Somatosensory System 3/4 Flashcards
The somatosensory system
Sensory stimuli that reach the conscious level of perception
Specialized cells that monitor specific conditions in the body or external environment
Somatic/General Senses = Somatosensory
Temp, pain, touch, pressure, vibration, proprioception
Simple receptors located anywhere on body
Special Senses: (these are not part of the somatosensory system)
Are located in sense organs such as the eye or ear
Olfaction, vision, gustation, hearing, equilibrium
Complex receptors located in specialized sense organs
from sensation –> perception
-role of receptor
Stimulus as physical energy –> sensory receptor
*Receptor acts as a transducer
Translate physical energy to something (convert a physical pressure into a neural signal)
Intracellular signal –> usually change in membrane potential
Stimulus –> threshold –> action potential to CNS
Integration in CNS –> cerebral cortex or acted on subconsciously
Sensory Receptors
- role/function
- specific or nah
Transduction – conversion of environmental stimulus into action potential by sensory receptor
Receptors specific for particular type of stimulus
Specificity is due to structure of receptor
A stimulus is a change in the environment that is detected by a receptor
Sensation: the awareness of changes in the internal and external environment
Perception: the conscious interpretation of those stimuli
Sensation **
Sensation: the awareness of changes in the internal and external environment
Perception: the conscious interpretation of those stimuli
Perception **
Perception: the conscious interpretation of those stimuli
Sensory Neurons
- location of cell bodies and recptors
- type
The cell bodies of the neurons are in the dorsal root ganglia
The receptors are in the periphery
The cells are pseudounipolar
Peripheral and central components are continuous- dendrites sort of merge into the axon
Classification by Location
*Exteroceptors
Respond to stimuli arising OUTSIDE the body
Receptors in the skin for touch, pressure, pain, and temperature
Most special sense organs
*Interoceptors (visceroceptors)
Respond to stimuli arising in internal viscera and blood vessels
Sensitive to chemical changes, tissue stretch, and temperature changes
What Must Sensory Cells Do
They must be able to convert stimulus energy into an electrical signal
Stimulus changes the permeability cation channels in the nerve ending –> depolarization
This depolarization is known as a receptor potential
Receptor potential proportional to stimulus magnitude
If receptor potential reaches threshold, then action potential are generated in the afferent fiber
Rate of APs proportional to magnitude of depolarization
Sensory Neuron/Fiber Differences
- Axon diameters vary
- Receptive fields differ
- Differences in the temporal dynamics of their responses
- Differences at the nerve ending in the periphery
which is the slowest transmission
Pain and temperature neurons-
Axon diameters vary
Sensory neurons of muscle- large diameter axons
Sensory afferent Ia fibers
(one-A)
Fastest
Touch neurons- medium diameter axons
Sensory afferent- Aβ fibers
*Pain and temperature neurons- small diameter
Sensory afferent- Aδ and C fibers
Slowest
***Two-Point Discrimination
At what distance can you determine two separate points as two separate points
Varies across the body based on receptive fields and innervation amount
density of receptors within the skin determines the sensitivity of the skin, so the fingers have the finest two point discrimination, indicating higher density of mechanoreceptors–and thus more cortical tissue dedicated to these areas
Fingertips: densely innervated by cells with small receptive fields –> only need a small difference to detect as two points
Back: sparsely innervated by cells with large receptive fields –> need much bigger difference to detect as two points
Sensory Neuron/Fiber Differences: Receptive fields differ
*Receptive fields differ
What area does it cover- how much of body does it cover
Receptive field- how much area is the receptor cell able to collect information from
Densely innervated areas, like fingertips, have cells with relatively small receptive fields
Areas with less innervation, like the back, have cells with larger receptive fields
Sensory Neuron/Fiber Differences: Differences in the temporal dynamics of their responses - rapidly adapting vs slowly adapting
Rapidly adapting: fire rapidly after stimulus onset and then fall quiet as stimulus is maintained
Useful for determining stimulus movement
*Slowly adapting: sustain their firing in response to the stimulus
Useful for spatial attributes of stimulus (size, shape)
Sensory Neuron/Fiber Differences: Differences at the nerve ending in the periphery
Encapsulated endings: nerve fiber ending is surrounded by a non-neural component, often connective tissue, that helps with the function
Free nerve ending: just a regular old nerve ending with nothing added on
Encapsulated endings (!)
Encapsulated endings: nerve fiber ending is surrounded by a non-neural component, often connective tissue, that helps with the function
Free nerve ending
-process/used for:
Free nerve ending: just a regular old nerve ending with nothing added on
process pain and temperature
Four Types of General Sensory Receptors
Pain: nociceptor (more about these next time)
Temperature: thermoreceptor (mostly ignored by Ch 9)
Physical: mechanoreceptor
Chemicals: chemoreceptors
All can be found in both somatic (exteroceptors) and visceral (interoceptors) locations except:
Proprioceptors (a mechanoreceptor) are somatic only
mechanoreceptor
Detect membrane distortion
- Three receptor types:
- Tactile Receptors
- Proprioceptors
- Baroreceptors
Mechanoreceptors- Tactile Receptors
6 functions
Detect membrane distortion
Detect touch, pressure and vibration on skin
Detect hair movement
Detect fine touch
Detect deep pressure
Respond to itch (respond among other to histamine) and light touch (detect changes in shape like bending)
Merkel Cells and Afferents
*Merkel cells are epithelial cells
Associated with sensory nerve endings to form a Merkel cell-neurite complex
The complex essentially functions as the receptor
Slowly adapting
*25% of the mechanosensory afferents in the hand
-Especially enriched in the fingertips
Only mechanoreceptor type to sample from epidermis
Express the mechanotransduction channel Piezo2
Both the Merkel cell and the fiber
*respond best to static stimuli, small receptive fields
• 25% of the mechanosensory afferents in the hand
• Especially enriched in the fingertips
• Only mechanoreceptor type to sample from epidermis
• Express the mechanotransduction channel Piezo2
• Both the Merkel cell and the fiber
• Ability to recognize the stimulus
• Merkel cell: signals static aspects of touch stimulus, like pressure
• The associated nerve fiber signals the dynamic aspects of the touch stimulus
• Have the highest spatial resolution of all the mechanosensory afferents
• Very sensitive to points, edges, and curvature
• Braile reading : Only slowly adapting Merkel cell afferents (top panel) provide a high-fidelity representation of the Braille pattern—that is, the individual Braille dots can be distinguished only in the pattern of Merkel afferent neural activity.
Meissner Afferents and Corpuscles
- slow vs rapid
- location
- shape
Rapidly adapting
Fire on an offset
(comparing to merkel)
About 40% of the mechanosensory afferents from the hand
Meissner corpuscles are in dermal papillae, close to the skin surface
The weird shape is due to connective tissue that forms a capsule
Encapsulating
Capsule contains flattened laminar cells and nerve terminals
The center of the capsule contained endings of 2-6 nerve fibers
When skin is indented, it changes the tension on the collagen fibers that deforms the corpuscle action potentials in afferent fiber
Larger receptive field than Merkel afferents, so lower spatial resolution
Can also detect low frequency vibration
Ex: when textured objects are moved against the skin
May be important for grip
Can detect slippage between your hand and an object
slide 25/26
Pacinian Afferents and Corpuscles
Rapidly adapting, more so than Meissner’s corpuscles
Also have a lower response threshold
Super rapidly adapting
Fire, but do not keep firing
About 10-15% of the mechanosensory afferents from the hand
Located: deep within the dermis (the deepest)
Look like cross-sections of tree trunks or onions
Concentric circular layers: Layers act as a filter, permitting only
Receptive fields are large
Well-suited to detect vibrations transmitted by objects
Hitting the hand, leaving the hand
Believed to be super important for tool use and writing
Ruffini Afferents and Corpuscles
-good at detecting : ?
Slowly adapting (respond the entire time the stimulus is there)
About 20% of the mechanosensory afferents from the hand
Located deep in the skin- But not as deep as the pacinian
*Also found in ligaments and tendons
Elongated spindle-shape
Long axis usually parallel stretch lines of skin
Thus they are really good at pereciving stretch of the skin… can tell if fingers are moving past eachother
Makes them sensitive to skin stretching due to moving fingers
They are probably also important for helping to convey information about the position and placement of the hands and fingers
This would overlay onto proprioception
Reading Braille
Only slowly adapting Merkel cell afferents (top panel) provide a high-fidelity representation of the Braille pattern—that is, the individual Braille dots can be distinguished only in the pattern of Merkel afferent neural activity.
Mechanoreceptors- Proprioceptors
Provide information about the placement of the body and its parts by reading information about mechanical forces
Mostly low threshold
Integrate their information with the vestibular system
Examples: muscle spindles, Golgi tendon organs, and join receptors
-Detect positions of joints and muscles
*Muscle spindles
Modified skeletal muscle cell
Monitor skeletal muscle length
*Golgi tendon organs
Dendrites around collagen fibers at the muscle-tendon junction
Monitor skeletal muscle tension
J*oint capsule receptors
Monitor pressure, tension and movement in the joint
Anatomy of the Muscle
Striated muscles are made of muscle fibers that have two parts, outer and inner:
Outer fiber = extrafusal fiber (does the work)
Inner fiber = intrafusal fiber
Muscle Spindles
collection of 6-8 specialized muscle fibers that are located within the muscle mass
- do not contribute significantly to the force generated by the muscle.
- are specialized receptors that signal (a) the length and (b) the rate of change of length (velocity) of the muscle.
- Modified skeletal muscle cell
- Monitor skeletal muscle length
Golgi Tendon Organ
A proprioceptor, sensory organ that receives information from the tendon, that senses TENSION.
Lift weights –> the golgi tendon organ tells you how much tension the muscle is exerting.
Too much muscle tension the golgi tendon organ inhibits the muscle from creating any force (via a reflex arc), thus protecting the you from injuring itself.
Decreases activity of alpha motor neuron
Because alpha motor neurons do all the “work”
2 Pathways We Cover for Tactile Information
- Dorsal column-medial lemniscal pathway: Tactile information from body
- Trigeminothalamic System: Tactile information from the face
Dorsal Column-Medial Lemniscal System
Transmits TACTILE information from the BODY to the CNS
Axons of mechanosensory afferents enter through the dorsal roots
Axons branch into ascending and descending branches
Synapse on projection neurons located deep in the spinal cord and on neurons further up in the brainstem
Spinal cord targets are in deep layers III, IV, V
- Leads to parallel columns*
- Dorsal column= ascending axons from sensory neurons
- Postsynaptic dorsal column projection= axons from projection neurons in spinal cord
*Dorsal columns are topographically organized
-Lower limbs more medial
Fasciculus gracilis or Gracile tract
-Upper limbs more lateral
Facsciculus cuneatus or Cuneate tract
SLIDE 54
Dorsal Column-Medial Lemniscal System: Dorsal columns are ____ organized
Dorsal columns are topographically organized
Lower limbs more medial: Fasciculus gracilis or Gracile tract
Upper limbs more lateral: Facsciculus cuneatus or Cuneate tract
Trigeminothalamic System
Tactile information from the face
The cell bodies of the mechanosensory neurons reside in the trigeminal ganglion
3 subdivisions to innervate specific areas of head and face- ophthalamic, maxillary, and mandibular
Axons continue enter the brainstem at the level of the pons
Synapse on the neurons in the principal nucleus of the trigeminal complex
Trigeminal complex has two major parts: principal nucleus and the spinal nucleus
Most of the afferents conveying low-threshold cutaneous information synapse in the principal nucleus
The spinal nucleus has multiple subnuclei and each receive collaterals from the mechanoreceptors, but also receive pain/temperature/non-discriminative touch information
Axons from the trigeminal complex cross the midline and ascend to the ventral posterior medial nucleus of the thalamus (VPM)
VPM sends axons through internal capsule to primary and secondary somatosensory cortices
Proprioceptive Information from the Body
- cell bodies
- axons
- lower limb information –>?
- upper limbs information –>?
Cell bodies live in dorsal root ganglia
Axons sent into spinal cord and branch into ascending and descending tracts.. And branch some more too
Branches extended to the ventral horn, where motor neurons live, to help mediate reflex arcs
Information from lower limbs synapses in Clarke’s nucleus, which is a region in the medial portion of the dorsal horn of the spinal cord
Enter around sacral region- travel with dorsal column to a higher level of the spinal cord, and then synapses here
Axons from Clarke’s nucleus ascend as the dorsal spinocerebellar tract
Axons sent to cerebellum
Axon collaterals will also be sent neurons near the gracile nucleus
Synapse on these proprioceptive neurons of the dorsal column nuclei
Information joins the medial lemniscus
Axons carrying information about the upper limbs enters the spinal cord and then travels via the dorsal column (cuneate tract) to the medulla
Synapse on proprioceptive neurons in the dorsal column nuclei, including in the external cuneate nucleus
Axons from these neurons go two places
Ipsilateral axons ascend to cerebellum
Other axons cross midline to join the medial lemniscus and go on to the VPL
Proprioceptive Information from the Face
-cell bodies
Also conveyed through the trigeminal nerve, but the cell bodies are not located in the trigeminal ganglion
Cell bodies are located in the mesencephalic trigeminal nucleus
Near the periaqueductal gray and midbrain
Details are fuzzy, but the information eventually reaches the thalamus and somatosensory cortices
Somatosensory Thalamus- VPL and VPM
Ventral Posterior Complex= VPL and VPM
Information arrives and synapses in the thalamus in a somatotopic manner
Medial lemniscus brings info from body and posterior head to VPL
Trigeminothalamic tracts bring info from the face to the VPM
Organized by where it is coming from in the body and head
Different types of mechanosensory input also remain separate and synapse on separate cells of the thalamus
Processing at the Perceptual Level
Interpretation of sensory input occurs in the cerebral cortex
The ability to identify the sensation depends on the specific location of the target neurons in the sensory cortex not on the nature of the message (all messages are action potentials)
Main Aspects of Sensory Perception
Perceptual detection – detecting that a stimulus has occurred and requires summation
Magnitude estimation – the ability to detect how intense the stimulus is
Spatial discrimination – identifying the site or pattern of the stimulus
*Feature abstraction – used to identify a substance that has specific texture or shape
*Quality discrimination – the ability to identify submodalities of a sensation (e.g., sweet or sour tastes)
*Pattern recognition – ability to recognize patterns in stimuli (e.g., melody, familiar face)
Somatosensation Perception
The specific sensation depends on the 2nd and 3rd order neurons
The ability to localize the specific location of a stimulus depends on the stimulation of a specific area in the primary somatosensory cortex
A sensory “homunculus” is a functional map of the primary somatosensory cortex
Somatosensation Perception
The specific sensation depends on the 2nd and 3rd order neurons
The ability to localize the specific location of a stimulus depends on the stimulation of a specific area in the primary somatosensory cortex
A sensory “homunculus” is a functional map of the primary somatosensory cortex
Activation of Pacinian Corpuscle
-shape
- Pacinian Afferents and Corpuscles
- Rapidly adapting, more so than Meissner’s corpuscles
- Also have a lower response threshold
- Super rapidly adapting
- Fire, but do not keep firing
- About 10-15% of the mechanosensory afferents from the hand
- Located deep within the dermis
- Look like cross-sections of tree trunks or onions
- Concentric circular layers
- Layers act as a filter, permitting only transient disturbances at high frequencies
- Rings of connective tissue => gives it the special abilities – acting like a filter
- Thus can detect a rapid stimulus I think
- Receptive fields are large
- Well-suited to detect vibrations transmitted by objects
- Hitting the hand, leaving the hand
- Because of the rings
- Believed to be super important for tool use and writing
Muscle spindles
slide 42 and before
Muscle spindles: stretch receptors that detect changes in the length of the muscle (how much it is stretched)
Involved in the sensation of position and movement of the body (proprioception)
Stretch and Speed of Stretch
-a type of Proprioceptor (which are mechanoreceptors)
Golgi Tendon Organ
A proprioceptor, sensory organ that receives information from the tendon, that senses TENSION.
Lift weights –> the golgi tendon organ tells you how much tension the muscle is exerting.
*Too much muscle tension –> the golgi tendon organ inhibits the muscle from creating any force (via a reflex arc), thus protecting the you from injuring itself.
Decreases activity of alpha motor neuron
Because alpha motor neurons do all the “work”
is equilibrium part of the somatosensory system
no
T/F you can have perception without sensation
FALSE! but, you can have sensation without perception
Four Types of Tactile Mechanoreceptors
Merkel, Meissner, Pacinian, Ruffini
All of the Tactile Mechanoreceptors have an axon diameter of _______ and conduction of ______, except for ______ which has an axon diameter of _____ and conduction of ______
All of the Tactile Mechanoreceptors have an axon diameter of 6-12um and conduction of 35-70 m/s, except for Merkel Cells which have an axon diameter of 7-11 um and conduction of 40-65 m/s
which tactile mechanoreceptors have small receptor fields?
Merkel, Meissner
which tactile mechanoreceptors have large receptor fields?
Pacinian, Ruffini
Exteroceptors
Respond to stimuli arising OUTSIDE the body
Receptors in the skin for touch, pressure, pain, and temperature
Most special sense organs
can mechanoreceptor be found in visceral locations?
yes ( think), however, proprioceptors (a mechanoreceptor) are found in somatic only
T/F the rate of action potentials are proportional to magnitude of depolarization
TRUE
** are rapidly adapting or slowly adapting useful for determining stimulus movement
rapidly
