The sensory nervous system uses sensory and motor inputs that are mediated by the peripheral nervous system. The central nervous system helps interpret the data that is received. The sensory system includes a sensory pathway that is made of the following: sensory reception, transduction, amplification and adaptation, transmission, integration, and perception. We will discuss these in further detail.
There is a hierarchical organization of this system that ranges from receptors to executive areas of the nervous system. The sensory system sends out two types of signals: somatosensory and viscerosensory signals. Somatosensory signals respond to stimuli from outside of the body, be that pain, touch, pressure, or temperature. These signals originate at the skin, bones, muscles, and joints of the body. The viscerosensory signals respond to internal stimuli, regarding consciously detectable information such as the stomach or urinary bladder. These signals are sent through neurons of the peripheral nervous system and central nervous system. The primary neurons receive the signal and send the information to the central nervous system. The secondary neurons conduct impulses from the spinal cord or brainstem and send them to the thalamus. Tertiary neurons conduct the impulses from the thalamus to the primary somatosensory cortex found in the cerebral cortex of the cerebrum. Signals are generated from outside the body but fibers are known to pick up the sensation. The fibers will receive stimuli and respond by only one means; pain receptors will only perceive pain whether the stimulus was temperature, chemical, touch, or damage. This concept is known as the “Labeled Line” principle.
Nerve fibers can be classified by two means: general classification (Erlanger-Gasser) and numerical classification. Both of these classifications apply to sensory fibers but general classification applies also to motor fibers. General classification has three types of fibers: Type A, Type B, and Type C. Type A fibers are large and medium-sized myelinated fibers of the spinal nerves. Type A fibers include alpha, perceives somatic motor and proprioception (related to the muscle spindle and Golgi tendon organ), beta, perceives touch and pressure, gamma, perceives motor to muscle spindles, and delta, perceives fast pain, cold, and touch. Type B fibers are small myelinated preganglionic autonomic fibers, these are usually part of the sympathetic division. Type C fibers are the most numerous as small unmyelinated nerve fibers that conduct low-velocity impulses. The numerical classification contains the fibers I (a and b), II, III, and IV. Ia classifies the muscle spindle and Ib classifies the Golgi tendon organ. II classifies the same as the beta in the general classification. III classifies the same as the delta in the general classification. IV classifies slow pain, hot, other receptors (usually mechanoreceptors), and reflex.
Sensory receptors can be classified by their structure. Simple receptors are free nerve endings. These receptors are numerous and are unmyelinated. Complex receptors are ensheathed by a connective tissue capsule where these encapsulated portions are unmyelinated. Special senses receptors contain specialized receptor cells.
Sensory receptors can also be classified by their function. Mechanoreceptors are receptors that are for skin tactile senses, deep tissue senses that include the muscle spindles and Golgi tendon receptors, hearing, equilibrium/balance, and arterial receptors. Muscle spindles are a small encapsulated group of specialized muscle fibers (receptors). The contractile elements are restricted to the ends of these fibers and none are in the middle. The contractile part is innervated by the gamma motor neurons that will result in Type A gamma fibers. The middle part is innervated by sensory neurons which will carry action potentials from the spindle to the central nervous system. Fibers that make up the muscle spindle include the intrafusal fibers and the extrafusal fibers. The intrafusal fibers are found inside the muscle spindle. These fibers are innervated by gamma motor neurons which give rise to Type A gamma fibers. The extrafusal fibers are a large majority of the fibers that allow the muscle to generate force. They can be found attached to the muscle-tendon. The extrafusal fibers are innervated by alpha motor neurons which will give rise to Type A alpha fibers. Also inside of the muscle fibers are stretch receptors which correct the length of the muscle. The Golgi tendon organ helps control muscle tension. This feature provides the nervous system with information on the degree of tension in each segment of each muscle. The Golgi tendon organ causes muscle relaxation and prevents excessive tension on the muscle. Thermoreceptors are receptors that perceive temperature and respond to warm and cold. These receptors are all free nerve endings. Nociceptors are free nerve endings that are receptors for pain. Photoreceptors are receptors for vision and can be described as Rods and cones. Chemoreceptors are receptors for a variety of stimuli: taste, smell, arterial oxygen, osmolality, blood CO2, and blood glucose, amino acids, and fatty acids.
The receptor potential is stimulated by an excitable receptor that changes the membrane electrical potential of the receptor. There are 4 ways in which a receptor potential can be generated: by mechanical stress, by temperature change, by chemical application, and by electromagnetic radiation.
Amplification is the intensity of the stimulus that is changed by the frequency and number of action potentials and receptors activated. The more intense the stimulus above the threshold level is, the greater the frequency of the action potential. The same goes for receptors. The more intense the stimulus, the more receptors are activated.
Sensory adaptation is the ability to diminish the extent of their depolarization despite sustained stimulus strength. Receptors that adapt more than others can be categorized into two areas: tonic receptors (slow) and phasic receptors (fast). Tonic receptors are slow adapting receptors that continue to transmit impulses as long as the stimulus is present. Phasic receptors are stimulated only when the strength of the stimulus changes.
