The nervous system A. Two organ systems, the nervous system and the endocrine system, coordinate organ system activities in response to changing environmental conditions.
1. The nervous system responds relatively fast but briefly to stimuli, whereas endocrine responses develop slower but last longer. 2. The nervous system is the most complex organ system.
3. The nervous system has three functions- 1- monitors the internal and external environments, 2- integrates sensory information and 3- coordinates voluntary and involuntary responses. 4. The functions of the two major anatomical subdivisions of the nervous system are shown in figure 8.1 pages 226.
5. The central nervous system (CNS): consisting of the brain and spinal cord, integrates and coordinates sensory data and motor commands. The CNS is also the site for intelligence memory and emotions. 6. All communication between the CNS and the rest of the body occurs over the peripheral nervous system (PNS). The PNS includes all the neural tissue OUTSIDE the CNS.
7. Its afferent division brings sensory information TO the CNS. 8. The efferent division carries motor commands AWAY from the CNS to the muscles and glands. 9. The PNS is divided into the somatic nervous system (SNS) and the autonomic nervous system (ANS)
10. The SNS provides control over skeletal muscle contractions. 11. The ANS provides automatic involuntary regulations of the smooth, cardiac muscles and glandular secretions. 12. The ANS includes a sympathetic and a parasympathetic division, which commonly have opposite effects.
13. Example: sympathetic division accelerates the heart rate and parasympathetic slows it down.
B. Cellular organization in Neural tissue 1. Neural tissue consists of two kinds of cells, neurons and neuroglia. 2. Neurons are the basic unit of the nervous system. All neural functions involve the communication of neurons with one another and with other cells.
3. The neuroglia regulates the environmental around the neurons; provide a support framework for neural tissue, and act as phagocytes.
C. The general structure of the neurons 1. The “model” neuron as cell body, several branching dendrites which receive incoming signals, an elongate axon which carries outgoing signals toward the one or more synaptic terminals.
2. Neurons can have a variety of shapes the most common type of neuron in the CNS is shown on page 227.
3. Neurons come in all shapes and sizes; they have enough features in common that we can draw a neuron.
a. The largest part of the neuron is the cell body. The cell body contains the nucleus and much of the cytoplasm. b. Spreading out from the cell body are short-branched extensions called dendrites. Dendrites carry impulses from the environment or from other neurons toward the cell body.
c. The long fiber that carries impulses away from the cell body is called the axon. d. The axon ends in a series of small swellings called axon terminates.
D. Structural classification of neurons 1. A multipolar neuron has multiple processes extending away from the cell body. These are very common in the CNS.
2. A unipolar neuron, the dendrites and axon are continuous, and the cell body lies off to one side. In a unipolar neuron, the action potential begins at the base of the dendrites and the rest of the process is considered an axon
3. Bipolar neurons have two processes, one dendrites and one axon, with the cell body between them. Bipolar neurons are rare but occur in special sense organs such as the eye and the ear.
E. Functional classification of Neurons 1. Neurons are sorted into three functional groups: sensory, motor and interneurons.
F.Sensory neurons of the afferent division convey information from both external and internal environments to other neurons inside the CNS. 1. Receptors may be grouped into three categories based on the information they carry.
2. The somatic sensory receptors carry two types of information; one about the outside world and the other about our position within it. 3. The external receptors provide information about the external environment in the form of touch, temperature and pressure sensations and more complex senses of sight, smell, hearing and touch.
4. The proprioceptors monitor the position and movement of skeletal muscles and joints. 5. The visceral receptors or internal receptors monitor activities of digestive, respiratory, cardiovascular, urinary and reproductive systems and provide sensation for taste deep pressure and pain.
G. Motor Neurons 1. The half million motor neurons of the efferent division carry instructions from the CNS to other tissues, organs or organ systems. 2. The peripheral targets are called effectors because they change their activities in response to the commands issued by the motor neurons.
3. The somatic motor neurons of the somatic nervous system and the visceral motor neurons of the autonomic nervous system. Example: cardiac tissue.
H. Interneurons 1. The 20 billion interneurons are located entirely within the brain and the spinal cord. 2. Interneurons are responsible for the connection between sensory and motor activity.
I. Neuroglia-are both found in both CNS and PNS, but the CNS has the greatest diversity of glial cells. There are four types of glial cells in the CNS. 1. Astrocytes- largest and the most numerous neuroglia. They secrete chemicals vital to the maintenance of the blood-brain barrier, which isolates the CNS from the general circulation.
2. Oligodendrocytes- has cytoplasmic extensions that wrap around axons creating a sheath called myelin. Myelin speeds up an impulse. 3. Microglia- are the smallest and rarest. They are phagocytic cells that eat cellular waste and pathogens.
II. Neuron Functions • The nerve impulse 1. The electrical activity in a nerve impulse is a flow of electrical charges along the cell membrane of a neuron.
2. Nerve cell has an electrical potential across its cell membrane because of the difference in the number of positively and negatively charged ions on each side of the cell. The potential is 70 millivolts. 3. Myelin improves the rate of impulses along an axon. Myelin is composed of 80 percent lipid and 20 percent protein forms an insulated sheath around the axon.
4. Most important feature is there are small nodes or gaps in thy myelin allowing the impulse to jump from note to node instead of moving along the membrane. Jumping greatly increases the speed of the impulse. 5. The minimum level of a stimulus that is required to activate a neuron is called a threshold. Any stimulus that is weaker than the threshold will produce no impulse. Any stimulus that is stronger than the threshold will have an impulse.
6. At the ends of the axons are receptors. Receptors are special sensory neurons in sense organs that receive stimuli from the external environment. 7. The points of contact at which the impulses are passed from one cell to another are known as synapses.
8. The axon terminals at a synapse contain tiny vesicles, or sacs. These tiny vesicles are filled with chemicals known as neurotransmitters. A neurotransmitter is a substance that is used by one neuron to signal another.
III. The brain • Major divisions of the brain 1.The adult brain has six major regions: ·Cerebrum ·Diencephalon ·Midbrain ·Pons ·Medulla oblongata ·Cerebellum
2. The brain is the main switching unit of the CNS. The spinal cord provides the link between the brain and the rest of the body. 3. The brain is wrapped in three layers of connective tissue known as meninges
4. The inner layer covers and is bound to the surface of the brain is called pia matter. 5. The outermost layer is called dura mater. 6. Between the pia mater and the dura mater is cobweb like layer called arachnoid. This area is filled with cerebrospinal fluid. The fluid protects the brain from injury. Acts like a shock absorber.
B. The cerebrum is the largest prominent part of the human brain. It is responsible for all the voluntary activities in the body. It is the site of intelligence. . Hearing and judgment. 1. Is divided into two hemispheres, the right and left. It is separated by a deep groove. The hemispheres are connected in a region known as the corpus callosum.
2. The cerebrum has numerous folds. These folds and grooves increase the surface area of the cerebrum. It allows more to fit in a small area. 3. Each hemisphere of the cerebrum is divided into regions called lobes. These lobes are named for the skull bones.
4. Each half of the cerebrum deals with the opposite side of the body. 5. It is thought that right hemisphere is associated with creativity and artistic ability, whereas the left hemisphere is associated with analytical and mathematical ability.
6. The cerebral cortex is the outer surface and it consists of gray matter. The inner surface is called the cerebral medulla. This consists of white matter.
C.The cerebellum is the second largest part of the brain is located at the back of the skull. 1. Its function is balance and coordination. 2. Major part of learning how to perform physical activities seems to be related to the cerebellum. Ex. Shooting a basketball.