Lecture 13: Somatic Sensation

Note that there is a reading for this lecture by Miguel Nicolelis in the Chemistry Library on Reserve. It is essential that you read it. I also take two figures from it which are described below.

There are several forms of somatic sensation, Exteroceptive, including mechanoreception, thermoreception and nociception, Proprioception, and Interoception. Exteroception refers to the sensations dealing with the external surface of the body; proprioception deals with the sensations from the muscles, joints, tendons, etc., that convey the sense of the body's position and movement in the world. Interoception deals with the internal organx of the viscera, some of which is conscious and some of which is not.

Figure 26.4

There are several kinds of somatosensory receptors. This portrays the morphology of the different types including their non-neural components that are essential for function. Pictured are the Meissner corpuscles, Merkel disks, Pacinian corpuscles and Ruffini endings. Finally, there are free nerve endings. We will discuss the morphology and how it impacts the function of the receptor in transducing the particular sensation.

Figure 26.5

Maps of the receptive fields of particular types of receptors in the hand. Meissner and Merkel corpuscles both have punctate receptors, while Pacinian and Ruffini corpuscles have bigger fields.

Figure 26.8

Proprioceptive afferents and their associated receptors - Golgi tendon organs (GTOs) and spindle organs. GTOs respond when the tendon is stretched when the muscle is contracted. Muscle spindle organs (types Ia and II) terminate on the non-contractive portions of the "intrafusal" muscle fibers. They are arranged in parallel with the muscle contractile fibers, the extrafusal fibers. Specialized motoneurons (g) provide the motor innervation to the intrafusal fibers. These will be discussed more during motor control lectures.

Figure 26.9

The classic dermatomes maps showing the distribution and segmentation of the spinal nerves from which they arise. There is considerable overlap between nerves arising from adjacent segments, but the map allows one to determine the location of injury.

Figure 26.10

Anatomy and cross section of the spinal cord at the cervical level. The gray matter can be divided into the dorsal and ventral horns. The dorsal horn is the sensory portion of the spinal cord, with sensory fibers entering through the dorsal root entry. The ventral horn contains the motor neurons which leave via the ventral roots. This diagrams the layers of the spinal cord.

Figure 26.11

Organization of the ascending somatosensory paths. There are two general pathways. A, the large diameter fibers synapse in the spinal cord, but don't terminate there. They continue on up to the dorsal column nuclei, the cuneate and gracili nuclei where they synapse and terminate. The second order neurons cross the midline and terminate in the ventral posterolateral region of the thalamus. These then go to the primary somatosensory cortex, SI. The other system, B, is composed of fibers which terminate in the spinal cord. The second order neurons cross the midline and synapse in the thalamus as well, but give off collaterals in the reticular formation and the pons.

Figure 26.14

Functional organization of the thalamic and cortical areas for somatosensory afferents. This diagrams the relationship between SI, the primary somatosensory cortex, and SII and association cortex in the posterior parietal lobe. This also diagrams the relationship between the thalamic nuclei to which the somatosensory neurons project and the cortex.

Figure 26.15

Somatopic organization of human SI showing the famous homunculus.

Figure 26.18

Overall organization of the pathway for somatosensory input to three cortical areas.

Figure 2 from Nicolelis

Nicolelis illustrates somatosensory function by studying the "whisker barrels" of rats. Each whisker has been believed to project through the dorsal column nuclei to the thalamus and to the cortex where they form columns, so-called whisker barrels. It has been believed that each thalamic neuron responds to a single whisker. However, Nicolelis shows quite convincingly that the responses of the thalamic neurons are far more dynamic than that. He records from as many as 50 cells simultaneously in awake behaving animals. In this figure two whiskers (D4 and E2) were stimulated, and the responses of 20 cells recorded over time. In the upper figure is the response to D4. Notice that the low numbered neurons respond vigorously at first, but all respond to a great extent, especially over time. When E2 is stimulated the same neurons respond with all giving a brisk response about 10 seconds after the stimulus, but they continue for a while. Generally, the spatiotemporal pattern varies with the stimulus location.

Figure 3 from Nicolelis

The spatiotemporal receptive field of a single thalamic neuron. This is 4D plot, showing the response of the neuron at different moments over time as a series of whiskers is stimulated. The plots at each moment are to the entire array of whiskers. At 5-10 milliseconds the response is best to several whiskers in the upper right quadrant. At 20-25 msec, the response is best to the left upper quadrant. This shifts continually until at 35-50 msec one sees an entirely different picture. Remember that this is the responses of a single neuron. Thus, a single neuron is not responding to just one whisker, but is responding to all the whiskers. Granted, this particular neuron is extreme, but all is neurons were of a far more dynamic and broader response than the typical picture in the book. Also, he found this at the cortex as well, and in primates as well as rats.