Introduction The knowledge of the world around us stimulates our sensory organs to provide us with the information of what is going on around us. All sensory information is picked up by the sensory receptors, specialised cells that monitor internal and external conditions. Examples of sensory organs are: SENSE ORGAN Eye Ear (Organ of Corti) Ear (Semicircular Canals) Ear (utricle and saccule) Olfactory mucous membrane Taste Buds Skin Skin Skin Skin Various Muscle Spindle Golgi tendon organ
RECEPTOR Rods and Cones Hair Cells Hair Cells Hair Cells Olfactory Neurons Taste Receptor Cells Nerve Endings Nerve Endings Nerve Endings Naked Nerve Endings Nerve Endings Nerve Endings Nerve Endings
SENSORY MODE Vision Hearing Rotational acceleration Linear acceleration Smell Taste Touch – Pressure Warmth Cold Pain Joint Movement and Position Muscle Length Muscle Tension
All sensory information arrives at the central nervous system (CNS) in the form of action potentials and the stronger the stimulus, the higher the frequency of action potentials. For example, touch, pressure, pain, temperature and taste sensations arrive at the primary sensory cortex. The CNS interprets the nature of the sensory information entirely on the basis of the area of the brain stimulated, it cannot tell between a ‘true’ sensation or a ‘false’ one. For example, when you rub your eyes, you may ‘see’ flashes of light. The stimulus was mechanical rather than visual; the activity was projected to the visual cortex and was experienced as a visual perception. Adaptation is a reduction in sensitivity in the presence of a CONSTANT stimulus. For e.g. ticking of the clock in the background, jumping into a hot bath or cold lake. After a short time, the body reduces the amount of information arriving at the cerebral cortex. Most sensory information is routed towards the spinal cord and triggers involuntary reflexes such as withdrawal reflex e.g. removing the hand from a hot stove. We can also adjust our sensitivity under conscious or unconscious direction such as ‘listening carefully’ which will increase our auditory sensitivity.
The skin over the entire body is supplied by somatic sensory nerves that carry nerve impulses from the skin to the CNS. Each spinal nerve contains sensory nerves that serve a specific area of the body. One of the cranial nerves (trigeminal nerve V) serves most of the skin of the face and scalp. The area of the skin that provides sensory information to the brain is called a ‘dermatome’. Knowing which dermatome goes to which spinal cord segment, would make it easier to locate the damaged regions of the spinal cord. For example, if the skin in a particular area is stimulated but you could not feel it, the nerves supplying that dermatome are probably damaged. There are overlaps in dermatome so if only one nerve supplying the dermatome is damaged then there will only be a small loss of sensation.
Sensations from the skin, body wall, muscles, bones, tendons and joints are termed ‘somatic sensation’. Some respond to mechanical stimulation of the skin, hairs and underlying tissues. There are distinct receptors for heat, cold, touch, pressure, limb position and pain. When the sensory nerves enter the CNS, they synapse neurons via the brain stem then the thalamus heading towards the ‘somatosensory cortex’. Note that the pathways cross from the side where the sensory nerves enter the CNS to the opposite side of the spinal cord or brain stem. Thus somatic receptors on the left side of the body go to the somatosensory cortex of the right cerebral hemisphere. The parts of the body that are most densely innervated are the fingers, thumb and lips and are represented by the largest areas of the somatosensory cortex. There is a considerable overlap of the body part representations. Within the broad areas, no one area exclusively controls the movement of a single body region.
The cerebral cortex plays a critical role...