Nineteen Flashcards
What are the two major components of the somatosensory system? What are these pathways called?
● A subsystem for the detection of:
mechanical stimulation - touch, vibration, pressure
mechanical forces - muscle length & tension, joint position (touch pathway)
A subsystem for the detection of
painful stimuli & temperature (pain/temp. pathway)
What are 3 sensorimotor modalites in the touch pathway? What are they? How are they tested? What is stereognosis? Graphesthesia?
- tactile discrimination or light touch
a) the awareness and precise localization on the body surface of a very delicate mechanical stimulus such as stroking the hairs or, in hairless areas, stroking with a wisp of cotton.
b) the ability to distinguish two separate points applied to the skin simultaneously (2-point sense).
c) the ability to recognize an object by touch alone, using its size, shape, texture, weight, etc. This is referred to as stereognosis and requires not only intact pathways but also memory.
d) the ability to identify letters or numbers drawn on the skin. This is referred to as graphesthesia and also requires intact pathways and memory.
- Deep touch/pressure or vibration sense - the awareness of various degrees of touch. These are tested with a tuning fork. Low frequency vibrations evoke a light,
fluttering sensation which can be precisely localized to the skin surface. High frequency vibrations evoke a diffuse humming feeling in deeper tissues. The former sensations are termed “flutter,” the latter “vibration.” - Proprioception or position sense - the awareness of the positions of the limbs and the direction and speed of movements of the limbs.
What kind of receptors are used in the touch pathway? What are they like? What are the fibers (axons) like in the touch pathway?
Tactile, pressure, and proprioceptive stimuli cause distortion of the various mechanoreceptors associated with these modalities. Mechanoreceptors are either encapsulated or free nerve endings in skin and muscles. It is generally accepted that large myelinated, fast conducting fibers carry the touch, pressure, and position sense impulses, and that these fibers are the peripheral branches of axons from the larger unipolar neurons of the spinal and appropriate cranial nerve ganglia.
Describe in detail the varoius tracts and synapses of the touch pathway.
The central branches of the axons of the unipolar neurons subserving tactile, vibration, and proprioception sensations enter the spinal cord through the dorsal roots and are funneled medially into the dorsal funiculus or column where they turn and ascend. Those entering below mid-thoracic levels form the gracile tract and those entering above mid-thoracic levels form the cuneate tract.
The fibers of the gracile and cuneate tracts ascend into the medulla without crossing and synapse on secondary neurons in their respective nuclei, the gracile and cuneate nuclei, the dorsal column nuclei. These secondary sensory centers give rise to bundles of axons that arch ventromedially as internal arcuate fibers, cross the midline as the sensory decussation, and ascend through the brainstem in the medial lemniscus.
After passing through the midbrain tegmentum, the medial lemniscus enters the diencephalon and terminates on neurons in the ventral posterolateral nucleus of the thalamus. From this nucleus axons enter the posterior limb of the internal capsule which carries these thalamocortical projections to the dorsal half of the postcentral gyrus and its extension medially, the posterior part of the paracentral lobule. The postcentral gyrus and posterior part of the paracentral lobule are the primary somatosensory (SI) cortex.
What are the two types of pain? What are they like? What elicits them? What are pain receptors called? What are they like? Where are they located? What are two types of temp. sensation? What are their receptors called? Where are they located and how are they distributed? What are they like? What are the fibers (axons) like for the pain/temp. pathway?
Pain sensations are of two types: fast and slow. Fast pain is of the sharp, pricking type and is well localized. Slow pain is of the dull, burning type and is poorly localized. Fast pain is tested by gently applying a sharp object, such as the tip of a safety-pin, to the surface of the skin. Slow pain results from tissue damage. The receptors for pain are termed nociceptors. Those associated with pin prick are in the epidermis. Those associated with slow pain are widely distributed.
Temperature sensations are of two types: cold and warm. The receptors for temperature are called thermoreceptors. Within the skin there are specific cold and warm points at
which only these sensations can be elicited. The thermoreceptors are located at these points and they occur as free nerve endings. The pain and temperature fibers are the thinner, slower conducting peripheral branches of the smaller unipolar ganglion cells.
Describe in detail the synapses and tracts of the spinothalamic pathway. What sensations does it serve?
The central branches of the axons of the unipolar neurons subserving fast pain and temperature sensations enter the spinal cord through the lateral parts of the dorsal roots which funnel them into the dorsolateral fasciculus (tract of Lissauer). Upon entering this tract, each axon bifurcates into an ascending and a descending branch. These branches extend for one or two segments and give off collaterals along their entire length.
The collaterals enter the gray matter and terminate in the dorsal horn (laminae I-VI). Secondary fast pain and temperature neurons are located in lamina I and laminae IV and V. Axons from these secondary pain and temperature neurons sweep ventromedially, decussate via the ventral white commissure which is ventral to the
central canal, and ascend in the anterior part of the lateral funiculus as the spinothalamic tract.
The spinothalamic tract ascends to the thalamus where most of its fibers terminate in the ventral posterolateral nucleus. Axons from the ventral posterolateral nucleus conduct fast pain and temperature impulses to the primary sensory cortex for the purpose of precise localization, sharpness, and intensity of pin prick and the warmth or coldness of temperature stimuli.
