Somatosensory system Flashcards
Special senses and general somatic senses
Special sense: vision, hearing, taste and smell
General somatic senses: touch, pain, temperature (sensory receptors for all)
What is Tactile stimuli
-External forces in physical contact with the skin that give rise to the sensations of touch, pressure, flutter, or vibration
Define the Tactile stimuli’s of:
Touch, pressure, flutter and vibration
Touch: minimal force on-or-by an object that produces very little distortion of the skin
Pressure: involves a greater force that displaces the skin and underlying tissue
Flutter or vibration: When the force of stimuli varies with time it produces more complex sensations such as object movement or object flutter (20 to 50 Hz) or vibration (100 to 300 Hz)
Discriminative touch
-fine touch. A sensory modality that provides detailed information (location, shape, texture, and movement) about a source of stimulation
Crude touch
-a sensory modality that allows a subject to sense that something has touched them, without being able to localize where they were touched, and provides poor localization and information.
Tactile receptors in the skin
-reason you can feel touch
-Low-threshold mechanoreceptors.
- 2 types of receptors are tactile receptors/ meissners corpuscle and lamellated corpuscle/ pacinian corpuscle
Primary sensory afferents (fibers) and their properties
-act as highways to pass the information from the receptor site to the central nervous system
Receptor types for primary sensory afferent nerves
A-beta fiber group: Myelinated. Functions in discriminative touch
A-delta fiber group: Myelinated, functions in pain and temp
C fiber group: UNmyelinated and therefore in C fibers the signal travels at a much slower speed. temp, itch, emotional touch, pain
How does the sensory conduction take place?
1) Hair follicle receptors have spiral nerve endings that wrap around the base of the hair
2) Bending the hair distorts the nerve ending
3) Mechanosensitive ion channels at nerve terminal respond to mechanical distortion
4) Channels are going to open and sodium ions will flow in. The accumulation of sodium in the nerve terminals raises the voltage potential. The sodium spreads through the axons and eventually reaches the spike generating region
- spike generating region is the first node between the two myelinated sites
Stages of polarization during the sensory conduction
1) When the receptor potential reaches the threshold (-70 mV usually), this is going to trigger the sodium channels to open. SO we get an increase in sodium ions in the nerve terminal, increasing the membrane potential. This is called the depolarization phase.
2) The membrane potential is going to eventually reach over 30 mV which leads to inactivation of the sodium ion channel. This is the repolarization phase when the sodium gates are closed but the potassium gates remain open.
3) During hyperpolarization both sodium and potassium gates are closed.
Process of receptor potential to action potential
-An applied stimulus causes a receptor potential to be generated.
-Terminal endings are depolarized.
-Receptor potentials spread to the axon trigger zone (spike generating region)
-A large enough receptor potential brings voltage-dependent sodium channels to be open.
-Action potential starts firing (when it meets threshold)
-Voltage-dependent sodium channels continue to open for a few milliseconds and then shut off automatically (entering refractory period)
-Cell membrane potential is restored
-Repeats as long as receptor potential stays large
Relation of receptor potential to action potentials
-Every action potential has around the same amplitude. The frequency of the AP (space between the spikes) is determined by how fast a sodium channel recovers and how soon the membrane potential can be restored (refractory period)
-If the receptor potential is great enough, the neuron fires again as soon as it repolarizes
-it continues to fire as long as the receptor potential is large enough to bring the membrane potential of the node to the firing level
-The more the receptor potential rises above the threshold level, the greater becomes the action potential FREQUENCY
Encoding of stimulus strength
Weak stimulus –> Lower receptor potential –> Lower AP frequency
Intense stimulus –> higher receptor potential –> higher AP frequency
Sigmoidal relation on a graph, not linear
Temporal summation
Increased strength of stimuli –> increased receptor potential –> increased frequency of AP –> increased increased intensity of sensation
Weber-Fechner law: R= K log S
Power law: R= KS^A
R is sensation felt
S is intensity of stimulus
Type of adaptation: Tonic receptor
-Slowly adapting receptor
-tonic receptor responds repetitively to a prolonged stimulus
-tonic receptors detect a steady stimulus (continuous stimulus strength)
-tonic receptors show little, slow and incomplete adaptation in response to a prolonged stimulus
-tonic receptor is better at coding the intensity of a stimulus for its entire duration; so continue ti transmit impulse to brain as long as the stimulus is present.
Type of adaptation: Phasic receptor
Phasic receptor is a rapidly adapting receptor. “Rate receptor” or “movement receptor”
-Phasic receptor shows a decline in action potential frequency with time in response to a constant stimulus. Phasic receptor primarily detects onset and offset of a stimulus
-phasic receptor or rapidly adapting receptor codes changes in stimulus intensity (strength) better but not duration
Phasic receptor
-Phasic receptor cannot be used to transmit continuous signals, because the receptor is stimulated only when stimulus strength changes; this is why it is called rate receptor, movement receptor, or phasic receptor. It reacts strongly when a change happens
Somatosensory receptor types
Meissner corpuscle, Merkel complex, Pacinian corpuscle, Ruffini corpuscle
Meissner corpuscle
-Consists of elongated, encapsulated stack of flattened epithelial cells with nerve endings interdigitated. between the cells.
-found in glabrous (i.e., hairless) skin, within the dermal papillae near the surface of skin.
-Long axis perpendicular to the skin
-pressure causes laminar cells to slide past one another, which distorts the membranes of axon terminals
-rapid adaptation receptor
Rapid adaptation
-Respond when a stimulus is changing
-good for sensing rapid changes in the intensity or location of a stimulus
-Meissners corpuscle
Merkel complex
-Unencapsulated and consists of a specialized receptor cell –> Merkel cell and disc-shaped nerve terminal
-Merkel cell has thick, short, finger-like protrusions to surrounding tissue
-located in basal epidermis
-good for sensing details of shape
-merkel cells and discs both respond to touch
-slow adaptation/ slowly adapting receptors
Pacinian corpuscle
-football shaped, encapsulated, and contains concentrically layered epithelial (laminar) cells
-like a slice of onion in cross section
-a single afferent terminal located at center
-outer layers of laminar cells contain fluid
-present over the entire body and in connective tissues (bone, body wall, body cavity, etc)
-in skin located deep in the subcutaneous tissue
-force causes displacement of fluid and lamellar cells
-very sensitive, adapt very rapidly
-Respond best to VIBRATION
-Commonly used clinically to test for first signs of neuropathy
Ruffini Corpuscle
-found deep in the skin and other connective tissue sites (mouth)
-cigar-shaped, encapsulated, and contains longitudinal strands of collagen fibers
-within the capsule, the afferent fiber branches repeatedly and its branches are intertwined with the collagen fibers
-oriented with long axes parallel to the surface of the skin
-most sensitive to skin stretch
-stretching the skin stretches the collagen fibers, which compresses axon terminals
-slowly adapting receptor
What is Receptive Field and what are its basic properties
The area from which a stimulus produces a response in that unit
Basic properties:
-Location
-Size
-> depends on location
-> depends on how widely the nerve endings spread out
Relation between size of receptive field and resolution
-The larger the receptive field the less sensitive the receptor is to localization of the stimulus –> low spatial resolution
-Axons innervating body parts like finger tips, lips, etc., tend to have small receptive fields
Rank somatosensory types from smallest to largest receptive field:
Meissner’s corpuscle: small
Merkel cells: small
Pacinian corpuscle (onion shape): large
Ruffini endings (football shape): large
