#14109 - Receptors And Transduction - Neuroscience 1

Receptors and transduction

-The ability to receive and process information about surroundings, and to respond to this, is essential for survival and reproduction.

-Sensory receptors are responsible for detecting various stimuli, with this information then potentially leading to either subconscious or conscious adjustments.

-The receptors convey four basic types of information about a stimulus: modality (type of energy), location, intensity and timing.

-A single stimulus usually activates many receptors.

-Information that is processed by the cerebral cortex leads to conscious perception of the stimuli and a conscious ‘decision’ about the appropriate response.

-Only a very small proportion of the total receptor stimulation leads to conscious perception of the stimulatory factor.

-Receptors can be defined as rapidly or slowly adapting. Rapidly adapting receptors respond to changes in stimulus intensity but there is very little firing if that stimulus is maintained.

-Slowly adapting receptors do not respond as rapidly when the stimulus strength changes but they continue to fire, indicating that the stimulus is still present.

-The main classes of receptor are: mechanoreceptors, thermoreceptors, chemoreceptors and nociceptors.

a) Mechanoreceptors: Touch, proprioception

-Receptors for touch and proprioception terminate in a nonneural capsule- sense mechanical stimuli that indent/physical deform the receptive surface

-These may be responsive to one or several of a very broad range of stimuli including vibrations or pressure on the skin, sound (vibrations), gravity, muscle stretch, vascular pressure, and speed of movement of a joint.

-Sense physical deformation of tissue- Mechanical distension such as pressure on skin/ stretch of muscles-transduced into electrical signal by deforming and opening stretch sensitive ion channels- increase Na+ , Ca2+ depolarise the membrane

-Mechanical stimuli: forces conveyed through lipid tension in cell membrane (osmotic swelling), forces conveyed through structural proteins linked to ion channels, indirectly activated by forces conveyed to a force sensor: use a second messenger pathway, slow to activate and inactivate-outlasts the stimulus, but sensory signal amplified

Skin

Meissner corpuscles

-Located superficially in dermal ridges of glabrous (hairless) skin such as fingertips- about 1/16^th the size of Pacinian corpuscles

-Detect stroking/fluttering- rapidly adaptive with small receptive fields- nerve ending is surrounded by Schwann cells which can modify the sensory input

-Can detect light touch

Merkel cells- innervated by slowly adapting type 1 fibres (fire in response to steady pressure on the skin). Detect initial contact of hand with objects, slippage of objects, motion of hand over textured surface, low frequency vibration

-Located superficially with attachments to basemement membrane underlying epidermis and are dense in fignertips

-Nerve terminal surrounded by an epitheial cell- derived from neural crest cells and contains many neuroactive substances. Slow receptive fields and are slowly adapting touch receptors

Pancinian corpuscles

-Largest and best studied of the mechanoreceptors in the skin- measure up to 2mm long an 1mm in diameter

-Function: Detect vibration

-Site: Lie deep in dermis- made of 20-70 concentric layers of connective tissue surrounding a central unmyelinated nerve terminal of a myelinated axon

-Mechanical stimuli compress these layers, eventually causing a deformation of membrane of nerve terminal which opens mechanosensitive ion channels- influx of positive charge and a resultant depolarisation

Rapidly adapting type 2 fibres (respond to motion on skin but not steady pressure), large receptors, detects vibration in tools, objects, probes held in hand

-Maintaining the compression doesn’t produce firing because the viscous fluid between the layers means that there is no further deformation of the membrane of the nerve terminal

-When stimulus is removed the layers change shape again and there is another depolarisation. Therefore the receptors are rapidly adapting which makes them more important in signalling changes in stimuli

-Layers of the corpuscle are essential to this rapid adaptation- in the 1960s-Werner Loewenstein and colleages showed that removing the layers still produced mechanosensitivity but the response was slowly adapting

-Receptors have large receptive fields

Ruffni endings

-Expanded free nerve endings found deep in the dermis in both hairy and glabrous skin

-Detect skin stretch- slowly adapting and have large receptive fields- important signalling stretch of the tissues underlying the dermis and are important in the modulation of grip

-They are also located in deep internal organs and help to sense pressure of one organ over another

Hairs of skin: Play an important role in sensation- their hair follicles are innervated by free nerve endings. When hair is removed, deformation of hair follicle triggers a receptor potential in nerve endings

-Receptors may be rapidly or slowly adapting

Extension: 2-point discrimination

-The ability to distinguish between two separate touches to the skin varies greatly across the body.

-In the fingertips, two points can be distinguished even when very close together; on the back the stimuli must be much further apart.

-One of the main reasons for this is that the fingertips are generally very richly innervated, particularly with receptors with small receptive fields. If two simultaneous stimuli fall within the same receptive field they will not be distinguishable.

-There is also a higher proportion of brain tissue involved in interpreting sensation from the fingertips than from other regions of the body, which may assist with processing.

ii) Proprioceptors: Muscle activity, joint positions

-Type of mechanoreceptors in muscles and joints- convey information about posture/movements of the body

- Types: Two types of muscle length sensors, Type 1a, 2 muscle spindle endings; 1 muscle force sensory, Golgi tendon organ, joint capsule receptor

-Muscle spindle-bundle of thin muscle fibres/intrafusal fibres-lie parallel to the larger fibers of the muscle and enclosed within a capsule. Intrafusal fibres entwined by a pair of sensory axons that detect muscle stretch- mechanoreceptive ion channels

-Golgi tendon organs- found between skeletal muscle and tendons- measure forces generated by muscle contraction- reflex

B) Nociceptors: pain

-respond to stimuli that can damage tissue- respond directly to mechanical and thermal stimuli, indirectly to other stimuli by chemcials released from cells in traumatised tissue

-2 broad classes based on myelination of afferent fibres: Adelta fibres produce short latency pain (shapr, pricking) also respond to heat that burn the skin, C fibres- dull burning pain, diffusely localised and poorly tolerated

-Type 1 Adelta nociceptors/ high threshold mechanoreceptors- respond to mechanical and chemical stimuli- but are also sensitive to high >50 degrees temperature – like other classes of nociceptors these cells sensitise to both mechanical and thermal stimuli in presence of tissue injury or inflammation

-Type 2 Adelta nociceptors respond to noxious thermal stimuli referentially to mechanical stimuli

C) Thermal receptors: Detect changes in skin Temperature

-Detect cold, cool, warm, hot – due to differences between external temperature of air/ objects conacting skin and normal skin temperature

-We are sensitive to sudden changes in skin temperature but unaware of wide swings in skin temperature- cutaneous blood vessels open and close

-Unware of changes in the range of 31-36. Below 31 degrees sensation progresses from cool to cold, 10-15 degrees- pain, above 36-hot, 45 degrees-pain

-Six different types of afferent fibres: Low threshold (Adelta), high threshold cold receptors, warm receptors (C fibres), 2 classes...