The Senses

sensesOur senses provide the input information that the brain needs to process a mental picture of the world and our surroundings.

Although, we typically define humans as only having five senses, as originally defined by Aristotle, medically it is agreed that there are at least nine different senses in humans. The relative proportion of the motor-sensory cortex, which lies between the frontal and occipital lobes, given over to our senses is reflected in the diagram called the motor-sensory Homuculus.

The word 'homunculus' is derived from the Latin meaning 'little human' and, in this context, it is attempting to show that if the human body were to be built in proportion to the amount of sensory and motor brain power needed to control them, the hands and mouth would be proportionally bigger, as shown. However, it is unclear as to the accuracy of this particular model, as you might expect the importance of sight to correspond to much bigger eyes. Still, it hopefully represents the general idea.

Sight

Stimulus: Light Waves
Sense Organ: Eye
Receptor: Rods and Cones of the Retina
Sensation: Colours, Patterns, Textures, Motion, Field Depth

The human eye is the organ that helps provide the sense of sight, which allows us to assimilate more about the surrounding world than any other sense. The eyes help see and interpret the shapes, colours, and dimensions of objects in the world by processing the light they reflect or emit. The eye is able to see in bright light or in dim light, but it cannot see objects when light is absent.

The process of vision allows light waves from an object to enter the eye first through the clear cornea and then through the pupil, the circular opening in the iris. The light waves are converged first by the cornea, and then further by the crystalline lens, to a point located immediately behind the back surface of the lens. At that point, the image becomes inverted. In the retina, light impulses are changed into electrical signals and then sent along the optic nerve and back to the occipital lobe of the brain, which interprets these electrical signals as visual images. Therefore, we do not 'see' with our eyes but, rather, with our brains; as our eyes merely assist in the conversion of light into electrical impulse, which then results in the 'visual picture' formed within the brain.

Hearing

Stimulus: Sound Waves
Sense Organ: Ear
Receptor: Hair Cells of the Inner Ear
Sensation: Noises, Tones

In human beings, hearing is performed by the ears, which also perform the function of balance, a sense in itself but not one traditionally listed. This is in common with most mammals. Normal human ears are said to be sensitive in the range of frequency of 30Hz to 12KHz. Some individuals are able to hear up to 22KHz, and high-quality sound reproduction equipment often goes up to 16KHz or even beyond. Frequencies within the range of human hearing are called audio. Frequencies higher than audio are called ultrasonic, while frequencies below audio are called infrasonic. There is some evidence of human ability to unconsciously detect ultrasound and infrasound. Infrasound has been found to affect the emotions.

Touch

Stimulus: External Contact
Sense Organ: Skin
Receptor: Nerve Endings in the Skin
Sensation: Touch, Pain, Warmth, Cold

Tactition is the sense of pressure perception. This definition is the one that differs the most from Aristotle's model, as it specifically excludes the perception of pain and temperature. Even within the limited field of pressure, there is still disagreement as to how many distinct senses there actually are. In the skin, for example, there are different receptors responsible for the detection of light versus heavy pressure, as well as brief versus sustained pressure. Adding to the complexity is the fact that there are also distinct receptors that detect pressure in the visceral organs, e.g. a full stomach, and endocrinal receptors that cause the feeling of tension, e.g. anxiety or excessive caffeine consumption.

Thermoception is the sense by which an organism perceives temperature. In larger animals, the skin does most thermoception. The details of how temperature receptors work is still being investigated. Mammals have at least two types of sensor; those that detect heat, i.e. temperatures above body temperature, and those that detect cold, i.e. temperatures below body temperature.

Smell

Stimulus: Volatile Substances
Sense Organ: Nose
Receptor: Hair Cells of Olfactory Membrane
Sensation: Odours (musky, flowery, burnt, mint)

Olfaction, the sense of smell, is the detection of chemicals dissolved in air. In vertebrates it is located in the nose. The importance and sensitivity of smell varies among different organisms; most mammals have a good sense of smell, whereas most birds do not. Among mammals, it is well developed in the carnivores, who must always be aware of each other, and in those, such as moles, who smell for their food. It is less well developed in the primates. Mammals generally have about 1000 genes for odour receptors. Each receptor cell in the nose expresses one of these genes. Each gene is expressed by thousands of cells, whose axons converge in the olfactory bulb. Humans have 347 functional odour receptor genes; this number was determined by analysing the genome in the Human Genome Project, although the number may vary among ethnic groups, as well as individuals.

Taste

Stimulus: Soluble Substances
Sense Organ: Tongue
Receptor: Taste Buds of the Tongue
Sensation: Flavours (sweet, sour, salty, bitter)

Taste is the direct detection of chemical composition, usually through contact with chemoreceptor cells. Taste is very similar to the sense of smell, in which chemical composition is detected by chemoreceptors. In humans, the sense of taste is conveyed via three of the twelve cranial nerves.

Vestibular Sense

Stimulus: Acceleration and Gravitational Forces
Sense Organ: Inner Ear
Receptor: Hair Cells of semicircular canals and vestibule
Sensation: Spatial Movement, gravitational pull

The vestibular sense interprets cues from gravity and acceleration. In practice, those cues are equivalent, meaning that the vestibular organ cannot tell them apart, which is not really so surprising given Einstein's equivalence principle. The vestibular organ is in the inner ear, but it has nothing to do with hearing. It consists of three semicircular canals filled with fluid. That fluid moves under acceleration or deceleration, but not under constant velocity. That fluid movement provides the cues for perceptions about gravity and acceleration.

Kinesthesis

Stimulus: Body Movement
Sense Organ: Muscles, Tendons and Joints
Receptor: Nerve Fibres in muscles, tendons and joints
Sensation: Movement and position of body parts

The kinesthetic sense receptors are located in the joints, muscles, and body. The kinesthetic sense allows us to locate parts of our bodies without having to see them. Adolescents, in the midst of growth spurt, may show clumsiness caused by the fact that they are growing so fast their kinesthetic sense has not had time to adjust to their new body dimensions. A more dramatic example of the kinesthetic sense is a phantom limb. Many amputees report that they can still feel their limbs even though they have been removed. Typical sensations are itching or burning pains. The limb is gone, but the brain still thinks it is there because kinesthetic senses are still being triggered.

Pain

Nociception is the term sometimes used to refer to the perception of physiological pain. Pain in this context can be defined as a harmful stimulus that signals tissue damage. So despite its unpleasantness, pain is nonetheless an important sense and a vital component of the body's defence system.

The very unpleasantness of pain encourages an animal to disengage from the action causing the pain. The interpretation of pain occurs in the brain, primarily in the thalamus. Interestingly, the brain cannot experience pain, thus headache is not pain in the brain itself. Some evolutionary biologists have speculated that this lack of nociceptive tissue in the brain might be due to the fact that any injury of sufficient magnitude to cause pain in the brain will incapacitate the organism and prevent it from taking appropriate action, which is the actual purpose of pain.