Human Physiology

Sensory Receptors are nerve endings containing transducers that respond to a stimulus of a specific nature (mechanical, vibratory, thermal, or chemical).

OVERVIEW. Conscious animals and humans perceive sensations including touch, vibration, position of the limb, temperature, sight and sound that inform them about events in their surroundings, the position of their limbs, and if they are touching objects in their environment. These different sensations are called 'modalities', and sensations occur because messages, trains of action potentials, are conducted from sensory receptors in skin, muscles tendons and joints, and the special sense organs- the eye and the ear.

* The diagram shows the mechanical stimulus to a Pacinian corpuscle, and the generator potential (depolarisation) in the nerve ending. This is a rapidly adapting receptor and responds to the onset and removal of the mechanical stimulus.

Sensory Receptors
Sensory Receptors are transducers in nerve endings that respond to a stimulus of a specific nature (mechanical, vibratory, thermal, or chemical). They signal the intensity of the stimulus by producing a generator potential (a depolarisation in the nerve ending) that increases the number of impulses generated per second as a function of the strength of the stimulus.
The depolarisation that occurs during the generator potential causes action potentials to occur because voltage-gated sodium channels open. Sensory receptors signal the intensity of the stimulus by increasing the number of impulses generated per second.
The term ‘adequate stimulus’ is used to describe the type of stimulus to which a sensory receptor is most sensitive. The 'threshold' is the intensity (strength) of stimulus that just manages to cause the receptor to become active, and is a useful measurement in that it tells you about how sensitive it is.
Sensory receptors are classified as mechanoreceptors, thermoreceptors (hot or cold), chemoreceptors (pO2, pH) and nociceptors.
Nociceptors are sensory endings that signal the presence of intense injurious stimuli, such as very large forces or extremes of temperatures that would give rise to the conscious sensation of pain.

* The graph shows the relationship between the size of the mechanical stimulus and the frequency of action potentials generated by a mechanoreceptor. The graph shows the mechanical threshold - the amount of force required to just excite the sensory receptor.

Threshold
Mechanoreceptors and nociceptors can both respond to mechanical stimuli (applied force), but the forces required to activate nociceptors are very much greater than for mechanoreceptors. The mechanoreceptors are said to have a low threshold, and nociceptors a high threshold to mechanical stimuli. Similarly thermoreceptors can sense small changes in temperature, but nociceptors have a higher threshold, and are activated when the temperature rise or fall causes tissue damage.
Other types of sensory receptor include sensors of the internal environment such as arterial baroreceptors and chemoreceptors, and distance receptors such as the eye and ear. Arterial baroreceptors and chemoreceptors do not give rise to a conscious sensation but are used to induce reflex changes.
The Specificity Theory states that peripheral receptors respond optimally to a specific type of stimulus; subsequent neurones in sensory pathways continue to maintain specificity in their afferent connections. Thus there are neural pathways in the CNS concerned with sensation that carry information from a single type of peripheral sensory receptor, and these are spectacularly accurate in transmitting information about the intensity of the stimulus.
As a consequence of the specificity of these pathways, the primary somatosensory receiving area of the cortex is divided into strips, each devoted to a major class of sensory receptor from skin, muscles, tendons and joints.

Threshold

Mechanoreceptors and nociceptors can both respond to mechanical stimuli (applied force), but the forces required to activate nociceptors are very much greater than for mechanoreceptors.

The mechanoreceptors are said to have a low threshold, and nociceptors a high threshold to mechanical stimuli.

Similarly thermoreceptors can sense small changes in temperature, but nociceptors have a higher threshold, and are activated when the temperature rise or fall causes tissue damage.

Other types of sensory receptor include sensors of the internal environment such as arterial baroreceptors and chemoreceptors, and distance receptors such as the eye and ear.

Arterial baroreceptors and chemoreceptors do not give rise to a conscious sensation but are used to induce reflex changes.

The Specificity Theory states that peripheral receptors respond optimally to a specific type of stimulus; subsequent neurones in sensory pathways continue to maintain specificity in their afferent connections.

Thus there are neural pathways in the CNS concerned with sensation that carry information from a single type of peripheral sensory receptor, and these are spectacularly accurate in transmitting information about the intensity of the stimulus.

As a consequence of the specificity of these pathways, the primary somatosensory receiving area of the cortex is divided into strips, each devoted to a major class of sensory receptor from skin, muscles, tendons and joints.

Adaptation
Adaptation is the term used to describe the time course of response of a sensory receptor to a natural stimulus.
If a physical stimulus of constant intensity is applied to a sensory receptor, some receptors respond only during the onset of the stimulus (and sometimes when the stimulus is withdrawn): these are rapidly adapting or phasic receptors, and monitor vibration and movement, as well as the timing of the onset of a stimulus.
Slowly adapting receptors generate trains of action potential throughout the period of natural stimulation, and the spike rate is a measure of the intensity of the stimulus.
The relationship between the intensity of the stimulus and the spike rate generated is not linear but follows a power function.
Many slowly adapting receptors also have a phasic response at the start of the natural stimulus. Rapidly adapting receptors respond to the onset of movement and to vibration and signal the timing and speed of movement.
The generator potentials of rapidly adapting receptors are short-lasting, whereas in slowly adapting receptors, the generator potential is prolonged, lasting the duration of the stimulus. Also the size of its depolarisation is a function of the stimulus strength.

Adaptation

Adaptation is the term used to describe the time course of response of a sensory receptor to a natural stimulus.

If a physical stimulus of constant intensity is applied to a sensory receptor, some receptors respond only during the onset of the stimulus (and sometimes when the stimulus is withdrawn): these are rapidly adapting or phasic receptors, and monitor vibration and movement, as well as the timing of the onset of a stimulus.

Slowly adapting receptors generate trains of action potential throughout the period of natural stimulation, and the spike rate is a measure of the intensity of the stimulus.

The relationship between the intensity of the stimulus and the spike rate generated is not linear but follows a power function.

Many slowly adapting receptors also have a phasic response at the start of the natural stimulus. Rapidly adapting receptors respond to the onset of movement and to vibration and signal the timing and speed of movement.

The generator potentials of rapidly adapting receptors are short-lasting, whereas in slowly adapting receptors, the generator potential is prolonged, lasting the duration of the stimulus. Also the size of its depolarisation is a function of the stimulus strength.

Structure and Function of Sensory Receptors
Sensory receptors with myelinated axons usually have a specialised structure at their peripheral endings. Most of these specialised endings are encapsulated.
There are two types that respond optimally to frequencies of vibration around 30 Hz and 200 Hz; these are the Meissner's corpuscles and Pacinian corpuscles seen in histological sections. * Diagram of a Pacinian Corpuscle showing lamellated structure like a small onion.
Unmyelinated axons are common in the peripheral nervous system and these sensory axons are concerned with nociception – signalling the presence of intense injurious stimuli
Nociceptors have free nerve endings – bare endings that, unlike the myelinated axons, do not terminate in encapsulated endings. Although they lack specific structural features, these endings do show some specific properties, such as sensitivity to large forces, or heat, or cold, or chemicals released within the tissues as a response to injury.
Some nociceptors respond to several forms of intense stimuli and are called 'polymodal' nociceptors.

Structure and Function of Sensory Receptors

Sensory receptors with myelinated axons usually have a specialised structure at their peripheral endings. Most of these specialised endings are encapsulated.

There are two types that respond optimally to frequencies of vibration around 30 Hz and 200 Hz; these are the Meissner's corpuscles and Pacinian corpuscles seen in histological sections.

* Diagram of a Pacinian Corpuscle showing lamellated structure like a small onion.

Unmyelinated axons are common in the peripheral nervous system and these sensory axons are concerned with nociception – signalling the presence of intense injurious stimuli

Nociceptors have free nerve endings – bare endings that, unlike the myelinated axons, do not terminate in encapsulated endings. Although they lack specific structural features, these endings do show some specific properties, such as sensitivity to large forces, or heat, or cold, or chemicals released within the tissues as a response to injury.

Some nociceptors respond to several forms of intense stimuli and are called 'polymodal' nociceptors.

Somatic Sensation
Touch is mediated by several types of sensory receptor. In skin, 4 types of sensory endings mediate the sensation of touch, and each has a different structure, such as the Merkel, Ruffini, Meissner and Pacinian endings, each with its own specific physiological signalling properties.
There are two types of slowly adapting receptors in skin: one responds to indentation, and the other to lateral stretch of the tissue; they correspond to Merkel cells and Ruffini end organs seen in histological sections.
Merkel cells are found in the base of the epidermis, and typically a number of Merkel cells are innervated by one afferent axon.
Ruffini end organs are encapsulated receptors that occur in the superficial layers of the dermis.
Rapidly adapting receptors respond to the onset of movement and to vibration and signal the timing and speed of movement. There are two types that respond optimally to frequencies of vibration around 30 Hz and 200 Hz; these are the Meissner's corpuscles and Pacinian corpuscles seen in histological sections.
In hairy skin, there are rapidly adapting receptors in hair follicles. In specialised hairs such as whiskers, the follicle also contains Merkel Cells which also behave as slowly adapting receptors.

Somatic Sensation

Touch is mediated by several types of sensory receptor. In skin, 4 types of sensory endings mediate the sensation of touch, and each has a different structure, such as the Merkel, Ruffini, Meissner and Pacinian endings, each with its own specific physiological signalling properties.

There are two types of slowly adapting receptors in skin: one responds to indentation, and the other to lateral stretch of the tissue; they correspond to Merkel cells and Ruffini end organs seen in histological sections.

Merkel cells are found in the base of the epidermis, and typically a number of Merkel cells are innervated by one afferent axon.

Ruffini end organs are encapsulated receptors that occur in the superficial layers of the dermis.

Rapidly adapting receptors respond to the onset of movement and to vibration and signal the timing and speed of movement. There are two types that respond optimally to frequencies of vibration around 30 Hz and 200 Hz; these are the Meissner's corpuscles and Pacinian corpuscles seen in histological sections.

In hairy skin, there are rapidly adapting receptors in hair follicles. In specialised hairs such as whiskers, the follicle also contains Merkel Cells which also behave as slowly adapting receptors.

Receptors in Muscle, Tendon and Joints
In muscles, tendons and joints the endings include the muscle spindle, the Golgi tendon organ, and joint position receptors; each of these has different physiologcial properties. The sense of proprioception of kinaesthesia depends on these sensory inputs to the CNS.
Most muscles have substantial numbers of muscle spindles. The are attached to the connective tissues surrounding each muscle fibre and contain thin, modified, fibres of skeletal muscle (intrafusal fibres) that run in parallel with the skeletal muscle fibres. The nerve endings are surrounded by a capusule.
Muscle Spindles signal their length to the CNS using the largest diameter, fastest conducting axons in the body (Ia fibres); these afferent fibres are responsible for the stretch reflex.
The structure of the spindle is complex, but in addition to the Ia fibres there are other sensory endings, and a motor supply to the intrafusal fibres (gamma efferent motoneurones).
Tendons have sensory receptors near the musculo-teninous junction, and these are 'in series' with the muscle fibres, and can monitor the tension produced by a muscle.
Joint receptors occur in the capsule of joints and can monitor the angle of the joint.

Receptors in Muscle, Tendon and Joints

In muscles, tendons and joints the endings include the muscle spindle, the Golgi tendon organ, and joint position receptors; each of these has different physiologcial properties. The sense of proprioception of kinaesthesia depends on these sensory inputs to the CNS.

Most muscles have substantial numbers of muscle spindles. The are attached to the connective tissues surrounding each muscle fibre and contain thin, modified, fibres of skeletal muscle (intrafusal fibres) that run in parallel with the skeletal muscle fibres. The nerve endings are surrounded by a capusule.

Muscle Spindles signal their length to the CNS using the largest diameter, fastest conducting axons in the body (Ia fibres); these afferent fibres are responsible for the stretch reflex.

The structure of the spindle is complex, but in addition to the Ia fibres there are other sensory endings, and a motor supply to the intrafusal fibres (gamma efferent motoneurones).

Tendons have sensory receptors near the musculo-teninous junction, and these are 'in series' with the muscle fibres, and can monitor the tension produced by a muscle.

Joint receptors occur in the capsule of joints and can monitor the angle of the joint.

The Human Brain

SENSORY RECEPTORS