Sensory Receptors

Somatic Nervous Pathway:

Types of Sensory Receptor:

• Somatic Sensory Receptors - modified or free nerve endings of sensory neurones.

• Special Sense Receptors - receptor cells which synapse with the sensory neurone; they release neurotransmitters to stimulate an AP.

Sensory receptors are located throughout the body, and are activated by specific stimuli.

Mechanoreceptors:

Mechanoreceptors are found mostly in the skin and some visceral organs, such as the heart. They react to physical distortion (e.g. touching the skin).

• Meissner’s Corpuscles - glabrous (smooth), with a low threshold for touch.

• Merkel’s Disk - glabrous, with a low threshold for static touch.

• Ruffini’s Corpuscles - glabrous and hairy, with a high threshold for stretching.

• Pacinian Corpuscles - largest and deepest.

• Free Nerve Endings - glabrous and hairy, with a very high threshold for touch.

Meissner’s and Pacinian Corpuscles are rapidly adapting; action potentials fire at the onset and offset of the stimulus only.

Merkel’s disks and Ruffini’s endings are slowly adapting; action potentials fire throughout the presence of the stimulus.

Types of Receptor:

• Thermoreceptors - found primarily in the skin, and respond to changes in temperature. They use two main free nerve ending ion channels: TRPV1, which detects temperature increase, and TRPM8, which detects temperature decrease.

• Nociceptors - found primarily in the skin, and respond to tissue-damaging stimuli. There are a few types of free nerve ending nociceptors: mechanical, chemical, and thermal.

• Proprioceptors - found in the muscles, tendons, ligaments, and joints. They respond to muscle tension and have three main types:

  • Muscle Spindles, such as stretch receptors.

  • Golgi Tendon Organs, which are responsible for muscle tension.

  • Proprioceptors in Joints - responsible for angle, direction, and velocity of movement in joints.

Types of Sensory Neurone:

• Aa - proprioception.

• AB - touch.

• AY - temperature and pain.

• C - temperature and pain.

Receptive Fields:

The receptive field is the area in which the specific stimulus will activate the sensory receptor.

• There is a high density of smaller receptive fields, which means that there is a higher spatial resolution of the stimulus.

  • For example, fingers and lips are highly discriminative, whereas the back, legs and arms are less discriminative.

Sensory Transduction:

Sensory transduction is the process of converting stimulus energy into electrical signals.

• The activation of receptors causes the movement of ions, which leads to a change in membrane potential.

Primary Afferent Neurones:

Axons of neurones which bring in information from the somatic sensory receptors are known as primary afferent neurons, and they transmit sensory information to the central nervous system for processing.

• Primary afferent neurones have their cell body in the dorsal root ganglia.

  • Their axons project into the dorsal horn of the spinal cord, through the dorsal roots.