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Nervous System

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University of Toronto Scarborough
Biological Sciences
Stephen Reid

Chapter 56: Nervous System STRUCTURES AND FUNCTIONS  The human nervous system is a highly specialized system responsible for the control and integration of the body’s many activities.  The nervous system can be divided into the central nervous system (CNS) and parts of the peripheral nervous system (PNS). o The central nervous system consists of the brain and spinal cord. o The peripheral nervous system consists of the cranial and spinal nerves and the autonomic nervous system (ANS).  The nervous system is made up of two types of cells: neurons and neuroglia. o The neurons of the nervous system come in many different shapes and sizes, but they all share common characteristics: (1) excitability, or the ability to generate a nerve impulse; (2) conductivity, or the ability to transmit the impulse to other portions of the cell; and (3) the ability to influence other neurons, muscle cells, and glandular cells by transmitting nerve impulses to them. o Neuroglia, or glial cells, provide support, nourishment, and protection to neurons.  Nerve impulses originate within a neuron as an action potential that moves along the body of the cell (axon) until it reaches the end of the nerve fiber. From there, it is transmitted across the junction between nerve cells by a chemical interaction and then, the impulse will move across the next neuron as an action potential. o A synapse is the structural and functional junction between two neurons. It is the point at which the nerve impulse is transmitted from one neuron to another or from neuron to glands or muscles. o A neurotransmitter is a chemical agent involved in the transmission of an impulse across the synaptic cleft. CENTRAL NERVOUS SYSTEM  The major structural components of the CNS are the spinal cord and brain. Spinal Cord  The spinal cord is continuous with the brainstem and exits from the cranial cavity through the foramen magnum. A cross section of the spinal cord reveals gray matter that is centrally located and is surrounded by white matter.  Specific ascending and descending pathways in the white matter can be identified. o In general, the ascending tracts carry specific sensory information to higher levels of the CNS. o Descending tracts carry impulses that are responsible for muscle movement.  Lower motor neurons are the final common pathway through which descending motor tracts influence skeletal muscle, the effector organ for movement. The cell bodies of these cells are located in spinal cord and the axons innervate the skeletal muscles. Brain  The brain consists of the cerebral hemispheres, cerebellum, and brainstem.  The cerebrum is composed of the right and left hemispheres. Both hemispheres can be further divided into four major lobes. o The frontal lobe controls higher cognitive function, memory retention, voluntary eye movements, voluntary motor movement, and expressive speech. o The temporal lobe contains Wernicke’s area, which is responsible for receptive speech and for integration of somatic, visual, and auditory data. o The parietal lobe is composed of the sensory cortex, controlling and interpreting spatial information. o Processing of sight take place in the occipital lobe.  The basal ganglia, thalamus, hypothalamus, and limbic system are also located in the cerebrum.  The brainstem includes the midbrain, pons, and medulla. The vital centers concerned with respiratory, vasomotor, and cardiac function are located in the medulla.  The brainstem contains the centers for sneezing, coughing, hiccupping, vomiting, sucking, and swallowing. Also located in the brainstem is the reticular formation, which relays sensory information, influences excitatory and inhibitory control of spinal motor neurons, and controls vasomotor and respiratory activity.  The cerebellum coordinates voluntary movement and to maintains trunk stability and equilibrium.  Cerebrospinal fluid circulates within the subarachnoid space that surrounds the brain, brainstem, and spinal cord. This fluid provides cushioning for the brain and spinal cord, allows fluid shifts from the cranial cavity to the spinal cavity, and carries nutrients. PERIPHERAL NERVOUS SYSTEM  The PNS includes all the neuronal structures that lie outside the CNS. It consists of the spinal and cranial nerves, their associated ganglia (groupings of cell bodies), and portions of the ANS.  The spinal nerve contains a pair of dorsal (afferent) sensory nerve fibers and ventral (efferent) motor fibers, which innervate a specific region of the neck, trunk, or limbs. This combined motor-sensory nerve is ca
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