Sensory Receptors Flashcards
What are the 3 main types of physiological receptors?
What do each detect?
1) Mechanoreceptors – tactile and position sensations
2) Thermoreceptors – detect heat and cold
3) Pain receptors – detect damaged tissues
What are the two types touch receptors?
- Pacini’s corpsucle
-Meissner’s corpsucle
What are the nerve endings of the pacini corpuscle receptor like?
- Oninon like encapsulation of nerve endings
Where are pacinis corpuscle receptors found?
In the deep layer of the epidermis
What frequencies and senses do pacini’s corpuscle receptors detect?
- Detects high frequency (40-500Hz) vibration
What fibres do pacinis corpuscle contain and where are they found
Aβ fibres, found in glabrous and hairy skin
How fast does the pacinis corpuscle adapt
Rapidly adapting due to a slick viscous fluid between layers
What is the action threshold of a pacini’s corpuscle?
Has a low activation threshold
What are the nerve endings of Meissner’s Corpuscle like?
Encapsulated nerve endings similar to but very small
How Meissner’s Corpuscle nerve endings found?
- Exist as stacks of discs interspersed with nerve branch endings
Where are Meissner’s Corpuscles found?
Found between dermal papillae
What frequencies and senses do Meissner’s Corpuscles detect?
Detects touch, flatter and low frequency vibration ( 2-40Hz)
What nerve fibres do Meissner’s Corpuscles contain
Aβ fibres
What type of skin are Meissner’s Corpuscles found
Glabrous skin types
How fast do Meissner’s Corpuscles adapt?
Rapidly adapting
What is the action threshold of Meissner’s Corpuscles ?
Low activation threshold
What are the 2 pressure/touch receptors?
- Hair follicles
- Merkel disks
What are the nerve endings of merkel discs like?
Non-encapsulated nerve endings
What do merkel disks consists of?
Consist of a specialised epithelial cell and nerve fibre
Where are Merkel disks found?
Found just under the skin surface
What do Merkel disks detect?
- Static touch
- Light pressure
What type of fibres do merkel discs contain?
Aβ fibres
What skin types are merkel discs found in?
Al skin types
How fast do merkel discs adapt?
Slowly adapting
What is the activation threshold of merkel disks?
Low activation threshold (sensitive)
Where are hair follicles found
Embedded in skin
Describe hair follicle nerve endings
Nerve ending wrapped around its follicle
What do hair follicles detect?
Muscular movements of the hair and external displacements of hair
What type of receptor is the Ruffini corpuscle?
Stretch Receptor
What type of nerve ending do ruffini corpuscles have?
Encapsulated nerve ending
What happens to the nerve endings of found in the ruffini corpuscle?
Nerve ending weave between collagen fibres which activate the nerve when they are pulled longitudinally
What do Ruffini corpuscles detect?
Skin stretch
Where is the ruffini corpuscle found?
located in the deeper layers of the skin as well as tendons and ligaments
What fibres are present in the ruffini corpuscle
Aβ fibres
What skin type are ruffini corpuscles found/
all skin types but especially abundant in hands and fingers as well as soles of feet
How fast do ruffini corpuscles adapt?
Slowly adapting
What is the action threshold of the ruffini corpuscle?
Low threshold activation (sensitive)
What is the role of muscle spindles?
Muscle spindles are the main proprioceptors that provide information about the state of musculature
Where are muscle spindles found?
Within muscle in parallel with skeletal muscle fibres
Where are muscle spindles numerous?
In fine motor control like the eyes and the hand?
What are the efferent and afferent fibres that innervate muscle spindles?
Muscles spindles are innervated by γ-motor neurons (efferent) and group 1a and II afferent fibres
What is the role the γ-motor neurons
γ-efferent activity regulates the sensitivity of the spindle
What is the role of 1a and II afferent fibres?
Afferents respond to muscle stretch
What is the role of Golgi tendon organs
Golgi tendon organs are the main proprioceptors that provide information about the state of musculature
Where are Golgi tendon organs found?
They lie within tendons in series with contractile fibres
What do golgi tendon organs respond to?
They respond to degree of tension within the muscle
What nerve fibres are golgi tendon organs innervated by?
Group IB afferent fibres
What do the Group IB afferent fibres do?
These relay information to the CNS (particularly the spinal cord and cerebellum)
What is the generator potential?
Generator potential is the potential caused by a stimulus to a nerve ending
What does the generator potential generate?
This generates action potentials in a sensory neuron
What is receptor potential?
Potential caused by a stimulus to receptor cell
What does the receptor potential affect?
The amount of neurotransmitter released by receptor cell
What 4 things can cause receptor potential?
1) Mechanical deformation of receptor e.g stretch receptor membrane, which will open ion channels
2) Chemical application to membrane – opens ion channels
3) Temperature change of membrane – changes permeability of membrane
4) Effect of EM radiation e.g light on retinal receptor
Is the nerve fibre of the pacini corpuscle mylelinated or unmyelinated?
In the Pacinian Corpuscle, the tip of the nerve fibre is unmyelinated, but is myelinated before leaving the corpuscl
Describe the 3 steps in generation of action potentials in a pacini corpuscle
1) Compression anywhere on the outside of the corpuscle elongates, indents/deforms the central fibre
2) Receptor potential induces local current flow (Na+ current), which spreads along the nerve fibre
3) At the 1st node of Ranvier, local current flow depolarizes fibre membrane at this node, which sets off action potentials to CNS
What occurs when receptor potential rises above a threshold
When receptor potential from a receptor rises above a threshold in the nerve fibre, action potentials fire
What happens to the amplitude of the receptor potential?
Amplitude of the receptor potential increases rapidly at first then less rapidly at high stimulus strength
What happens to the frequency of action potential as receptor potential rises more above threshold level
, the greater the action potential frequency
Aps generates in a sensory nerve at a frequency directly related to stimulus size
What do slowly adapting receptors detect?
Continuous stimulus strength (known as tonic receptors)
What do rapidly adapting receptors detect?
Rapidly adapting receptors detect change in stimulus strength
(known as rate/movement/phasic receptors)
How does the firing of AP during the generator potential differ between the slow and rapidly adapting receptors?
Slowly Adapting Receptors (Tonic Receptors)
These receptors continue to fire action potentials as long as the stimulus is present.
The firing rate may decrease gradually over time, but APs persist, providing continuous information about the stimulus.
Rapidly Adapting Receptors (Phasic Receptors)
These receptors fire APs only at the beginning (onset)
What are the two different types of nerve fibres?
Myelinated and Unmyelinated
What is the fibre diameter of Aα fibres
12-20 micrometers
What is conduction velocity of Aα nerve fibres?
80-120 m/s
What is the numerical form of Aα fibres
Ia and Ib
List the myelinated nerve fibres
Aα
Aβ
Aγ
Aδ
B
Are C fibres myelinated?
No, unmyelinated
What is the numerical form of Aβ fibres?
II
What is the fibre diameter of Aβ fibres
5-12 micrometers
What is the conduction velocity of Aβ fibres?
35-75 m/s
What is the numerical form of Aγ fibres
II
What is the fibre diameter of Aγ fibres?
3-9 micrometers
What is the conduction velocity of Aγ fibres?
15-30 m/s
What is the numerical form of Aδ fibres
III
What is the fibre diameter of Aδ?
1-5 micrometers
What is the conduction velocity of Aδ fibres?
5-30
What is the fibre diameter of B fibres?
1-3 micrometers
What is the conduction velocity of B fibres?
3-15
What is the numerical form of C fibres?
IV
What is the fibre diameter of C fibres?
0.2-1.5 micrometers
What is the conduction velocity of C fibres?
0.5-2
What 3 things does precision localisation of a particular stimulus is determined by?
1) Size of individual nerve fibre receptive field
2) Density sensory units
3) Amount of overlap in nearby receptive fields
How do the sizes of Pacini’s Corpuscles and Messiner’s corpuscles receptor fields differ?
Pacini’s corpuscles have a broad receptive field
Meissner’s corpuscles are very small receptive field
Small receptive fields allow for high spatial resolution
What is lateral inhibition?
Lateral inhibition is when information from afferent neurons whose receptors are at the edge of a stimulus are strongly inhibited compared to information from the stimulus’ centre
This aids in enabling localisation stimulus
What is the dorsal column?
The dorsal column (fasciculus gracilis) is the sensory input on the dorsal aspect of the spinal cord
Describe the 4 steps in the dorsal column/medial lemniscus tract
1) Sensory information from mechanoreceptors feed into dorsal root ganglion (neuron 1) which feed into the dorsal column (red for lower limbs – T6-T12, L1-L5, S1-S5. Blue for upper limbs C1–C8 and T1-T5)
2) Second-order neurons (neuron 2) in the nucleus gracilis and cutaneous cross in the midline to form the medial lemniscus in the medulla regions
3) This sensory information scends through the medulla, pons and midbrain and terminates in the contralateral ventral posterolateral nucleus of the thalamus
4) Axons of third-order (neuron 3) in thalamus travel in the internal capsule and terminate in the somatosensory cerebral cortex
What is the direct spinothalamic tract for?
The direct spinothalamic tract is for peripheral receptors sensing temperature, pain and simple tactile sensations (tickle and itch)
Describe the 5 steps of the spinothalamic tract
1) First order neurons from dorsal root ganglion cells (neuron 1) synapse with neurons of nucleus proprius
2) Second-order neurons (neuron 2) cross via the anterior white commissure and enter the contralateral white matter
3) Sensory information ascends in the spinothalamic tract (anterolateral tract)
4) Third order neurons (neuron 3) located in the central posterolateral nucleus of the thalamus
5) Axons of the third order neurons project to primary sensory cortex
What does the somatosensory homunculus show?
A somatosensory homunculus shows that areas where we need lots of input or lots of sensory input coming in (e.g lips, fingers,) are given a lot of cortex in order to process this information, while areas like the back and top of thighs get less cortex given to them
Where does sensory information pass through?
Sensory information passes through the thalamus to the primary sensory cortex
Where is the primary sensory cortex located
This is located in a strip posterior to the post central sulcus of the brain
What does a homunculus show?
A homunculus shows sensory information projected in a topographical manor to this area
