GOAL: TO DEVELOP GENERAL APPRECIATION OF THE NEUROLOGICAL BASIS FOR SOMESTHESIS.

1. Apply knowledge of A/P to the evaluation of the peripheral speech mechanism.

2. Differentiate the types of somesthetic senses

3. Describe the three neuron organization of the somesthetic system.

1. There are Three types of somesthesis : mechanoreception; thermoreception; nocireception.



1. Mechanoreception is the sense of gross position changes body tissues.



1. Neurons are mechanically displaced by some external force.



2. Mechanoreception includes several kinds of displacement



1. The TACTILE ("touch") sensations of touch, pressure or vibration.



2. The KINESTHETIC or PROPRIOCEPTIVE sense of position and movement.



2. Thermoreception is the sensation of heat or cold.



3. Nocireception is pain associated with tissue destruction.



2. There are gradations of the somesthetic sensory experience.



1. Not all sensations fit into the groups of "pain, temperature. touch."



1. For example: into which of the above categories would you place: itch or tickle, or stereognosis?



2. Perhaps each individual has special percepitions of these stimuli.



2. The somesthetic experience is graded because stimuli impinge on receptors in groups.



1. Stimulation of a single receptor is rare.



2. Reality of touch is quite complex, and temporal and spatial summation result in great variations in the type of sensation.

1. Special somesthetic receptors fall into two braod categories: exterioceptors and enteroceptors.



1. Exterioceptors are located in skin, joints, muscles and tendons



1. CUTANEOUS RECEPTORS are located just beneath the skin.



1. They sense exterioceptive changes: changes in the external surface of the body.



2. These sensations include touch pressure, pain, vibration, and thermal sensitivity.



2. PROPRIOCEPTORS are located in the striated muscles, ligaments and joints.



1. They provide continuous feedback on muscle stretch status.



2. This feedback in extremely important because it enables modification of movement in progress.



2. Exteroceptors are linked to the CNS by SOMATIC AFFERENT NEURONS.



1. Not all of the input is conscious



2. Responses to the unconscious input may be reflexive.



3. ENTEROCEPTORS respond to changes in the internal viscera. , blood vessels.



1. They are associated with autonomic NS.



2. Include vasosensory receptors.



1. Vasosensory receptors respond to tension between oxygen and carbon dioxide levels in the blood stream.



2. Their input to the afferent ANS gives initiates the ventilation response.



4. Somesthetic receptors can be categorized according to their sensitivity and adaptability.



1. Sensitive receptors are readily excited.



1. They readily adapt and then their response drops off.



2. The most sensitive and adaptive of the receptors are ENCAPSULATED.



1. Encapsulated receptors have concentric layers of tissue around the nerve ending.



2. They generate an action potential when capsule is deformed.



3. Encapsulated receptors are located in the lips. fingers as well as other body parts. (Examples of encapsulated receptors are Meissner Corpuscles & Pacinian Corpuscles).



3. Moderately Sensitive are "Expanded Tip Receptors." These receptors convey sensation from the dermatomes and the joiints. They are moderatey adapting.



4. The least adaptive receptors are free of surrounding capsules and not as sensitive. Accordingly, they are called "Free" receptors.



1. Free receptors are arborized throughout skin and viscera.



2. They sense gross temperature variations and pain.





2. Cutaneous somesthesis is segmented in DERMATOMES.



1. Dermatomes are topical regions of touch sensitivity.



2. They correspond to spinal or cranial nerve segments.





3. All somesthesis is transmitted to the CNS via the THREE NEURON arrangement.

1. FIRST ORDER neurons begin at the receptors.



1. Receptors have their cell bodies in the dorsal root ganglia of the cord.



2. They synapse in posterior horn gray of the spinal cord or in the lower levels of the brainstem.



2. SECOND ORDER neurons decussate (cross to the other side of the CNS).



1. Through the anterior white commissure of the cord or through the lower part of the brainstem.



2. They turn rostrally...



3. and ascend to thalamus.



3. THIRD ORDER neurons in ventral or posterior nuclear group to radiate to somesthetic cortex.



1. Primary reception is in the postcentral gyrus.



2. Association occurs in the paracentral gyrus...



3. ...of the parietal lobe.