1. Anatomy & Physiology

2. Introduction  The nervous system is the master controlling and communicating system of the body  Every thought, action, and emotion reflects its activity  To carry out its normal role, the nervous system has three overlapping functions: 1) It uses millions of sensory receptors to monitor changes occurring both inside and outside the body Gathered information is called sensory input

3. 2) It processes and interprets the sensory input and makes decisions about what should be done at each moment (integration) 3) It then effects a response by activating muscles or glands; the response is called motor output

4. Organization of the Nervous System - Structural Classification  Includes all nervous system organs  Consists of two subdivisions: 1) central nervous system, 2) peripheral nervous system  Central Nervous System (CNS) Consists of the brain and spinal cord which act as the integrating and command centers Interpret incoming sensory information and issue instructions based on past experiences and current conditions

5.  Peripheral Nervous System (PNS) The part of the nervous system outside of the CNS Consists mainly of the nerves that extend from the brain and spinal cord Spinal nerves carry impulses to and from the spinal cord Cranial nerves carry impulses to and from the brain They link all parts of the body by carrying impulses from the sensory receptors to the CNS and from the CNS to the appropriate glands or muscles

6. Organization of the Nervous System - Functional Classification  Functional Classification is concerned only with the PNS  The PNS is divided into two principal subdivisions:  1) Sensory (afferent) division Consists of nerve fibers that convey impulses to the CNS from sensory receptors located in parts of the body

7. The somatic sensory (afferent) fibers transmit impulses from the skin, skeletal muscles, and joints Visceral (afferent) fibers transmit impulses from the visceral organs  2) Motor (efferent) division Carry impulses from the CNS to effector organs, muscles and glands These impulses activate muscles and glands (bring about a motor response) The motor division has two subdivisions: ○ 1) Somatic nervous system – voluntary control of skeletal muscles Called voluntary nervous system

8. ○ 2) Autonomic nervous system - regulates involuntary activities of the body Called involuntary nervous system Has two parts: sympathetic and parasympathetic

10. Nervous Tissue: Structure and Function  There are two principal types of cells in nervous tissue: 1) supporting cells 2) neurons  1) Supporting Cells Supporting cells in the CNS are lumped together as neuroglia (glia) Neuroglia includes many types of cells that generally support, insulate, and protect the delicate neurons

11. A) Astrocytes (star shaped cells) account for nearly half of the neural tissue ○ They have numerous projections with swollen ends that cling to neurons, bracing them and anchoring them to the blood capillaries ○ Form a living barrier between capillaries and neurons, and they play a role in making exchanges between the two ○ Help protect neurons from harmful substances that might be in the blood ○ Also help control the chemical environment in the brain by picking up excess ions and recapturing released neurotransmitters

12. B) Microglia are phagocytes that dispose of debris (dead brain cells and bacteria) C) Ependymal cells line the cavities of the brain and the spinal cord ○ The beating of their cilia helps to circulate the cerebrospinal fluid that fills those cavities and forms a protective cushion around the CNS D) Oligodendrocytes wrap their flat extensions tightly around the nerve fibers, producing fatty insulating coverings called myelin sheaths  Neuroglia are not able to transmit nerve impulses and they never lose their ability to divide  Most brain tumors are gliomas (tumors formed by neuroglia cells)

13.  Supporting cells in the PNS come in two major varieties – Schwann cells and satellite cells Schwann cells form the myelin sheaths around nerve fibers that are found in the PNS Satellite cells act as protective, cushioning cells

14.  2) Neurons Neurons (nerve cells) are highly specialized to transmit messages (nerve impulses) from one part of the body to another Neurons can differ structurally, they have many common features: ○ A) Cell body – contains the nucleus and is the metabolic center of the cell ○ B) Processes (fibers) – extend from the cell body Vary in length from microscopic to 3-4 feet ○ Processes that conduct electrical impulses toward the cell body are dendrites

15. ○ Processes that generate nerve impulses away from the cell body are axons ○ Neurons will have hundreds of branching dendrites, but each neuron only has one axon ○ The axons branch profusely at their terminal ends, forming hundreds to thousands of axonal terminals ○ When the impulses reach the axon terminals, they stimulate the release of chemicals called neurotransmitters into the synaptic cleft Most long nerve fibers are covered with a whitish, fatty material, called myelin Myelin protects and insulates the fibers and increases the transmission rate of nerve impulses

16. Axons outside the CNS are myelinated by Schwann cells ○ Cells wrap themselves tightly around the axon jelly-roll fashion forming the myelin sheath ○ The myelin sheath is formed by many individual Schwann cells, it has gaps or indentations, called nodes of Ranvier Myelinated fibers are also found in the CNS but the oligodendrocytes form the myelin sheath ○ Oligodendrocytes have many flat extensions and can coil around as many as 60 different fibers at the same time

17.  The importance of myelin insulation to nerve transmission is best illustrated by observing what happens when it is not there In people with MS, the myelin sheaths around the fibers are gradually destroyed, converted to hardened sheaths When this happens the current is short- circuited, and the affected person loses the ability to control their muscles MS is an autoimmune disease in which a protein component of the sheath is attacked

18.  Clusters of neuron cell bodies and collections of nerve fibers are named differently when they are in the CNS than when they are part of the PNS Cell bodies are found in the CNS in clusters called nuclei Small collections of cell bodies called ganglia are found in a few sites outside the CNS in the PNS Bundles of nerve fibers running through the CNS are called tracts, whereas in the PNS they are called nerves White matter consists of dense collections of myelinated fibers (tracts), gray matter contains mostly unmyelinated fibers and cell bodies

20. Classification  Neurons many be classified either according to how they function or according to their structure  Functional Classification Groups neurons according to the direction the nerve impulse is traveling relative to the CNS Neurons carrying impulses from sensory receptors (internal organs or skin) to the CNS are sensory (afferent) neurons

21. ○ The cell bodies of sensory neurons are always found in a ganglion outside the CNS ○ The dendrite endings of the sensory neurons are usually associated with specialized receptors that are activated by specific changes occurring nearby Cutaneous sense organs – skin sensory receptors Propriceptors – receptors in the muscles and tendons Neurons carrying impulses from the CNS to the viscera and/or muscles and glands are motor (efferent) neurons ○ The cell bodies of motor neurons are always located in the CNS

22. Association neurons (interneurons) connect the motor and sensory neurons in the neural pathways ○ Their cell bodies are always located in the CNS  Structural Classification Based on the number of processes extending from the cell body Multipolar neuron has several processes ○ All motor and association neurons are multipolar (most common structural type) Bipolar neurons – neurons with two processes – an axon and a dendrite

23. ○ These are rare in adults, found only in some special sense organs in the eye and ear Unipolar neurons have a single process emerging from a cell body ○ The process is very short and divides almost immediately into proximal and distal fibers ○ Only the small branches at the end of the proximal process are dendrites ○ The remainder of the process function as axons ○ Sensory neurons found in the PNS ganglia are unipolar

25. Physiology – Nerve Impulses  Neurons have two major functional properties: A) irritability and B) conductivity  A) Irritability – unmyelinated fibers The plasma membrane of a resting, or inactive, neuron is polarized (fewer positive ions sitting on the inner face of the neuron’s plasma membrane than there are on the outer surface)

26. ○ The major positive ions inside the cell are potassium ions ○ The major positive ions outside the cell are sodium ○ As long as the inside remains more negative as compared to the outside, the neuron will stay inactive Most neurons are excited by neurotransmitters released by other neurons Others are stimulated by light, sound or pressure Regardless of the stimulation the result is that the permeability properties of the cell’s plasma membrane changes

27. Sodium ions gates open and sodium will diffuse quickly into the neuron ○ The inward rush of sodium ions changes the polarity of the membrane (depolarization) Depolarization activates the neuron to transmit an action potential Remember the nerve impulse is an all-or-none response Now the potassium ions will diffuse out of the neuron restoring the electrical conditions to the polarized (resting) state ○ This is called repolarization The levels of sodium and potassium are restored to their normal levels by the sodium- potassium ATPase pump

28.  A) Irritability – myelinated fibers Fibers with myelin sheaths conduct impulses much faster The nerve impulse literally jumps from node to node along the length of the fiber No current can flow across the axonal membrane where there is fatty myelin insulation This is called saltatory conduction

29.  B) Conductivity When the action potential reaches the axonal endings a neurotransmitter is released The chemical diffuses across the synapse and binds to receptors on the membrane of the next neuron If enough of the neurotransmitter is released then sodium entry, depolarization, action potential generation, and repolarization occurs in the neuron beyond the synapse The neurotransmitter is then removed from the synapse either by re-uptake into the axonal terminal or by enzymatic breakdown

31. Physiology – Reflex Arc  Reflexes are rapid, predictable, and involuntary responses to stimuli  Reflexes occur over neural pathways called reflex arcs  Autonomic reflexes regulate the activity of smooth muscles, the heart, and glands Regulate body functions such as digestion, elimination, blood pressure, and sweating Examples: salivary reflex and pupillary reflex

32.  Somatic reflexes include all reflexes that stimulate the skeletal muscles  All reflexes have a minimum of five elements: 1) A sensory receptor 2) An effector organ 3 & 4) afferent and efferent neurons to connect the two 5) CNS integration center (synapse between the afferent and efferent neurons)  The knee-jerk reflex is an example of a two-neuron reflex arc (most simple type in humans)

33.  Many spinal reflexes involve only spinal cord neurons and occur without brain involvement  Some reflexes require the brain because many different types of information have to be evaluated to arrive at the right response