The Nervous System

1. The Nervous System General Function and Nervous System Cells Lecture 1

2. The Nervous System as a Whole • General Function • The Nervous System serves as the chief coordinating agency for all systems. • It must detect and respond to both external and internal stimuli so that the body can adapt to new conditions.

3. Structural Divisions • The Central Nervous system (CNS) includes the brain and spinal cord. • The Peripheral Nervous system (PNS) is composed of all the nerves outside the CNS. • It includes all the cranial nerves that carry impulses to and from the brain and all the spinal nerves that carry messages to and from the spinal cord.

4. Functional Divisions • The Nervous System is divided according to whether control is voluntary or involuntary, and to according to what type of tissue is stimulated. • Any tissue or organ that carries out a command from the Nervous System is called an effector, all of which are muscles or glands. • The Somatic Nervous System (SNS) is controlled voluntarily and all the effectors are skeletal muscles. • The Autonomic Nervous System (ANS) is the involuntary division. • It is also called the Visceral Nervous System because it controls smooth muscle, cardiac muscle, and glands, much of which makes up the viscera (or organs).

5. Aging of the Nervous System • The Nervous System is one of the first systems to develop in the embryo. • By the beginning of the third week of development, the rudiments of the brain and spinal cord have appeared. • Beginning with maturity, the Nervous System begins to undergo changes. • The brain begins to decrease in size and weight due to a loss of cells, especially in the cerebral cortex. • These losses are accompanied by a decrease in synapses and neurotransmitters.

6. The speed of processing information decreases and movements are slowed. • Memory diminishes, especially for recent events. • Arteriosclerosis, narrowing of the blood vessels, reduces blood flow to the brain and degeneration of blood vessels increases the chances for a stroke to occur.

7. Nervous System Cells: Structure and Function • Neuroglia: Connective Tissue Cells • There are different types of neuroglia cells, each with a specialized function. • Some functions are • To protect nervous tissue • To support nervous tissue and bind it to other structures • To aid in the repair of cells • To act as phagocytes and remove pathogens and impurities • To regulate the composition of fluids around and between cells

8. Unlike nerve cells, neuroglia cells continue to multiply through out life. • Because of their capacity to reproduce, most tumors of the Nervous System are tumors of neuroglial tissue.

9. Neurons (Nerve Cells) • Structure • The cell body: the main portion of each neuron; it contains the nucleus and other common cellular organelles. • Dendrites: neuron fibers that conduct impulses to the cell body. • Most dendrites are highly branched. • They function as receptors; the receive the stimulus that begins a neural pathway. • Axons: neuron fibers that conduct impulses away from the cell body. • These impulses may be delivered to another neuron or to an effector. An axon is a long, single fiber that may have branches at its end.

10. Neurons

11. Myelin Sheath • The myelin sheath is a fatty material that covers some axons. It insulates and protects the fiber. • In the PNS, this covering is produced by neuroglia called Schwann cells. • Schwann cells wrap around the axon depositing layers of myelin. • When the sheath is complete, small spaces remain between the individual cells. These tiny gaps, called nodes, are important in speeding the conduction of nerve impulses.

12. The Neurilemma • The outermost membranes of the Schwann cell forms a layer called the neurilemma. • This covering is involved in the repair of some peripheral nerves. • Under certain conditions, damaged nerve cell fibers may regenerate by growing into the sleeve formed by the neurilemma. The repair is a slow and uncertain process. • Cells of the brain and spinal cord are myelinated by oligodendrocytes. They lack a neurilemma and when injured, permanent damage results.

13. White Fibers vs. Gray Fibers • Myelinated axons are called white fibers and are found in the white matter of the brain, spinal cord, and nerve trunks in all parts of the body. • The fibers and cell bodies of the gray fibers are not covered with myelin and are found in the gray matter of the brain and spinal cord.

14. Types of Neurons • Sensory neurons (afferent neurons) conduct impulses to the spinal cord and brain. They receive stimuli from the environment. • Motor neurons (efferent neurons) carry impulses from the brain and spinal cord to the effectors (muscles and glands). • Interneurons (association neurons or central neurons) relay information within the spinal cord and brain.

15. Nerves and Tracts • Nerve: a bundle of fibers located within the PNS. • A few of the cranial nerves only contain sensory fibers for conducting impulses toward the brain. These are described as sensory nerves. • A few of the cranial nerves contain only motor fibers for conducting impulses from the brain. These are classified as motor nerves. • Most of the cranial nerves and all of the spinal nerves contain both sensory and motor fibers and are called mixed nerves. • Tract: a bundle of fibers located within CNS.