Somatosensory system. psy 445 w01
I. Somatosensory sensation is based on depolarization of a primary
afferent fiber that ends in skin, muscle or joint. If this depolarization
reaches threshold, ap travels to spinal cord.
A. sensory endings are axons of dorsal root ganglion cells.
B. the central branch of the axon of dorsal root ganglion
cells goes to spinal cord.
II. What determines the nature of the sensation? proprioception,
mechanoreceptors, thermal, pain (nocioceptors)
A. What you feel depends on type of sensory ending and
the connections of the afferent nerve to the brain and spinal cord.
B. afferent nerves connected to different types of sensory
endings vary in conduction velocity.
III. General properties of somatosensory receptors: (discuss reason
for each of these properties).
A. modality
B. receptive field
1. location
2. size
C. adaptation
IV. modalities
A. Touch or Mechanoceptors:
B. Proprioception: muscles, joints, tendons
C. Thermal: Ad and C (both types are connected to both
cold and heat).
D. nocioceptors: Adelta and C, free nerve endings
V. Pathways to the cortex.
A. dorsal column pathway: large diameter axons serving
muscle spindles, joint afferents, and light touch terminate in bottom portion
of dorsal horn
1. collaterals of these axons ascend to brain stem where
they synapse in gracile and cuneate nuclei
2. axons of cells in these nuclei cross and go to ventral
posterior nucleus of thalamus.
3. Thalamic nuclei send axons to layer IV of S1.
B. anterolateral pathway: small fibers serving pain and
temperature terminate in upper part of dorsal horn
1. most axons in this pathway cross in cord, but there
are a few uncrossed fibers
2. spinothalamic end in VPL, posterior nucleus and intralaminar
nucleus.
a. pathway through VPL probably mediates sharp pain.
b. pathway through posterior nucleus and intralaminar
nucleus to association areas of cortex probably mediate slow burning pain.
3. spinomesencephalic end in tectum and periaqueductal
gray.
4. Consider hemisection of cord: lose pain and temp on
contralateral side, touch and proprioception ipsilateral.
VI. mapping.
A. there are topographic maps in gracile and cuneate nuclei,
in ventral posterior nuclei, and in S1.
B. touch animal and find stimulus location that produces
largest evoked potential.
C. actually 4 maps in S1, each respond primarily to different
submodality: muscle receptors, slow adapting skin receptors, and rapidly
adapting skin receptors, and joint receptors. but there is some mixing
of modalities.
VII. Cortex methods of study: receptive fields, lesions.
VIII. Properties of single cells in areas 3a and 3b.
A. columnar organization
B. each has a receptive field.
C. receptive fields have excitatory and inhibitory
regions, function of inhibition.
D. variation in receptive field size as a function of location.
For example, trunk:fingers-100:1
IX. Some S1 cells (esp in areas 1 and 2) have more complex properties.
These get input from areas 3a and 3b as well as thalamic input. Used to
make more abstract somatosensory distinctions, e.g. shape, texture, movement,
3 D shape.
A. these cells have larger receptive fields.
B. some sensitive only to movement. Others to direction
or orientation.
C. If inhibit area 2 with GABA agonist, lose ability to
make skilled movements
X. Lesions
A. Loss of S1: lose localization, 2 point discrimination, not
crude touch and not temp or pain. loss of stereognosis
B. loss of area 2 lose coordination. Point out motor deficit
even though not a motor area.