Sensing

Senses are transducers from the physical world to the realm of the mind. Another broadly acceptable definition of a sense is: A system that consists of a group of sensory cell types, responding to a specific physical phenomenon, and corresponding to a particular group of regions within the brain where the signals are received and interpreted. 

Disputes about the number of senses typically arise around classification of the various cell types, and their mapping to regions of the brain.

Sensory Modalities

A sensory modality (also called a stimulus modality) is an aspect of a stimulus or what is perceived after a stimulus. The term sensory modality is often used interchangeably with sense. The basic sensory modalities include: light, sound, taste, temperature, pressure, and smell.

Light Modality

The sensory modality for vision is light. To perceive a light stimulus, the eye must first refract the light so that it directly hits the retina. The transduction of light into neural activity occurs via the photoreceptors in the retina. 

When a particle of light hits the photoreceptors of the eye, the photopigment of the photoreceptor undergoes a chemical change leading to a chain of chemical reactions occur. A message is sent to a neuron called the bipolar cell through the use of a nerve impulse. Finally, a message is sent to the ganglion cell and then, finally, the brain.

Sound Modality

The sensory modality for audition is sound. Sound is created through air pressure. A vibrating object compresses the surrounding molecules of air as it moves towards a given point, and expands the molecules as it moves away from the point. 

The eardrum is stimulated by vibrations in the air. It collects and sends these vibrations to receptor cells. The ossicles (three tiny bones in the middle ear) pass the vibrations to the fluid-filled cochlea (a spiral, shell-shaped auditory organ of the inner ear). The vibrations move through the liquid in the cochlea where the receptive organ is able to sense it.

Taste Modality

Taste stimuli are encountered by receptor cells located in taste buds on the tongue and pharynx. Receptor cells disseminate onto different neurons and convey the message of a particular taste in a single medullar nucleus. 

Taste perception is created by combining multiple sensory inputs. Different modalities help determine the perception of taste.

Temperature Modality

Temperature modality excites or elicits a symptom through cold or hot temperature. The cutaneous somatosensory system detects changes in temperature. 

Thermal stimuli from a homeostatic set point excite temperature specific sensory nerves in the skin. Specific thermosensory fibers respond to warmth and to cold. 

Pressure Modality

Tactile stimulation can be direct, such as through bodily contact, or indirect, such as through the use of a tool or probe. Tactual perception gives information regarding cutaneous stimuli (pressure, vibration, and temperature), kinesthetic stimuli (limb movement), and proprioceptive stimuli (position of the body). 

Smell Modality

The sense of smell is called olfaction. Materials constantly shed molecules, which float into the nose or are taken in through breathing. Inside the nasal chambers is the neuroepithelium lining. 

It contains the receptors responsible for detecting molecules that are small enough to smell. These receptor neurons then synapse at the olfactory cranial nerve, which sends the information to the olfactory bulbs in the brain for initial processing. 

Multimodal Perception 

Multimodal perception is the ability of the mammalian nervous system to combine all of the different inputs of the sensory system to result in an enhanced detection or identification of a particular stimulus. 

Integration of all sensory modalities occurs when multimodal neurons receive sensory information that overlaps with different modalities. Multimodal perception comes into effect when a unimodal stimulus fails to produce a response.

Multisensory perception

This is a diagram of how multimodal perception is created by the overlapping and combining of different inputs from the sensory systems.

This is a diagram of how multimodal perception is created by the overlapping and combining of different inputs from the visual, auditory, and somatosensory systems, each labeled in its own box. Inside each box are the organs and their locations that are connected to the sensory system, with all three boxes being connected to a red dot in the center, labeled multisensory.

Additional Senses

Balance (or equilibrioception) is the sense that allows an organism to sense body movement, direction, and acceleration, and also attain and maintain postural equilibrium and balance. The organ of equilibrioception is the vestibular labyrinthine system found in both of the inner ears. 

In technical terms, this organ is responsible for two senses: angular momentum and acceleration (known together as equilibrioception). The vestibular nerve conducts information from sensory receptors in three ampulla, each of which sense fluid motion in three semicircular canals caused by a three-dimensional rotation of the head. 

The vestibular nerve also conducts information from the utricle and the saccule; these contain hair-like sensory receptors that bend under the weight of otoliths (small crystals of calcium carbonate) that provide the inertia needed to detect head rotation, linear acceleration, and the direction of gravitational force.

Inner ear

Inner ear anatomy showing utricle, saccule, and vestibular nerve.

This is two color drawings. One is a cutaway view of the ear, showing the ear canal and the bony labyrinth. The second is a closeup of the bony labyrinth, with all its parts identified, and the membranous labyrinth outlined within it.

Thermoception is the sense of heat or absence of heat (cold) by the skin and internal skin passages. Perceiving changes in temperature is referred to as heat flux (the rate of heat flow) in these areas. 

There are specialized receptors for cold (declining temperature) and heat. The cold receptors infer wind direction, an important part in the animal's sense of smell. The heat receptors are sensitive to infrared radiation and can occur in specialized organs, for instance in pit vipers. 

The thermoceptors in the skin are quite different from the homeostatic thermoceptors in the brain (hypothalamus), which provides feedback on internal body temperature.

Proprioception, the kinesthetic sense, provides the parietal cortex of the brain with information on the relative positions of the parts of the body. Neurologists test this sense by telling patients to close their eyes and touch their own nose with the tip of a finger. Assuming proper proprioceptive function, at no time will the person lose awareness of where their hand actually is, even though it is not being detected by any of the other senses. Proprioception and touch are related in subtle ways, and their impairment results in deep and surprising deficits in perception and action.

Nociception (physiological pain) signals nerve or other tissue damage. The three types of pain receptors are cutaneous (skin), somatic (joints and bones), and visceral (body organs). 

It was previously believed that pain was simply the overloading of pressure receptors, but research in the first half of the 20th century showed that pain is a distinct phenomenon that intertwines with all of the other senses, including touch.

Chronoception refers to how the passage of time is perceived and experienced. Although the sense of time is not associated with a specific sensory system, psychological and neuro-scientific research indicates that human brains do have a system governing the perception of time. 

It is composed of a highly distributed system involving the cerebral cortex, cerebellum and basal ganglia. One particular component, the suprachiasmatic nucleus, is responsible for the circadian (daily) rhythm, while other cell clusters appear capable of shorter-range (ultradian) timekeeping.