Somatic sensation enables our body to feel, to ache, to chill, and to know what its parts are
doing. It is sensitive to many kinds of stimuli. The somatic sensory system is different from other
sensory systems in two interesting ways:
its receptors are distributed throughout the body rather than being concentrated at small,
it responds to many different kids of stimuli (grouped into four senses touch,
temperature, pain, and body position)
A single stimulus usually activates many receptors. The central nervous system interprets the
activity of the vast receptor array and uses its to generate coherent perceptions.
Touch begins at the skin. There are two major types of skin: hairy and glabrous (hairless). Skin
has an outer layer, the epidermis, and an inner layer, the dermis. Skin performs an essential
protective function, and it prevents the evaporation of body fluids into the dry environment we
Mechanoreceptors of the Skin
Most of the sensory receptors in the somatic sensory system are mechanoreceptors, which
are sensitive to physical distortion such as bending or stretching. These receptors monitor:
contact with the skin
pressure in the heart and blood vessels
stretching of the digestive organs and urinary bladder
force against the teeth
Each mechanoreceptor contains unmyelinated axon branches which have mechanosensitive
ion channels whose gating depends on stretching or changes in tension of the surrounding
See Fig. 12.1. The largest and best studied receptor is the Pacinian corpuscle which lies deep
in the dermis. Ruffinis endings are found in both hairy and glabrous skin (slightly smaller than
the Pacinian corpuscles). Meissners corpuscles are located in the ridges of glabrous skin.
Merkels disks are located within the epidermis and consist of a nerve terminal and flattened,
non-neural epithelial cells. In Krause end bulbs borders regions of dry skin and mucous
Skin can be vibrated, pressed, pricked, and stroked, and its hair can be bent or pulled. The skin
is able to tell these different stimuli apart. We have mechanoreceptors that vary in their
preferred stimulus frequencies, pressures, and receptive field sizes. Mechanoreceptors also
vary in the persistence of their responses to long lasting stimuli.
For some animals, hair is a major sensory system. Hairs grow from follicles embedded in the
skin. Each follicle is richly innervated by free nerve endings that either wrap around it or run
parallel to it. There are several types of hair follicles. In all types, the bending of the hair causes: deformation of the follicle and surrounding skin tissues
This stretches, bends, or flattens the nearby nerve endings which increases or
decreases their action potential firing frequency.
Mechanoreceptors of hair follicles may be either slowly adapting or rapidly adapting.
The different mechanical sensitivities of mechanoreceptors mediate different sensations.
Pacinian corpuscles: 200 300 Hz
Meissners corpuscles: 50 Hz
Vibration and the Pacinian Corpuscles
The selectivity of a mechanoreceptive axon depends primarily on the structure of its special
ending. Pacinian corpuscle has a football-shaped capsule with 20-70 concentric layers of
connective tissue with a nerve terminal in the middle. When the capsule is compressed, energy
is transferred to the nerve terminal, its membrane is deformed, and mechanosensitive channels
open. Current flowing through the channels generates a depolarizing receptor potential (see Fig.
12.5). If the depolarization is large enough, the axon will fire an action potential.
These capsule layers are slick and if the stimulus pressure is maintained, the layers slip
past one another and transfer the stimulus energy in such a way that the axon terminal
is no longer deformed and the receptor potential dissipates.
When the pressure is released, the events reverse themselves. The terminal depolarizes
again and may fire another action potential.
Two-point discrimination varies at least twentyfold across the body. Fingertips have the highest
resolutions. Several reasons explain why the fingertip is so much better than, say, the elbow for
There is a much higher density of mechanoreceptors in the skin of the fingertips than
The fingertips are enriched in receptor types that have small receptive fields
There is more brain tissue devoted to the sensory information of each square millimetre
of fingertip than elsewhere
There may be special neural mechanisms devoted to high-resolution discriminations
Primary Afferent Axons
The skin is richly innervated by axons that course through the vast network of peripheral nerves
on their way to the CNS. Axons bringing information from the somatic sensory receptors to the
spinal cord or brain stem are the primary afferent axons. The primary afferent axons enter the
spinal cord through the dorsal roots which lie in the dorsal root ganglia (see Fig. 12.8).
Primary afferent axons have widely varying diameters and the size correlates with the type of
sensory receptor to which they are attached (see Fig. 12.9). The diameter of an axon (with its
myelin) determines its speed of action potential conduction. The smallest axons (C fibers) have
no myelin and mediate pain and temperature sensation. They are the slowest of axons.
Touch sensation, mediated by the cutaneous mechanoreceptors, are conveyed by the
relatively large A axons.The Spinal Cord
Segmental Organization of the Spinal Cord
The arrangement of paired dorsal and ventral roots (see Fig. 12.8) is repeated 30 times down
the length of the human spinal cord. Each spinal nerve, consisting of dorsal root and ventral root
axons, passes through a notch between the vertebrae of the spinal column.
See Fig. 12.10, the 30 spinal segments are divided into four groups: cervical (C) 1-8, thoracic
(T) 1-12, lumbar (L) 1-5, and sacral (S) 1-5.
The area of skin innervated by the right and left dorsal roots of a single spinal segment is called
a dermatome. There is a one-to-one correspondence between dermatomes and spinal
segments. When mapped, the dermatomes delineate a set of bands on the body surface (see
Fig. 12.11 & 12.12).
When a dorsal root is cut, the corresponding dermatome on that side of the body does not lose
all sensation. The residual somatic sensation is explained by the fact that the adjacent dorsal
roots innervate overlapping areas. To lose all sensation, three adjacent dorsal roots must be
The spinal cord in the adult ends at about the level of the third lumbar vertebra (see Fig. 12.10).
The bundles of spinal nerves streaming down within the lumbar and sacral vertebral column are
called the cauda equina. It is filled with cerebral spinal fluid (CSF).
Sensory Organization of the Spinal Cord
The spinal cord