Somatosensory System Flashcards
Lecture 10/11/12
Intrafusal fibers
Non-force producing muscle fibers (lengthen and stretch with changes in muscle length)
Afferent axons
SENSORY INFO- innervate endings of muscle spindle and send feedback to the spinal cord
Muscle spindles
Specialized sensory receptors (provide feedback on muscle stretching to CNS)
Efferent axons
MOTOR COMMANDS- yMN’s innervate polar (contractile) ends of intrafusal fibers and send excitatory inputs from spinal cord
Gamma motor neuron
stimulates intrafusal muscle fibers
Sensory endings
sense length of intrafusal muscle fibers (part of afferent axons)
Center of muscle spindle
Non-contractile region
Capsule
connective tissue surrounding intrafusal fibers and sensory receptors that compose spindle
Extrafusal (skeletal muscle) fibers
Fibers that cause muscle contraction (parallel with intrafusal fibers)
Nuclear bag fibres
instrafusal fibres w/ nuclei arranged in a central ‘bag’ region
Nuclear chain fibres
intrafusal fibres w/ nuclei arranged along a straight chain
primary afferents (group 1a)
wrap around all types (bag/chain) types of intrafusal fibres
secondary afferents (group 2)
‘flower spray endings’ onto ONLY nuclear chain type intrafusal fibers
Secondary muscle spindle ending
Combination of a secondary afferent and the intrafusal fibres it contacts
Primary muscle spindle ending
Combination of a primary afferent and the intrafusal fibres it wraps around
Larger stretches of the muscle lead to…
Larger receptor potentials
Dynamic phase
when the muscle is being stretched
The difference in the firing rate of muscle spindles
provides the CNS with sensory input proportional to the velocity
Primary endings are more sensitive to…
the dynamic phase of movement (velocity)
Secondary endings are more sensitive to…
muscle length
Human microneurography
method of inserting an electrode directly into a peripheral nerve to record action potentials from sensory and motor neurons
Do secondary and primary endings respond equally as much to the dynamic phase?
No, primary endings are more responsive to velocity
Gamma motor neurons (yMN’s)
stimulated at the same time as the alpha-MN, the intrafusal fibers contract along with the extrafusal (skeletal) muscle fibers WHICH ALLOWSTHE MUSCLE SPINDLE TO KEEP RESPONDING DURING CONTRACTIONS
Without yMN
the spindle would stop providing feedback where your limbs are (floppy/slack fibers)
Co-activation
when alpha (extrafusal) and gamma (intrafusal) motor neurons respond together (same message from CNS)
Proprioception
our bodies understanding of where it is in space (position)
Kinesthesia
our brains understanding of movement (basically proprioception of movement)
Mechanotransduction
mechanical stimulus (energy) is converted to electrical info (voltage change) in sensory afferent neurons
cutaneous receptive field
region of the skin capable of evoking action potentials in a given cutaneous afferent
non-hairy skin
glabrous
Type 1 cutaneous receptors are…
superficial
Type 2 cutaneous receptors are…
deep
Which receptors are slow adapting?
Merkel cells and Ruffini endings
Which receptors are fast adapting?
Meissner’s corpuscles and pacinian corpuscles
Place the receptors in order from most sensitive to indentation to least
1) Pacinian corpuscles (FA2)
2) Meissner’s corpuscles (FA1)
3) Merkel cells (SA1)
4) Ruffini endings (SA2)
If a receptor has a low mechanical threshold what does this imply?
It is easily activated and sensitive to indentation (force)
Which receptor is known for irregular discharge when stimulated?
Merkel cells (SA1)
Qualities of Merkel cells (SA1)
-irregular discharge when stimulated
-highly sensitive to edges/curvature
-30um threshold
Qualities of Ruffini endings (SA2)
-regular discharge
-very sensitive to skin stretch
-high (300um) threshold
-NOT IN NON-HUMAN PRIMATES
Slow adapting receptors will…
continue to fire until stimulus is removed
Fast adapting receptors will…
adapt to changes in indentation (held stimulus will stop firing)
Qualities of Meissner’s corpuscle (FA1)
-codes for velocity of skin indentation (motion)
-40% of innervation in hand
-sensitive to low frequency vibration (30-50Hz)
-6um threshold
Qualities of Pacinian corpuscle (FA2)
-codes for ACCELERATION (change in indentation rate/velocity)
-picks up high frequencies (50-500Hz)
-extremely low threshold (0.08um)… most sensitive to indentation
Type 1 receptors have _______ mechanoreceptor(s)/hotspots per axon
multiple
Type 2 receptors have _______ mechanoreceptor(s)/hotspots per axon
one
Which type of receptor has larger receptive fields?
Deep (Type 2)
Which type of receptor has a higher innervation ratio?
Superficial (Type 1)
Which receptors are highest in density on the tips of fingers?
SA1 and FA1
Which receptors are equally distributed?
SA2 and FA2
What do merkel cells (SA1) respond most to?
Edges, curvature, and texture
What do Meissner’s corpuscles (FA1) respond most to?
slip/motion detection, grip control
What do Ruffini endings (SA2) respond most to?
skin stretch
What do Pacinian corpuscles (FA2) respond most to?
feeling vibration through objects, perception of fine texture
Vibration sensitivity
-receptors are very sensitive to alternating inputs
-sensitive to acceleration of skin indentation
Spatial code
-CNS can determine the location of stimuli based on which afferents specifically are active
How do thermally-gated receptors work?
Heat deforms the protein channel allowing ions to flow in
When will nociceptors turn on in relation to thermoreceptors?
When thermoreceptors peak/asymptote
When do nociceptors turn on?
become active once the heat becomes tissue damaging
Why are spicy foods hot?
Capsaicin artificially creates a sensation of heat (chemical signal)
Capsaicin
lipid based so it diffuses easily across cell membrane and binds inside cell (chemically igniting feeling of heat)
What are Golgi tendon organ’s (GTO’s) attached to?
Tendon on one side, muscle fibres on the other
What is one property of collagen receptors in GTO’s
They remain taut when stretched (crush 1b afferents)
Is it a 1:1 ratio of muscle fibre to GTO
No! multiple muscle fibres attach to each GTO
What are GTO’s helpful for detecting?
Active force production, not passive stretch
On a graph, what are GTO’s similar to?
GTO firing closely follows EMG reading
What are joint receptors good at detecting?
Extreme stretch and pressure (know ends of ROM)
What is one fault of joint receptors?
Don’t give distinguishable signals (flexion/extension), not much info about angles, not great for proprioception
