Somatosensory+System+IV

=**Somatosensory System IV**=


 * 1. Point out the locations of the four areas of primary somatosensory cortex.**

The primary somatosensory cortex is located on the postcentral gyrus in the anterior parietal lobe and has 4 cortical areas arranged in a mediolateralsomatotopic direction. From rostral to caudal, these areas are called area 3a, 3b, 1 and 2. Each of these areas contains a separate map of the body and face which is somatotopically organized such that the lower limb, upper limb, and face are organized medial to lateral.

Each of the four areas has different somatotopic representations and processes different input modalities: Area 3a processes muscle and join inputs. Area 3b processes cutaneous tactile inputs. Area 1 processes tactile and pain inputs. Area 2 processes cutaneous tactile, muscle, and joint inputs.

Signals processed across the areas of the primary somatosensory cortex and neurons in one cortical area can communicate with neurons of another cortical area. After processing in the primary cortical areas, signals are processed in higher order somatosensory cortical areas.


 * 2. Point out the names and locations of the cortical components of the processing route through posterior parietal cortex.**

The for route of higher order processing outside the primary cortical areas projects from the primary somatosensory cortex to cortical areas in the posterior parietal lobe; the second route projects to the cortical areas in the lateral sulcus.

The route to the posterior parietal lobe involves connection between cortical area 5, 7, 39, and 40 where multisynaptic processing occurs.


 * 3. Point out the names and locations of the cortical components of the processing route through the lateral sulcus cortex.**

Primary somatosensory cortical projections to cortical areas in the lateral suclus go towards the secondary somatosensory cortex, insular area, and retroinsular area.


 * 4. Describe two ways that signals from a given body location are processed differently in different cortical areas.**

In addition to inputs from primary somatosensory cortex, the posterior parietal areas receive integrated visual and auditory inputs. Projections of the posterior parietal cortical areas also go towards motor areas in the frontal cortex and the cingulated cortex on the medial wall, resulting in multisensory information integration and processing. There are also feedback connections between areas of this route. Projections to the lateral sulcus are directed toward limbic structures such as the amygdale and hippocampus where they interact with emotional (fear) and memory structures, respectively. There are interconnections between this processing route and the posterior parietal lobe.


 * 5. Point out differences in size and laterality of receptive fields of primary sensory neurons versus primary cortical area 3b neurons versus posterior parietal area 7 neurons, and what this infers about spatial coding.**

The size of receptive fields of single neurons in different cortical areas varies. For example, the receptive field of somatosensory neurons in central structures such as primary sensory, thalamic, and cortical area 3b neurons are relatively small, whereas receptive fields get larger for neurons int eh posterior parietal (e.g. area 7) and lateral suclus (e.g. secondary somatosensory and retroinsular) cortical areas. Unilateral receptive fields are usual in primary somatosensory cortical areas, but bilateral receptive fields are common in non-primary cortical areas (e.g., double arrow fields in secondary somatosensory cortex and area 7). The size and laterality of a receptive field are indicators of the spatial detail with which body inputs are coded by activity of an individual neuron. Generally, it is thought that the smaller the receptive field, the more detailed its spatial coding. The size and specificity of cortical representations vary in different cortical areas. A representation or map is a group of neurons that is activated by (i.e., have receptive fields on) one part of the body. e.g., the size and specificity of cortical maps of the thumb vary in different areas of the primary cortex.

The size of the cortical representation of a given part of the body reflects the number of neurons that can be activated when stimulated. The larger the representation, the largest the number of neurons that can be activated, and (presumably) the more detailed the cortical processing of those inputs. Specificity of a representation indicates whether inputs from one or more parts of the body activate neurons (e.g., one ore more than one finger). Representations with different specificities are presumably involved in different functions.

In summary, different-sized receptive fields and different-sized cortical representations in different cortical reas indicate these areas are performing different functions.


 * 6. Explain how body maps of each half of the body are connected.**

The primary somatosensory, posterior parietal, and lateral sulcus cortical areas of the two cerebral hemispheres are reciprocally interconnected through the corpus callosum. Since signal processing along the ascending neuraxis into primary somatosensory cortex is ipsilateral, interhemispheric connections through the corpus callosum are important for constructing a unified cortical image of inputs from both halves of the body. This uniting of each half of the body largely occurs in the posterior parietal and lateral sulcus cortex where bilateral receptive fields are more common.


 * 7. Explain ways that cortical representations of the skin are affected by normal use.**

The size of a map or cortical area of cortex with represents inputs from a specific skin region is partly determined by how the skin region i used. For example, increase use and acctivity of digits 2 and 3 enlarges the cortical maps of these fingers at the expense of maps of the less utilized fingers. e.g., the cortical area representing a Braille reading finger in a blind person is larger than normal; somatosensory and motor cortical areas may be involved in these enlargements.


 * 8. Describe changes in cortical maps after injury, and after regeneration, of nerves.**

Changes in cortical maps occur following injuries of the body and its innervation. Limb amputation results in changes in the maps of intact inputs from adjacent cortical areas.

Zones in cortical maps that lose normal inputs after peripheral injury can acquire substitute inputs from neighboring body regions that are innervated by adjacent uninjured nerves. Allodynia, hyperalgesia, phantom sensations, and other sensory abnormalities seen after peripheral injuries are presumably caused by these cortical changes. Injuries of somatosensory structures other than peripheral nerves such as dorsal roots or spinal cord can cause similar substitutions of intact for lost inputs in cortical areas.

Cortical map changes that occur after nerve injury can be reversed when injured nerve fibers regenerate back to the skin and reactivate the cortex. However, regeneration may result in a cortical map organization that is different from the original organization. When regeneration is optimal, normal cortical map organization can be recovered.

Thus, following nerve injury and regeneration, cortical map changes may or may not result in recovery of pre-injury organization and optimizing axon regeneration is important for cortical recovery.


 * 9. Point out potential presynaptic and postsynaptic mechanisms that are throught to underlie somatosensory system plasticity.**

(1) If strong neuron inputs that usually activate post-synaptic neurons are lost, the post-synaptic neurons may become activated by normally ignore weaker signals that were formerly overshadowed, resulting in a substitution of strong inputs by the once-weaker remaining inputs and changes in the convergent or divergence synaptic connections.

(2) Increase release of excitatory neurotransmitter from presynaptic terminals of neurons that are related to the substitute projection system may increase the postsynaptic response to these inputs.

(3) Sprouting of more presynaptic terminals of neurons that are related to the substitute projection system may increase the numbers of synapses and synaptic strength.

(4) Changes in inputs can result in phosphorylation of proteins that serve as receptors at synapses, altering sensitivity of receptors to excitatory transmitter and strengthening postsynaptic responses.


 * 10. Describe difficulties in determining cause and effect relationships between cortical and sensory changes.**

Patterns of functional activation in cortex produce sensory feelings and changes in cortical functional organization presumably underlie changes in feelings of the body after injury, use, etc. However, it is difficult to determine what cortical changes cause what sensory changes. Clinical use of brain mapping to help patients with sensory disorders like phantom pain requires developing such a cause and effect understanding of cortical and sensory changes.

For example, it is difficult to tell if amputation causes cortical changes which causes phantom pain or if amputation causes phantom pain which causes cortical changes. Alternatively, it is possible that amputation produces an unknown affect that results in both phantom pain and cortical changes but phantom pain and cortical changes are not themselves related to each other in any cause and effect manner.


 * 11. Define terms used to refer to disturbances of somatosensory sensation.**

Allodynia – pain sensation to a stimulus that is normally not painful Analgesia – loss of pain sensibility Anesthesia – complete loss of all modalities of sensation Astereognosis – inability to recognize objects by touch Atopognposis – inability to localize tactile stimuli Causalgia – burning pains due to nerve injury, often associated with changes in the appearance of skin or nails. Hypalgesia – diminished sensibility to pain Hyperalgesia – increase sensibility to pain Hyperesthesia – increased tactile sensibility Hypesthesia – diminished tactile sensibility Neuralgia – painful spasms occurring along distribution of nerve Neuritis – toxic, traumatic, or infectious inflammation of a nerve, characterized by pain or tenderness in the receptive field of nerve Paresthesia – spontaneously occurring abnormal numbness, tingling, or prickling sensations