1.4.2 Somatosensory Receptors Flashcards
What is the role of the muscle spindle afferents?
- Feedback to cerebellum about muscle length and velocity
- Sensation of joint position via three types of input:
- Receptors at extremes of joint (not good sensors)
- Tendon vibration
- Tendon tension
What are the three different categories of peripheral somatosensory receptors?
Mechanoreceptors (mechanical force), thermoreceptors (change in temp), and nociceptors (potentially damaging/noxious stimuli)
What is true about the size and sensitivity of receptive fields?
A region within the sense organ in which the receptor is located and in which it has the capacity to transduce stimuli.
Receptors can have receptive fields of vastly differing sizes. Mechanoreceptors in the skin can have receptive fields less than a millimeter, or as large as an entire dermatome (see below) depending upon the area of skin in which deformation will result in a change in the receptors activity.
Peripheral innervation density is proportional to the sensitivity of that area and directly proportional to the size of that peripheral regions representation in the central map. Peripheral receptive field size is directly proportional to the sensitivity of that area and inversely proportional to the amount of area a peripheral region gets in the central representation (map) i.e., small receptive fields, dense innervation = large area in the central map.
What is the role of the golgi tendon organ (GTO)? What innervates the GTO?
Monitors stresses and forces (tension) at the tendon; group Ib afferents
Inhibitory neuron to decrease muscle force - GTO reflex aids in maintaining a constant force such as when maintaining grip on a cup
What are the different nerve fiber types of first-order afferent axons, their relative conduction velocity, and their role in sensation?
Aα – muscle information,
Aβ – touch,
Aδ – fast pain,
C – unmyelinated, pain and temp
What is the sequence of events that result in proprioception by the GTO?
extrafusal muscle fibers contract -> stretches elastic fibers of GTO -> Ib afferent nerve endings are deformed -> increases depolarization of nerve ending -> increase in Ib afferent firing
How is the somatosensory representation in the periphery retained in the brain?
-Richly innervated areas have larger cortical representation (homunculus)
What is the function of the gamma motoneuron?
It causes Ia and II afferent to increase firing at constant length
Increases sensitivity of spindle Ia and group II afferents to length
What are the characteristics of Pacinian receptors?
Pacinian - deep tissue pressure, set of nerve endings wrapped like a onion to be sheltered from light touch, slippage of layers cause sensation of pressure
Senses vibration at high frequencies
What is adaptation?
A decrease in neural response to maintained stimulation
Fill in the blanks covered by the green. Determine is these are rapidly or slowly adapting receptors.
Red: Rapidly adapting
Blue: Slow adapting
Describe the differenct between primary and secondary afferent innervation of muscle spindles?
-Primary = annulospiral = Group Ia (A-alpha)
Encircles equatorial region of all bag and chain fibers
-Secondary = flower spray = Group II (A-beta)
Arise from juxtaequatorial region of chain fibers
What are the characteristics of Meissner’s corpuscle?
Meissner’s corpuscle - looks like a stack of discs, give high density innervation to finger tips and toes
Also low freq vibration
Noxious skin stimulation - prostaglandins, histamine, substance P
Areas of the body in which the touch sensations of the skin are carried by a single pair of spinal nerves
“Somatotopic” Dermatomes
What type of somatosensation do muscle spindles assist with?
Proprioception
What are the three main somatosensory modalities?
Discriminative touch, pain and temp, and unconscious proprioception
What are the three main fiber types of primary afferent neurons from spinal nerves?
A(beta), A(delta), and C fibers
What are the characteristics of thermal or pain receptors?
Thermal or pain - course into dermal/epidermal border - highly branched, can have large receptive fields
Fill in this diagram
What are the two sites of axon projection of the muscle spindle afferents?
Alpha motor neuron and spinocerebellar neurons of Clarke’s Column
Tell me about muscle spindle anatomy
Here’s what you need to know about spindle anatomy!
Two types of intrafusal muscle fibers: bag and chain.
- Intrafusal fiber nuclei located in the equatorial region.
- Polar regions are striated muscle.
- Equatorial region is non-contractile and behaves like a spring.
Two types of afferent innervation: primary and secondary afferents
- Primary afferent = annulospiral afferent = Group Ia afferent
- Secondary afferent = flower spray afferent = Group II afferent
- Primary encircles equatorial region of all bag and chain fibers
- Secondary endings arise from the juxtaequatorial region of chain fibers
Two types of efferent innervation: dynamic gamma and static gamma
- Dynamic gamma efferents innervate the polar regions of only bag intrafusal fibers
- Static gamma efferents innervate the polar regions of primarily chain fibers.
What is the sequence of events that leads to the firing of muscle spindle neurons?
Muscle stretch (lengthens) -> Stretches (lengthens) the muscle spindle (parallel arrangement) -> deforms the Ia and II nerve endings -> Na++ channels open -> depolarization of nerve ending membrane -> increase in Ia and II firing rate (# of APs per second)
