PNS/Reflexes
I. Types of sensory receptors based on structure
A. Free dendritic endings- ex., nociceptors
B. specialized receptors: could be
dendrites covered by connective tissue sheath: “encapsulated”- ex,
Meissner’s corpuscles
more than one cell interact to form a very specialized receptor-, ex.
Merckel's (touch), most special sensory receptors
II. Types of receptors based on where they are bringing information from
A. Exteroceptors
B. Interoceptors
C. Proprioceptors
III. Types of receptors based on the stimulus they respond to
A. Thermoreceptors- monitor temperature. Located in the dermis and
hypothalamus.
B. Mechanoreceptors- run the gamut: some are free dendritic endings, some are
more specialized.
1. Tactile- respond to pressure & vibration, ex., in the skin
2. Baroreceptors- respond to pressure against tubes/stretch of tubes (ex,
blood pressure, stomach distention)
3. Proprioceptors- monitor stretch and position of tendons, muscles, joint
capsules
C. Chemoreceptors- monitor fluid (solute) content, ex. blood and CSF. Locations
will be discussed in the vessels chapter; but, for ex., chemoreceptors in the
medulla monitor CSF.
D. Photoreceptors- respond to light energy. In the retina of the eyes
E. Nociceptors- pain receptors. One of the Nt commonly released by nociceptors
is glutamate.
can be mechanoreceptors (respond to stretch, etc), chemoreceptors
(respond to "help" signals by damaged cells), or thermoreceptors (respond
to extreme temperatures)
free dendritic endings, located virtually all over
some transmit pain signals via type A fibers: this produces a sharp pain.
Pain signals transmitted via type C fibers produce a throbbing/burning
pain.
pain receptors are tonic and do not exhibit peripheral adaptation; but
central adaptation can reduce the perception of pain (see below).
IV. Adaptation - when you are exposed to a constant stimulus (ex, a scent), your
perception of that stimulus can sometimes diminish over time. One of two things can
happen to produce this effect
A. Peripheral adaptation- some sensory neurons can adapt to constant stimuli; that
is, they begin to require stronger stimulus input to depolarize
Phasic receptors are usually "off," and are turned "on" by a threshold
stimulus or change in stimulus level. Phasic receptors can typically exhibit
peripheral adaptation quickly (ex, thermoreceptors)
Tonic receptors are usually "on," constantly providing information about
the intensity of a stimulus. Tonic receptors typically do not exhibit
peripheral adaptation quickly, if at all (ex., nociceptors)
B. Central Adaptation- peripheral neurons send signals, but those signals don't
make it to awareness. Control by the thalamus is one way central adaptation is
achieved. "Getting used to" a strong scent is an example of central adaptation,
although this is achieved by inhibition from other portions of the brain, not the
thalamus. I was very happy for central adaptation when I worked at a Humane
Society for a couple of years ;).
V. Nerves
A. Structure- Bundles- endo, peri, and epineuria, fascicles
B. Regeneration
C. Cranial Nerves – Know for Lab Practical 4 (see handout)
D. Spinal Nerves- nerves originating from the spinal cord. Spinal nerves are
mixed (carry both sensory and motor neurons). Remember that as a spinal nerve
"approaches" the spinal cord, the sensory and motor neurons separate from each
other into roots, so that sensory neurons synapse in specific area and motor
neurons synapse in a specific area of the cord (where?). The spinal nerves branch
to serve different areas of the body. Some merge together, forming plexuses.
Know for Lab Practical 4:
>Cervical Plexus- nerves serve head, neck, chest, diaphragm
>Brachial Plexus- nerves serve shoulders, arms, hands and chest
>LumboSacral Plexus- nerves serve pelvic region, legs and feet
>Thoracic Nerves- serve the trunk
E. Dermatomes- Know how they are related to the disease Shingles.
VI. Spinal Reflexes- stereotyped responses to stimuli in which motor neurons are
stimulated without input from the brain: fast, survival mechanisms.
A. Monosynaptic Reflexes; for example, the stretch reflex
B. Polysynaptic Reflexes; for example, the withdrawal reflex
C. Reciprocal inhibition (antagonists are inhibited)