23 Touch: Central Processing

Receptive Fields and Lateral Inhibition

The receptive fields of the sensory neurons become more complex as information moves up the pathway. We saw in the last lesson that mechanoreceptors have receptive fields that, when touched, activate the neuron. The mechanoreceptors synapse on neurons in the dorsal column, and those neurons have more complex receptive fields. The dorsal column nuclei have receptive fields that are divided into center and surround regions. The center of the receptive field is a result of direct innervation from the 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 / surround structure is like that of bipolar and ganglion cells in the vision system.

Illustration of activating the center of a dorsal column neuron receptive field. Details in caption.
Figure 23.1. The receptive field of a dorsal column neuron has an excitatory center that is generated by the mechanoreceptors that synapse directly on the dorsal column neuron. A) When no stimulus is present, the dorsal column neuron fires at a baseline rate. B) When a stimulus touches the center of the receptive field of Cell E, 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.

The surround region of the receptive field is a result of indirect communication between the receptor neurons and the dorsal column neurons via inhibitory interneurons. The surround has an inhibitory effect on the dorsal column neuron. If a stimulus touches the skin in the surround of a dorsal column neuron’s receptive field, the neuron will decrease its firing rate.

Illustration of activating the surround of a dorsal column neuron receptive field. Details in caption.
Figure 23.2. The receptive field of a dorsal column nucleus has an inhibitory surround, which is a result of the indirect connections between mechanoreceptors and the dorsal column neuron via inhibitory interneurons. A) When no stimulus is present, the dorsal column neurons fire at a baseline rate. B) When a stimulus touches the surround of the receptive field of Cell E, the firing rate decreases. Note that the stimulus is in the surround of Cell E’s receptive field but is also in the center of Cell D, so the firing rate of Cell D 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.

Lateral Inhibition

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

Illustration of actual versus perceived stimulus strength due to lateral inhibition. Details in caption.
Figure 23.3. Lateral inhibition heightens the perception of edges or points on the skin. The point of a blunt probe pressing on the receptive field of Cell B will cause an increase in the firing rate of Cell E, but will also cause a decrease in the firing rate of Cells D and F. This increases the perceived difference between the point and the area next to the point that is not being 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.

Pathway to Brain

Dorsal Column-Medial Lemniscus Pathway

Primary afferent sensory fibers have their cell bodies located in the dorsal root ganglion, a structure that lies just outside of the spinal cord. The axons of these first-order neurons enter the ipsilateral dorsal side of the spinal cord. Some axon collaterals terminate in the spinal cord and are important for reflexes. The main axon branch ascends the spinal cord toward the brain, via the dorsal column, terminating in the dorsal column nuclei located in the brainstem. The axons of sensory neurons in the lower body remain separate from the axons of sensory neurons in the upper body throughout the pathway. These two populations of neurons synapse in different regions of the brainstem. The lower body axons terminate in the gracile nucleus, whereas the upper body axons terminate in cuneate nucleus. Projections from the second-order neurons in the dorsal column nuclei cross the midline, or decussate, and ascend via a white matter tract called the medial lemniscus. The axons terminate in the ventral posterior lateral nucleus of the thalamus. The thalamic neurons then project to the primary somatosensory cortex located in the postcentral gyrus in the parietal lobe.

Illustrated pathway of the touch pathway from the sensory neuron in the body to the somatosensory cortex. Details in caption and text.
Figure 23.4 Somatosensory information from the neck and body travels through the dorsal column – medial lemniscus pathway, named for structures within the pathway. Axons enter the spinal cord and ascend through the dorsal column to the medulla where decussation, or crossing the midline, occurs. Information continues to the thalamus via the medial lemniscus, and then reaches the somatosensory cortex. Details of the pathway are found 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.

View the thalamus using the BrainFacts.org 3D Brain

View the primary somatosensory cortex using the BrainFacts.org 3D Brain

Trigeminal Pathway

Sensory receptors in the face and head send information to the brain via cranial nerve V, the trigeminal nerve. The first-order neurons have their cell bodies in the trigeminal ganglion, located just outside of the brainstem, and they project to the ipsilateral trigeminal nucleus in the pons. The second-order neurons cross the midline and project up to the ventral posterior medial nucleus of the thalamus. These neurons then send projections to the face region of the somatosensory cortex.

Illustrated pathway of the touch pathway from the sensory neuron in the face to the somatosensory cortex. Details in caption and text.
Figure 23.5. Somatosensory information from the head and face travels through the trigeminal pathway. Axons enter the brainstem at the level of the pons and decussate before traveling to the thalamus and somatosensory cortex. Details of the pathway are found 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.
Touch pathways in text. Details in caption.
Figure 23.6. To compare the two pathways, sensory information comes in from the periphery. For the body, the peripheral axon branch travels via a spinal nerve to the cell body located in the dorsal root ganglion, which sits just outside the spinal cord. The central axon branch then enters the spinal cord and ascends via the dorsal column to the dorsal column nuclei in the brainstem. The second-order neuron crosses the midline and then projects to the ventral posterior lateral nucleus of the thalamus via the medial lemniscus tract. The thalamic third-order neuron projects to the primary somatosensory cortex in the parietal lobe. For sensory information from the face, the peripheral axon branch travels to the trigeminal ganglion via cranial nerve V. The ganglion sits outside of the brainstem, and the axons then enter the brainstem and synapse on the trigeminal nucleus. The second-order neuron travels to the ventral posterior medial 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.

Primary Somatosensory Cortex

Anatomy

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

Illustration of the divisions of the postcentral gyrus. Details in caption and text.
Figure 23.8. The somatosensory cortex, located in the postcentral gyrus, just posterior to the central sulcus, is divided into 4 areas: 3a, 3b, 1, and 2. The posterior parietal cortex, an output region of the somatosensory cortex, lies just posterior to the postcentral gyrus and is divided into 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.

Somatotopic Map

The receptive fields of each higher order neuron increases in size and complexity, but even cortical neurons are associated with a specific region of the body. Cortical neurons are organized by the region of the body they represent, so neurons that respond to sensation in the fingers are located close to the neurons that respond to sensation in the hand. Remember from above that axons in the dorsal column from the lower body run next to, but remain separate from, the axons from the upper body. This separation, which occurs for all body regions and at all levels of the pathway, 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.

Regions with high receptor density in the skin, and, therefore, fine two-point discrimination, have more cortical space devoted to them. This means that the cortical representation of the body is not true to actual physical proportions. A homunculus is a cartoon representation of what a body would look like if actual body size was proportional to the cortical representation. The hands and lips would be excessively large while the torso, arms, and legs, would be relatively small.

Illustration of somatosensory cortex showing location of body regions on the cortex. Details in caption.
Figure 23.8. The body is mapped onto the somatosensory cortex. Regions with high touch sensitivity, and therefore high mechanoreceptor density, have more cortical space dedicated to their processing. The feet and legs are represented in the medial superior region of the cortex; the face is represented on the lateral side of the cortex; the hand and fingers fall in between. ‘Somatotopic map’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Higher-Level Processing of Touch Information

The primary somatosensory cortex sends projections to other parietal lobe regions for higher-level processing of touch information.

Secondary Somatosensory Cortex

The secondary somatosensory cortex (SII) is located in the inferior parietal lobe, just above the lateral fissure. This region, like the dorsal stream of visual processing, is responsible for object recognition, discerning texture, shape, and size. The SII also has receptive fields that represent bilateral regions of the body, so both hemispheres will be activated by touch on either side of the body. The SII sends projections to the posterior parietal cortex, the premotor cortex, the amygdala, and the hippocampus.

Posterior Parietal Cortex

The posterior parietal cortex recognizes touch characteristics like orientation and movement. It is also important for combining the touch and motor components of actions like grasping. The posterior parietal cortex outputs to the frontal motor cortex.

Illustration of the brain showing the location of the primary and secondary somatosensory cortices and the posterior parietal cortex. Details in caption.
Figure 23.9. The primary somatosensory cortex is located in the postcentral gyrus. The posterior parietal cortex regions important for somatosensory lie caudal to the postcentral gyrus in the superior parietal lobe. The secondary somatosensory cortex is located dorsal to the lateral fissure, caudal to the postcentral gyrus. ‘Somatosensory Streams” by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

View the posterior parietal cortex using the BrainFacts.org 3D Brain

Cortical Plasticity

In adulthood, the brain is plastic, meaning synaptic connections can rearrange under certain conditions. Amputation or loss of a finger, for example, will lead to the associated cortical space to be functionally remapped by input from neighboring regions of the hand. The cortical neurons do not die, they begin to be activated by a different region of the body. Likewise, cortical representation can expand with use or practice. Repeated training of certain fingers can lead to an increase in cortical space mapped to those digits. Cortical plasticity is believed to underly the phenomenon of the perception of phantom limbs after amputation. In these cases, subjects that have lost a region of their body can sometimes still “feel” the missing part.

Illustration of somatotopic map before and after amputation. Details in caption.
Figure 23.10. It is possible to map cortical space to regions of skin. A) The cortical space mapped to each finger for an imaginary individual is shown as an example. B) If this individual were to lose a finger, in this case digit 3 or the middle finger, and the cortical space was remapped after time had passed, the region that had once responded to touch on digit 3 would instead respond to touch on either digit 2 or 4. The brain does not let that cortical space die or go to waste; it rearranges connections to make use of all the neurons. Based on Merzenich et al., 1984. ‘Cortical Plasticity’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Key Takeaways

  • Receptive fields become more complex as information moves through the touch pathway
  • Lateral inhibition enhances edges and borders by affecting the perceived stimulus strength
  • Mechanoreceptor afferents synapse in the dorsal column nuclei in the medulla. Information then decussates and synapses in the ventral posterior nucleus of the thalamus before traveling to the primary somatosensory cortex
    • Sensory axons from the lower body synapses in the gracile nucleus in the dorsal column
    • Sensory axons from the upper body synapses in the cuneate nucleus in the dorsal column
    • Information from the neck and body synapse in the ventral posterior lateral nucleus of the thalamus
    • Information from the head and face synapse in the ventral posterior medial nucleus of the thalamus
  • The primary somatosensory cortex is organized in a somatotopic map
  • The cortex is plastic and connections can change with experience

Test Yourself!

Additional Review

  1. What is the anatomical name of the primary somatosensory cortex?
  2. After somatosensory information leaves the brainstem, it must relay through which structure before reaching the primary somatosensory cortex?

Answers

Video Version of Lesson

License

Share This Book