Chapter 8.
Nervous System
The nervous system may be divided into two main portions: central nervous system (the brain and the spinal cord), and peripheral nervous system (the cranial nerves, spinal nerves and the autonomic nervous system).
The central nervous system is comprised of the brain and spinal cord. The brain is the most vital and essential organ in the body. It is the center that receives impulses or sensations that are stored and interpreted by the mind. The accumulation of these stored impulses forms the basis of memory.
The brain is an organ that serves as the center of the nervous system. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in human body. The brain exerts centralized control over the body’s other organs. It acts on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones.
The brain receives impulses, and also transmits or sends impulses through the nerves to different parts of the body. The brain is the place of consciousness, thought, memory, speech, hearing, vision, and the will to carry out purposeful actions. The brain is responsible for interpreting our environment for us. The brain and spinal cord are protected immediately by the meninges (membranes) and spinal fluid, and outwardly by the skull and vertebral column.
The spinal cord is a long, thin, tubular structure (situated in the spinal canal, formed by the vertebrae) made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column. It encloses the central canal of the spinal cord, which contains cerebrospinal fluid. It is around 18 in. in men and around 17 in. long in women.
The spinal cord functions primarily in the transmission of nerve signals from the motor cortex to the body, and from the afferent fibers of the sensory neurons to the sensory cortex. It is also a center for coordinating many reflexes and contains reflex arcs that can independently control reflexes. The spinal cord is, and extends from the brainstem to the third lumbar vertebrae. If it is damaged, paralysis results following traumatic injury.
The central nervous system is mostly concerned with the interpretation of sensory impulses and in sending motor impulses, which passed to voluntary or striated muscle in the body, whereas the autonomic (or involuntary) nervous system is concerned with the control of all of the involuntary, on striped or on striated muscle in the body.
There are 31 pairs of spinal nerves that come off the spinal cord at intervals. They are classified according to the region from which they arise. Impulses travel back and forth to all areas of the body. There are 12 pairs of cranial nerves that arise from the brain and brainstem. Some of them are sensory, bringing impulses to the brain, while others are motor, caring impulses from the brain to the extremities. A few of the cranial nerves are mixed, and contain both sensory and motor fibers of the body.
The peripheral nervous system is comprised of nerves and ganglia outside of the brain and spinal cord. Ganglia are knots of nerve cell bodies. The peripheral, involuntary nervous system governs skeletal or striated muscle.
An essential feature of the involuntary system (autonomic) is a series of ganglia, which are classified into three groups: the cranial, they sympathetic, and the parasympathetic autonomic system.
Parasympathetic fibers carry impulses that slow the heart rate, while sympathetic fibers will speed up the heart rate. The secretions of the ductless glands, such as the adrenaline gland, which produces adrenaline, influence the autonomic nervous system.
The autonomic nervous system is a control system that acts largely unconsciously and regulates bodily functions such as the heart rate, digestion, respiratory rate, pupillary response, urination, sexual arousal and the skin. It is divided into the sympathetic and parasympathetic systems. This system is the primary mechanism in control of the fight-or-flight response.
The sympathetic division typically functions in actions requiring quick responses. The parasympathetic division functions with actions that do not require immediate reaction. The sympathetic system is often considered the “fight or flight” system, while the parasympathetic system is often considered the “rest and digest” or “feed and breed” system.
Please watch the following videos:
- Nervous System Overview (about 14 minutes)
- The Brain (about 14 minutes)
- The Sensory System (about 11 minutes)
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