#10 Flashcards
There are four major modalities of somatic sensation
- Discriminative touch – for recognition of size, shape, and texture of objects, and
their movement across the skin - Proprioception – for sensation of static position and movement of the limbs and body
- Temperature sense – for sensation of “cool” and “warm” changes in temperature
- Nociception – for signaling of tissue damage or chemical irritation (pain or itch)
Each major modality of somatic sensation is mediated by a distinct system of receptors and pathways to the brain,
but all share a common class of sensory neuron:
: the dorsal root ganglion cell
X are the “ first order neuron” in the somatosensory pathway
Dorsal root ganglion
Peripheral terminals of DRG neurons
two types:
- Terminal is encapsulated by a nonneural structure (such as a Pacinian corpuscle or Golgi tendon organ) – for sensations of touch and proprioception
- Terminal is a bare nerve ending – for thermal and painful sensations
sensory receptors encode four elements attributes of stimuli:
modality, location, intensity, duration
Somatosensory systems process information in a series of relay nuclei. These pathways
have a serial organization with a distinct functional hierarchy, as follows:
I. First-order neuron (the sensory receptor) is the DRG cell; primary afferent fibers from multiple DRG
neurons converge and synapse on…
II. Second-order neurons located in a segment of spinal cord gray matter or in the brain stem; these neurons cross and project to…
III. Third-order neurons located in the thalamus, which axons project to higher-order neurons in the cortex where perception is experienced on a conscious level.
The receptive field of a DRG neuron
it’s the patch of skin that will
trigger an action potential if presented with an adequate stimulus
Every DRG neuron projects to and excites a second-order neuron (which receives a
cluster of DRG inputs from a patch of skin).
Meissner’s corpuscles and Pacinian corpuscles both contribute to
Merkel’s disks respond to a
(pressure sense), and Ruffini endings are thought to respond to b
vibration sense
meissner: 20-50 Hz
Pacinian: 250-350 Hz
a: indentation of the skin
b: skin stretch
- Among mechanoreceptors, a are slow-adapting.
These receptors generate action potentials throughout the period of skin
indentation, signaling the magnitude/intensity of stimulation. Objects indenting
the skin are detected by receptors that adapt slowly. - b are fast-adapting. They only
respond at the beginning and end of a skin indentation, signaling the rate or
velocity of stimulation. Vibration is detected by receptors that adapt quickly.
a: Merkel’s disks and Ruffini endings
b: Pacinian corpuscles and Meissner’s corpuscles
Proprioception, literally “sense of self” (from Latin ‘proprius’ = ‘own’) is governed by
proprioceptors. Proprioceptors sense the modalities of a, b,
and c. The information encoded by these receptors is conveyed in both conscious and nonconscious pathways. Conscious proprioception impinges on consciousness and
therefore relays in the d. Nonconscious proprioception mainly drives reflexes in
the e and signals the f about body position (does not use the
d!)
a: muscle stretch
b: muscle tension
c: joint angle
d: thalamus
e: spinal cord
f: cerebellum
muscle spindles
signal muscle length and speed at which muscle is stretched
type Ia fibers: dynamic fast adapting, respond to change in length of muscle
Type IIa fibers: static, active during the steady state after a change in length
Only “cool” (5-40°C) and “warm” (30-45°C) sensations are detected by a.
Above or below these limits, humans perceive “cold” and “hot” as pain detected by b
a: thermoreceptor
b: thermal nociceptors
Visceral afferents are structurally indistinguishable from somatic afferent neurons
— Cell bodies located in the DRG or CN ganglia
— Most visceral nerve terminals are bare
— Majority of fibers are thin Ad and C fibers
Fine touch, consciousness lives in
medulla
most myelineted sensory receptor type?
proprioceptors of skeletal muscle
