Somatosensory systems and Muscle receptors Flashcards
Sensory receptors
group of cells that transform a type of physical energy into an electrical signal
2 structure types of sensory receptors
free nerve ending and specialized receptor
free nerve ending
a sensory neuron (usually pseudounipolar) in a sensory ganglion whose peripheral process is specialized for reception – the cell bodies usually located in dorsal root ganglion
Associated with temperature sensation and pain
specialized receptor
an actual receptor cell connected to the peripheral process of a pseudounipolar neuron in the dorsal root ganglion
Associated with touch reception
Mechanoreceptors
respond to mechanical deformation, detect vibration, tapping,
pressure, stretch, etc.
everywhere!!
Chemoreceptors
respond to the presence of certain molecules, detect chemical changes
skin, organs, taste/smell buds
Thermoreceptors
respond to non-painful temperature changes, warm/cold receptors
in all tissues except CNS
Nociceptors
respond when the magnitude of any given energy type stimulus is sufficient to cause tissue damage/pain
Photoreceptors
respond to light energy (rods and cones)
FASTEST
identify which group size of fibers each receptor is attached to
Mechanoreceptors: Group I (a and b), Group II (A beta) and Group III (A delta)
Chemoreceptors: Group IV (C)
Thermoreceptors: Group III (A delta) and Group IV (C)
Nocireceptors: Group III (A delta) and Group IV (C)
Photoreceptors: not connected to groups, but to specialized fast conducting retinal ganglion axons
relationship between fiber size and velocity and which afferent fiber types conduct w the fastest velocity
Fibers with the largest diameter have the greatest conduction velocities and those with the smallest diameter have the slowest conduction velocities.
Therefore, Group I mechanoreceptors are the
fastest.
Intensity
Frequency coding – how many action potentials
Population coding – how many receptors are activated
(weak stimulus will activate fewer receptive fields; strong stimulus will activate more)
Quality
Receptor specificity – each receptor type responds to one stimulus type
Somatotopy – homunculus
Duration
how long you feel the given stimulus
Adaptation – there are slow and rapid adapting receptors (rapidly adapting receptors allow for better detection of changes in environmental stimuli)
Location
Labeled line theory
Receptive fields: the territory that will activate an axon and its branches given a stimulus. A weak stimulus will likely only activate one receptive field. A stronger stimulus is more likely to activate more receptive fields
Somatotopy
Somatotopy
- a sensory map
- organization of the CNS pathways, describing the relationship between a particular part of the body and a particular area of the brain.
- allow our cortex to localize a sensory experience
Homunculus organization
Face and UE = lateral cortex, which is supplied by
MCA
LE = medial cortex, which is supplied by ACA
sensory threshold and absolute
- how we process and modulate info (so we don’t perceive every sensation)
It is not fixed—can be changed by expectation, awareness, or distraction.
absolute - lowest stimulus intensity that a subject can detect; reflects perception of the stimulus
Intrafusal muscle fibers
- run parallel to extrafusal
- in the muscle spindle
- DO NOT cause contraction
- serve a role in receiving certain types of sensations from muscle
- stretch receptors: change in muscle length and rate of movement
- innervated by gamma motor neurons
Extrafusal muscle fibers
- striated muscle, actin, myosin
- make muscles contract
- innervated by alpha motor neurons which prompt the contractions
Gamma neurons are connected with
intrafusal fibers
Alpha neurons are connected with
extrafusal fibers
alpha and gamma neurons are located in
the ventral horn
what types of stimulation do intrafusal muscle spindle fibers respond to/which class/type of receptor
Muscle stretching. Connected to Group 1a and Group II afferent fibers. A muscle spindle is a sensory receptor.
sensation carried from muscle spindle to CNS via Ia and II sensory nerve fibers
Proprioceptive information regarding length of muscle
(1a and II afferents) and velocity of movement (1a
afferents only).
general and specific functions of the muscle spindle
- convey info about muscle length or stretch and the rate of change of muscle length (velocity of movement)
- also provide the mechanism for the stretch/myotatic reflex, contribute to presetting and regulating muscle stiffness/tone, and play a role in coordinating and smoothing muscle contractions
function of gamma motor neurons
stimulate contraction at each end of intrafusal fibers to stretch the otherwise saggy fiber and produce action potentials in the Ia and II afferents during active shortening—thus they increase muscle spindle sensitivity
function of alpha motor neurons
stimulate actual muscle contraction.
what stimulus does the golgi tendon organ response to and which class of sensory afferent fiber is it connected to
- proprioceptive info about tension/force
- connected to afferent 1b fibers
(no efferent component, not making contraction happen)
function of golgi tendon organ
a sensory receptor that receives info about muscle force and is sensitive to small changes in that force/tension
- associated w/ 1b afferents
connections of 1a sensory fibers and how they contribute to reflexes
provide info about rate of change and length of a muscle connects to static and dynamic fibers
In response to fast stretch, 1a afferents from the muscle spindle excite the alpha motor neuron causing a muscle contraction, the alpha motor neurons connected to the synergist muscles cause them to contract, the alpha motor neurons connected to the antagonist muscles cause them to relax via inhibitory interneuron, and finally 1a fibers also send information to the cortex for conscious proprioception and to the cerebellum for unconscious proprioception.
connections of 1b sensory fibers and how they contribute to reflexes
In response to a force generated in a muscle 1b afferents inhibit the alpha motor neurons of the muscle that it arises from and the synergists of the muscle in which force is building. Alpha motor neurons of the antagonist muscles are excited by 1b fibers. 1b fibers also send proprioceptive information regarding tension in the muscle to the cerebellum for unconscious proprioception that influences movement automatically and to the cortex for conscious proprioception.
general and specific functions of the GTO
- Proprioceptive information about tension/force produced by muscle.
- critical role: providing exquisite info about tension
- specific: operates as a tension feedback system to prevent or reduce too much tension, decrease force at end range movements, somehow act in inhibition/excitation during walking, and help regulate force of muscle contraction via a feedback loop.
three neural mechanisms that potentially contribute to spasticity
- decrease in alpha motor neuron inhibition
- collateral sprouting
- denervation hypersensitivity
decrease in alpha motor neuron inhibition is caused by
a lesion in descending pathways (input for presynaptic inhibitor is gone, so then you get uncontrolled responses to muscle stretch)
collateral sprouting
more terminal branches sprout after an injury and take over vacant synapses after a lesion in the descending pathways – the result is a stretch that evokes a bigger than normal response
denervation hypersensitivity
in which cells that lose some presynaptic terminals become more sensitive to transmitter effects from the remaining excitatory pathways which therefore produces a stronger effect
what causes hypotonia and how does it present clinically
- Caused by certain upper motor neuron lesions and all lower motor neuron lesions. - Presents as decreased muscle tone and difficulty producing and sustaining muscle contractions.
clinical implications of using muscle receptors to inhibit spasticity or excite a hypotonic muscle
Increased activation/excitation of alpha motor neurons can inhibit spasticity and excite hypotonic muscles.
labelled line theory
no matter how you stimulate a pathway you will have the same perception (if you hit yourself in the eye, you may see light)
Ia and II afferent fibers carry
proprioceptive information regarding length of a muscle
Ia afferent fibers also carry proprioceptive info regarding the
velocity of movement
characteristics of Group I
(Ia and Ib) = A alpha
myelinated and not found in skin
largest diameter
highest conduction velocity
characteristics of Group II
A beta
myelinated
half the diameter and conduction velocity of group I
characteristics of Group III
A delta
smallest myelinated
characteristics of Group IV
C fibers
unmyelinated
smallest diameter and slowest conduction velocity
transduction
receptors convert physical energy into electrical signals
the local stimulus depolarizes the membrane and produces a
receptor or generator potential
generator potentials are
locally graded potentials, which are dependent on stimulus intensity (amplitude is proportional to this)
afferents are stimulated by relatively discrete areas of the sensory field called
receptive fields
receptive fields are small in areas such as the ___, and huge in ___
small - fingers and face
huge - back and thigh (don’t require much precision)
intensity, quality, location, duration are:
4 characteristics of stimuli
muscle spindle
in the muscle belly, muscle length and rate of change (velocity)
Ia & II afferents
gamma motor neurons, think stretch reflex
golgi tendon organ
in the tendon
force/tension
Ib afferents
no motor component
spasticity
velocity dependent; increase in tonic stretch reflexes and exaggerated deep tendon reflexes (because of the hyper excitability of the stretch reflex)
receptor group meaning
transmits a sensation from the muscle
receptor type meaning
transmits a sensation from the skin
