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Chapter 5

PSYB65 Chapter 5.docx

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Ted Petit

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Chapter 5: The Sensorimotor System
Somatosensory Receptors
Much of the somatosensory information that we receive about the world comes from sensory receptors in the
oVariety of different sensory receptors present in the skin
Three types of somatic information
oNociception – sensations of pain and temperature
oHapsis – sensations of fine touch and pressure
oProprioception – awareness of the body and its position in space
oRegardless of function, most of the sensory receptors in the skin are mechanoreceptors which
react to distortion such as bending or stretching
Most are axons that have mechanosensitive ion channels on them
Axons that contain these ion channels are primary afferent axons that enter the spinal
cord through the dorsal roots
Cell bodies of the primary afferent axons reside in the dorsal root ganglia of the spinal
Spinal cord is organized into dorsal and ventral root ganglia
oDorsal root ganglia are somatosensory
oVentral root ganglia are motor
o30 pairs of spinal nerves, each of which is made up of dorsal and ventral roots that exit the spinal
cord through a notch in the vertebrae of the spine
Spinal segments can be divided into four groups on the basis of where the nerves
oEach of the 30 dorsal roots of the spinal cord innervates different areas of the skin called
oWhen a dorsal root is cut, spinal cord can no longer obtain information from that nerve
oNot all sensation from that dermatome is lost
Extensive overlap between dermatomes
To lose complete sensation in one dermatome, must cut three dorsal roots
One serving the dermatome and the dorsal roots above and below it
Somatosensory Pathways in the Brain
Two main pathways named for their position in the spinal cord and the connections made:
oDorsal spinothalamic tract
Responsible for transmitting information about proprioception and hapsis
Enters spinal cord through the dorsal root ganglion and synapses ipsilaterally in the
dorsal column nuclei of the spinal cord
Axons of the medial lemniscus synapse in the ventrolateral thalamus (sends projections to
both the motor and somatosensory cortex)
oVentral spinothalamic tract
Nociceptive information travels separately
Enters the spinal cord through the dorsal root ganglion and ascends the spinal cord
In the brainstem, these axons join the medial lemniscus and ascend to the ventrolateral
Some of these neurons send projections to the somatosensory cortex
oAlthough 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 results in a loss of sensorimotor function below the site of
If spinal cord is not completely transected (cut through), nociception is lost of 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
Posterior Parietal Association Cortex
Active whenever brain is interacting with space or with spatial information
Plays important role in determining both the original position of the body and objects around the body in
Receives input from a variety of sensory systems, including proprioception, hapsis, and vision
oUsing this information, PPAC is responsible for creating mental picture of the boy in space
oBrodmann’s area 5 receives inputs from primary somatosensory cortical areas
oBA3, BA1, BA2, and BA7 receive higher-order visual information
oDamage to these areas of the parietal lobes tend to have difficulties with spatial relations
Also tend to have disturbances of body image
Fail to recognize parts of their body as belonging to themselves
Involved in processing of spatial relations of both the body and objects surrounding the body
oCritical role in the production of accurate movements
oExtensive interconnections between the posterior parietal association cortex and the dorsolateral
prefrontal association cortex allow this information to guide movements
Has extensive reciprocal connections with areas that are lower in motor hierarchy
oEg. Secondary and primary motor cortex
Dorsolateral Prefrontal Cortex
Involved with the decision to execute voluntary movements
Actively directs lower areas in the motor hierarchy
Assessing the likely outcome of planned movements
Secondary Motor Cortex
Include the supplementary motor area, premotor cortex, and the cingulate motor areas
Regardless of the specific site, all areas of the secondary motor area are reciprocally connected to one another
Secondary motor cortex sends direct projections to brainstem nuclei
Electrical stimulation of any of these areas results in complex motor movements
oAll play a role in voluntary motor production

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Chapter 5: The Sensorimotor System  5.1:ENSORIMOTOR  SYSTEM  Somatosensory Receptors • Much of the somatosensory information that we receive about the world comes from sensory receptors in the  skin o Variety of different sensory receptors present in the skin  • 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  o Regardless of function, most of the sensory receptors in the skin are mechanoreceptors which  react to distortion such as bending or stretching   Most are axons that have mechanosensitive ion channels on them   Axons that contain these ion channels are primary afferent axons that enter the spinal  cord through the dorsal roots   Cell bodies of the primary afferent axons reside in the dorsal root ganglia of the spinal  cord  • Spinal cord is organized into dorsal and ventral root ganglia o Dorsal root ganglia are somatosensory  o Ventral root ganglia are motor  o 30 pairs of spinal nerves, each of which is made up of dorsal and ventral roots that exit the spinal  cord through a notch in the vertebrae of the spine   Spinal segments can be divided into four groups on the basis of where the nerves  originate  • Cervical • Thoracic • Lumbar • Sacral  o Each of the 30 dorsal roots of the spinal cord innervates different areas of the skin called  dermatomes  o When a dorsal root is cut, spinal cord can no longer obtain information from that nerve  o Not all sensation from that dermatome is lost  Extensive overlap between dermatomes   To lose complete sensation in one dermatome, must cut three dorsal roots  • One serving the dermatome and the dorsal roots above and below it  Somatosensory Pathways in the Brain  • Two main pathways named for their position in the spinal cord and the connections made: o Dorsal spinothalamic tract  Responsible for transmitting information about proprioception and hapsis  Enters spinal cord through the dorsal root ganglion and synapses ipsilaterally in the  dorsal column nuclei of the spinal cord   Axons of the medial lemniscus synapse in the ventrolateral thalamus (sends projections to  both the motor and somatosensory cortex)  o Ventral spinothalamic tract  Nociceptive information travels separately   Enters the spinal cord through the dorsal root ganglion and ascends the spinal cord  contralaterally   In the brainstem, these axons join the medial lemniscus and ascend to the ventrolateral  thalamus   Some of these neurons send projections to the somatosensory cortex  o 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 results in a loss of sensorimotor function below the site of  injury   If spinal cord is not completely transected (cut through), nociception is lost of 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  Posterior Parietal Association Cortex • Active whenever brain is interacting with space or with spatial information  • Plays important role in determining both the original position of the body and objects around the body in  space • Receives input from a variety of sensory systems, including proprioception, hapsis, and vision  o Using this information, PPAC is responsible for creating mental picture of the boy in space  o Brodmann’s area 5 receives inputs from primary somatosensory cortical areas o BA3, BA1, BA2, and BA7 receive higher­order visual information  o Damage to these areas of the parietal lobes tend to have difficulties with spatial relations   Also tend to have disturbances of body image   Fail to recognize parts of their body as belonging to themselves  • Involved in processing of spatial relations of both the body and objects surrounding the body  o Critical role in the production of accurate movements  o Extensive interconnections between the posterior parietal association cortex and the dorsolateral  prefrontal association cortex allow this information to guide movements  • Has extensive reciprocal connections with areas that are lower in motor hierarchy o Eg. Secondary and primary motor cortex Dorsolateral Prefrontal Cortex  • Involved with the decision to execute voluntary movements  • Actively directs lower areas in the motor hierarchy • Assessing the likely outcome of planned movements  Secondary Motor Cortex • Include the supplementary motor area, premotor cortex, and the cingulate motor areas  • Regardless of the specific site, all areas of the secondary motor area are reciprocally connected to one another  • Secondary motor cortex sends direct projections to brainstem nuclei • Electrical stimulation of any of these areas results in complex motor movements o All play a role in voluntary motor production  • All areas of the SMC appear to be bilaterally active before and during voluntary movements  o Involved in the planning and execution of motor movements  o No evidence that these motor programs must actually occur  • Large similarities are observed between areas of the secondary motor cortex o Still assumed that each has a unique role in the programming and execution of motor programs Primary Motor Cortex • Controls movements of the muscles o Plans out the coordinated activity of the muscles  • Stereognosis ability to identify objects by touch  o Most likely mediated by the connections between the primary motor cortex and somatosensory  areas  • Damage just limits speed, accuracy, and force with which an individual makes a movement, along with  causing astereognosia  • Somatosensory and motor representations for the same body part are side by side o Representation of the sensory properties of the left hand is next to the motor representation of the  left hand  • There seems a network of neurons distributed throughout the premotor cortex that become active when the  index finger is moved  o Overlapping representation of fingers is not confined to the PMC  Observed in PSC as well  Basal Ganglia and Cerebellum • Basal ganglia are subdivision of the telencephalon and composed of three structures: o Caudate nucleus  Extends from the putamen in a C­shaped curling, tail like structure  o Putamen or striatum  o Globus pallidus   This and putamen can be collectively called lentiform nucleus   Similar in shape and size to thalamus   Encased in the putamen  • Critically important for initiating movements and maintaining muscle tone • Caudate and putamen receive vast majority of the cortical inputs to the basal ganglia as well as the  projections from the substantial nigra  • Huntington’s chorea results from damage to striatum – the caudate nucleus and the putamen  o Striatum receives most of the motor input for the basal ganglia, and the globus pallidus serves as  the primary output  o Most of these connections with thalamus which in turn connects to motor and nonmotor structures  such as the cingulate cortex and prefrontal cortex  Presumed to be involved in response selection  • Cerebellum plays very important role in the modulation of motor movements and the acquisition of motor  skill  o Three zones: o Lateral zone o Intermediate zone o Vermis  o Lateral and intermediate zone are represented in each cerebellar hemisphere   Vermis lies between the hemispheres  o Within the zones are three nuclei of the cerebellum, collectively referred to as deep cerebellar  nuclei  o Fastigial nuclei receives projections from the vermis  o Interpositus nuclei receive projections from the intermediate zone o Dentate nuclei receive projections from the lateral zone  o Vermis involved with maintaining posture and coordinating whole­body movements o Intermediate zone is specialized for guiding skilled limb movement  o Lateral zone involved in coordinating multijoint movements   Play an important role in the acquisition of motor skills and its role in higher cognitive  functions such as language  Spinal Motor Pathways  • Ventromedial system  o Brainstem projections of this system form three tracts:   Vestibulospinal tract (maintaining balance)   Reticulospinal tract (maintaining posture)   Tectospinal tract (controlling the head and eye movements)  o Cortical projection of this system forms the ventral corticospinal tract (controlling muscles of the  trunk and upper legs, collectively used in tasks such as walking)  • Lateral system  o One tract projecting from the brainstem and one tract projecting from the cortex o Brainstem projections form rubrospinal tract (making movements of the limbs and hands)  o Cortical projections form lateral corticospinal tract (controlling limb movement and extremities  such as fingers and toes)   5.2:ENSORIMOTOR  DISORDERS   Cortical Sensorimotor Disorders  Apraxia  • Actions often unorganized and inappropriate  • Deficit in performing skilled voluntary movements o Cannot be attributable to primary sensory problems, paralysis or muscle weakness, or some other  motor disturbance  • Four major classes of apraxia: o Ideomotor  Cannot execute or imitate simple gestures in response to a command   Test for this by asking person to wave goodbye, salute, etc. • Individual being asked to make gestures that are intransitive (one does not  manipulate an object)   Impair one’s ability to perform transitive gestures   Test ability complete relatively complicated serial acts   Apraxia typically occurs following LH damage • Can compromise language functions  o Ideational  Confusion or loss of knowledge about an object’s use   Impairs ability to identify whether another person is using the object appropriately   Distinction between ideomotor apraxia is hard to make  o Constructional  Inability to construct a complex object  Deficit in processing the spatial aspects of the task but the movements that are required  can be performed correctly  Stem from inability to use visuperceptual information to guide voluntary action  o Oral  Inability to perform skilled movements of the face, lips, cheeks, tongue, pharynx, or  larynx following a command   Impaired with mimicking meaningless oral gestures as well  When objects are introduced as props for the commanded movement, their performance  improves • Normally the right side that is affected and deficit is called unilateral apraxia  o Can also impair movements on both sides of the body and is called bilateral apraxia  Limb Apraxia  • Impairment in fine or precise movements of the limbs  • Impaired when using common objects such as scissors  • Types of actions that are affected can be relatively complex or simple  • One common type of error involves substitution of a body part for the object whose 
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