Somatosensory receptors Flashcards
How we feel? - Sensation
Superficial- Touch, Pain, Temperature Two-point discrimination
Deep- Muscle & joint position sense Vibration
Visceral- Hunger, nausea & visceral pain
Special sense- Smell, vision, hearing, taste, equilibrium
Sensory system
Perception, Conduction and Integration of sensory inputs
- external
-internal enviroment
Peripheral receptors
Exteroceptors (skin)
-Stimuli outside the body
Pain, temperature, touch, pressure
Proprioceptors (muscles, tendons,joints)
-Signal awareness of body position and movements
Movement, Joint position
Enteroceptors (viscera)
Monitor events within the body
Sensory receptors can be classified by stimulus
Somatic, visual, auditory, vestibular, taste and olfactory system
Chemoreceptors – detect chemicals
Photoreceptors – a specialized neuron able to detect and react to light during vision
Mechanoreceptors – detect mechanical forces
(movement, tension and pressure)
Thermoreceptors – detect changes in temperature
Nociceptors – detect pain
Receptors are widely distributed throughout the body
Classification of Sensory Receptors
▫ Modality: Type of Stimulus
▫ Chemicals (chemoreceptors)
▫ Light (photoreceptors)
▫ Pressure (mechanoreceptors)
▫ Temperature (thermoreceptors)
▫ Pain (nociceptors)
Somatic sensations: Modalities
Descriminative touch
Crude (nondiscriminative touch)
Pain = nociceptive ( fast / low)
Flutter- vibration
Proprioception
Thermal (hot / cold)
Receptors are widely distributed throughout the body
Classification of Sensory Receptors
▫ Intensity
The lowest level of strength a stimulus must reach to produce a Sensory threshold
▫ Duration
▫ Time the sensory stimulation
continues
▫ Adaptation - less sensitive to the stimulus
Adaptation (Tonic/Phasic)
-Slowly adapting (Tonic) = Fire continuously throughout the stimulus (duration and intensity of stimulus
-Rapidly adapting (Phasic) = Signal the onset and cessation of the stimuli (activity reflects the rate of application of the stimulus)
Sensory Receptors: continuation
▫ Location
▫ Site and ability to distinguish
between stimuli
▫ Cutaneous (skin)
▫ Muscle spindles contain mechanoreceptors that detect stretch in muscles
Morphology
▫ Free nerve endings
▫ Nociceptors
▫ Thermoreceptors
▫ Encapsulated receptors
General Senses
Light Touch
▫ Two-point discrimination
▫ Stereognosis
ability to recognize objects by touch alone
▫ Graphesthesia
ability to recognize numbers or letters drawn
on the skin, it requires memory
- Pressure
▫ Referred to deep touch - Vibration Sense
▫ Requires intact pathway from deep structures
▫ Low frequency 128 vibrations/second are associated with the light touch pathways
Two Point Discrimination
The minimum distance needed between two stimuli to perceive them as two units.
Tactile acuity threshold are determined by Merkel’s receptors which are densely packed in the fingerprints.
Receptive Field (rf)
The space or region over which a stimulus alters neuronal activity
Stimulus Transduction
Sensory receptors convert a stimulus into a neural activity
Stimulus –>Mechanoreceptors (Stimulated by mechanical opening of ion channels) –> Receptor potential (Tonic/Phasic)—> stimulus transduction
Sensory receptor types
Simple receptors - are neurons w. free nerve endings
Complex neural receptors - have nerve ending enclosed in connective tissue capsules
Special sense receptors- are cells that release neurotransmitter onto sensory neurons, initiating an action potential (ej. hair cell)
Neuron types
Bipolar
Pseudo-unipolar***
Multipolar
Peripheral components
Primary Afferent fibers consists of:
- Peripheral process –
extending from posterior root ganglion that contact a mechanoreceptor or end as free nerve endings (Unmyelinated fibers) - Pseudounipolar cell body – in the dorsal root ganglion (Unmyelinated fibers)
- Central process – to CNS (Myelinated)
Primary Sensory Neuron
First order neurons
Dorsal root (spinal) ganglia and the homologous ganglia of the:
Trigeminal (CN V)
Facial (CN VII)
Glossopharyngeal (CN IX)
Vagus (CN X) nerve
RECEPTORS CLASSIFICATION
- In base of localization:
▫ exteroceptors: in skin; stimulating by changes in external modalities: tactile, pressure, pain, temperature
▫ proprioceptors: in muscles, fascia, ligaments, articulations; stimulated by body changes; proprioception
▫ enteroceptors: in viscera (GVA); we will NOT discuss this topic
Receptors can be classified based on
cutaneous fibers or contribution to a compound action potential
- Cutaneous fibers:
* Presence or absence of myelin
* Conduction velocity (I, II, III, IV) * Axon diamete - Contribution to a compound action potential (A, B and C waves).
***estudiar tabla Slide 21
Skin
larger organ in the body
Epidermis
▫ Outer layer composed of
dead skin cells
- Dermis
▫ Below the epidermis
▫ Composed of mechanoreceptors that respond to pressure, stretching and vibration
Tactile Receptors
Mechanoreceptors differ by:
- Morphology (structure and location)
- Physiologically (size and receptive fields)
- Functional information that they encode
Epidermis - touch / pressure / vibration
Dermis - sensitive
Each mechanoreceptors has different receptive field and respond different to tactile sensation
Encapsulated
* Meissner’s corpuscle
* Pacinian’s corpuscle
* Ruffini’s ending
Unencapsulated
* Merkel’s disks/cells
* Hair follicle receptors
Tactile Receptors: Meissner’s Corpuscles
Found in glabrous skin
* Fingers, hand (palm), foot (plantar), toes
* Stacks of horizontal flattened epithelial cells (encapsulated)
* Low-threshold
* Rapidly adapting (RA) – fire at onset and offset of stimulation
* Sensitive to light touch
* Response for abrupt changes in the shape and edges of objects
* Vibration below 100 Hz
* Small receptive field
Fine touch perception, which is essential for tactile discrimination, and reading Braille.
Pacinian Corpuscles
Encapsulated mechanoreceptors, located deeper in the skin
▫ Found in Hairy & Glabrous skin
▫ Skin of hands, feet, nipples, mammary glands, walls of mesentery, periosteum, joint capsules
* Concentric lamellae of flattened cells that are supported by collagenous tissue
* Low threshold
* Rapidly adapting (RA)
* Large receptive fields
* Sensitive to rapid indentation of skin, vibration and pressure.
* Sensation caused by a high frequency vibration (100-400 Hz).
e.g. rough versus smooth surface texture
Ruffini’s Corpuscles (Endings)
Located in the deep layer of the skin (dermis)
▫ Found in Hairy & Glabrous skin
▫ Skin of hands, feet, nipples,
mammary glands
* Widely distributed
* Low threshold
* Slowly adapting (SA)
* Detect skin stretching and pressure (magnitude & direction; mechanical information within joints)
* Also acts as thermoreceptors
- In a case of a deep burn to the body, there will be no pain as these receptors will be burned off
Merkel’s Receptors (Discs)
Nonencapsulated
* Located in the basal cell layer of the epidermis, below glabrous skin of:
▫ Lips
▫ Extremities (distal part)
▫ Genitalia (external)
* Low threshold
* Slow adapting (SA)
* Sensitive to pressure and low- frequency vibration
▫ Signal discrete indentation
* Each ending consists of a Merkel cell in close apposition with an enlarged nerve terminal, up to 90, sometimes called as a Merkel cell-neurite complex.
* Small receptive fields
Hair Follicle Receptors
A mesh-like arrangement of axons around a hair follicle
- Provide information to the brain about discrete tactile stimulation
- Rapid and Slow adapting
- Sensitive to touch
Cutaneous Mechanoreceptors and Their Associated Fiber Types and Sensations
Accuracy of stimulus depends on:
1. Density of receptors
2. Size of receptive fields
SLIDE 30 ver tabla***
TACTILE SENSATION
Meissner (Adaptive rate: rapid / receptive fiel: small)
Pacinian (Adaptive rate: rapid / receptive fiel: large)
important for sensing movement of an object across the skin
Merkel (Adaptive rate: slow / receptive fiel: small)
Ruffini (Adaptive rate: slow / receptive fiel: large
Provides input related to displacement and velocity of the stimulus
Signaling the pressure and shape of the object through the skin
PROPRIOCEPTION
Distributed throughout the musculoskeletal system
- Respond to mechanical forces (position and movement of extremities),
within the body itself. - Play an important role in posture and movement
- The body’s ability to transmit position sense (from proprioceptors), interpret the information received and respond
- Afferent nerves that receive and send information from skin, tendons, joints and muscles to the CNS
PROPRIOCEPTION
Types of Proprioception:
▫ Consciously
▫ Unconsciously
Modalities:
▫ Limb position sense ▫ Kinesthesia
joint position
direction
velocity of joint movements
Proprioceptors provide sensory information to the cerebral cortex:
Muscles and tendons have receptors that detect:
Muscle length = muscle spindles
Muscle strength = Golgi tendon organs
Muscle pain is detected by free nerve endings
Proprioceptors in the joints detect information regarding its position
Conscious Awareness: Receptors
Joint Receptors & Ligaments
* Free nerve endings
* Encapsulated receptors
▫ Low-threshold mechanoreceptors
▫ Rapid adapting
Provide information regarding the dynamic aspect of kinesthesia
Movement, direction and velocity of its movement
▫ Slowly adapting
Provide information about static aspect of kinesthesia Position judgment
NonConscious Awareness: Receptors
Muscle spindle
Detect changes in muscle length
* Present in skeletal muscles
▫ Extrafusal fibers
Innervated by alpha motor
neurons
Each spindle consist of a connective tissue capsule containing 8 to 10 intrafusal fibers, which are parallel to extrafusal fibers.
▫ Innervated by spinal gamma motor neurons
Types of Intrafusal fibers
Nuclear chain fiber
§ Single row of central nuclei
§ Smaller and shorter than the nuclear bag fiber
Nuclear bag fiber
§ Cluster of nuclei located in a bag-like dilation at the center of the fiber
§ Larges intrafusal fiber
Afferents from intrafusal fibers
- Annulospiral endings (primary afferents; type Ia sensory endings)
§ Innervate the middle portion of both nuclear chain and bag fibers - Flower-spray endings (secondary afferents)
§ Located at the end of the nuclear bag fiber
Muscles spindles detect changes in muscle length.
Efferent axons of gamma motor neurons (ventral horn of spinal cord) innervate the polar end.
Motor innervation to intrafusal fibers
Efferent axons of gamma (ɣ) motor neurons (ventral horn of spinal cord) innervate the polar end, causing intrafusal fibers to contract and remain responsive.
Muscle spindle density
Varies among different muscles.
Those muscles that requires precise movements, such as the extraocular muscles, have higher density of muscle spindles than muscles responsible for gross movements.
Stretch reflex (myotonic reflex)
intrafusal fiber –> (a-Alpha) motor neuron –> reflex-induced contraction of extrafusal fiber
Golgi Tendon Organs
Located at the tendon-muscle junction
- Slowly adapting mechanoreceptor that
are stimulated by tension in thew tendon - High-threshold
- Arranged in series with the extrafusal fibers, consisting of a mesh-like weave of collagenous bundles within a thin capsule
- Sensory neuron conduct impulses to spinal cord
- Innervated by type Ib afferent fibers
- Senses changes in tendon tension/force
Joint Receptors
Within and around capsule joints–>Nerve endings–>Pain information
Connective tissue external to the capsule–>Ruffini endings Pacinian—>Respond to start and stop of movement & Position of the joint
Pain Receptors
Found in cutaneous as well as in deep structures
Mechanical–>Aδ afferents (thinly myelinated)–>Sharp, pricking–>fast and acute pain sensations
Thermal and Mechano-thermal–>Aδ afferents (thinly myelinated)–>Slow burning, cold sharp, pricking–>Fast and acute pain sensations
Polymodal–>Unmyelinated (C)–>Hot, burning sensation, cold, mechanical stimuli–>Tissue damage Slow, chronic pain sensation
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