Download Nolte Chapter 9 – Sensory Receptors and the Peripheral Nervous

Nolte Chapter 9 – Sensory Receptors and the Peripheral Nervous System
The intensity and duration of a stimulus are indicated by size and duration of the receptor potential
produce.
All receptors show adaptation, which means they become less sensitive during the course of a
maintained stimulus. Those that adapt relatively little are called slow adapting. Those that adapt a great
deal are rapidly adapting.
Some mechanoreceptros have channels that are directly sensitive to mechanical distortion, and other
have channels directly gated by some molecule or ion or by temperature changes.
If a particular sensory receptor is physically small relative to its length constant - that is, if it contacts
the next cell in its neuronal pathway close to the site of transduction – the receptor potential itself can
adequately modulate the rate of transmitter release at the synaptic terminal. This in turn causes a
postsynaptic potential, and typically a change in action potential frequency in the second cell.
The action potential frequency is modulated by the receptor potential, known as generator potentials.
Cutaneous receptors are either encapsulated or nonencapsulated. Receptors with layered capsules are
rapidly adapting. Nonencapsulated are either free nerve endings or accessory structures that are
associated with the ending, but do not surround it.
Nonencapsulated endings
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spiral endings that wrap around the base of the hair
o bending of the hair deforms the sensory ending and mechanically distorts mechanically
sensitive channels.
o most hair receptors are rapidly adapting; they respond well to something brushing
across the skin, but not to stead pressure.
Merkel cells
o in the basal layer of the epidermis
o sensitive to deformation and uses this synapse to pass information about mechanical
stimuli
o slowly adapting
Encapsulated Endings
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Meissner corpuscles
o in the dermal paoillae of hairless skin just beneath the epidermis.
o oriented with their long axis perpendicular to the surface of the skin.
o myelinated with schwann cells like a stack of pancakes where one or more myelinated
fibers approach the base of the corpuscle in a winding fashion, in and around the
schwann.
 this allows for vertical pressure to compress the nerve endings between the
stacks.
o These are rapidly adapting
o responsible for tactile discrimation since they are primarily on the skin of the fingers.
Pacinian corpuscles
o
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subcutaneous and look like an onion with many concentric layers of very thin epithelial
cells and fluid spaces between adjacent layers
o they are rapidly adapting.
 quickly applied forces are transmitted through the interior of the capsule and
reach the ending, but maintained forces are not, as a result of the elastic
properties of the capsular layer.
o allows for the feeling of vibration
 vibration will cause a steady train of umpulses from the endings
Ruffini endings
o in the dermis and subcutaneous and connective tissue sites
o thin, cirgar-shaped capsules that are traversed longitudinally by strands of collagenous
connective tissue.
o a sensory fiber enters the capsule and branches profusely.
o this is slowly adapting
o the squeezing of sensory terminals between strands of connective tissue when tension
is applied elicits firing. (sensitive to stretch)
Most tissues also contain free nerve endings that normally don’t respond until after tissue damage.
These are known as silent or sleeping nociceptors.
Temperature sensitivity is governed by cation channels that each have a specific range of temperatures
that increases the probability of its openings. These same channels have binding sites for various
botanical molecules, leading to the warm and cool feelings of chili peppers and menthol.
Nociceptors can detect stimuli that provide nxious levels of heat or cold or chemicals that are released
by damaged tissue. While many are specialized, polymodal nociceptors respond to all.
Sharp pain is carried by thinly myelinated fibers (Adelta fibers) and is known as first pain.
Dull aching pain, second pain, is from the non-myelinated C fibers.
Nociceptors are both afferents and efferents. They also can have their sensitivity modulated by the CNS
or by damaged tissue(like how burns release Potassium ions nearby and sensitize nociceptive endings).
Stimuli that are normal, but then become painful (think slapping on sunburn) is known as hyperalgesia.
Even light tough could cause pain and is known as allodynia.
In what is known as the axon reflex, nociceptors can be efferents and cause flaring and edema by
releasing glutamate not only ony second-order neurons but also by releasing neurotransmitters onto
themselves. Neuropeptides released in this peripheral fashion causes the dilation of arterioles(flare) and
the leakage of plasma from venules (edema) and also recruit phagocytes.
the more densely innervated areas can subserve subtler tactile discrimination than less densely
innervated.
Muscle Sensation
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Muscle spindles detect muscle length
o
o
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they are long and thin and scattered throughout every striated muscle.
Whenever a muscle is stretched, it stretched the intrafusal msucles as well, which is
where muscle spindles innervate.
o works in the same way as mechanical receptive channels get stretched and their
channels distorted.
Intrafusal muscles are of two types
o nuclear chain
 nuclei are lined up in a single file
o nuclear bag
 chains with breasts/bags
Types of sensory endings in muscle spindles
o primary ending
 single, very large nerve fiber that enters the capsule and then branches. each
branch wraps around the central region of an intrafusal fiber(mainly bags),
frequently in a spiral fasion (which is why they are also called annulospiral
endings)
 selectively sensitive to onset of muscle stretch
o secondary endings
 primarily innervate the chain fibers as flower-spray endings
 less sensitive to onset, but more so duration
Motor Innervation
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Muscle spindles receive motor innervation that supply extrafusal muscle fibers
alpha motor neurons are the main ones
gamma motor neurons are the smaller ones
o their activity “prestretches” the central receptive region of the muscle spindle and
causes some background activity while the muscle is contracted (since when the muscle
is contracted, the spindles won’t exactly be sensitive to stretch). they generate tension
on the nuclear regions so that is taught even though the muscle is contracted.
 basically, regulates the sensitivity of a muscle spindle so that the sensitivity of a
muscle spindle so that this sensitivity can be maintained during contraction.
Golgi tendon organs detect muscle tension
o they are found at the junctions between muscles and tendons
o they are similar to ruffini endings.
o they are interwoven collagen bundles surrounded by a thin capsule.
o Large sensory fibers enter the capsule and branch into fince processes that are inserted
among the collagen bundles. Tension on the capsule along its long axis squeezes these
fine processes, and the resulting distortion stimulates them.
o They are slowly adapting and great for when fine adjustments in muscle tension need to
be made.
kinesthesia is the conscious awareness of movement and seems to come from muscle receptors
whereas joint and cutaneous receptors are of more limited importance. mainly spindles, since their
stimulation causes illusions of movement.
Visceral Receptors
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most visceral responsed are supplied by thinly myelinated and unmyleinated fibers that
terminate as free nerve endings with or without accessory structures and are processed at a
subconscious level
they include mechanireceptors in the walls of hollow organs that detect pressure,
chemoreceptors that change blood pH, and nociceptors that relay pain.
o most are only sensitive when an organ is inflamed and many can serve as dualmechanisms(half of the time mechanoreceptors, half the time nociceptors)
Miessner corpuscles show that mechanical indentations can trigger action potentials that feel like touch
just like an electrical stimulation of that same neuron can yield. Since they are rapidly adapting, you can
feel multiple touches with multiple stimulations. However, you dot hat for a merkel neuron(slow
adapting) and it feels as though its maintained pressure. Do the same for pacininian and its interpreted
as vibration(since its very fast adapting).
Sensation from Ruffini’s only come touches of the skin and not direct stimulation, probably because the
CNS relies on many at once.
Peripheral nerves have their cell bodies in the dorsal root ganglia.
Epineurium is a loss connective tissue sheath that surrounds peripheral nerves.
Perineurium is a thin concentrically arranged set of cells with interspersed collaged and tight junctions
that isolate epineural space and serve as a blood-nerve barrier.
Larger fibers conduct action potentials faster than slow ones. Conduction velocities positively correlate
with both myelination and axonal diameter size.
A fibers are myelinated sensory and motor fibers. A delta convey sharp pain. alpha motor neurons are in
this class as well and the largest. Sensory are in generally in this group.
B fibers are myelinated visceral – preganglionic B fibers
C are non-myelated pain fibers.