The sensory system consists of sensory receptors at the peripheral endings of afferent neurones, the ascending pathways in the spinal cord and the brain centres responsible for sensory processing and perception. Hence, it spans both the central nervous system (CNS) and the peripheral nervous system (PNS).
Sensory receptors exist internally and externally around the body and are activated via different stimuli. They are designed to respond to the body’s interaction with the external environment or its internal state. Sensory receptors are specialised to respond to stimuli of the following: light, chemical, mechanical, thermal or nociceptive stimuli. The activated sensory receptor generates an action potential, which propagates along the axon to reach the CNS.
In this article, we shall consider the different types of sensory receptors and some of their properties.
A stimulus to a sensory receptor results in a change in cell wall’s ionic permeability, which leads to a generation of an action potential. This process of converting a sensory signal into an electrical signal is known as sensory transduction. In general, a higher intensity stimulus will generate a higher frequency of action potentials along the neuron. However, different types of receptors will adapt to prolonged stimulation in different ways:
- Tonic receptors are slow adapting receptors. They will respond to the stimulus as long as it persists, and produce a continuous frequency of action potentials. Hence, they convey information about the duration of the stimulus.
- Phasic receptors are rapidly adapting receptors. They will respond quickly to stimuli but stop responding upon continual stimulation. Therefore, action potential frequency decreases during prolonged stimulation. This class of receptor conveys information about the changes to the stimulus such as intensity.
Sensory modalities can be thought of as subtypes of sensory experiences, such as pain, temperature, pressure etc. Each sensory modality is perceived by a class of specialised receptors:
Nociceptors are receptors, which respond to noxious stimuli (stimuli that would cause tissue injury if they were to persist) and their activation results in the sensation of pain.
The receptors are free nerve endings, found on the ends of the type Aδ fibres and type C fibres that transmit the pain sensation.
These are further categorised into:
- Mechanical – stimulated by the distension of skin (stretch) and pressure eg. in inflammation
- Thermal – stimulated by extremities of temperature
- Chemical – stimulated by exogenous and endogenous chemical agents, such as prostanoids, histamines etc
- Polymodal – can respond to more than one stimuli
More information about pain pathways can be found here.
These are located in joint capsules, ligaments, tendons, muscle and skin, and respond to deformation by the means of pressure, touch, vibration or stretch.
- Merkel’s discs: These are tonic receptors present in skin, near the border of the dermis and epidermis. They respond to pressure and are particularly sensitive to edges, corners and points. They play a key role in differentiating textures.
- Meissner’s corpuscles: These are phasic receptors present in the dermis of the skin, namely on the palms of the hands, soles of the feet, lips and tongue. They detect the initial contact with objects or slipping of the objects held in hand.
- Pacinian corpuscles: These are phasic receptors are present in the dermis, hypodermis, ligaments and external genitalia. They respond to high-frequency vibration and are crucial in our ability to detect vibrations transmitted through objects in our hands.
- Ruffini corpuscles: These are tonic receptors present in the dermis, ligaments and tendons. They are the least understood of the mechanoreceptors. They respond to stretch and signal position and movements of fingers.
- Muscle spindles and Golgi tendon organs: These exist in skeletal muscle and detect stretch. The detected stimuli contribute to the generation of reflexes at the level of the spinal cord. Additionally, the signals are transmitted to the dorsal column medial lemniscal pathway (DCML), subsequently reaching the cerebral cortex and providing information about posture, position and orientation of limbs and joints in space – proprioception.
Thermoreceptors are found within the skin, liver, skeletal muscle and hypothalamus. They respond to changes in temperature. Those responding to warm temperatures are present within type C fibres, whereas those responding to cold are present within of both type C and type Aδ fibres. Cold thermoreceptors are thought to be around three times more common than those responding to warmth.
A warm stimulus results in both an increase in firing for warm receptors and a decrease in firing for cold receptors (and vice versa for cool stimuli). Initially, the response of receptors changes very quickly based on minute temperature changes, however, after a time, this phasic activity switches to a tonic response, allowing adaptation to the temperature if it remains constant.
Temperature stimuli are transmitted to the central nervous system via the lateral spinothalamic tract, a part of the anterolateral system. Further information on the ascending pathways, including this system, can be found here.
All modalities have different qualities that can be felt. For example, pain can be sharp/aching, the temperature can be hot/warm/cold. The quality felt depends on the subtype of the activated receptor, whereas its intensity depends on the strength of the stimulus.
Clinical Relevance – Hyperalgesia
Hyperalgesia is an increased sensitivity to pain, which may be due to damage to nociceptors or peripheral nerves. It is present in many disease states, for example, in inflammation and it may act as physiological protection from further damage.
Primary hyperalgesia is thought to be due to nociceptor sensitisation and occurs in diseases such as rheumatoid arthritis. Secondary hyperalgesia is thought to be a result of central sensitisation and is generally found in neuropathic pain.
It is also important to be aware of hyperalgesia as it may be opioid-induced in some patients and is a common reason for a reduction in the efficacy of opioid-based medications for pain relief. As it is distinct from opioid tolerance, increasing the dose of these medications does not improve the pain, but worsens it, increasing the sensitivity even further.