Peripheral Nervous System PDF

 Two parts › Central Nervous System (CNS);  Brain and spinal cord › Peripheral Nervous System (PNS);  Connects body to brain & spinal cord  12 pairs of nerves from your brain (cranial nerves)  31 pairs from your spinal cord (spinal nerves)...

 Two parts › Central Nervous System (CNS);  Brain and spinal cord › Peripheral Nervous System (PNS);  Connects body to brain & spinal cord  12 pairs of nerves from your brain (cranial nerves)  31 pairs from your spinal cord (spinal nerves)  Bundles of sensory and motor neurons held together by connective tissue  The peripheral nervous system includes the nervous structures outside the brain and spinal cord  Peripheral nerves allow the CNS to receive information and take action  Functional components of the PNS; › Sensory inputs and motor outputs categorized as somatic or visceral › Sensory inputs also classified as general or special   What is the Anatomic Organization of PNS Neurons? › Ganglia—Ganglia are collections, or small knots, of nerve cell bodies outside the CNS. › Nerve—Bundle of axons supported by connective tissue  Spinal nerves  To/from spinal cord  Cranial nerves  To/from brain Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings Neurons and Glial Cells  The basic unit of the nervous system is the individual nerve cell, or neuron.  Neurons are the basic building block of the nervous system.  Neurons occur in a wide variety of sizes and shapes.  Nevertheless, all neurons share features that allow cell-to-cell communication.  Anatomy  A neuron can be divided into four anatomically distinct regions: (1) a cell body, (2) dendrites, (3) an axon, and (4) axon terminals.  Anatomy 1. Cell body: The cell body (soma) houses the nucleus and components required for protein synthesis and other normal cellular housekeeping functions.  2. The dendrites are a series of highly branched outgrowths of the cell body. They and the cell body receive most of the inputs from other neurons, the dendrites being more important in this role than the cell body.  The branching dendrites (some neurons may have as many as 400,000!) increase the cell’s surface area. Thus, dendrites increase a cell’s capacity to receive signals from many other neurons.  3. Axon  The axon, sometimes also called a nerve fiber, is a single long process that extends from the cell body and carries output to its target cells.  Axons range in length from a few micrometers to over a meter.  The portion of the axon closest to the cell body plus the part of the cell body where the axon is joined is known as the initial segment, (or axon hillock).  The initial segment is the “trigger zone” where, in most neurons, the electrical signals are generated.  4. Axon Terminal  The axon divides into presynaptic terminals, each ending in a number of synaptic knobs which are also called terminal buttons or boutons. They contain granules or vesicles in which the synaptic transmitters secreted by the nerves are stored. Nerve terminal: The nerve terminal is specialized to convert an electrical signal (an action potential) into a chemical signal for dispatch to one or more recipients. The junction between the terminal and its target is called a synapse. The presynaptic and postsynaptic cell membranes are separated by a 30–50 nm synaptic cleft. Facing the terminal across the cleft may be any of a number of different postsynaptic effector cells, including myocytes, secretory cells, or even a dendrite extending from the cell body of another neuron.  Neurons are secretory cells, but they differ from other secretory cells in that the secretory zone is generally at the end of the axon, far removed from the cell body.  The apparatus for protein synthesis is located for the most part in the cell body, with transport of proteins and polypeptides to the axonal ending by axoplasmic flow. Thus, the cell body maintains the functional and anatomic integrity of the axon; if the axon is cut, the part distal to the cut degenerates (wallerian degeneration).  Axonal transport occurs along microtubules that run along the length of the axon and requires two molecular motors, dynein and kinesin.  Anterograde transport moves from the cell body toward the axon terminals. It has both fast and slow components; › fast axonal transport occurs at about 200 - 400 mm/day, and › slow axonal transport occurs at 0.5 to 10 mm/day.  The substances and organelles being moved are linked by proteins to microtubules in the cell body and axon. The microtubules serve as the “rails” along which the transport occurs. The linking proteins (dynein and kinesin) act as the “motors” of axon transport and, as ATPase enzymes, they also transfer energy from ATP to the “motors.”  Retrograde transport, which is in the opposite direction (from the nerve ending to the cell body), occurs along microtubules at about 200 mm/day. Synaptic vesicles recycle in the membrane, but some used vesicles are carried back to the cell body and deposited in lysosomes.  By this route, growth factors and other chemical signals picked up at the terminals can affect the neuron’s morphology, biochemistry, and connectivity.  This is also the route by which certain harmful substances, such as tetanus toxin, herpes, and other viruses can be taken up by the peripheral axon terminals and enter the central nervous system.  Classification of Neurons and Nerves Neurons may be classified according to their function or structure. 1. The structural classification of neurons is based on the number of processes that extend from the cell body of the neuron -Unipolar neurons -Bipolar neurons -Multipolar neurons  Unipolar neurons have one process, with different segments serving as receptive surfaces and releasing terminals.  Bipolar neurons have two specialized processes: a dendrite that carries information to the cell and an axon that transmits information from the cell. - Some sensory neurons are in a subclass of bipolar cells called pseudo-unipolar cells. As the cell develops, a single process splits into two, both of which function as axons—one going to skin or muscle and another to the spinal cord.  Multipolar neurons have one axon and many dendrites. › Examples include motor neurons, hippocampal pyramidal cells with dendrites in the apex and base, and cerebellar Purkinje cells with an extensive dendritic tree in a single plane.  Neurons can be divided into three functional classes: afferent neurons, efferent neurons, and interneurons.  Afferent neurons convey information from the tissues and organs of the body into the central nervous system.  Efferent neurons convey information from the central nervous system out to effector cells (particularly muscle or gland cells or other neurons).  Interneurons connect neurons within the central nervous system. General organizational scheme of the nervous system.  Neurons account for only about 10 percent of the cells in the central nervous system. The remainder are glial cells, also called neuroglia.  However, because the neurons branch more extensively than glia do, neurons occupy about 50 percent of the volume of the brain and spinal cord.  Glial cells surround the soma, axon, and dendrites of neurons and physically and metabolically support neurons. Glial roles include;  maintaining the ionic milieu of nerve cells  modulating the rate of nerve signal propagation  modulating synaptic action by controlling the uptake of neurotransmitters at or near the synaptic cleft  providing a scaffold for some aspects of neural development  Types of glial cells 1. Microglia 2. Astrocytes 3. Oligodendrocytes 4. Schwann cells 5. Ependymal cells  Types of glial cells 1. Microglia a. act as phagocytes b. part of brain’s immune system  Types of glial cells 2. Astrocytes Astrocytes (aster = star) are large stellate cells with numerous cytoplasmic processes that radiate outward. They are the mostabundant of the glial cells in the CNS, constituting up to 90% of the nervous tissue in some areas of the brain. some of the proposed functions of astrocytes: 1. Astrocytes take up K+ from the extracellular fluid. 2. Astrocytes take up some neurotransmitters released from the axon terminals of neurons. 3. The astrocyte end-feet surrounding blood capillaries take up glucose from the blood. 4. Astrocytes appear to be needed for the formation of synapses in the CNS. 5. Astrocytes induce the formation of the blood-brain barrier. 1. Astrocytes take up K+ from the extracellular fluid.  Types of glial cells 3. Oligodendrocytes myelinate axons of central nervous system 4. Schwann cells myelinate axons of peripheral nervous system  The axons of most but not all neurons are covered by myelin, which consists of 20 to 200 layers of highly modified plasma membrane wrapped around the axon by a nearby supporting cell.  In the brain and spinal cord these myelin- forming cells are the oligodendrocytes.  Each oligodendrocyte may branch to form myelin on as many as 40 axons.  In the peripheral nervous system single myelin- forming cells, called Schwann cells, form one individual myelin sheath.  The spaces between adjacent sections of myelin where the axon’s plasma membrane is exposed to extracellular fluid are the nodes of Ranvier.  The myelin sheath speeds up conduction of the electrical signals along the axon and conserves energy. 5. Ependymal cells Ependymal cells form the epithelium lining the ventricular spaces of the brain that contain CSF. Many substances diffuse readily across the ependyma, which lies between the extracellular space of the brain and the CSF. CSF is secreted in large part by specialized ependymal cells of the choroid plexuses located in the ventricular system.  Nerve fibers are classified according to their conduction velocity, which depends on the size of the fibers and the presence or absence of myelination.  Conduction is increased by the fiber diameter (Briefly, the larger the fiber, the higher the conduction velocity).  Conduction velocity also is increased by the presence of a myelin sheath around the nerve fiber. Thus, large myelinated nerve fibers have the fastest conduction velocities, and small unmyelinated nerve fibers have the slowest conduction velocities.  Two classification systems, which are based on differences in conduction velocity, are used: › The first system applies to both sensory (afferent) and motor (efferent) nerve fibers and uses a lettered nomenclature of A, B, and C. › The second system applies only to sensory nerve fibers and uses a Roman numeral nomenclature of I, II, III, and IV.  General somatic senses – include touch, pain, vibration, pressure, temperature  Proprioceptive senses – detect stretch in tendons and muscle provide information on body position, orientation and movement of body in space  Special Senses – hearing, balance, vision, olfaction (smell), gustation (taste) Sensory fibers which carry impulses from skin, skeletal muscles, and joints Classification of Receptor  There are five major divisions of these sensory receptors based on stimuli that they respond to: 1. Mechanoreceptors 2. Thermoreceptors 3. Nociceptors 4. Photoreceptors 5. Chemoreceptors  Sensory  Mechanoreceptors are activated by pressure or changes in pressure.  Mechanoreceptors include, but are not limited to the;  Pacinian corpuscles in subcutaneous tissue  Meissner's corpuscles in nonhairy skin (touch)  Baroreceptors in the carotid sinus (blood pressure)  Hair cells on the organ of Corti (audition) and in the Semicircular canals (vestibular system)  Photoreceptors are activated by light and are involved in vision.  Chemoreceptors are activated by chemicals and are involved in olfaction, taste, and detection of oxygen and carbon dioxide in the control of breathing.  Thermoreceptors are activated by temperature or changes in temperature.  Nociceptors are activated by extremes of pressure, temperature or noxious chemicals. › General somatic motor  Signals contraction of skeletal muscles  Under our voluntary control › Visceral motor  Makes up autonomic nervous system (ANS)  Regulates the contraction of smooth and cardiac muscle, controls function of visceral organs  The human body contains 12 pairs of cranial nerves  Cranial nerves I and II attach to the forebrain  All others attach to the brain stem  Primarily serve head and neck structures  The vagus nerve (X) extends into the abdomen  The spinal cord is the most caudal portion of the CNS, extending from the base of the skull to the first lumbar vertebra.  The spinal cord is segmented, with 31 pairs of spinal nerves that contain both sensory (afferent) nerves and motor (efferent) nerves. Functions: 1. Sensory and motor innervation of entire body inferior to the head through the spinal nerves 2. Two-way conduction pathway between the body and the brain 3. Major center for reflexes  In general, the eight cervical nerves (C1-8) control the muscles and glands and receive sensory input from the neck, shoulder, arm, and hand.  The 12 thoracic nerves (T1- 12) are associated with the chest and abdominal walls.  The five lumbar nerves (L1-5) are associated with the hip and leg, and the five sacral nerves (S1-5) are associated with the genitals and lower digestive tract.  Sensory nerves carry information to the spinal cord from the skin, joints, muscles, and visceral organs in the periphery via dorsal root and cranial nerve ganglia  Motor nerves carry information from the spinal cord to the periphery and include both somatic motor nerves (which innervate skeletal muscle) and motor nerves of the autonomic nervous system (which innervate cardiac muscle, smooth muscle, glands, and secretory cells)