basal
ganglia as well as projections from the substantia nigra
o Globus pallidus: a component of the basal ganglia; acts as the primary output nucleus of
the basal ganglia
o Striatum: collective term for the caudate nucleus and the putamen; receives most of the
motor input for the basal ganglia
o Lentiform nucleus: collective term for the globus pallidus and the putamen
The globus pallidus is next to the egg shaped thalami. The thalami are encased in the putamen,
and the caudate extends from the putamen in a C-shape tail
The basal ganglia are critically important for initiating movements and maintaining muscle tone
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