13: Somatic Senses, Integration and Motor Responses
The somatic nervous system (SNS) is the part of the peripheral nervous system responsible for our conscious perception of the environment through general and special senses as well as for our voluntary motor responses to that perception by means of skeletal muscles. In this chapter, you will look at the peripheral sensory structures that receive input from environmental stimuli, how the information is integrated in the CNS and at the motor neurons of the CNS that react to those stimuli sending motor responses to skeletal muscles.
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- 13.1: Introduction to the Somatic Nervous System
- The somatic nervous system includes all the afferent, sensory receptors from the skin, skeletal muscles, joints and special senses which are responsible for our conscious perception of the environment. It also includes the efferent, motor division which reaches the skeletal muscles and is responsible for our voluntary motor response.
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- 13.2: Touch, Taste and Smell
- Sensory receptors can be classified based on cell type, location or type of stimuli. The general sense of somatosensation (touch) contains receptors spread throughout the body. Taste and smell belong to the special senses and rely on chemoreceptor cells. Taste transduction depends on gustatory receptor cells of the taste buds within papillae. Smell transduction depends on olfactory receptor neurons within the olfactory epithelium.
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- 13.3: Hearing, Balance and Vision
- Hearing, balance and vision belong to the special senses. Both hearing and balance depend on hair cells within the inner ear. The cochlea is responsible for hearing. The vestibule detects linear acceleration of the head. The semicircular canals sense the rotation of the head. Vision rely on photoreceptor cells within the retina of the eye that detect light and colors. The eyes are protected by the bones of the orbits, the eyelids, the tarsal glands, and the lacrimal apparatus.
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- 13.4: Integration of Somatic Functions
- Sensory information conveyed through spinal nerves will project to the opposite side of the brain. The cranial nerves are connected to the same side of the brain from which the sensory information originates, except for the optic nerve. Sensory input to the brain enters through pathways that travel through either the spinal cord (for somatosensory input from the body) or the brainstem (for everything else, except the visual and olfactory systems) to reach the diencephalon and thalamus.
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- 13.5: Somatic Motor Responses
- The prefrontal cortex, secondary motor cortices and primary motor cortex plan, coordinate and execute movement of skeletal muscles. Axons project through the corticobulbar and corticospinal tracts to synapse on lower motor neurons in the brainstem and ventral horn of the spinal cord, respectively. The extrapyramidal system includes projections from the brainstem and higher centers that influence movement. Somatic reflexes include the withdrawal reflex and stretch reflex.