lec 7 - receptors and reflexes Flashcards
Special sensation:
vision, smell, taste, hearing, and vestibular sense
General sensation:
touch/tactile, pressure, pain/nociception, temperature/thermal, vibration, and proprioception
Touch/tactile:
crude (can’t localize) or discriminative (localizable)
Pressure:
degree of mechanical distortion of skin
Pain/nociception:
nociception is the transduction of noxious/painful stimuli, while pain is the CNS-generated experience of the painful stimulus
Temperature/thermal:
hot, warm, cool, cold, external temperature of the environment, and internal body temperature
Vibration:
detection of different frequencies (typically measured in Hertz (Hz) with a tuning fork)
Proprioception:
body position (primarily, joints and limbs), can be conscious or unconscious
Mechanoreceptors:
sense touch, pressure, vibration, and stretch
ex) Merkel’s discs, Ruffini’s endings, and Meissner’s and Pacinian corpuscles
Proprioceptors:
specialized mechanoreceptors that are in muscles, tendons, and joints to detect body positioning
sense limb and joint position and muscle stretch and tension
detect mechanical distortion of tissue (e.g., muscle stretch, muscle tension, change in joint angle, etc.)
include muscle spindles and Golgi tendon organs (GTOs)
Thermoreceptors:
sense warm and cool stimuli
detect mild-to-moderate temperatures (i.e., warm and cool) that do not cause tissue damage
Different receptors for warm vs. cool temperatures
Cutaneous thermoreceptors are in ___, while other thermoreceptors are in ___
skin
viscera and muscle
Extreme heat or cold that causes tissue damage is detected by ___
nociceptors
Nociceptors:
sense noxious/painful stimuli and tissue damage (may be thermal, pressure, chemical, etc.)
Cutaneous nociceptors are in the ____, while other nociceptors are in _____
epidermis of the skin
viscera, muscles, bones, and joints
Different types of noxious stimuli are carried via different primary afferent sensory neurons:
Acute, sharp pain is carried via A𝛿 fibers
> Lightly myelinated axons
Chronic pain is carried via C fibers
> Unmyelinated axons
Chemoreceptors:
sense changes in blood oxygen and pH as well as taste (taste molecules) and smell (odorants)
> olfaction
gustation
Photoreceptors:
sense/detect light levels
rods and cones
in the retina
Olfaction:
odorant binds to odorant receptors in nasal mucosa
Gustation:
taste molecule binds to taste receptors on tongue
Chemoreceptors in ____ and ____ detect blood pH and solute concentrations
carotid body
aortic arch
Types of mechanoreceptors:
> Merkel’s discs
Meissner’s corpuscles
Pacinian corpuscles
Ruffini’s endings
Merkel’s discs
Function: detect pressure, vibration, touch
Location: epidermis of skin
Meissner’s corpuscles
Function: detect pressure, vibration, fine/discriminative touch
Location: dermis of hairless skin (fingertips and soles of feet)
Pacinian corpuscles
Function: detect pressure and vibration
Location: dermis and hypodermis of skin, bones, tendons, ligaments, and joints
Ruffini’s endings
Function: detect pressure and stretch
Location: dermis and hypodermis of skin and joints
Skeletal Muscle Fiber Structure
Most is composed of large extrafusal muscle fibers, which are innervated by large diameter ⍺-motor neurons
Small intrafusal muscle fibers are arranged parallel to extrafusal muscle fibers, which are innervated by small diameter 𝛾-motor neurons
> Muscle spindles wrap around intrafusal muscle fibers
Muscle Spindles
wrap around intrafusal muscle fibers
monitor muscle length and the rate of change in muscle length (i.e., stretch)
activated by stretching of muscle fibers
utilize 1a primary afferent fibers
Golgi Tendon Organs (GTOs)
> in muscle tendons
> perpendicular to intrafusal and extrafusal muscle fibers
> detect and monitor muscle tension
> Prevent muscle damage from too much tension
> utilize 1b primary afferent fibers
Primary Afferent Sensory Neuron Types
A-⍺ I (1a)
A-⍺ II (1b)
A-β (2)
A-𝛿 (3)
B
C(4)
A-⍺ I (1a) fibers
receptors: Muscle spindles
functions: Sense muscle stretch & proprioception
myelination: Myelinated
fiber diameter: Largest
conduction velocity: Fastest
A-⍺ II (1b)
receptors: Golgi tendon organs (GTOs)
functions: Sense muscle tension & proprioception
myelination: Myelinated
fiber diameter: Large
conduction velocity: Fast
A-β (2)
receptors: Mechanoreceptors & muscle spindles
functions: Sense touch, stretch, pressure, vibration, & proprioception
myelination: Myelinated
fiber diameter: Medium
conduction velocity: Moderate
A-𝛿 (3)
receptors: Nociceptors
functions: Sense sharp, acute noxious stimuli (acute pain)
myelination: Lightly myelinated
fiber diameter: Small
conduction velocity: Moderate
B
receptors: Mechanoreceptors & chemoreceptors
functions: Sense stretch & chemical environment of viscera
myelination: Moderately myelinated
fiber diameter: Small
conduction velocity: Moderate
C (4)
receptors: Nociceptors
functions: Sense dull, chronic noxious stimuli (chronic pain)
myelination: Unmyelinated
fiber diameter: Smallest
conduction velocity: Slowest
A-𝛿 and C Fibers
can synapse with 2nd order neurons in marginal zone or substantia gelatinosa
Marginal zone neurons:
> project to contralateral anterolateral system/ spinothalamic tract
> conveys pain, temperature, and crude touch to cortex
Substantia gelantinosa:
contains dendrites of neurons whose cell body is in nucleus proprius
Nucleus proprius neurons project to contralateral anterolateral system/ spinothalamic tract
A-β and A-⍺ Fibers:
can synapse on interneurons or LMNs in ventral horn or enter the dorsal column
Synapses with interneurons and LMNs in motor nuclei of ventral horn are involved in ___
spinal reflexes
Fibers that ascend in ipsilateral dorsal column form:
dorsal column medial lemniscus pathway that conveys somatosensory information to cortex
Components of a Spinal Reflex
> Peripheral receptors detect sensations
> Primary afferent fibers synapse with LMNs or interneurons
> LMN axon exits ventral horn and enters ventral rootlet
> Effector (e.g., muscle) responds to stimulus
Primary afferent fibers synapse with LMNs or interneurons =
Pseudounipolar sensory neuron cell body in dorsal root ganglion
Integration by interneurons (not required for some reflexes)
Stretch/Myotatic/Deep Tendon Reflex
Function: monitor muscle length/stretch
Receptor: muscle spindles
Primary afferent fiber type: 1a
Synapses: 1a fiber synapses directly with an ⍺-LMN in ventral horn and with an interneuron
Result/Effector: ⍺-LMN in ventral horn causes agonist muscle to contract and interneuron inhibits the ⍺-LMN innervating the antagonist muscle
Common Stretch/Myotatic Reflex Testing
Elbow flexion
> Biceps: C5 and C6
> Brachioradialis: C6
Elbow extension
> Triceps: C7
Finger flexion: C8 and T1
Knee extension
> Quadriceps/Patellar/Knee jerk: L4
Ankle plantar flexion
> Achilles/Ankle jerk: S1
GTO/Inverse myotatic reflex
Function: monitor muscle tension/load
Receptor: Golgi tendon organ (GTO)
Primary afferent fiber type: 1b
Synapses: 1b fiber synapses with an excitatory and an inhibitory interneuron
Result/Effector: inhibitory interneuron inhibits the ⍺-LMN innervating the agonist muscle (agonist relaxes) and excitatory interneuron excites the ⍺-LMN innervating the antagonist muscle (antagonist contracts)
Nociceptive Flexor/Withdrawal Reflex
Function: detect noxious/painful stimuli
Receptor: cutaneous nociceptors
Primary afferent fiber type: A-𝛿 and C
Synapses: A-𝛿 and C fibers synapse with excitatory interneuron
Result/Effector: excitatory interneuron excites ⍺-LMN innervating agonist muscle (agonist contracts)
A-𝛿 and C fibers also synapse with neurons in marginal zone and substantia gelatinosa (and nucleus proprius) to convey pain information to cortex via anterolateral system/spinothalamic tract
Nociceptive crossed extensor reflex
Function: detect noxious/painful stimuli
Receptor: cutaneous nociceptors
Primary afferent fiber type: A-𝛿 and C
Synapses: A-𝛿 and C fibers synapse with excitatory interneurons and inhibitory interneurons
Result/Effector: excitatory interneuron excites ⍺-LMN innervating agonist muscle (agonist contracts) and inhibitory interneuron inhibits ⍺-LMN innervating antagonist muscle (antagonist relaxes)
Nociceptive crossed extensor reflex
excitatory interneurons =
Interneurons also decussate to innervate ⍺-LMNs in contralateral ventral horn to cause appropriate response in opposite limb
A-𝛿 and C fibers also synapse with neurons in marginal zone and substantia gelatinosa (and nucleus proprius) to convey pain information to cortex via anterolateral system/spinothalamic tract
Central Pain Pathways and Pain Modulation
Gate control theory
Periaqueductal gray area/matter (PAG)
Gate control theory
Sensory input from A-β fibers reduces pain transmission through dorsal horn of spinal cord
A-β fiber collateral branches synapse on inhibitory interneurons that inhibit pain transmission
Transcutaneous electrical nerve stimulation (TENS) devices reduce chronic pain via this mechanism (activating A-β fibers)
Pain modulation involves:
interactions between local circuits at the level of the spinal cord dorsal horn and long-range modulatory inputs from the brainstem and cerebral cortex
These signals can upregulate or downregulate pain processing and pain perception
This is why shaking your hand after hitting your finger, or touching the skin around a cut reduces pain sensation
Gate control theory
> Nociceptive fibers and somatosensory fibers converge on and stimulate the “gate cell”, which is an inhibitory interneuron that inhibits 2nd order neurons in the dorsal horn
> When non-noxious, somatosensory information activates A-beta fibers, the collateral branches of A-beta fibers stimulate an inhibitory interneuron in the dorsal horn to block the pain transmission
Periaqueductal gray area/matter (PAG)
Contains a large amount of opioid receptors
Receives input from anterolateral system/spinothalamic tract, amygdala, hypothalamus, and cerebral cortex
Activates brainstem regions (e.g., raphe nuclei and locus coeruleus) to inhibit pain transmission
Descending fibers from brainstem can directly modulate pain transmission or activate enkephalinergic interneurons to inhibit pain transmission
Descending fibers from brainstem (activated by PAG) can exert presynaptic or postsynaptic inhibition of pain signals
Presynaptic inhibition of 1st order neuron conveying noxious stimulus (2 mechanisms)
Postsynaptic inhibition of 2nd order neuron conveying noxious stimulus
Presynaptic inhibition of 1st order neuron conveying noxious stimulus (2 mechanisms) =
Direct inhibitory synapse on 1st order neuron by descending fibers
Descending fibers synapse with enkephalinergic interneuron that inhibits 1st order neuron
Postsynaptic inhibition of 2nd order neuron conveying noxious stimulus =
Direct inhibitory synapse on 2nd order neuron by descending fibers
Sensory Disturbances
Anesthesia
Hemianesthesia
Paresthesia
Dysesthesia
Analgesia
Anesthesia:
absence of sensation
Hemianesthesia:
affects one side of the body, common after stroke
Paresthesia:
sensations (e.g., tingling) perceived without specific stimulation, may be tactile, thermal or painful, may be episodic or constant
Dysesthesia:
painful sensations (e.g., burning, shooting, pins and needles) elicited by a nonpainful cutaneous stimulus such as a light touch or gentle touch over affected areas of the body
Analgesia:
absence of pain sensation
