Sensation and Sensory Processing

Understand stimulus features encoded by the sensory system
List the classification of somatic senses and discuss the somatosensory receptor types

The stimulus features encoded by the sensory system include modality (i.e. the type of sensation), spatial information (where it is), intensity (how strong the stimulus is) and quality (other characteristics, such as the pitch of sound or the sharpness of pain). The types of stimuli that we have receptors for include mechanical, chemical, photic (visual), thermal, and noxious (pain).

There are many different types of receptors for touch that you might need to know about. Free nerve endings are slowly-adapting, unencapsulated receptors that can detect a range of stimuli, such as temperature and pain. Merkel's disks are also slowly-adapting, unencapsulated receptors that sense continuous light touch. There are three main types of nerve endings that are encapsulated (wrapped in glial cells or connective tissue). Meissner's corpuscles are superficial, rapidly-adapting nerve endings that respond to altered touch and spatial characteristics. Ruffini's corpuscles are deep, slowly-adapting nerve endings that respond to heavy touch and stretch of joints. Finally, Pacinian corpuscles are deep, rapidly-adapting nerve endings that respond to deep pressure and vibration. Mechanosensitive receptors all have stretch-gated ion channels that might be excitatory or inhibitory, depending on which ions they let through.

As well as receptors for touch, there are also receptors for temperature. The main ones you need to know about are TRPM8, which detects cold, and TRPV1/3 and TREK1, which detect heat.

Understand detection and transmission of sensory stimuli

Stimuli are detected by specialised cells. Each type of stimulus has a special type of cell- for instance, mechanoreceptors detect mechanical stimuli. Some receptor cells are nerves, whereas others are special cells that can communicate with a neuron. When sensory neurons are activated, they transmit action potentials to the central nervous system (brain and spinal cord). You can read about how action potentials work here.

Some neurons are rapidly-adapting, or phasic. These neurons respond better to dynamic stimuli (i.e. stimuli that are rapidly changing), rather than continuous stimuli. Slowly-adapting (tonic) neurons are the opposite- they respond much better to continuous stimuli than to dynamic stimuli.

Another important aspect of sensation is stimulus acuity, which allows for fine discrimination between two sensations that occur close together. Stimulus acuity depends on the number of receptors and the size of their receptor fields, as well as some other factors. Central convergence, in which multiple receptors can synapse onto the same second-order neuron, can decrease stimulus acuity, whereas lateral inhibition, in which a strongly-activated neuron inhibits surrounding neurons, can increase stimulus acuity.

Discuss the ‘labelled line’ concept of sensory transmission

The "labelled line" principle states that sensory nerve fibres transmit only one modality of sensation to a specific location. Hence, if a pain fibre is stimulated, you will feel pain, no matter what stimulus caused the pain or which receptors picked it up. All the brain cares is that a pain fibre was stimulated.

Understand encoding of sensory stimulus intensity

The more intense a stimulus is, the more rapidly the sensory neurons fire. The higher rate of firing is then picked up by the brain as being a more intense stimulus.

Describe the ascending sensory pathways, the somatosensory cortex and topographic maps

• Nervous System II

Describe nociceptors & nociception

Nociception is the sensation of pain. Nociceptors are receptors that respond to noxious stimuli and give the perception of pain. Some nociceptors are simply normal receptors but with a high threshold. For example, light pressure might activate only the mechanoreceptors that let you sense that you are experiencing pressure. However, as the pressure increases, nociceptors responsible for pain may then reach their threshold, so you are now experiencing pain. There are also chemicals that are able to stimulate nociceptors directly, such as histamine.