Sensory innervation

Types of sensory receptor (Modality)

  • Photoreceptor- light
  • Mechanoreceptors- pressure/mechanical energy
  • Thermoreceptor- heat energy/cold
  • Osmoreceptors- solute concentrations
  • Chemoreceptors- specific chemical (e.g O2/CO2)
  • Nociceptors- pain/tissue damage/displacement

Graded vs Action Potentials in sensory systems

  • A stimulus will produce open channels at the receptor, resulting in an inward flux of Na (in most cases) and depolarisation (a receptor potential)
  • A receptor potential is a type of ‘graded potential’– the stronger the stimulus, the greater the graded potential.  Receptor potentials also have no refractory period, so sustained output is possible.
  • Action potentials are generated in the afferent neuron
    • this may be the same cell or a different cell
      • If it is the same cell, a myelinated cell, this occurs at the nearest node of ranvier where there ion flow is possible (opening of VG Na channels)
      • If it is the same cell, a specialised cell, this occurs at the adjacent membrane
      • If this is a different cell, this occurs by synaptic transmission
        • Note none of these are at the axon hillock (as with efferent/inter neurons)

Afferent neurons and Adaptation

  • The stronger the stimulus, the larger the graded receptor potential, the more frequent the action potentials of the afferent neuron (you cannot get a ‘larger AP’), the more neurotransmitter is released at the next synapse
  • (see below- Input-output relations for the exact relationship)

  • Stronger stimuli also usually affect larger areas and so incorporate more receptors

Adaptation is the term to describe the different responses of different receptors to a similar stimuli.  Receptors can generally be split into one of two types:

  • Tonic
    • A tonic receptor is a sensory receptor that adapts slowly to a stimulus and continues to produce action potentials over the duration of the stimulus. In this way it conveys information about the duration of the stimulus.  Some tonic receptors are permanently active and indicate a background level. Examples of such tonic receptors are pain receptors, joint capsule, and muscle spindle
  • Phasic
    • A phasic receptor is a sensory receptor that adapts rapidly to a stimulus. The response of the cell diminishes very quickly and then stops. It does not provide information on the duration of the stimulus; instead some of them convey information on rapid changes in stimulus intensity and rate. An example of a phasic receptor is the Pacinian corpuscle.

The mechanism behind adaptation is a mixture of the structural properties of the receptor cell; the molecular/bio- pathways of the receptor; the electrical properties of the receptor or even modulation by higher order cells.

In the image above

  1. (A) represents slowly adapting tonicresponses: continuous information is being sent to the CNS while the receptor is deformed.  This is usually used to sense information about position or degree of stretch/force
    1. e.g. stretch receptors
  2. (B) represents rapidly adapting (RA) phasicresponses: detects changes in stimulus strength e.g. rate of deformity (the quicker the change the fewer the impulses, but the more impulses per unit time).
    1. This is used in some muscle spindle afferents; hair follicle afferents; and receptors detecting rate of movement of an object across the skin
  3. (C) represents very rapidly adapting phasic responses: responds only to very fast changes in movement e.g. fast acceleration/vibration
    1. Pacinian corpuscles (1-2 APs only)

Pacinian Corpuscles (a little bit)

Pacinian Corpuscles are specialised receptors used in detecting vibration and deep pressure.  They consist of layers of connective tissue surrounding the peripheral terminal of the afferent neuron.  If the skin is displaced, these layers slide against each other and a signal is generated.  However, even if the stimulus is maintained, the signal will be lost because the layers of receptor are no longer moving against each other.

Receptive fields

A receptive field of an afferent neuron is the region of space in which the presence of a stimulus will induce the production of a signal in that neuron.  For touch, this is the area of skin one neuron is responsible for.

  • Importantly, one region of skin, however, may be part of overlapping receptive fields of more than one neuron.  This increases the chance of a stimulus being ‘recognised’ by the body.

Different kinds of receptor have different receptive fields too, which reflect their function.  In touch, the superficial receptors that are responsible for detecting light touch (e.g. Merkel’s discs- slowly adapting; and Meissner’s corpuscles- rapidly adapting), have small, well-defined receptive fields.

In contrast, the deep receptors responsible for vibration/deep touch/stretch (e.g. Pacinian corpuscles- rapidly adapting; and Ruffini endings- slowly adapting) have large, overlapping receptive fields, which is more sensitive at its centre.

Input-output relationships in sensory systems

The Dynamic Range of a sensory system (receptor and afferent neuron) is the range of stimulus it will respond to.

  • Below this range, there is no response.  This is usually because:
    • the stimulus is not strong enough to generate threshold potential in the afferent neuron
  • Above this range, likewise, an increase in stimulus intensity will not alter the response.  This can be because of 3 things (in theory):
    • there are a limited number of stimulus gated ion channels- once all are open, the receptor is effectively saturated of stimulus
    • the receptor potential can only reach a certain size because it is generated in response to opening of non-specific ion channels (limited by the equilibrium potential of the receptor)
      • (in reality, this makes little difference because threshold is usually reached)
    • the receptor potential can only generate so many AP’s- limited by the refractory period
      • This is the MAIN limit on signalling

Relationship between stimulus and outcome within the dynamic range

  • The amplitude of the the receptor potential is proportional to the logarithm of stimulus intensity (i.e. more responsive to changes in weak stimuli than strong)
  • Combining this with the adaptation properties of sensory systems (i.e. less responsive to changes in weak stimuli…), we get a response that is maximal in the middle range.
    • Because different cells have different adaptations, different cells also have different middle ranges.
    • In combination, all these cells work together to sense a wide range of stimulus intensity.
      • this is known as range fractionation

see also Fibre types

see also 2-point discrimination, vibration and temperature sensation

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