Peripheral Receptors Flashcards
What are the components of peripheral nerves?
- Axons
- Supporting Schwann cells
- Connective tissue (including fibroblasts)
- Blood vessels
Describe the dimensions of a peripheral nerve
- Range in diameter from ~0.2-20µm
- Axons above ~1-1.5µm are myelinated
- Those below this threshold remain unmyelinated (but not generally ‘naked’
Describe the characteristics of primary spindle endings and golgi tendon organs
- 12-20µm diameter
- Myelinated
- Condcution velocity = 80-120 m/sec
- Class - Aα
Describe the characteristics of secondary spindle endings, deep pressure receptors and cutaneous tactile receptors
- 6-12µm diameter
- Myelinated
- Conduction speed = 35-75 m/sec
- Class - Aβ
Describe the characteristics of crude touch fibres and pain and temperature receptors
- 1-6µm diameter
- Myelinated
- Conduction speed = 5-30 m/sec
- Class - A𝝳
Describe the characteristics of slow pain fibres
- 0.5-1.5µm diameter
- Unmyelinated
- 0.5-2 m/sec
- Class - C
Describe motor to extrafusal fibres in skeletal muscle
- 12-20µm diameter
- Myelinated
- 80-120 m/sec
- Class - Aα
Describe motor to intrafusal muscle fibres in skeletal muscle spindles
- 3-6µm diameter
- Myelinated
- 15-30 m/sec conduction speed
- Class - Aγ
Describe preganglionic autonomic efferents
- 1-3µm diameter
- Myelinated
- 3-14 m/sec
- Class - B
Describe postganglionic sympathetic efferents
- 0.5-1.5µm diameter
- Unmyelinated
- 0.5-2 m/sec
- Class - C
What is the sensory threshold?
The minimum strength of a stimulus which a receptor can detect
What is receptor adaptation?
In skin receptors (and some others) if a stimulus is continuous for a period of time, the neural response becomes diminished.
Some receptors adapt slowly, others adapt rapidly.
What controls the intensity of a stimulus?
Intensity of the stimulus is encoded for by the frequency of the action potentials.
This is frequency modulation.
What are tactile receptors associated with?
Touch, pressure and vibration
List the types of tactile receptors
- Hair follicle receptors
- Meissner’s corpuscles
- Pacinian corpuscles
- Ruffini endings
- Merkel’s nerve endings
- Free nerve endings
Which tactile receptors are encapsulated nerve endings?
- Meissner’s corpuscles
- Pacinian corpuscles
- Ruffini endings
- Surrounded by a connective tissue capsule
Describe hair follicle receptors
- Formed from axons that surround or run parallel to hair follicles.
- These are rapidly adapting receptors that respond to the bending of the hair shaft and so serve as receptors for light touch.
Describe Meissner’s corpuscles
- Found in many areas, but concentrated in glabrous skin (fingers, toes, plantar surfaces of feet, palms).
- Composed of a stack of epithelial cells with 1-2 unmyelinated axons spiralling through them.
- They are surrounded by a connective tissue sheath.
- They are low threshold and rapidly adapting.
- Sensitive to touch and vibration.
Where are Pacinian corpuscles found?
- Deep in the dermis or in the hypodermis
- Can also be found in the:
- Joint capsules
- Mesentery
- Periosteum surrounding bone
Describe the characteristics of Pacinian corpuscles
- More than 1000µm long
- Concentric layers of flat cells with fluid and connective tissue between the layers.
- The innervating fibre is myelinated, but loses its myelination once it enters the Pacinian corpuscle and lies at its centre.
- Respond to pressure and vibration
- Low threshold and very rapidly adapting
Describe Ruffini endings
- Small receptors (1-2µm)
- Widely distributed and found in the dermis
- Myelinated axon enters the capsule (loses its myelin as it does so) and ramifies amongst the collagen fibrils.
- Low threshold, slowly adapting
- Respond to the stretch of the collagen fibres and thus the skin
Describe Merkel’s discs
- Found in:
- Lips
- Fingers
- Toes
- Genitalia
- Specialised large epithelial cell in the basal layer of the epidermis which is on contact with a disc-like ending of an axon.
- Sensitive to pressure
- Slowly adapting
- Low threshold
Where are free nerve endings found?
- Skin (dermis, hypodermis, epidermis)
- Organ capsules
- Ligaments
- Meninges
- Perichondrium
- Parietal peritoneum
- Endomysium of muscle
- Cornea
Describe free nerve endings
- Can be myelinated or unmyelinated
- Respond to several sitimuli:
- Light touch
- Heat / cold
- Tissue damage
- Many act as nociceptors
Which receptors are responsible for proprioceptive information?
- Free nerve endings
- Muscle spindles
- Golgi tendon organs
Describe muscle spindles
- Special sense organs in muscle which provide information on the amount of stretch in the muscle and velocity of change of length in the muscle.
- Contain special muscle fibres - intrafusal fibres
- Normal contractile muscle fibres are termed extrafusal fibres
What are the 2 types of intrafusal muscle fibres?
- Nuclear chain fibres
- Nuclear bag fibres
Describe nuclear chain fibres
Homogeneous diameter and have their nuclei in a line
Describe nuclear bag fibres
Have a swelling in the middle where their nuclei congregate
Compare the innervation of the intrafusal muscle fibres to the extrafusal fibres
- Intrafusal fibres are contractile and are innervated by smaller motor neurons (AƔ fibres or ‘gamma motor neurons).
- Extrafusal fibres are innervated by Aα fibres (or ‘alpha’ motor neurons)
What are the 2 types of sensory axons within the spindle?
- Annulospiral endings
- Flower spray endings
Describe the golgi tendon organ
- Spindle shaped, found in tendons near the myotendinous junction.
- Thin capsule with a sensory axon that enters, ramifies and becomes interwoven between collagen fibres.
- Golgi tendon organ fibres are slowly adapting and are stimulated by tension in the collagen fibres (which reflects the tension in the tendon and therefore the attached muscle).
Which 3 cells are involved in forming the neuromuscular junction?
- Motor neuron
- Muscle fibres
- Schwann cell
Describe the motor neuron at the neuromuscular junction
The terminal of the motor neuron axon is the presynaptic element in the synapse. It contains vesicles of neurotransmitter - termed synaptic vesicles.
Describe the muscle fibres at the neuromuscular junction
Specialised region of the muscle fibre that receives the nerve terminal is called the sole plate, and forms the postsynaptic element of the synapse.
Each terminal sits in a depression in the muscle fibre surface called the primary synaptic cleft.
Describe the Schwann cell at the neuromuscular junction
- Schwann cells are the main glial (support) cells of the peripheral nervous system.
- Perisynaptic - covers over the cell process forming the synapse.
Describe the neuromuscular junction
- Almost all skeletal muscle fibres have a single neuromuscular junction.
- Here, motor axons terminate at a motor end plate and action potentials arriving in the axon cause release of acetylcholine and initiate an action potential in the sarcolemma.
What is the motor plate?
A slightly raised complex at the site where the motor axon terminal meets the muscle fibre.
