Chapter 8 notes The Nervous System PDF

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Chapter 8 Nervous System 3 Functions of the Nervous System: 1. Detects changes and feel sensations 2. Initiates responses to changes 3. Organizes and stores information Divisions of the Nervous System: has two divisions: - Central nervous system (C N S)—brain and spinal cord -

Peripheral nervous system (P N S)—cranial nerves and spinal nerves: o Includes nerves to and from skin and skeletal muscles. Also includes a subdivision nervous system called the autonomic nervous system (ANS), (fight or flight) with nerves to visceral effectors, and the enteric nervous system, which is located in the wall of the alimentary tube

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Peripheral nervous system relays information to and from the central nervous system the brain is the center of activity that integrates this information, initiates responses, and makes us the individuals we are.

Nerve Tissue—neurons, Schwann cells, and CNS neuroglia -

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Neuron (Nerve cell)— 3 main parts: I. Dendrites: processes (extensions) that transmit electrical impulses toward the cell body. II. Cell body: contains the nucleus and is essential for the continued life of the neuron o neuron cell bodies are found in the central nervous system or close to it in the trunk of the body. III. Axon: neuron transmits impulses away from the cell body, perhaps to another neuron, or to an effector cell of muscle or glandular tissue. Goes to the dendrites to the cell body and then the axon. o One-way street Synapse—neurotransmitter: o Synapse- small space which an electrical impulse cannot cross. o between the axon of one neuron and the dendrite or cell body of the next neuron, impulse transmission depends on chemicals called neurotransmitters. o A neurotransmitter carries the impulse across a synapse and is then destroyed by a chemical inactivator or absorbed (reuptake). o Synapses make impulse transmission one way in the living person. Neuron cell bodies are in the CNS or close to the CNS: Peripheral Nerves System – o The axons and dendrites are “wrapped” in specialize cells referred to a Schwann cell:

Schwann cells—only found in peripheral neurons .

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Specialized cells grow to surround the neuron processes, enclosing them in several layers of Schwann cell membrane o layers are the myelin sheath; myelin is a phospholipid that electrically insulates neurons from one another. Their nuclei and cytoplasm form the neurolemma: which becomes very important if nerves are damaged. o Neurolemma covers the myelin sheath o Neurolemma helps repair damage nerves If a peripheral nerve is severed and reattached precisely by microsurgery, the axons and dendrites may regenerate through the tunnels formed by the neurolemmas. Schwann cells are also believed to produce a chemical growth factor that stimulates regeneration.

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Neuroglia (glial cells)- (has many kinds of cells) only found in the central nervous - Specialized cells found only in the brain and spinal cord. I.

Oligodendrocytes form the myelin sheath in the CNS. o Produce the segments of the myelin sheath to electrically insulate neurons of the CNS

II.

Microglia- phagocytosis o Capable of movement and phagocytosis of pathogens and damaged tissue  Phagocytosis eats or engulf any pathogen that harms the body.

III.

Astrocytes form the blood–brain barrier. o Provide a framework for the developing fetal brain and support established neurons o Regulate localized blood flow in response to brain activity o Help maintain K+ level o Contribute to the blood–brain barrier  prevents potentially harmful waste products in the blood from diffusing out into brain tissue.  prevents potentially harmful waste products in the blood from diffusing out into brain tissue.  capillaries of the brain also contribute to this barrier; their cells are tightly packed edge to edge, making brain capillaries less permeable than other capillaries. e.g. Some harmful chemicals, such as alcohol and nicotine, do cross the blood– brain barrier, and the rabies virus is a pathogen that can cross it.

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Ependyma o Line the ventricles of the brain and central canal of the spinal cord; many of the cells have cilia; involved in circulation of cerebrospinal fluid

The Nerve Impulse: - A neuron not carrying an impulse: polarization o The membrane has a (+) charge outside and a (negative) inside. I. Polarization—neuron membrane has a positive (+) charge outside and a negative (–) charge inside; many Na+ ions outside and many K+ions inside.

Chapter 8 Nervous System II.

Depolarization—entry of N a+ ions, reversal of charges: on either side of the membrane; the outside becomes negative relative to the inside.  outside now has a negative (-) charge, and the inside has a positive (+) charge  depolarization takes place, the neuron membrane becomes very permeable to K+ ions, which rush out of the cell.

III.

Repolarization: is the exit of K+ ions and return to a (+) charge outside.  restores the positive charge outside and the negative charge inside o The term action potential refers to depolarization followed by repolarization  Then the sodium and potassium pumps return Na+ions outside and K+ions inside, and the neuron is ready to respond to another stimulus and transmit another impulse.  action potential in response to a stimulus takes place very rapidly and is measured in milliseconds.

Saltatory conduction—myelinated fibers; only the nodes of Ranvier depolarize and repolarize. Effect on speed: of impulses Electrical nerve impulses are all exactly alike; their different effects depend on their various destinations (muscles, glands, or other neurons). Synapses- small gap or space between the axon of one neuron and the dendrites or cell body of the next neuron o Neurons that transmit impulses to other neurons (or to effector cells) do not actually touch one another o Within the synaptic knob (terminal end) of the presynaptic axon is a chemical neurotransmitter that is released into the synapse by the arrival of an electrical nerve impulse o neurotransmitter diffuses across the synapse, combines with specific receptor sites on the cell membrane of the postsynaptic neuron, and there generates an electrical impulse that is, in turn, carried by this neuron’s axon to the next synapse, and so forth. 

A chemical inactivator at the cell body or dendrite of the postsynaptic neuron quickly inactivates the neurotransmitter. - Prevents unwanted, continuous impulses, unless a new impulse from the first neuron releases more neurotransmitter. -

Hyperpolarization- the neuron membrane becomes even more positive outside as K+ ions leave the cell or Cl–ions enter the cell, and therefore the neuron does not transmit an electrical impulse. Such inhibitory synapses are important, for example, for slowing the heart rate and for balancing the excitatory impulses transmitted to skeletal muscles

Neurotransmitters released only by a neuron’s axon e.g. acetylcholine, dopamine, norepinephrine, glutamate, and serotonin, GABA, epinephrine

Chapter 8 Nervous System

Types of Neurons (nerve fibers): 3 Types I. Sensory (afferent) neurons—carry impulses from receptors to the CNS e.g. optic nerves for vision and olfactory nerves for smell o Somatic: from skin, skeletal muscles, joints. o Visceral: from internal organs. II. Motor (efferent) neurons—carry impulses from the CNS to effectors e.g. mainly muscles and glands o Somatic: to skeletal muscle o Visceral: to smooth muscle, cardiac muscle, or glands.  Visceral motor neurons make up the autonomic nervous system. III. Interneurons—entirely within the C N S Nerves and Nerve Tracts: Sensory nerve—made only of sensory neurons. Motor nerve—made only of motor neurons. Mixed nerve—made of both sensory and motor neurons. Nerve tract—a nerve within the CNS; also called white matter The Spinal Cord: - Functions: Transmits impulses to and from the brain; o Integrates spinal cord reflexes - Location: Within the vertebral canal, extends from the foramen magnum to the second lumbar vertebra - Spinal cord ends between L1 and L2 - Cross-section: Gray matter is internal, shaped like an H, made of cell bodies: of motor neurons and interneurons o White matter is external, made of myelinated axons and dendrites, arranged in nerve tracts. - Ascending tracts—white matter; carry sensory impulses to the brain: - Descending tracts—carry motor impulses away from the brain: - Central canal—opens into the ventricles of the brain; contains cerebrospinal fluid: Spinal Nerves: 8 cervical pairs— to head, neck, shoulder, arms, and diaphragm 12 thoracic pairs— to trunk 5 lumbar pairs— to hip, pelvic cavity, and leg 5 sacral pairs— to hip, pelvic cavity, and leg 1 coccygeal pair— Cauda equina—lumbar and sacral nerves: that extend below the end of the spinal cord. Each spinal nerve has two roots: Dorsal (sensory)— neurons that carry impulses into the spinal cord. Dorsal root ganglion— contains cell bodies of sensory neurons Ventral root (motor)— the two roots unite to form a mixed spinal nerve.

Chapter 8 Nervous System it is made of the axons of motor neurons carrying impulses from the spinal cord to muscles or glands Outside the spinal cord the two roots unite— o

Spinal Cord Reflexes: Spinal Cord Reflexes—do not depend directly on the brain Reflex—an involuntary response to a stimulus Reflex arc—the pathway of impulses: during a reflex o Receptors: detect a change (the stimulus) and generate impulses. o Sensory neurons: transmit impulses from receptors to the CNS. o C N S: contains one or more synapses (interneurons may be part of the pathway). o Motor neurons: transmit impulses from the CNS to the effector. o Effector: performs its characteristic action. Stretch reflex—a muscle that is stretched will contract. o Example: patellar reflex, help keep us upright against gravity o Purpose: clinically to assess neurologic functioning, as are many other reflexes Flexor reflex—a painful stimulus causes withdrawal. o Example: If you inadvertently touch a hot stove, you automatically pull your hand away. o Purpose: the stimulus is something painful and potentially harmful, and the response is to pull away from it. The Brain—the many parts function as an integrated whole. Ventricles—four cavities: two lateral and the third and fourth ventricles – each contains a choroid plexus that forms cerebrospinal fluid o Choroid plexus— capillary network in each ventricle, which forma the cerebrospinal fluid from blood plasma. Medulla—posterior to the pons o Has centers to regulate vital functions: i. Cardiac centers regulate heart rate ii. Respiratory centers regulate breathing iii. Vasomotor centers regulate blood pressure o Has centers for reflexes: regulates reflexes of coughing, sneezing, swallowing, and vomiting. Pons—works with the medulla: contains two respiratory centers that work with those in the medulla to produce a normal breathing rhythm. The many other neurons in the pons (ponsis from the Latin for “bridge”) connect the medulla with other parts of the brain. Midbrain—extends from the pons to the hypothalamus hypothalamus and encloses the cerebral aqueduct, a tunnel that connects the third and fourth ventricles. Several different kinds of reflexes are integrated in the midbrain, including visual and auditory reflexes. If you see a wasp flying toward you, you automatically duck or twist away; this is a visual reflex. Keeping an eye on that wasp (actually both eyes) until it flies away is called tracking; the coordinated eye movements that make this possible are also a reflex.

Chapter 8 Nervous System Other examples are following the flight of a home run ball or a long touchdown pass. The ability to read depends in part on the coordinated movement of the eyeballs from word to word; this is scanning and is another reflex mediated by the midbrain. Turning your head (ear) to a sound is an example of an auditory reflex, as is the “startle” reflex, a jump or cringe upon hearing a sudden loud sound such as thunder. The midbrain is also concerned with what are called righting reflexes, those that keep the head upright and maintain balance or equilibrium. When a cat falls from a tree and lands on its feet (if it was not too high up), its midbrain helped get its head upright, and other parts of the brain then swung the trunk and legs into position for a landing. Human midbrains are not nearly that efficient.

Contains reflex centers: auditory reflexes, and righting (equilibrium) reflexes. Cerebellum—posterior to the pons and medulla, has two hemispheres Its functions are concerned with voluntary movement. o regulates coordination of voluntary movement, muscle tone, stopping movements, and equilibrium; contributes to sensations involving texture and weight. Hypothalamus—above the pituitary gland and below the thalamus o 1. Produces antidiuretic hormone (A D H) and oxytocin: which increases water reabsorption by the kidneys; oxytocin which promotes uterine contractions for labor and delivery o 2. Produces releasing hormones: that regulate the secretions of the anterior pituitary gland o 3. Regulates body temperature: promoting responses such as sweating in a warm environment or shivering in a cold environment o 4. Regulates food intake: respond to changes in blood nutrient levels, to chemicals secreted by fat cells, and to hormones secreted by the gastrointestinal tract o 5. Regulates the autonomic nervous system and visceral responses: emotional situations o 6. Regulates body rhythms: Acts as a biological clock - Thalamus—below the cerebrum and above the hypothalamus o 1. Groups sensory impulses: as to body part before relaying them to the cerebrum; awareness of pain but inability to localize o 2. Suppresses unimportant sensations: permit concentration o 3. Contributes to alertness and memory: awareness - Cerebrum—two hemispheres o Corpus callosum: permits communication between the hemispheres. o Cerebral cortex—gray matter: which consists of cell bodies of neurons and is folded extensively into convolutions. o White matter: consists of nerve tracts that connect the lobes of the cerebrum to one another and to other parts of the brain. - Frontal lobes: motor areas: initiate voluntary movement o Premotor areas: regulates sequences of movements for learned skills o Prefrontal areas: for aspects of social behavior o Broca’s motor speech area: (left hemisphere) regulates the movements involved in speech.

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Parietal lobes o General sensory area: feels and interprets the cutaneous senses and conscious muscle sense o Speech areas: (left hemisphere) for thought before speech. o Part of taste areas: extends into temporal lobe, for sense of taste Temporal lobes o Auditory areas: for hearing and interpretation o Olfactory areas: for sense of smell and interpretation o Speech areas: for thought before speech. Occipital lobes o Visual areas: for vision; interpretation areas for spatial relationships. Association areas: in all lobes, for abstract thinking, reasoning, learning, memory, and personality. The hippocampi are essential for the formation of memories. Neural plasticity is the ability of the brain to adapt to changing needs. o Basal ganglia—masses of gray matter within the white matter: within the cerebral hemispheres; regulate accessory movements such as gestures and facial expressions, as well as muscle tone.

Meninges and Cerebrospinal Fluid (CSF): - Three meningeal layers— made of connective tissue o Dura mater: outer o Arachnoid membrane: middle o Pia mater: inner  all three enclose the brain the brain and spinal cord. - Subarachnoid space contains C S F, the tissue fluid of the C N S; it is produced in the ventricles of the brain; o Functions of C S F: formed continuously in the ventricles of the brain by choroid plexuses, from blood plasma. -

Circulation of C S F—made from blood plasma and is returned to the blood o C S F is formed in the ventricles and circulates in the central canal of the spinal cord and the cranial and spinal subarachnoid spaces. o C S F is reabsorbed from the cranial subarachnoid space, through the arachnoid villi, into the blood in the cranial venous sinuses. As tissue fluid, CSF brings nutrients to CNS neurons and removes waste products. CSF also acts as a shock absorber to cushion the CNS. Cranial Nerves: - Twelve pairs of nerves that emerge from the brain - A way to remember: Oh Oh Oh To Tickle A Female’s Vagina Gave Victor A Hard-on o

Most are mixed nerves; some are sensory. I. Olfactory- sense of smell II. Optic- sense of light III. Oculomotor- movement of the eyeball; constriction of pupil in bright light or for near vision IV. Trochlear- movement of eyeball

Chapter 8 Nervous System V. VI. VII. VIII. IX. X.

XI. XII.

Trigeminal- sensation in face, scalp, and teeth; contraction of chewing muscles Abducens- movement of the eyeball Facial- sense of taste; contraction of facial muscles; secretion of saliva Acoustic (vestibulocochlear)- sense of hearing; sense of equilibrium Glossopharyngeal- sense of taste; sensory for cardiac, respiratory, and blood pressure reflexes; contraction of pharynx; secretion of saliva Vagus- sensory in cardiac, respiratory, and blood pressure reflexes; sensory and motor to larynx (speaking); decreases heart rate; contraction of alimentary tube (peristalsis); increases digestive secretions Accessory- contraction of neck and shoulder muscles; motor to larynx (speaking) Hypoglossal- movement of the tongue

The Autonomic Nervous System (A N S)—consists of motor neurons to visceral effectors - Has two divisions—sympathetic and parasympathetic o their functioning is integrated by the hypothalamus. Consists of motor neurons to visceral effectors: smooth muscle, cardiac muscle, and glands. -

An A N S pathway has two neurons that synapse in a ganglion (see Table 8–5). o Preganglionic neuron—from the C N S to a ganglion o Postganglionic neuron—from the ganglion to an effector: o Sympathetic ganglia— are in two chains just outside the vertebral column o Parasympathetic ganglia— are very near or in the visceral effectors.

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Autonomic Nervous System (see Table 8–6 for specific functions) o Sympathetic division—dominates in stressful situations: responses prepare the body to meet physical demands. (fight our flight)  Preganglionic neuron cell bodies are in the thoracic and lumbar spinal cord  Most ganglia are located in two chains outside the spinal column  One preganglionic neuron synapse with many postganglionic neurons, which go to many effectors  The neurotransmitters acetylcholine and norepinephrine are released o

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Parasympathetic division—dominates in relaxed situations: to permit normal functioning. ( rest and digest)  Preganglionic neuron cell bodies are in the brain and sacral spinal cord  Ganglia are located near or in the visceral effector  One preganglionic neuron synapse with only a few postganglionic neurons, which go to many effector  All neurons release the neurotransmitter acetylchline Neurotransmitters:  Acetylcholine: is released by all preganglionic neurons and by parasympathetic postganglionic neurons; the inactivator is cholinesterase

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Norepinephrine: is released by most sympathetic postganglionic neurons; the inactivator is COMT or MAO.

Table 8-6 Functions of the autonomic nervous system ORGAN SYMPATHETIC RESPONSE Heart (cardiac muscle)

Increase rate

Bronchioles (smooth muscle)

Dilate

Iris (smooth muscle) Salivary glands Stomach and intestines (smooth muscle) Stomach and intestines (glands) Internal anal sphincter Urinary bladder (smooth muscle) Internal urethral sphincter Liver Pancreas

Pupil dilates Decrease secretion Decrease peristalsis

Sweat glands Blood vessels (their smooth mu...