sensory receptors mw %% (+ Flashcards
Sensory receptors
- Are nerve endings, often with specialized non-neural structures
- They are transducers that convert different forms of energy into frequency of Action Potentials (APs).
Terminology
- Sensory modality – a type of stimulus activating a particular receptor: eg. touch.
- An adequate stimulus is the type of energy a receptor normally responds to
- Sensory receptors are highly sensitive to one specific energy form but activated by other intense stimuli (poke in the eye - “see stars”)
Types of sensory receptors
- Mechanoreceptors: stimulated by mechanical stimuli - pressure, stretch, or deformation. Detect many stimuli
- Proprioceptors: are mechanoreceptors in joints and muscles that signal information related to body or limb position
- Nociceptors: respond to painful stimuli - tissue damage and heat
Cutaneous Mechanoreceptors and Proprioceptors
- Are good examples of the principles of peripheral sensory processing
- Transduction in ALL sensory receptors involves opening or closing of ion channels
- An adequate stimulus causes a graded membrane potential change called a a generator potential
- The adequate stimulus in cutaneous mechanoreceptors and proprioceptors is - membrane deformation.
- This activates stretch-sensitive ion channels causing ion flow across the membrane.
The receptor potential is graded to stimulus intensity
- A stimulus – causes local current to flow to the part of the membrane with voltage-gated ion channels.
- This generates action potentials (APs).
- The larger the stimulus, the larger the receptor potential and the HIGHER THE FREQUENCY of APs in a sensory nerve.
- This is frequency coding of stimulus intensity
Mechanoreceptors and APs
- For some mechanoreceptors: if the stimulus persists – APs persist
- Often we are interested in novel and changing events
- Aware of putting on our clothes, after that their mechanical stimulation is not important - until we take them off!
ADAPTATION
- Some mechanoreceptors ADAPT to a maintained stimulus and only signal change – eg. the onset of stimulation.
- It responds only to a change or a novel event.
Different receptors and extents of adaptation
- Rapidly/Moderately-adapting receptors include Pacinian corpuscles
- Slowly-adapting receptors.
- Nociceptors which are free nerve endings detecting painful stimuli - do not adapt.
Features of the Pacinian corpuscle
- A myelinated nerve with a naked nerve ending
- enclosed by a connective tissue capsule of layered membrane lamellae separated by fluid (like an onion)
How does the Pacinian corpuscle respond?
- A mechanical stimulus deforms the capsule and nerve ending
- This stretches the nerve ending and opens ion channels
- Na+ influx causes local depolarisation – a generator potential
- APs are generated and fire at the myelinated nerve
The Pacinian corpuscle and adaptation
- Shows rapid adaptation
- Fluid redistribution in the capsule – rapidly dissipates stimulus - removes mechanical stretch of nerve ending – APs stop firing.
- Withdrawal of the stimulus - capsule springs back - AP fire again
- Detects ON and OFF phases of a mechanical stimulus
Importance of lamellae
Capsule intact:
- Normal, rapidly adapting ON/OFF response
Capsule removed:
- Bare nerve ending loses much of adaptation
- Continues to produce a receptor/generator potential
The non-neural accessory structure is critical to how this sensory receptor works – in general they enhance sensory function
Sensory receptors and receptive fields
- Possess receptive fields: the particular region of the sensory space (e.g., the body surface, or the visual field) in which a stimulus will modify the firing of that neuron.
Our ability to tell 2 points on the skin apart
Depends on 2 things:
1) receptive field size
2) neuronal convergence
- it is determined by a 2 point discrimination test.
- So convergence and a large secondary receptive field indicates a relatively insensitive area.
What is acuity?
The ability to locate a stimulus on the skin and differentiate it from another closeby.
How can it be so precise?
Lateral Inhibition is important
- Information from neurons with sensory receptors at the edge of a stimulus is strongly inhibited, compared with information from the centre of the stimulus.
- The contrast between relevant and irrelevant information is enhanced
- Lateral inhibition occurs in the spinal cord for cutaneous information.
Componenets of proprioceptors
- Muscle spindles – monitor muscle length and rate of change of muscle length and so they control reflexes and voluntary movements.
- Golgi tendon organs - monitor tension on tendons
- Tension is produced by muscle contraction, so monitoring muscle tension.
- Joint receptors - monitor joint angle, rate of angular movement and tension on the joint.
Structure of muscle spindle (pic)
Structrure of Golgi tendon
- The top one is muscle spindle structure
- The bottom one is Golgi tendon
Intrafusal fibre types
- Nuclear bag fibres - bag shaped and nuclei collected together
- Nuclear chain fibres - nuclei lined up in a chain.
Golgi tendon organ (GTO)
- Nerve endings of GTO mingle with the tendon bundles at ends of muscles.
- They are stretch receptors and monitor stretch of tendon.
- Tendons are fairly inelastic, so passive stretch does not affect them much (unlike muscle spindles). Muscles have to develop tension by contracting to stretch the tendons.
The organization of muscle proprioceptors
A)Muscle spindles lie in parallel and GTO’s in series with the extrafusal muscle fibres
B)Isometric muscle contractions increase tension in GTOs and Ib sensory axons fire. But activated muscles stay the same length (isometric), so 1a afferents do not fire.
What is the relevance of the gamma motor innervation of the muscle spindles?
- If it was not present, then when muscle contracts, muscle spindle would be floppy and spindle discharges could stop
- The brain would not be informed about muscle length
- A lack of information flow to the brain about muscle length could prevent use of that muscle
