 The nervous system has three specific functions:  1. Sensory input. Sensory receptors present in skin and organs respond to external and internal stimuli by generating nerve impulses that travel to the brain and spinal cord.  2. Integration. The brain and spinal cord sum up the data received from all over the body and send out nerve impulses.  3. Motor output. The nerve impulses from the brain and spinal cord go to the effectors, which are muscles and glands. Muscle contractions and gland secretions are responses to stimuli received by sensory receptors.

The nervous system has two major divisions:  The central nervous system (CNS); - Includes the brain and spinal cord. - Have a central location.. they lie in the midline of the body.

 The peripheral nervous system (PNS) ; - which is further divided into the somatic division and the autonomic division. - includes all the cranial and spinal nerves.

 nervous tissue is made up of just two principal types of cells:  (1) neurons, also called nerve cells, which transmit nerve impulses;  (2) neuroglia, which supports and nourishes neurons

 three parts: a cell body {contains the nucleus as well as other organelles}., dendrite(s), and an axon.  In motor neurons, the dendrites are the many short extensions that receive signals from sensory receptors or other neurons.  The cell body {contains the nucleus as well as other organelles}.  At the dendrites, signals can result in nerve impulses that are then conducted by an axon.  The axon is the portion of a neuron that conducts nerve impulses.

 Long axons { called a nerve fiber } are covered by a white myelin sheath formed from the membranes of tightly spiraled neuroglia.  In the PNS, a neuroglial cell called a neurolemmocyte (Schwann cell) performs this function, leaving gaps called neurofibril nodes (nodes of Ranvier).  Another type of neuroglial cell performs a similar function in the CNS.

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 Motor neurons;  take nerve impulses from the CNS to muscles or glands.  are said to be multipolar because they have many dendrites and a single axon  cause muscle fibers to contract or glands to secrete, and therefore they are said to innervate these structures.

 Sensory neurons  take nerve impulses from sensory receptors to the CNS.  The sensory receptor, which is the distal end of the long axon of a sensory neuron, may be as simple as a naked nerve ending (a pain receptor), or it may be a part of a highly complex organ, such as the eye or ear.  Almost all sensory neurons have a structure that is termed unipolar

 In unipolar neurons, the extension from the cell body divides into a branch that comes to the periphery and another that goes to the CNS. Because both branches are long and myelinated and transmit nerve impulses, it is now generally accepted to refer to them collectively as an axon.

 Interneurons, also known as association neurons, occur entirely within the CNS.  Interneurons, which are typically multipolar  convey nerve impulses between various parts of the CNS. Some lie between sensory neurons and motor neurons, and some take messages from one side of the spinal cord to the other or from the brain to the cord, and vice versa.  They also form complex pathways in the brain where processes accounting for thinking, memory, and language occur.

 When axons are resting, they are not conducting nerve impulses. When they are active, axons are conducting nerve impulses, also called action potentials.  Resting Potential When an axon is resting, its membrane is polarized; that is, the outside is positive compared to the inside, which is negative.

 A protein carrier in the membrane, called the sodium- potassium pump, pumps sodium (Na) out of the axon and potassium (K) into the axon.  Another factor that causes the inside of the axon to be negative compared to the outside is the presence of large, negatively charged protein ions inside an axon.  The polarity across an axon that is not conducting nerve impulses is called the resting potential

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When the nerve fiber is conducting a nerve impulse (action potential), a change in polarity occurs across the axon’s membrane.  First, the inside of an axon becomes positive compared to the outside (this is called depolarization), and then the inside becomes negative again (this is called repolarization).  An action potential requires two types of channels in the membrane: One channel can allow Na ions to pass through the membrane, and the other can allow K ions to pass through the membrane. During depolarization, Na ions move to the inside of the axon, and during repolarization, K ions move to the outside.

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 Every axon branches into many fine endings, each tipped by a small swelling called an axon terminal.  Each swelling lies very close to either the dendrite or the cell body of another neuron. This region of close proximity is called a synapse  At a synapse, the membrane of the first neuron is called the presynaptic membrane, and the membrane of the next neuron is called the postsynaptic membrane.  The small gap between is the synaptic cleft.

 Transmission across a synapse is carried out by molecules called neurotransmitters, which are stored in synaptic vesicles in the axon terminals.  When nerve impulses traveling along an axon reach an axon terminal, channels for calcium ions (Ca2) open, and calcium enters the terminal.  This sudden rise in Ca2 stimulates synaptic vesicles to merge with the presynaptic membrane, and neurotransmitter molecules are released into the synaptic cleft.  They diffuse across the cleft to the postsynaptic membrane, where they bind with specific receptor proteins.

 Depending on the type of neurotransmitter and the type of receptor, the response of the postsynaptic neuron can be toward excitation or toward inhibition.

 The CNS, consisting of the brain and spinal cord, is composed of gray matter and white matter.  Gray matter is gray because it contains cell bodies and short, nonmyelinated fibers.  White matter is white because it contains myelinated axons that run together in bundles called tracts.

 Both the spinal cord and the brain are wrapped in protective membranes known as meninges  The dura mater ; The outer meninx, white, fibrous connective tissue that lies next to the skull and vertebrae.  The arachnoid; the next meninx, consists of weblike connective tissue, The subarachnoid space is filled with cerebrospinal fluid.  The pia mater is very thin and closely follows the contours of the brain and spinal cord

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 The spinal cord is a cylinder of nervous tissue that begins at the base of the brain and extends through a large opening in the skull called the foramen magnum.  The spinal cord is protected by the vertebral column, which is composed of individual vertebrae. The cord passes through the vertebral canal formed by openings in the vertebrae.

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 The spinal cord provides a means of communication between the brain and the peripheral nerves that leave the cord.  When someone touches your hand, sensory receptors generate nerve impulses that pass through sensory fibers to the spinal cord and up one of several ascending tracts to a sensory area of the brain.  When you voluntarily move your limbs, motor impulses originating in the brain pass down one of several descending tracts to the spinal cord and out to your muscles by way of motor fibers.

 The Cerebrum  is the largest portion of the brain in humans.  is the last center to receive sensory input and carry out integration before commanding voluntary motor responses.  The cerebrum carries out the higher thought processes required for learning and memory and for language and speech.

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 The Cerebral Hemispheres The cerebrum has two halves called the left and right cerebral hemispheres.  A deep groove, the longitudinal fissure, divides the left and right cerebral hemispheres. Still, the two cerebral hemispheres are connected by a bridge of white matter within the corpus callosum.  Convolutions called gyri are separated by shallow grooves called sulci.  The sulci divide each hemisphere into lobes

 The frontal lobe is anterior to the parietal lobe, which is anterior to the occipital lobe.  The temporal lobe is the lateral portion of the cerebral hemisphere.  The cerebral cortex is a thin but highly convoluted outer layer of gray matter that covers the cerebral hemispheres.  The cerebral cortex contains over one billion cell bodies and is the region of the brain that accounts for sensation, voluntary movement.

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 The Diencephalon Contains;  The hypothalamus is an integrating center that helps maintain homeostasis by regulating hunger, sleep, thirst, body temperature, and water balance.  The thalamus is on the receiving end for all sensory input except smell. The thalamus integrates this information and sends it on to the appropriate portions of the cerebrum.

 The Cerebellum  The cerebellum has two portions that are joined by a narrow median portion.  The cerebellum receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts.  the cerebellum sends motor impulses by way of the brain stem to the skeletal muscles.  the cerebellum maintains posture and balance.

 The Brain Stem  The brain stem contains the midbrain, the pons, and the medulla oblongata  The medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and vasoconstriction.

 The peripheral nervous system (PNS) lies outside the central nervous system  is composed of nerves and ganglia.  Nerves are bundles of myelinated axons.  Ganglia are swellings associated with nerves that contain collections of cell bodies.

 The cranial nerves are attached to the brain, Humans have 12 pairs of cranial nerves Most of the cranial nerves belong to the somatic system.  the spinal nerves are attached to the spinal cord. Humans have 31 pairs of spinal nerves; one of each pair is on either side of the spinal cord

 The PNS is subdivided into the somatic system and the autonomic system.  The somatic system serves the skin, skeletal muscles, and tendons. It includes nerves that take sensory information from external sensory receptors to the CNS.  The autonomic system, regulates the activity of cardiac and smooth muscles and glands. ** Sympathetic Division ** Parasympathetic Division

PARASYMPATHETIC DIVISIONSYMPATHETIC DIVISION Brain Constricts pupil Stimulates saliva production Constricts bronchi Slows heart Stimulates stomach, pancreas, and intestines Stimulates urination Promotes erection of genitals Spinal cord Eye Salivary glands Lung Heart Liver Stomach Adrenal gland Pancreas Intestines Bladder Genitals Dilates pupil Inhibits saliva production Relaxes bronchi Accelerates heart Stimulates epinephrine and norepi- nephrine release Stimulates glucose release Inhibits stomach, pancreas, and intestines Inhibits urination Promotes ejacu- lation and vaginal contractions