Ch. 9: Somatosensory System Flashcards
What does “sensation” entail?
The ability to TRANSDUCE, ENCODE and ultimately PERCEIVE information generated by STIMULI arising from BOTH EXTERNAL AND INTERNAL environments.
Somatosensory afferents convey information from where to where?
FROM the SKIN SURFACE to CENTRAL CIRCUITS
Where do the CELL BODIES of somatosensory afferent fibers (SENSORY RECEPTORS FOR THE BODY) – which are conveying information about THE BODY – reside?
They reside within the DORSAL ROOT GANGLIA – which lie along the spinal cord
Where are the CELL BODIES which convey information about the HEAD?
They are found primarily in the TRIGEMINAL GANGLIA
What kind of neurons in the dorsal root ganglia give rise to peripheral processes that ramify (FORM BRANCHES) within the skin (or muscles or joints) and central processes that synapse with neurons in the spinal cord and at higher levels of the nervous system?
PSEUDO UNIPOLAR NEURONS
The peripheral processes of mechanoreceptor afferents are encapsulated by what? And what does that mean?
The peripheral processes of mechanoreceptor afferents are encapsulated by specialized receptor cells ; AFFERENTS CARRYING PAIN AND TEMPERATURE INFORMATION TERMINATE IN THE PERIPHERY AS FREE ENDINGS
What is “sensation”?
The ability to feel something physically, especially by touch
What does SURVIVAL depend on (2)?
- Sensation
2. Perception
What is perception (2)?
- The conscious interpretation of the stimuli
- Awareness of the elements of environment through physical sensation
Explain the transduction in a mechanosensory afferent. E.g. for a Pacinian corpuscle:
FOR EXAMPLE:
Pressing the skin (deforms encapsulated afferent fiber)–> ACTIVATES RECEPTORS ( opens cation channels –> DEPOLARIZATION of the afferent fibers –> send Action Potentials to brain (if afferent is sufficiently depolarized / meets threshold –> signal is INTERPRETED
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First, the deformation fo the capsule leads to a stretching of the membrane of the afferent fiber, increasing the probability of opening mechano-transduction channels in the membrane.
Next, the opening of the CATION channels leads to DEPOLARIZATION of the afferent fiber (receptor potential).
If the afferent is sufficiently depolarized, an action potential is generated and propagates to central targets.
If the stimulus is moderate, and the threshold is not met will the signal be interpreted by the brain?
No. Because the stimulus does not meet threshold, there is no AP generated, and stimulus will not be felt / interpreted by the brain.
The somatosensory system is the combination of what TWO Sensory functions?
PROPRIOCEPTION and TOUCH
What is the name of the receptor type for the sensory function PROPROPCEPTION?
MUSCLE SPINDLE
What type of Receptor for the sensory function TOUCH (4)?
- MERKEL
- MEISSNER
- PACINIAN
- RUFFINI CELLS
What is the receptor type for the sensory function PAIN AND TEMPERATURE?
FREE NERVE ENDINGS
What is the name of the receptor for the sensory function PAIN, TEMPERATURE, ITCH, NON-DISCRIMITIVE TOUCH ?
FREE NERVE ENDINGS (UNMYELINATED)
Which afferent axon type is utilized for chronic pain? And what are the characteristics of the axon (i.e. axon diameter and velocity [fast / slow])
C fibers : chronic pain – dull pain, very slow transmission with a very small diameter.
Where will it evoke stronger responses when applying stimulation to the RECEPTIVE FIELD?
Stimulation applied to the centre of the receptive field tends to evoke stronger responses. Stimuli applied at more eccentric locations within the receptive field.
Is the two-point discrimination threshold finer in the fingers? Or in the wrist?
IN THE FINGERS ; the two-point discrimination threshold is finer than that in the wrist because of the SIZES of the AFFERENT RECEPTIVE FIELDS or The separation distance necessary to produce two distinct FOCI – More afferents innervating the lower arm than fingertips
Are the two-point discrimination thresholds always consistently the same on the right and the left side of the body?
NO – not always the same thresholds.
Where is somatic acuity highest? In the Fingers, toes and face OR the arms leg and torso?
Somatic acuity is highest in the fingers, toes and face.
When are slowly adapting receptors responding to a stimulus?
Slowly adapting sensory receptors CONTINUOUSLY respond to a stimulus
When are RAPIDLY ADAPTING RECEPTORS responding to stimuli?
Rapidly adapting sensory receptors respond only at the ONSET and at the OFFSET of STIMULATION
Which sensory receptors (fast or slow) are providing information about the static qualities of a stimulus? Dynamic qualities of a stimulus?
STATIC via slowly adapting sensory receptors
DYNAMIC via rapidly adapting sensory receptors
Does static stimuli infer that there is no movement?
NO. Static could imply no movement, or CONSTANT movement
What type of neuron transmits the pressure sensation and with what type of fibers?
A BETA Fibres VIA PSEUDO-unipolar neuron
Is pain transmitted, like pressure via a pseudo-unipolar neuron?
NO, pain is transmitted by a different neuron
What three tactile receptors are located superficially in the epidermis?
- Meissner’s corpuscles (fine touch, RAR [rapidly adapting receptors])
- Merkel’s discs (touch, light pressure, SAR [slowly adapting receptors])
- Hair end-organ (hair movement, RAR)
What two tactile receptors are located in the deeper layers of the skin?
- Ruffini’s end-organ (heavy and prolonged touch and pressure, SAR)
- Pacinian corpuscles (tissue vibration, rapid changes – deep pressure, stretch (RAR)
Which receptors provide the best representation of the Braille pattern?
MERKEL cell (touch and light pressure)
What is a dermatome?
An area of the skin supplied by nerves form a single root
Do dermatomes overlap? Why / why not?
Dermatomes overlap substantially – injury to an INDIVIDUAL DORSAL ROOT does not lead to COMPLETE loss of sensation.
Dermatome overlap is more extensive for which senses? Less extensive for which senses?
More extensive for TOUCH, PRESSURE and VIBRATION
Less extensive for PAIN and TEMPERATURE
How are dermatomes used clinically?
Dermatomes are used in clinical assessment of Spinal cord injured patients.
What chart or SCALE can be used to determine neurological symptoms when working with SCI patients – especially in rehab centres?
ASIA scale
Where do sensory fibers decussate (cross-over)?
In the MEDULLA (brain stem)
What type of information does the DORSAL COLUMN-MEDIAL LEMNISCAL PATHWAY carry and where does it carry it?
The dorsal column-lemniscal pathway carries conscious proprioceptive and MECHANO-SENSORY information FROM the posterior third of the head and the rest of the body.
It is a MAIN TOUCH PATHWAY
If there is no pain, the dorsal column – medial lemniscal pathway is a how many neuron pathway?
THREE NEURON PATHWAY:
Stimulus –> MEDULLA –> THALAMUS –> SENSORY CORTEX (3 neuron system)
Where is the information supplied by somatosensory receptors organized?
In the VENTRO POSTERIOR (VP) complex of the THALAMUS
What is VPL?
Ventral Posterior Lateral nucleus
Where does the VPL nucleus receive projections from?
The medial lemniscus carrying SOMATO -Sensory and conscious proprioceptive info
Where does the VPM nucleus receive projections from?
The trigeminal lemniscus carrying info from the face
What is the VP complex comprised of?
The ventroposterior nuclear complex is comprised of the VPM and the VPL which transmit information from the face and somatosensory information from the rest of the body (respectively)
Where is the primary somatosensory cortex located in the brain?
Medial to lateral in the parietal lobe
Is the secondary somatosensory cortex found medially or laterally in the parietal lobe area of the brain?
The secondary somatosensory cortex is found lateral parietal lobe of brain
What is the popular cartoon representation of the somatotopic body parts called?
HOMUNCULUS
The homunculus was constructed based on what?
The homunculus is bases on the mapping of the somatosensory cortex.
What parts of the body have the largest amount of somatosensory cortex devoted to their regions?
The HANDS AND FACE
In the study involving the owl monkey – their 3rd digit was amputated. What happened to the cortical region of digit 3 immediately after the lesion, and a few weeks after the lesion?
Immediately the cortical region for digit 3 is UNRESPONSIVE however, a few weeks later, the cortical neurons that formally responded to stimulation of digit 3 responded to stimulation of digits 2 and 4
When the owl monkey was trained in a task requiring heavy usage of digits 2, 3 and 4 – What happened after several months of “practice”?
A larger region of the cortex contained neurons activated by the digits used in the task.
