General senses

Sensory receptors

Sensory receptors are specialized structures that detect and convert environmental stimuli into nerve impulses. They are classified according to the type of stimulus they respond to: mechanoreceptors detect physical forces like touch and pressure, thermoreceptors sense temperature changes, nociceptors (free nerve endings) detect pain, chemoreceptors react to chemical stimuli, and photoreceptors in the retina are sensitive to light. The process of transduction involves converting these stimuli into electrical signals (action potentials), which are then transmitted through sensory neurons to the central nervous system for processing and interpretation.

Sensory receptors exhibit adaptation, a process where their response to a constant stimulus diminishes over time, allowing the nervous system to prioritize new or changing stimuli. This adaptation can be fast, as in the case of touch receptors, or slow, as observed in nociceptors. Sensory information is further refined through spatial and temporal summation. Spatial summation occurs when multiple receptors are stimulated simultaneously, amplifying the overall signal. Temporal summation, on the other hand, involves repeated stimulation of a single receptor, increasing the intensity of the signal over time. Together, these mechanisms enable the nervous system to efficiently interpret a wide range of sensory inputs.

Mechanoreceptive somatic senses

Mechanoreceptive somatic senses are responsible for detecting mechanical stimuli such as touch, pressure and vibration. These sensations are detected by specialized receptors in the skin and deeper tissues that respond to physical deformation. Key receptors include tactile (Meissner’s) corpuscles, which detect light touch and texture, Merkel discs (also called tactile menisci or epithelial cells), which sense sustained pressure and fine detail and bulbous (Ruffini) corpuscles, which respond to skin stretch. Lamellar (Pacinian) corpuscles, located deeper in the skin, are particularly sensitive to vibration and rapid changes in pressure. Each of these mechanoreceptors transforms mechanical stimuli into electrical signals, which are then transmitted via sensory neurons to the central nervous system.

Pain and thermal sensations

Pain and thermal sensations are essential components of the body's sensory system, enabling the detection of harmful stimuli and temperature changes. Pain, or nociception, is detected by nociceptors (free nerve endings), which are specialized sensory receptors that respond to potentially damaging stimuli such as extreme heat, cold, mechanical injury, or chemical irritants. Nociceptors are found throughout the skin, muscles, joints and internal organs. When activated, they transmit signals to the brain, where they are interpreted as pain, helping the body to react and avoid further damage. Pain can be categorized into two main types:

1. Fast, sharp pain: This immediate and localized pain is mediated by A-delta fibers, which are thin, myelinated fibers that rapidly transmit signals to the brain, enabling a quick response to acute injury.
2. Slow, aching pain: This prolonged, diffuse pain is transmitted by C fibers, which are unmyelinated and conduct signals more slowly, providing a lingering awareness of injury and promoting protective behavior to aid healing.

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Thermal sensations are detected by thermoreceptors, specialized sensory receptors in the skin that respond to changes in temperature. There are two main types of thermoreceptors: cold receptors and warmth receptors. Cold receptors are activated by cooler temperatures, typically between 10°C and 35°C (50°F and 95°F), while warmth receptors respond to higher temperatures, usually between 30°C and 45°C (86°F and 113°F). 

When temperatures exceed normal thresholds, nociceptors may be activated, leading to sensations of pain, such as the burning sensation from extreme heat or the stinging pain from extreme cold. This interaction between thermal and pain sensations helps the body maintain homeostasis and protect itself from environmental hazards.

Somatosensory pathways

Somatosensory pathways are the neural routes that transmit sensory information from the body to the brain for interpretation. They convey signals related to touch, pressure, pain, temperature and proprioception from sensory receptors in the skin, muscles and joints to the central nervous system. There are two main somatosensory pathways: the posterior column-medial lemniscus (PCML) pathway and the spinothalamic tract.

The posterior column-medial lemniscus pathway transmits signals related to fine touch, vibration and proprioception. Sensory information enters the spinal cord and ascends through the dorsal columns to the medulla, where it synapses and decussates (crosses over) to the opposite side of the body. From the medulla, the signals continue to the thalamus and finally reach the somatosensory cortex in the brain for interpretation.

In contrast, the spinothalamic tract conveys information about pain, temperature and crude touch. Sensory signals in this pathway enter the spinal cord, cross to the opposite side almost immediately, and ascend through the spinal cord to the thalamus. From the thalamus, the signals are relayed to the somatosensory cortex.

These pathways ensure that sensory information is accurately processed, allowing the brain to perceive and respond to environmental changes, essential for reflexes, motor coordination and spatial awareness.

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Articles within this topic:

• Ruffini corpuscle
• Sensory receptors