Neurobiology of Sensory Motor Systems and Internal Environment

Chapter Outline

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  Sensory system

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  Motor system

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  Regulation of internal environment

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  Limbic system

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  Autonomic system

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  Neuroendocrine system

  • Learning Objectives: To learn basic anatomy, physiology and clinical implications of sensory, motor, autonomic and limbic system.

Introduction

• Sensory System: The sensory system carries sensory information from external and internal environment. Different types of receptors carry different sensations like touch, pain, position and vibration through sensory nerve fibres. Impulses that arise by stimulation of receptors are carried by sensory nerves, and then conveyed through fiber tracts to higher centers in the brain.The sensory system can be classified as somatic and visceral sensation, with general and special varieties of each. General somatic afferent fibers carry touch, pain, position and vibration sense. General visceral afferent fibers carry impulses from internal organs. Special somatic and special visceral afferent fibers mediate special senses- smell, vision, taste, hearing, and vestibular sensation.

The different sensory modalities are carried over nerve fibers of different size. Sensory impulses are carried to the dorsal (posterior) root ganglia, and then into the spinal cord. After one or more synapses, the impulses ascend through specific fiber tracts to reach sensory areas of the brain. Fine touch, position, and vibration sensations are carried through the posterior column/medial lemniscus system. Pain and temperature senses are carried through spinothalamic tracts. The major sensory pathways are depicted in Figure1.

Figure 1 depicts the light touch, pressure, position, and vibration pathways from the body and face are indicated by the dashed line; the pain and temperature fibers from the body and face are indicated by the solid line. Fibers from these various sources ultimately converge on the ventral posterior nuclei of the thalamus, which projects via the thalamic radiations to the primary sensory cortex in the postcentral gyrus. V, trigeminal; VPL, ventral posterior lateral; VPM, ventral posterior medial.

• Sensory Receptors: There are many different types of receptors in the skin, subcutaneous tissues, muscles and internal organs. Receptors convert various types of sensory information into nerve impulses. Sensory end-organs are found in the skin and mucous membranes throughout the body. They are denser on the tongue, lips, genitalia, and fingertips and farther apart on the upper arms, buttocks, and trunk. Receptor stimulation causes a change in the permeability of its membrane that gives rise to a local, non propagated potential known as receptor potential. Receptor potentials induce action potentials in the nerve. Receptors may be free nerve endings, ormay be encapsulated or specialized sense organs.

Examples:

• Free Nerve Endings: Fine, unmyelinated terminal fibers mediate pain (nociceptors). Merkel cell endings (tactile discs or menisci) are specialized nerve endings that function as mechanoreceptors which sense pressure.Other examples of receptors include Golgi tendon organs, muscle spindles, Ruffini's endings, and Meissner's and Pacinian corpuscles.

• Dermatomes: Sensory nerve roots supply cutaneous innervation to specific dermatomes. There is significant dermatomal overlap; hence clinical deficit from an isolated root lesion is typically much more restricted than that expected from the anatomical geography of the dermatome.

Figure 1.

Sensory pathways