Electrical recordings of action potential trains in single afferent nerve fibres ('single units') show that the information carried by each sensory axon relates to a specific type of natural stimulus ('modality' - mechanical, thermal, etc); their transducers are fairly specific for a particular form of natural stimulation. The 'adequate stimulus' is the specific natural stimulus (mechanical, thermal or noxious, etc) to which the transducers are most sensitive.
The adequate stimulus can be used to classify sensory receptors. Most sensory endiings respond to one main type of stimulus, such as light touch, stretch, or temperature. Thus for the sensation of touch, there are around four different types of sensory neurones responding to:
- indentation of the skin
- lateral stretch of the skin
- vibration (optimally at 30 Hz)
- vibration (optimally at 250 Hz)
Others respond to hair movement, and the hair receptors are most highly developed in the vibrissae of animals such as cats.
Cutaneous receptors are classified into three main groups:
Mechanoreceptors respond to mechanical stress or strain, or vibration.
Thermoreceptors respond to the normal range of temperatures in the skin.
Nociceptors respond to damaging stimuli such as painful mechanical chemical or thermal stimuli. The skin proteins begin to denature around 45 degees centigrade, and heat nociceptors respond to temperatures above 45 degrees C.
The threshold stimulus is the intensity of the natural stimulus that causes the afferent endings to start producing action potentials. So the temperature threshold of thermoreceotprs is less than that of nociceptors. Also the threshold of touch afferents is less than that of the mechanical nociceptors that only respond to damaging stimuli.
The receptive field is the area of skin that is innervated by a single sensory neurone. Some receptive fields are small, and a few may be described as punctate. But many axons innervate a moderately large area of skin, and the receptive fields of single axons overlap with each other; the exact location of a stimulus is worked out by the CNS as a result of comparisons of the spatial distribution of activity in a large number of axons.
Overlapping receptive fields in the skin can also be demonstrated when a nerve is cut. The area of skin innervated by a single nerve trunk does not become insensitive following section of one dorsal root, because adjacent roots also innervate that area. Spatial discrimination may change, but the fact remains that the areas of skin innervated by single axons or nerve trunks overlap with others.
Rate of adaptation
- A tonic (or slowly adapting) receptor is a sensory receptor that continues to produce action potentials throughout the duration of the natural stimulus. As a result the signals arriving at the CNS provide information about the duration and intensity of the stimulus. Some tonic receptors have a resting discharge and indicate a background level of the natural stimulus (such as temperature or stretch.
- A phasic (or rapidly adapting) receptor is a sensory receptor that responds to the application or removal of a natural stimulus but action potentials do not continue throughout the duration of the stimulus. Instead, the activity reflects the rate of application of the stimulus, and the timing of movements. Phasic receptors are particularly adapted to monitor vibration, and the Pacinian Corpuscle is an example that responds to vibrations particulalry around 200-300 Hz. Other phasic receptor in skin respond optimally to frequencies of around 30 Hz.
The receptor (generator) potential is the depolarising potential produced by the transducers at sensory nerve endings that initiates trains of action potentials in the afferent axon. It can be seen that in phasic (rapidly adapting) receptors the generator potential lasts only for the period during which the stimulus is applied.
It can be seen from the diagram that many slowly adapting receptors also show some phasic activity at the time the stimulus is applied. Whereas in slowly adapting (tonic) receptors the generator potential is maintained throughout the natural stimulus.
Some receptors monitor events inside the body : interoceptors include baroreceptors, chemoreceptors and osmoreceptors that sense arterial pressure, arterial blood gases or the osmolality of blood. This information is used to regulate the functions of internal organs, but does not enter consciousness.
Others, in muscle tendons and joints, are proprioceptors, responsible for the sense of kinaesthesia- the sense of position of the limbs.
The intensity of a natural stimulus is signalled by the frequency of action potentials generated during the tonic phase of their discharge.
In rapidly adapting receptors, the rate of discharge of action potentials is related to the change in the generator potential, which is dependent on the rate of application of the stimulus.
In slowly adapting receptors, the generator potential is maintained and that causes a train of impulses to occur in which the rate of discharge is related to the depolarisation. Most tonic, slowly adapting receptors, show some phasic activity related to the speed of onset of the stimulus.
Classification of axons according to diameter and conduction velocity Top
Axons may be classified according to their diameter or their conduction velocity: for myelinated axons, there is a relationship between the two, and as a rule of thumb, the conduction velocity in m/sec is roughly the six times the axonal diameter.
The two forms of classification use different symbols: Groups A, B and C based on diameter, and Groups I, II, III and IV, based on conduction velocity. The former has been used in studies of skin nerves, and the latter in studies of muscle nerve afferents.
Classification by diameter : the A, B, C system
A fibres have large diameters, and C fibres are amall unmyelinated axons.
The A fibres are divided into four groups:
- A alpha
- A beta
- A gamma
- A delta
A fibres have diameters between 2 (A delta) and 20 A alpha
C fibres are unmyelinated (diameters of 0.2-1 micrometers)
Examples are ;
- alpha motoneurones
- gamma efferents
- A-delta and C-fibre nociceptors
Classification by Conduction Velocity: the I, II, II, IV system
Myelinated fibres in Muscle afferent nerves are divided into four goups depending on conduction velocity
- Group I - these are subdivided into Ia and Ib
- Group II
- Group III
- Group IV
Group I have higher conduction velocities than the other groups, with Group IV being the slowest.
- Ia - Muscle spindle primary endings
- Ib - Golgi Tendon Organs
- II- muscle spindle secondary endings
- III- pressure pain receptors in muscle
- IV- nociceptors