Where Is The Primary Somatosensory Cortex Located – The receptive fields of sensory neurons become more complex as information moves up the pathway. In the last lesson we saw that mechanoreceptors have receptive fields that activate the neuron when touched. Mechanoreceptors synapse on neurons in the spinal column, and these neurons have more complex receptive fields. Dorsal column nuclei have receptive fields divided into central and peripheral regions. The center of the receptive field is the result of direct innervation of mechanoreceptors. If a stimulus touches the skin in the center of a dorsal column neuron’s receptive field, the neuron will increase its firing rate. The center/periphery structure is similar to the bipolar and ganglion cells of the visual system.

Figure 25.1. The receptive field of the dorsal column neuron has an excitatory center formed by mechanoreceptors that synapse directly onto the dorsal column neuron. A) In the absence of a stimulus, the dorsal column neuron fires at its initial rate. B) When a stimulus touches the center of the receptive field of the E cell, the firing rate increases. ‘Touch Receptive Field Center’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share Alike (CC BY-NC-SA) 4.0 International License.

Where Is The Primary Somatosensory Cortex Located

The region surrounding the receptive field is the result of indirect communication between receptive neurons and spinal column neurons via inhibitory interneurons. The envelope has an inhibitory effect on the spinal column neuron. If a stimulus touches the skin around the receptive field of a dorsal column neuron, the neuron will decrease its firing rate.

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Figure 25.2. The receptive field of the dorsal column nucleus has an inhibitory surround, which is due to indirect connections between the mechanoreceptors and the neurons of the dorsal column via inhibitory interneurons. A) In the absence of a stimulus, spinal column neurons fire at their initial rate. B) When a stimulus touches the vicinity of the receptive field of the E cell, the firing rate decreases. Note that the stimulus is around the receptive field of the E cell but also in the center of the D cell, so the firing rate of the D cell will increase. ‘Touch Receptive Field Surround’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share Alike (CC BY-NC-SA) 4.0 International License.

The center-surround structure of the receptive field is critical for lateral inhibition to occur. Lateral inhibition is the ability of sensory systems to enhance the perception of the edges of stimuli. At a point or edge of a stimulus, due to inhibitory interneurons, the strength of the perceived stimulus will be enhanced compared to the strength of the actual stimulus.

Figure 25.3. Lateral inhibition increases the perception of edges or points on the surface. A blunt probe point pressing on the receptive field of the B cell will cause an increase in the firing rate of the E cell, but will also decrease the firing rate of the D and F cells. This increases the perceived difference between the dots. and the area next to the point that is not stimulated. ‘Touch Lateral Inhibition’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share Alike (CC BY-NC-SA) 4.0 International License.

The right side of the brain processes touches the left side of the body; The left side of the brain touches the right side of the body.

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Primary sensory afferent fibers have their cell bodies located in the dorsal root ganglion, a structure located outside the spinal cord. The axons of these primary neurons enter the ipsilateral dorsal aspect of the spinal cord. Some collateral axons terminate in the spinal cord and are important for reflexes. The main branch of the axons travels up the spinal cord towards the brain, through the spinal column, ending in the spinal column nuclei located in the brainstem. The axons of sensory neurons in the lower body remain separated from the axons of sensory neurons in the upper body all the way. These two neuronal populations synapse in different regions of the brainstem. Axons of the lower body terminate in the gracile nucleus, while axons of the upper body terminate in the cuneate nucleus. Projections of secondary neurons from the dorsal column nuclei cross the midline, or decussate, and ascend through the white matter called the medial lemniscus. Axons terminate in the posterior ventral side of the thalamus. Thalamic neurons then project to the primary somatosensory cortex located in the postcentral gyrus of the parietal lobe.

Figure 25.4 Somatosensory information from the neck and body travels through the spinal column – the mid-lemniscal pathway, on behalf of the structures within the pathway. Axons enter the spinal cord and ascend the spinal column to the medulla where the decussation or midline is crossed. The information continues to the thalamus via the medial lemniscus, and then to the somatosensory cortex. Path details are in the text. ‘Touch Pathway from Body’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share Alike (CC BY-NC-SA) 4.0 International License.

Sensory receptors in the face and head send information to the brain via cranial nerve V, the trigeminal nerve. Primary neurons have their cell bodies in the trigeminal ganglion, which is located outside the brainstem, and project to the ipsilateral trigeminal nucleus in the floor. Second-order neurons cross the midline and project to the medial ventral nucleus of the thalamus. These neurons then send projections to the facial region of the somatosensory cortex.

Figure 25.5. Somatosensory information from the head and face travels through the trigeminal pathway. Axons enter the brainstem at the level of the foramen and decussate before traveling to the thalamus and somatosensory cortex. Path details are in the text. ‘Touch Pathway from Face’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share Alike (CC BY-NC-SA) 4.0 International License.

Solved The Gyrus Anterior To The Arrows Contains The Primary

Figure 25.6. To compare the two pathways, sensory information is input from the periphery. In the body, the axon branch travels through a peripheral nerve to the cell body located outside the spinal cord. The central axon branch then enters the spinal cord and ascends through the spinal column to the dorsal column nuclei of the brainstem. The second-order neuron crosses the midline and then projects to the ventral nucleus of the posterior side of the thalamus via the medial lemniscus. The third order thalamic neuron projects to the primary somatosensory cortex of the parietal lobe. For facial sensory information, the axon branch travels to the trigeminal via cranial nerve V. The ganglion is outside the brainstem, and axons then enter the brainstem and synapse in the trigeminal nucleus. The second-order neuron travels to the posterior middle ventral nucleus of the thalamus, and the third-order neuron projects to the primary somatosensory cortex in the parietal lobe. ‘Touch Pathways’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share Alike (CC BY-NC-SA) 4.0 International License.

The primary somatosensory cortex is divided into four regions, each with its own input and function: areas 3a, 3b, 1 and 2. Most tactile information from mechanoreceptors enters region 3b, while most proprioceptive information from muscles enters region 3a. These regions then send and receive information from areas 1 and 2. As the processing of somatosensory information progresses, the stimuli required to activate neurons become more complex. For example, area 1 is involved in detecting texture, and area 2 is involved in detecting the size and shape of an object. The posterior parietal cortex, an important output arm of the somatosensory cortex, is caudal to the postcentral gyrus; Areas 5 and 7 are downstream structures that continue to process touch.

Figure 25.8. The somatosensory cortex, located in the postcentral gyrus, posterior to the central foramen, is divided into 4 areas: 3a, 3b, 1 and 2. The posterior parietal cortex, the output region of the somatosensory cortex, is posterior. It is divided into the postcentral gyrus and areas 5 and 7. ‘Postcentral Gyrus’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share Alike (CC BY-NC-SA) 4.0 International License.

The receptive fields of each higher-order neuron increase in size and complexity, but cortical neurons are also associated with a specific region of the body. Cortical neurons are organized according to the body region they represent, so neurons that respond to sensation in the fingers are close to neurons that respond to sensation in the hand. Remember that the dorsal axons of the upper body run next to, but remain separate from, the axons of the upper body. This differentiation, which occurs in all body regions and at all pathway levels, creates a somatotopic map of the body in the primary somatosensory cortex. Each area of ​​the somatosensory cortex (Figure 23.7) has its own, but similar, map of the body.

Execution Of Movement

Regions in the skin with high receptor density, and thus fine two-point discrimination, are afforded more cortical space. That means cortical

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