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Chapter 5

PSYB65 Chapter 5 Detailed chapter notes


Department
Psychology
Course Code
PSYB65H3
Professor
Ted Petit
Chapter
5

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Chapter 5: The Sensorimotor System
Module 5.1 Sensorimotor System
- Used to moving - utilizes a parallel and hierarchical system that relies heavily on functional
segregation
Somatosensory Receptors
Types of receptors are usually functionally grouped into three types of somatic information:
1. Nociception - sensations of pain and temperature
2. Hapsis - sensations of fine touch and pressure
3. Proprioception - awareness of the body and its position in base
- Most of the sensory receptors in the skin are mechanoreceptors
Receptors that react to distortion such as bending or stretching
- There are many different types of mechanoreceptors throughout the body - most are axons
that have mechanosensitive ion channels on them
Primary afferent axons that enter the spinal cord through the dorsal roots
Cell bodies of primary afferent axons reside in the dorsal root ganglia of spinal cord
- Spinal cord is organized into dorsal and ventral root ganglia:
Dorsal root ganglia: somatosensory
Ventral root ganglia: motor
-30 pairs of spinal nerves - each makes up a dorsal and ventral roots that exit the spinal cord
through a notch in the vertebrae of the spine
Spinal segments divided into 4 groups on the basis of where the nerves originate:
Cervical (C) - 1-8
Thoracic (T) - 1-12
Lumbar (L) - 1-5
Sacral (S) - 1-5
Each of the 30 dorsal roots of the spinal cord innervates different areas of the skin
(dermatomes)
When a dorsal root is cut, the spinal cord can no longer obtain information from that
nerve; however not all sensation from that dermatome is lost - extensive overlap
between dermatomes
To lose complete sensation in one dermatome, must cut 3 dorsal roots: the one
serving the dermatome and the dorsal roots above and below it
Somatosensory Pathways in the Brain
- Two main somatosensory pathways that are named for their position in the spinal cord and
the connections made:
Dorsal spinothalamic tract
Ventral spinothalamic tract
1. Dorsal Spinothalamic tract
- Responsible for transmitting information about proprioception and hapsis
- Enters the spinal cord through the dorsal root ganglion and synapses ipsilaterally into the
dorsal column nuclei of the spinal cord
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Axons of dorsal column nuclei ascend through the spinal cord until the brainstem, where
they decussate or cross, and continue to ascend through the brainstem in a pathway
called the medial lemniscus
Axons of the medial lemniscus synapse in the ventrolateral thalamus, which sends
projections to both the motor and somatosensory cortex
2. Ventral Spinothalamic tract
-Nociceptive information
- Enters the spinal cord through the dorsal root ganglion and ascends the spinal cord
contralaterally
- In brainstem, axons join the medial lemniscus and ascend to the ventrolateral thalamus
- Some of these neurons send projections to the somatosenory cortex
- Because 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 spinal cord results in a loss of sensorimotor function below the site of injury
If the spinal cord is not completely cut through (transected),
Nociception is lost for the side of the body contralateral to the injury
Hapsis is lost for the side of the body ipsilateral to the injury
**Dorsal Spinothalamic tract crosses contralerally at the brain stem
Ventral Spinothalamic tract crosses contralaterally at the spinal cord
Association Cortex
Posterior Parietal Association Cortex
- Important role in determining both the original position of the body and objects around
the body in space
- Receives information from a variety of sensory systems, including proprioception, hapsis,
and vision
Brodmann’s area 5 (BA5) - receives input from primary somatosensory cortical areas
BA3, BA1, BA2, BA7 - receive high-order visual information
Damage to these areas of the parietal lobe tend to result in difficulties with spatial
relations and disturbances of body images
- Uses the information to create a mental picture of the body in space
- Important in production of accurate movements
- Extensive interconnection between the posterior parietal association cortex and the
dorsolateral prefrontal association cortex allows information to guide movements
- Has extensive reciprocal connections with areas that are lower in the motor hierarchy, such as
secondary and primary motor cortex
Dorsolateral Prefrontal Cortex
- Involved with the decision to execute voluntary movements
Actively directs lower areas in the motor hierarchy, such as the secondary and primary
motor cortex
- Information provided to the dorsolateral prefrontal association cortex by the posterior parietal
association cortex plays a large role in the decision to make the movement
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- Stimulation of the upper levels of dorsolateral prefrontal cortex is associated with what
movements to make; and stimulation of lower levels of dorsolateral prefrontal cortex is
associated with how the movements will be made
Secondary Motor Cortex
- Includes the supplementary motor area, the premotor cortex, and the cingulated motor areas
- All areas of the secondary motor area are reciprocally connected to each other
- Sends direct projections to brainstem nuclei
- Electrical stimulation of any of these areas result in complex motor movements - voluntary
motor production
All areas of the secondary motor cortex appear to be bilaterally active before and during
voluntary movements - involved in planning and execution of motor movements
- Assumed that each area of the secondary motor cortex has a unique role in the programming
and execution of motor programs and execution of motor programs
Activation of supplementary motor area - self-generated movements
Governed by internal feedback (ie. Rhythmic tapping of the finger)
Activation of premotor cortex - externally generated movements
Externally controlled or triggered (ie. Tapping the finger at the same speed as a
metronome)
Cingulate activation is often observed when there is conflict about whether or not action
should be taken
Primary Motor Cortex
- Controls the movements of the muscles and plans out the coordinated activity of the muscles
- Area of the brain that is directly in front of the central fissure (precentral gyrus), which is
right next to the primary somatosensory cortex (postcentral gyrus)
- Heavy interconnections with somatosensory cortex - ability to modify motor programs on the
basis of sensory feedback
Stereognosis - the ability to identify objects by touch
Mediated by the connections between primary motor cortex and somatosensory
areas
- Damage results in the reduction of speed, accuracy, and force with which an individual
makes a movement, along with causing astereognosia
Summary of the Motor Cortices:
- Secondary motor cortex takes the goal that was directed by the cortical association areas and
plans out the actions that must be taken
- Primary motor cortex uses plans of secondary motor cortex to determine the sequence of
muscle movements to be made
- Each area of the secondary motor cortex has its own unique function
- Functional and organizational similarities and the degree of interconnection between the
primary motor cortex and the primary somatosensory cortex - voluntary movement is
sensorimotor
Basal Ganglia and Cerebellum
Basal Ganglia
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