10/23 - Sensory System - Peripheral Receptors Flashcards
“soma”
“soma” = “body,” in Greek
The cell bodies of all neurons involved in primary processing of sensory information are located ______ .
in a GANGLION located somewhere in the periphery.
For the body, the cell bodies of all neurons involved in primary processing of sensory information are located _____ .
DORSAL ROOT GANGLIA.
For structures in the head. the cell bodies of all neurons involved in primary processing of sensory information are located ______ .
in ganglia related to each cranial nerve.
For example, neurons carrying sensations from the face will be in the TRIGEMINAL GANGLION.
Somatosensory System
Collective category for all sensations that are NOT vision, hearing, taste, olfaction or vestibular sense of balance (i.e., “Special Senses”)
Receptors are distributed throughout the body – external & internal.
Somatosensory System:
4 Senses
Touch
Body Position
Pain
Temperature
Divide into 2 subgroups:
Touch & Body Position
Pain & Temperature
largest organ in body
Skin = largest organ in body (3000 mm2, 6ft person)
All of the skin is innervated, but to varying degrees
The Mechanical Senses
Touch (Somatosensory)
Proprioception
Thermal
TEMPERATURE
Chemical
Pain
Itch
Itch is often considered a category of pain.
Different forms of energy elicit distinct types of somatosensory perception
Same type of physical energy can elicit different sensations depending on type & location of receptor & stimulus intensity.
Different forms of physical energy: mechanical, thermal, and chemical give rise to distinct types of precepts which group into broad categories known as submodalities: touch, proprioception, thermal, pain & itch, and arguably certain visceral sensations are all different submodalities.
For example, gentle mechanical stimuli give rise to touch sensations, like tapping, stroking or pressure, whereas very intense mechanical stimuli can give rise to pain.
Mechanoreceptors
low threshold
can detect minute forces impinging on skin (exteroceptive)
Our ability to sense touch relies on mechanoreceptors.
These receptors can detect very minute forces impinging on the skin.
Proprioceptors
low threshold
can detect small changes in muscle tension
Receptor
located on primary afferent ending itself
stretch sensitive ion channel
Cell body
in DRG or Trigeminal ganglion
DRG NEURON
Peripheral process:
Large diameter, heavily myelinated axons carry information from muscle spindles/joint receptors (proprioception)
Slightly smaller diameter, myelinated axons, carry information from cutaneous receptors (touch, vibration)
DRG NEURON
Central process
Enters through dorsal root and courses directly to posterior column where they ascend to the brainstem forming the fasciculus gracilis or fasciculus cuneatus.
What sensory axons are the largest sensory axons?
Proprioception axons
Consequently, they are also the fastest conducting.
Trigeminal Ganglion
Large structure that contains sensory neurons that send axons into the 3 divisions of the trigeminal nerve:
V1, V2, and V3.
V1 of the Trigeminal
Sensory innervation of orbit including eyeball.
Touch, pressure, pain. Also area around orbit externally including forehead.
Some trigeminal ganglion sensory neurons send their axons into V1. These axons enter the orbit above the muscles that elevate the eyelid or move the eye. They carry the sensory information from the orbit as well as the eye itself.
Think of touching the eye with your finger or putting in a contact lens. Also, putting drops in your eyes. The touch and cool sensations are carried to the CNS via branches of V1.
V2 of the Trigeminal
Sensory innervation of area around zygomatic arch, nasal cavity and upper teeth.
Some trigeminal ganglion sensory neurons send their axons into V2. These axons leave the skull through a separate opening and innervate structures such as the upper teeth, the area of the face over the zygomatic arch, the nasal cavity, and the hard and soft palates.
V3 of the Trigeminal
Sensory innervation area around mandible and chin, including lower teeth and tongue.
Some trigeminal ganglion sensory neurons send their axons into V3 which leaves through another opening.
This branch innervates the teeth in the mandible and the skin over the cheek, the tongue (general sensory only) and the chin.
This branch of the trigeminal nerve also contains axons that innervate muscles of mastication.
The neurons giving rise to these axons are located within the brainstem.
In addition to the peripheral processes (V1, V2, V3), the trigeminal ganglion cells give rise to a ____ that enters the CNS carrying sensory information from different regions of the face.
In addition to the peripheral processes (V1, V2, V3), the trigeminal ganglion cells give rise to a Central Process that enters the CNS carrying sensory information from different regions of the face.
Peripheral somatosensory nerve fibers vary in size
Specialization starts in the periphery.
Fibers of different diameter are associated with different submodalities.
A alpha
Aα
Group I
Largest in axon diameter of the 4 types.
Proprioceptors of Skeletal Muscle
Primary receptors of muscle spindle.
Golgi tendon organ.
A beta
Aβ
Group II
Mechanoreceptors of Skin
Touch
Secondary receptors of muscle spindle.
All cutaneous mechanoreceptors.
A delta
Aδ
Group III
Free nerve endings of touch, pain, and pressure.
Nociceptors of neospinothalamic tract.
Cold thermoreceptors.
C
Group IV
Nociceptors of paleospinothalamic tract
Warmth receptors
MECHANORECEPTORS
Receptors that are sensitive to physical distortion – bending, stretching, pressure, vibration.
In addition to the skin, they are present in organs such as the heart, intestines, bladder, and even teeth.
Consist of unmyelinated distal segment that is often surrounded by a fibrous capsule or related to some other specialization. The axons have mechanosensitive ion channels which depend on stretching or changes in tension for activation.
Sensory receptors transduce physical stimulus into electrical signal called the receptor or generator potential in response to skin distortion.
What are mechanoreceptors sensitive to?
Receptors that are sensitive to physical distortion
bending, stretching, pressure, vibration.
Where are mechanoreceptors found?
In addition to the skin, they are present in organs such as the heart, intestines, bladder, and even teeth.
How do mechanoreceptors respond to stimuli?
Sensory receptors transduce physical stimulus into electrical signal called the receptor or generator potential in response to skin distortion.
What components are mechanoreceptors composed of?
Consist of unmyelinated distal segment that is often surrounded by a fibrous capsule or related to some other specialization.
The axons have mechanosensitive ion channels which depend on stretching or changes in tension for activation.
Because they are unmyelinated, they are easily stimulated.
glabrous skin
external skin that is naturally hairless.
It is found on the ventral portion of the fingers, palmar surfaces of hands, soles of feet, lips.
What 2 Mechanoreceptors do NOT have a layered structure associated with it.
Ruffini and Merkel
What 2 Mechanoreceptors are situated superficially in the skin.
Merkel’s disks & Meissner’s corpuscles
Because of this superficial placement, they have limited branching, and are densely packed.
Ruffini endings
profusely branched & interspersed among collagen fibers of the extracellular matrix-
The lack of a layered structure, combined with the limited elasticity of collagen gives rise to the Slow Adapting nature of these receptors.
Merkel cells
simple cup shaped endings
They are so-named becuase they are closely associated with a specialized epi cell, MC, whose function is unknown.
Their relatively “naked” state thought to account for Slow Adapting nature.
Because pressure spreads out as move away from source ___ and ___ can be activated by stimuli arising from more distant stimuli.
Ruffini endings and
Pacinian corpuscles
What 2 Mechanoreceptors are situated deeper in the skin.
Pacinian corpuscles & ruffini’s endings deep
Because they are deep, they can also be sparse.
