25-01-22 - Sensory Receptors Flashcards
Learning outcomes
- List the main classes of sensory receptor found in the body
- Explain the specificity of the modal nature of sensory receptors
- Give examples of the main types of mechanosensory ending found in hairy and glabrous skin and explain how their structures relate to their function
- Recall the role of receptor/generator potential, and explain its characteristics
- Explain how an appropriate stimulus produces a receptor/generator potential that is localized to the sensory ending and whose amplitude is related to the strength of stimulation
- Explain how changes in the receptor/generator potential are involved in the phenomenon known as adaptation
- Discuss the functional significance of differing receptive field sizes both in terms of our sensation and in the context of brain organisation
- Recall that sensory nerves are arranged segmentally and that this is reflected in their distribution to the body surface.
What are the 3 main types of physiological receptors?
What do each detect?
• 3 main types of physiological receptors
1) Mechanoreceptors – tactile and position sensations
2) Thermoreceptors – detect heat and cold
3) Pain receptors – detect damaged tissues
What are the 2 types of touch receptors?
What are their nerve endings like?
Where are they found?
What frequencies and senses do they each detect?
What fibres do they each contain?
What skin types are they found in?
How fast do they each adapt?
What is their action threshold like?
• 2 types of touch receptors:
1) Pacini’s corpuscle
• Largest mechanoreceptor (2mm long)
• Onion like encapsulation of nerve endings
• Found in the deep layer of the epidermis
• Detects high frequency (40-500Hz) vibration
• Aβ fibres, found in glabrous and hairy skin
• Rapidly adapting due to a slick viscous fluid between layers
• Has a low activation threshold (sensitive)
2) Meissner’s Corpuscle
• Encapsulated nerve endings similar to Pacini’s. but much smaller
• Exist as stacks of discs interspersed with nerve branch endings
• Found between dermal papillae
• Detects touch, flutter and low frequency vibration (2-40Hz)
• Aβ fibres – glabrous skin types
• Rapidly adapting
• Low activation threshold
What are the 2 pressure/touch receptors?
How do their nerve endings exist?
What do they consist of?
Where are they found?
What type of senses do they detect?
What type of fibres do they contain?
What type of skin types are they found in?
How rapidly do they adapt?
What is their activation threshold like?
• Pressure/touch receptors:
1) Merkel disks
• Non-encapsulated nerve endings
• Consist of a specialised epithelial cell + nerve fibre
• Found just under the skin surface in for example the finger tips
• Detects static touch and light pressure
• Aβ fibres - all skin types
• Slowly adapting
• Low activation threshold (sensitive)
2) Hair follicles
• Embedded in skin
• Innervated by nerve ending wrapped around its follicle
• Detect muscular movements of the hair (erector muscle) and external displacements of hair
What is the stretch receptor?
How do their nerve endings exist?
What senses do they detect?
What type of fibres do they contain?
What skin types are they found in?
How fast do they adapt?
What is their threshold activation like?
• Stretch receptor:
1) Ruffini Corpuscle
• Encapsulated nerve ending
• Nerve ending weave between collagen fibres which activate the nerve when they are pulled longitudinally
• Responds to skin stretch and is located in the deeper layers of the skin as well as tendons and ligaments
• Aβ fibres - all skin types but especially abundant in hands and fingers as well as soles of feet
• Slowly adapting
• Low threshold activation (sensitive)
What is the role of muscle spindles?
Where are they found?
Where are they particularly numerous?
What are the efferent and afferent fibres that innervate muscle spindles?
What is the role of each?
- Muscle spindles are the main proprioceptors that provide information about the state of musculature
- Muscle spindles lie within muscle in parallel with skeletal muscle fibres
- Particularly numerous in fine motor control muscles like the eyes and hands
- Muscles spindles are innervated by γ-motor neurons (efferent) and group 1a and II afferent fibres
- γ-efferent activity regulates the sensitivity of the spindle
- Afferents respond to muscle stretch
What is the role of Golgi tendon organs?
Where are they found?
What do they respond to?
What nerve fibres are they innervated by?
What do these nerve fibres do?
- Golgi tendon organs are the main proprioceptors that provide information about the state of musculature
- Golgi tendon organs lie within tendons in series with contractile fibres
- They respond to degree of tension within the muscle
- Golgi tendon organs are innervated by Group IB afferent fibres
- These relay information to the CNS (particularly the spinal cord and cerebellum)
What is the generator potential?
What does this generate?
What is receptor potential?
What does this affect?
What 4 things can cause receptor potential?
- Generator potential is the potential caused by a stimulus to a nerve ending
- This generates action potentials in a sensory neuron
- Receptor potential is the potential caused by a stimulus to a receptor cell
- Affects amount of neurotransmitter released by receptor cell
• Receptor potential can be caused by:
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 myelinated or unmyelinated in a Pacini Corpuscle?
Describe the 3 steps in generation of action potentials in a pacini corpuscle
- In the Pacinian Corpuscle, the tip of the nerve fibre is unmyelinated, but is myelinated before leaving the corpuscle
- 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 certain threshold?
What happens to the amplitude of the receptor potential?
What happens to the frequency of action potential as receptor potential rises more above threshold level?
- When receptor potential from a receptor rises above a threshold in the nerve fibre, action potentials fire
- Amplitude of the receptor potential increases rapidly at first then less rapidly at high stimulus strength
- The more the receptor potential rises 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?
What can they be known as?
What do rapidly adapting receptors detect?
How does the firing of AP during the generator potential differ between the slow and rapidly adapting receptors?
- Slowly adapting receptors detect continuous stimulus strength (known as tonic receptors)
- Rate of firing action potentials is constant throughout generator potential
- Rapidly adapting receptors detect change in stimulus strength (information about a rate of change - known as rate/movement/phasic receptors)
- High rate of firing of action potentials at the height of the generator potential, which dissipates rapidly
Summary of Mechanoreceptors
What are the 2 different nerve fibre types?
What are the axon diameters of the different fibres’ types?
What are the axon conduction velocities?
What are examples of each type of nerve fibre?
What 3 things does precision localisation of a particular stimulus is determined by?
• 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?
What do small receptive field allow for?
- Pacini’s Corpuscles and Meissner’s Corpuscle are both sensitive, but have different receptive field sizes
- Pacini’s corpuscles have a broad receptive field – we’ll know something is going on, but couldn’t tell exactly where pressure is occurring
- Meissner’s corpuscles and Merkel’s discs are very small – allows us to determine exactly where a particular thing is happening
- Small receptive fields allow for high spatial resolution
What is two-point discrimination?
Where is two-point discrimination smaller?
- Two-point discrimination is the smallest distance between 2 points where you can determine 2 points are hitting you
- Two-point discrimination is smaller in things that interact with the environment, such as the hands and fingers, which have a lot of receptors in them
What is lateral inhibition?
What does this aid in?
- 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?
Where does it exist?
Describe the 4 steps in the dorsal column/medial lemniscus tract
- The dorsal column (fasciculus gracilis) is the sensory input on the dorsal aspect of the spinal cord
- 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?
Describe the 5 steps of the spinothalamic tract
- The direct spinothalamic tract is for peripheral receptors sensing temperature, pain and simple tactile sensations (tickle and itch)
- 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?
Where does sensory information pass through?
Where is this located?
What does a 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
- Sensory information passes through the thalamus to the primary sensory cortex
- This is located in a strip posterior to the post central sulcus of the brain
- A homunculus shows sensory information projected in a topographical manor to this area
