2/5: Sensory Physiology Flashcards
What does the afferent pathway inter the CNS via?
The posterior/dorsal roots
What are labeled lines?
Receptors and neurons in the CNS are dedicated to carrying particular sensory information
What are the schematics of sensory (afferent) pathways?
Receptor
First order neuron
Second order neuron
Third order neuron
Primary somatosensory cortex (parietal lobe)
What does the second order neuron do?
Decussation (crossing over) occurs, though where this occurs within the CNS varies based on the sensory modality
What does the dorsal column medial lemniscal pathway do?
Touch, pressure, and proprioception
What does the spinothalamic tract do?
Pain, temperature
Input to the nervous system is provided by _______ receptors
Sensory
What are the 5 functional types of sensory receptors?
- Mechanoreceptors
- Chemoreceptors
- Thermoreceptors
- Nociceptors
- Photoreceptors
What are mechanoreceptors?
compression/stretch
Includes receptors in skin, muscle spindles, hearing, equilibrium, arterial pressure, etc
What are chemoreceptors?
Ligands
incldues receptors for taste, smell, arterial oxygen, osmolarity, blood gas, blood glucose, etc
What are thermoreceptors?
Cold/warm
What are nociceptors?
Damage
What are photoreceptors?
Electromagnetic receptors
Light
What is an adequate stimulus?
Particular form of energy to which a receptor is most sensitive
What do receptors respond to?
Forms of energy, but the threshold for these nonspecific responses is much higher
Sensory receptors are specific for a ___________________
Particular energy type (or modality)
Activation of any sensory receptor changes _____________ and produces what is called ____________
Membrane potential (transduction); receptor potential (a graded potential)
Describe how the pacinian corpuscle works as a mechanoreceptor?
When the surrounding capsule is distressed, the
central nerve fiber is in turn distorted, opening ion channels. The greater the
depression, the more ion channels will open, so a larger GP will be produced which leads to more APs
What does an increase in stimulus strength also increase?
The amplitude of receptor potentials, but this is not a direct (linear) relationship
What two things does the intensity (or strength) of stimulus perception determined by?
- The frequency of action potentials (temporal summation)
- APs are all or none, a stronger stimulus will NOT generate a larger AP
- AP frequency (how close together in time the AP’s are) - The total number of receptors activate (Spatial summation)
- stronger stimulus = more receptors
- number of receptors activated
A stronger stimulus intensity produces a __________ receptor (graded) potential, _______ action potentials (temporal summation), and causes _______ neurotransmitter release into the synapse
Larger; more; more
What are two types of receptor adaptation?
- Perceptual threshold
- Adaptation can occur at the receptor or in the neuron
What is perceptual threshold?
If a stimulus of constant strength is maintained on a sensory receptor, the frequency of the action potentials in its sensory nerve declines over time.
What is adaptation?
In response to continual,
high impulse sensory stimuli, the response of almost all receptors decreases, but to varying degrees
- second you put a necklace on, you feel it due to adaptation
- jumping into a cold pool
What are the two sepcific types of adaptation receptors?
Tonic = slow adapting
- receptors that adapt after they initially respond
- pain receptors are tonic
Phasic = fast adapting
What is receptor adaptation?
Decrease in amplitude of receptor potential over time in the presence of a constant stimulus
- corresponding decrease in frequency of APs
- decreases perception of stimulus
What do tonic receptors help differentiate?
Stimulus intensity
- slow adapting
What are examples of tonic receptors?
Golgi tendon organs, NOCICEPTORS, chemoreceptors, baroreceptors
Phasic receptors help differentiate stimulus ___________
Duration
What are examples of phasic receptors?
Pacinian corpuscles, vestibular receptors in inner ear, TEMP RECEPTORS
What does a precise modality activate?
Specific receptors and postsynaptic cells
- This information continues on a predicted pathway such that particular kinds
of information are conveyed via specific nerve fibers to specific regions of the CNS that are programmed for perception of that modality
What does the homunculus correlate to?
The anatomical regions of the CNS with where interpretation and awareness of sensations are perceived
Where are the two first order neuron locations?
Dorsal column pathway
Spinaothalamic tract
Where does the second order neuron occur?
In the dorsal column it crosses into the lower medulla
In the spinothalamic tract is crosses over immediately in the upper limb
Where does the third order neuron occur?
In thalamus for both pathways
What is a receptive field?
Region where a single fiber’s afferent receptors (primarily on dendrites) are located
The overlap between receptive fields of adjacent neurons improves what?
Localization ability - two point discrimination
What is acuity?
The precision with which a stimulus is perceived
What enhances acuity?
The process of lateral inhibition
What improves discrimination?
Lateral inhibition
What can sensory stimulation of a single point on the skin elicit?
Excitation in one tract of post-synaptic cells while simultaneously inhibiting lateral neurons
Where does lateral inhibition occur?
In the CNS
Why is lateral inhibition advantageous?
It diminishes lateral spread of an excitatory signal and increases degree of contrasts in the sensory pattern perceived in the cortex
What 3 synapses does lateral inhibition occur at?
- Medulla
- Thalamus
- Cerebral cortex
Nerve fibers that have _______ diameter axons and/or that are ________ have faster conduction velocities
Large; myelinated
What fibers are fast fibers?
A fibers
What fibers are slow fibers?
C fibers
Which do you perceive first - a temperature or a vibration (mechanical) sensation? Why?
Vibration mechanical because it is an A beta fiber that is faster
What are examples of slow pain?
Crude touch and pressure
Tickle
Aching pain
Warmth
What fiber is temperature?
A delta & C fibers
What do mechanoreceptors include?
Tactile and position sensations that are stimulated by mechanical displacement of somet issues in the body
What are the three examples of tactile sensations?
- Touch sensations from
stimulation of tactile receptors
in the skin or tissues beneath
the skin. - Pressure sensations from
deformation of deeper tissues. - Vibration sensations from
rapidly repetitive sensory
signals
What are two examples of position sensations?
- Static position sense is
conscious perception of
orientation of different parts of the body with respect to one another. - Rate of movement sense is also called kinesthesia or
dynamic proprioception
What are examples of mechanoreceptors?
Free nerve endings
Pacinian corpuscle
Meissner’s corpuscle
Merkel’s disks
Ruffini endings
Hair end-organs
What do free nerve endings detect?
Touch and pressure (temp and pain)W
Where are free nerve endings?
Skin, cornea, dental pulp, GI tract
What kind of adaptation are free nerve endings?
Slow adaptation/tonic
What do pacinian corpuscles detect?
Deep pressure, vibration
Where are pacinian corpuscles?
Subcutaneous tissue, viscera, joints
What kind of adaptation are pacinian corpuscles?
Rapid adaptation/phasic
What do meissner’s/tactile corpuscles detect?
Light touch, pressure, vibtation
Where are meissner’s/tactile corpuscles found on?
Glabrous skin (hairless - palms, inside of forearm)
LOCALIZATION
What kind of adaptation are meissner’s/tactile corpuscles?
Rapid adaptation/phasic
The density of meissner’s corpuscles in skin can be determined by?
Two-point discrimination tests
- the number of tactile corpuscles in skin normally declines during adult life
What do merkel’s disks detect?
Localize continuous pressure and sensing an object’s texture
What skin are merkel’s disks on?
All skin
What kind of adaptation do merkel’s disks have?
Slow adaptation/tonic
What are ruffini’s endings sensitive to?
Stretch or indentation; proprioception
Where are ruffini’s endings found?
Deep layers of skin, joints, surrounding tooth roots
What kind of adaptation do ruffini’s endings have?
Slow adaptation/tonic
What receptors are periodontal mechanoreceptors in the periodontal ligament?
Ruffini like receptors
What are hair end-organs sensitive to?
Hair movement
Where are hair end-organs found?
Base of hair follicle
What kind of adaptation are hair end-organs?
Rapid adaptation/phasic
What kind of receptors have smallest receptive fields so most precise localization?
Meissner’s corpuscle
Merkel cells
Memorize where each somatic receptor is located, size, their receptive fields, and their adaptation type
What is the medial lemniscus?
An ascending bundle of heavily myelinated axons that cross over in the medulla
Where does the ascending sensory tract ascend?
The spinal cord on the same side as the stimulus
Where does the second order neuron cross over?
In the medulla oblongata
What does the spinothalamic/anterolateral pathway detect?
Pain, thermoreceptors, crude tactile, tickle, itch and sexual sensations
What does the spinothalamic/anterolateral pathway transmit?
A broad spectrum of sensory modalities
Where does the second order neuron cross over?
crosses over in the spinal cord and ascends in either the anterior or lateral spinothalamic tract on the opposite side of the spinal cord from where the stimulus occurred
With a UNILATERAL
lesion of the spinal cord, one would expect to lose
_____ lateral touch/pressure & _____ lateral pain/temperature sensations below the injury level
Ipsi (same side); contra (opposite sides)
What areas are poscentral gyrus?
Areas 1-3
What areas are sensory association areas?
Areas 5 and 7
What does damage to the somatosensory association area result in?
the inability to recognize complex objects and forms felt on the opposite side of the body. Also loses sense of form of their own body on the opposite side; mainly oblivious to the opposite side of the body
What is proprioception?
Awareness of the body’s position in space
What are the two types of proprioception?
Static vs dynamic
What are the receptor types of proprioception?
- Photoreceptors
- Touch and pressure receptors in skin, joints, ligaments (ex: periodontal ligament)
- Skeletal muscle receptors (unconscious proprioception)
a. muscle spindles
b. golgi tendon organs - Vestibular receptors
What are the two types of somatosensory mechanoreceptors in the mouth/face?
A. extensive innervation
B. Cutaneous and mucosal receptors
What are extensive innervation?
- Incredibly high sensitivity to stimuli
- Relatively large region of the cortex receives
information (Sensory Homunculus)
What are cutaneous and mucosal receptors?
- Meissner, Merkel, Ruffini, and free nerve endings
a. No Pacinian Corpuscles - Also send proprioceptive information
- Mechanoreceptors can convey taste perception
Where are periodontal mechanoreceptors?
In the periodontal ligament
What are periodontal mechanoreceptors made of?
Mostly complex Ruffini-like receptors; also free
nerve endings. Axons are large & myelinated.
Describe the adaptation of periodontal mechanoreceptors?
Adaptation is both slow and fast
What do the location of receptors allow for?
respond to any forces applied to the crown of the teeth (when biting and chewing)
What does a brisk tap on the tooth do?
inhibits jaw closing muscles (ex. jaw opens to prevent damage)
What does the weak tap on the tooth do?
activates jaw closing muscles (ex. to hold onto food and adjust chewing force)
