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PSYB65 Chpt 5.docx

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Ted Petit

The Sensorimotor System Sunday, October 28, 2012 2:41 PM 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 movements 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 coffee). 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. Somatosensory Receptors: 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. 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 nerve 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 hapsis. 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 injury. Association Cortex: 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 pathways. 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 areas. 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
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