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Chapter 9 Nervous System Human Anatomy & Physiology P. Wilson 1 Organization of Nervous System Human Anatomy & Physiology P. Wilson 2 Organization of Nervous System Human Anatomy & Physiology P. Wilson 3 II. A & B: There are 2 types of cells in the nervous system: • Neurons are the structural & functional units of the nervous system • Glial cells (aka neuroglial cells) provide support for the neurons. • In fact, neurons could not exists without the glial cells performing their various functions. Human Anatomy & Physiology P. Wilson 4 II. C: Organs of the Nervous System The organs of the nervous system can be divided into 2 groups: 1. The central nervous system (CNS) consists of the brain and the spinal cord. 2. The peripheral nervous system (PNS) consists of nerves (peripheral nerves) that connect the CNS to other body parts. Human Anatomy & Physiology P. Wilson 5 II. D: Functions of the Nervous System Together the CNS and PNS allow us to think, remember, move, to be aware of, and to respond, to conditions in the world in which we live. This is possible because the nervous system provides sensory, integrative, and motor functions to the body. Human Anatomy & Physiology P. Wilson 6 III. A: Sensory Functions • Sensory receptors at the end of peripheral nerves gather information by detecting changes to the body’s environment (both external & internal changes) • then convert the information into nerve impulses that travel to the CNS. Human Anatomy & Physiology P. Wilson 7 III. A: Integrative functions • To integrate means to combine parts into a whole, so … • In the brain, sensory nerve impulses are integrated into perceptions ( a sensation that is an interpretation of the combined & coordinated impulses) Human Anatomy & Physiology P. Wilson 8 III. A: Motor functions • Once a decision (conscious or unconscious) is made, motor impulses travel to effector organs and a response to the sensory input is stimulated. • Motor responses are carried out by effectors (a muscle or gland that effects change in the body). Human Anatomy & Physiology P. Wilson 9 III. B: PNS & Motor Functions The motor functions of the PNS can be divided into 2 categories: • the somatic system is under conscious control; the somatic motor pathways lead to the skin and to skeletal muscles (aka voluntary muscles) • the autonomic system controls effectors that are involuntary – the heart, smooth muscles in blood vessels and in viscera, and glands Human Anatomy & Physiology P. Wilson 10 IV. Neuroglial Cells • Neuroglial cells perform functions that are vital to neurons by filling spaces, support neurons by providing nutrition and structural frameworks, producing myelin, and performing phagocytosis. Human Anatomy & Physiology P. Wilson 11 Human Anatomy & Physiology P. Wilson 12 IV. CNS Glial Cells There are 4 types of glial cells in the CNS: • microglia cells are small cells that are scattered throughout the CNS an function to phagocytize bacterial cells and cellular debris • oligodendrocytes occur in rows along nerve fibers and provide insulating layers of myelin in the brain and spinal cord Human Anatomy & Physiology P. Wilson 13 IV. CNS Glial Cells (glial cells in the CNS continued) • astrocytes, found between blood vessels & nerves, support structures, aid in metabolism, and respond to brain injury by filling in spaces (also, form scar tissue following injuries to the CNS) • ependymal cells form epithelial-like membranes found in specialized parts of the brain and spinal cord; they help regulate the production of spinal fluid Human Anatomy & Physiology P. Wilson 14 IV. Glial Cells in the PNS Schwann cells are found in the PNS and form a covering called the myelin sheath around neural axons (Iike a band-aide around a finger) The outer, living layer of the Schwann cells (neurilemma) plays an important role in the regeneration of damaged peripheral nerves (the axons can regenerate) In contrast, when neurons of the CNS (myelinated by oligodendrocytes) are damaged the axons do not generally regenerate. Human Anatomy & Physiology P. Wilson 15 Human Anatomy & Physiology P. Wilson 16 Head lines!! p. 206 • Excess neuroglial cells cause harm… • Why are babies so uncoordinated?!... • Neuron cell body the size of a tennis ball?!... • Grey matter vs white matter… Human Anatomy & Physiology P. Wilson 17 V. Neurons • Neurons vary on size and shape but ALL neurons have 3 parts: a cell body dendrites an axon • Mature neuron cells do not divide BUT some parts of the nervous system contain neural stem cells that can differentiate into neurons or neuroglia. Human Anatomy & Physiology P. Wilson 18 Human Anatomy & Physiology P. Wilson 19 Neuron Cell Body • The cell body of a neuron consists of: granular cytoplasm, cell membrane, various organelles including a nucleus (with a prominent nucleolus), mitochondria, & Nissl bodies (similar to the rough endoplasmic reticulum) with attached ribosomes (Do you remember the function of ribosomes?) Human Anatomy & Physiology P. Wilson 20 Dendrites • The receptive part of the neuron consists of short, highly branched structures called dendrites. • Dendrites are receive nerve impulses from the axons of other neuron cells and transmit the impulse to the cell body. Human Anatomy & Physiology P. Wilson 21 Axon • A single axon carries the nerve impulse away from the cell body. • Large axons of the PNS are enclosed in myelin sheaths composed of many Schwann cells. • The portions of the Schwann cell that contains most of the cytoplasm & the nucleus remain outside the sheath and form the neurilemma, which assists in the repair of damaged axons. • Gaps in the myelin sheath are called nodes of Ranvier; these “gaps” facilitate rapid conduction of nerve impulses Human Anatomy & Physiology P. Wilson 22 Myelinated vs Unmyelinated • Myelinated axons have myelin sheaths are appear white. • Masses of myelinated axons and their cell bodies form the white matter within the CNS • Unmyelinated axons lack myelin sheaths are appear gray. • Masses of unmyelinated axons and their cell bodies form the gray matter within the CNS Human Anatomy & Physiology P. Wilson 23 Neurons Classified by Structure • Bipolar neurons have only 2 processes arising from each end of their cell body; one is a dendrite and one is an axon. both processes are structurally similar Neurons within specialized parts of the eyes, nose, & ears are bipolar Human Anatomy & Physiology P. Wilson 24 Neurons Classified by Structure • Unipolar neurons have a single process extending from their cell body; a short distance from the cell body this process divides into 2 branches. The cell bodies of these neurons aggregate in groups called ganglia which are located outside the CNS; both branches really function as 1 axon the peripheral branch is associated with dendrites near a peripheral body part and has a “trigger zone” – the initial portion of the axon the other branch enters the brain or spinal cord Human Anatomy & Physiology P. Wilson 25 Neurons Classified by Structure • Multipolar neurons have many process arising from their cell body – many dendrites and one axon; most of the neurons whose cell bodies lie in the brain and spinal column are multipolar. Human Anatomy & Physiology P. Wilson 26 Human Anatomy & Physiology P. Wilson 27 Neurons Classified by Function • Sensory (afferent) neurons carry nerve impulses from peripheral receptors to the CNS. afferent neurons are usually unipolar (some are bipolar) sensory neurons are located in peripheral body parts and have specialized receptor ends at the tips of their dendrites or their dendrites are closely associated with receptor cells in the skin or sensory organs Human Anatomy & Physiology P. Wilson 28 Neurons Classified by Function • Interneurons carry nerve impulses from one part of the CNS to another. interneurons are multipolar interneurons lie entirely within the brain & spinal cord (the CNS) Human Anatomy & Physiology P. Wilson 29 Neurons Classified by Function • Motor (efferent) neurons carry nerve impulses from the CNS to effectors. efferent neurons are multipolar carry nerve impulses to stimulate muscles to contract and/or glands to release secretions Human Anatomy & Physiology P. Wilson 30 CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. VI. Cell Membrane Potential A. A cell membrane is usually polarized (electrically charged), with an excess of negative charges on the inside of the membrane. Polarization is important to the conduction of nerve impulses. • This separation of charge (or potential difference) is called the resting potential. 9 - 31 VI. Cell Membrane Potential B. The + ions involved in enabling the conduction of a nerve impulse are Na+ and K+ ions. • There is a high concentration of Na+ ions outside the cell while there is a high concentration of K+ ions inside the cell (along with many negative ions contained in the cytoplasm). • K+ ions pass through the cell membrane more easily than Na+ ions; this make K+ ions a major contributor to cell polarization, depolarization, and repolarization Human Anatomy & Physiology P. Wilson 32 CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. 9 - 33 VI. Cell Membrane Potential C. Conduction of a nerve impulse • Resting potential: higher concentration of Na+ ions outside, higher concentration of K+ ions inside. • A threshold stimulus causes sodium channels to open, allowing Na+ ions to diffuse in the cell membrane depolarizes • SOON after, potassium channels open and K+ ions move out the cell membrane repolarizes This rapid change is called an action potential. Human Anatomy & Physiology P. Wilson 34 VI. Cell Membrane Potential C. A nerve impulse is created as depolarization creates a wave of action potentials (a localized bioelectric current) that move down the axon. Human Anatomy & Physiology P. Wilson 35 VII. Nerve Impulse A. Nodes of Ravier: • Unmyelinated fibers conduct impulses over their entire membrane surface. • Myelinated fibers conduct impulses from node of Ranvier to node of Ranvier; a phenomenon called saltatory conduction. • Therefore, impulses move faster along myelinated fibers that along unmyelinated fibers. Human Anatomy & Physiology P. Wilson 36 VII. Nerve Impulse B. All-or-nothing Response • • • When the threshold stimulus is applied to an axon it responds completely by conducting the impulse (all-or-nothing response). The result is that all impulses carried on a particular axon is of the same strength. Greater intensity of stimulation triggers more impulses per second, not stronger impulses. Human Anatomy & Physiology P. Wilson 37 VII. Nerve Impulse C. Refractory Period • There is a brief period, following a nerve impulse, when a threshold stimulus will not trigger another impulse on an axon – this is called the refractory period. • This limits the frequency of impulses in an axon. Frequency as high as 700 impulses per second is possible but 100 impulses per second is more common. Human Anatomy & Physiology P. Wilson 38 VIII. The Synapse A. Synaptic Transmission 2. When an impulse (an action potential) reaches the synaptic knobs of an axon, synaptic vesicles release neurotransmitter into the synaptic cleft. – The neurotransmitter reacts with specific receptors on the postsynaptic membrane. Human Anatomy & Physiology P. Wilson 39 VIII. The Synapse 3. About 50 types of neurotransmitters have been identified • acetylcholine • monoamines such dopamine, serotonin, epinephrine, norepinephrine • several amino acids, and a large group of neuropeptides Human Anatomy & Physiology P. Wilson 40 VIII. The Synapse 3. Stimulation of the nerve fiber stops when the neurotransmitter is degraded by an enzyme or reabsorbed by the presynaptic neuron. Human Anatomy & Physiology P. Wilson 41 IX. Impulse Processing Neuronal Pools Neurons within the CNS are organized into neuronal pools with varying numbers of cells. Each pool receives input from afferent nerves and processes the information according to the special characteristics of the pool. Human Anatomy & Physiology P. Wilson 42 IX. Impulse Processing Facilitation A particular neuron of a pool may receive excitatory or inhibitory stimulation; if the net effect is excitatory but sub-threshold, the neuron becomes more excitable to incoming stimulation (a condition called facilitation). Human Anatomy & Physiology P. Wilson 43 IX. Impulse Processing Convergence A single neuron within a pool may receive impulses from two or more fibers (convergence), which makes it possible for the neuron to summate impulses from different sources. Human Anatomy & Physiology P. Wilson 44 IX. Impulse Processing Divergence Impulses leaving a neuron in a pool may be passed into several output fibers (divergence), a pattern that serves to amplify an impulse. Human Anatomy & Physiology P. Wilson 45 X. Nerves • A nerve is a bundle of nerve fibers held together by layers of connective tissue. nerves that carry impulses to the brain or spinal cord (CNS) are referred to as sensory nerves nerves that carry impulses to the muscles or glands are referred to as motor nerves most nerves include both sensory & motor fibers and are called mixed nerves Human Anatomy & Physiology P. Wilson 46 Nerve Pathways • The routes that nerve impulses follow as they travel through the nervous system are referred to as nerve pathways. • A reflex (the simplest of nerve pathways) is an automatic subconscious response to stimuli (changes within or without the body). Human Anatomy & Physiology P. Wilson 47 Nerve Pathways • A reflex (the simplest of nerve pathways) is an automatic, involuntary (made without conscious thought) response to stimuli (changes within or without the body). reflexes help maintain homeostasis by controlling involuntary processes such as heart rate & breathing reflexes also carry out the automatic actions of swallowing, sneezing, coughing, & vomiting Human Anatomy & Physiology P. Wilson 48 Reflex Arc A reflex arc begins with a sensory receptor (dendrites of a sensory neuron or a specialized receptor cell in a sensory organ) a sensory neuron carries the impulse to the CNS an interneuron in the CNS conducts the impulse to a motor neuron the motor neuron transmits impulse from CNS out to an effector which responds by producing a reflex (behavioral action). Human Anatomy & Physiology P. Wilson 49 Knee Jerk! • The knee-jerk reflex is an example or a simple reflex that utilizes only 2 neurons: a sensory neuron communicates directly with a motor neuron (no interneuron as a go-between!!) sensory neuron motor neuron Human Anatomy & Physiology P. Wilson 50 CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. 9 - 51 XII. Bony Coverings • The brain is covered by the cranial cavity of the skull. • The spinal cord is covered by the spinal or vertebral column, Human Anatomy & Physiology P. Wilson 52 Meninges – Dura Mater • The dura mater is the outermost layer of the meninges; it is formed of tough, white fibrous connective tissue is attached to the inside of the skull contains blood vessels & nerves forms partitions that support & protect the brain & spinal cord. Human Anatomy & Physiology P. Wilson 53 Meninges – Arachniod Mater • The arachnoid mater is the middle layer of the meninges; it is a thin, web-like membrane that spreads over the surface of the brain & spinal cord • The subarachnoid space (between this mater and pia mater) contains the cerebrospinal fluid (CSF) CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. 9 - 55 Meninges – Pia Mater • The innermost meninges is the pia mater. • It is very thin; contains numerous nerves & blood vessels. • Nourishes the cells of the brain Spinal Cord • The spinal cord is continuous with the brain, but… • The superior boundary of the spinal cord is the inferior end of the cranium. • The inferior boundary of the spinal cord is at the disk between the L1 (1st lumbar) and L2 (2nd lumbar) vertebrae. • There are 31 pairs of spinal nerves. CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. 9 - 58 Spinal Cord • The spinal cord has 2 major functions: 1. conducting nerve impulses 2. Serving as a center for spinal reflexes • The nerve tracts that carry sensory information to the brain are called ascending tracts. Injury to ascending tracts results in a loss of sensation in body parts distal to the injury Spinal Cord • The nerve tracts that conduct motor impulses from the brain to muscles and glands are called descending tracts. Injury to descending tracts results in a loss of motor function in body parts distal to the injury 2 Types of Descending Spinal Tracts • Pyramidal tracts (aka corticospinal tracts) • Carry impulses that control skeletal muscle movements. • Extrapyramidal tracts control motor activities associated with maintaining balance and posture The Brain The major parts of the brain are: • the cerebrum (the largest portion of the brain) • the cerebellum • the diencephalon • the brain stem CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. 9 - 63 Structure of the Cerebrum • Two large masses – left & right cerebral hemispheres connected by a bridge of neural fibers called the corpus callosum. • Surface has many ridges or convolutions (aka gyri) separated by grooves. • A shallow groove is called a sulcus; a deep groove is called a fissure. Lobes of the Cerebrum • Frontal lobe • Parietal lobe • Temporal lobe • Occipital lobe • Insula The first 4 lobes listed correspond to the bones of the skull with the same names Cerebrum • The outermost layer of the cerebrum is the cerebral cortex, a thin layer of gray matter (myelinated or unmyelinated?). • The cerebral cortex contains nearly 75% of all the neuron cell bodies in the nervous system. • Beneath the cortex is a mass of white matter that makes up the bulk of the cerebrum. Cerebrum Cerebral functions include: • Sensory – interpret sensory input & give rise to sensations or feelings • Motor – generate nerve impulses that control muscle activity • Associative – interpreting, reasoning, memory, & other higher brain functions Cerebrum Functional areas include: • Motor major motor areas in frontal lobe just in front of central sulcus control skeletal muscles speech movements controlled by Broca’s area frontal eye field controls voluntary movements of eyes and eyelids Cerebrum Functional areas include: • Sensory – parietal lobes, posterior occipital lobes, temporal lobes cutaneous sensations from parietal lobe visual areas in posterior occipital lobe auditory areas in the temporal lobe taste areas are near the base of central sulci smell arises from areas deep within the cerebrum Cerebrum Functional areas include: • Associative – anterior portions of frontal lobe, lateral portions of parietal, temporal, & occipital lobes memory, reasoning & problem solving, judgment, emotion CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. 9 - 71 Dominant or Non-dominant Dominant Hemisphere • Used for languagerelated activities such as speech, reading, writing, and for complex intellectual skills that require verbal, analytical, and computational skills. • In 90% of population left hemisphere is dominant. Non-dominant Hemisphere • Used for motor tasks that require orientation of the body in space, understanding & interpreting musical patterns, and non-verbal visual experiences. • Also controls intuitive thinking and emotional thinking. Human Anatomy & Physiology P. Wilson 72 Cerebral Spinal Fluid (CSF) • The pia mater contains specialized cauliflower-like masses of specialized capillaries that secrete CSF. • The brain & spinal cord float in the CSF which supports and protects them by acting as shock absorbers. • CSF also maintains electrolyte balance and serves as a pathway to the blood for excretion of cellular waste. Human Anatomy & Physiology P. Wilson 73 Diencephalon Thalamus • Functions in sorting & directing sensory information arriving from other parts of the nervous system. • Perform services of both messenger & editor. (It is the executive secretary for the cerebrum!) • Produces an awareness of pain, touch, & temperature Human Anatomy & Physiology P. Wilson 74 Diencephalon Hypothalamus • Maintains homeostasis by regulating heart rate & rhythm, hunger & weight gain, various other visceral functions • Regulates secretions of various hormones, including those that stimulate sleep and wakefulness Human Anatomy & Physiology P. Wilson 75 Limbic System • The limbic system includes the thalamus, hypothalamus. It: controls emotional experience and expression recognizes threats to the organisms and produces emotions such as fear, anger, & pleasure, which leads to behaviors that increase chances for survival Human Anatomy & Physiology P. Wilson 76 Brain Stem • Lies between the brain and spinal cord and connects the two structures. • Three parts: midbrain pons medulla oblongata Human Anatomy & Physiology P. Wilson 77 Midbrain • It is like a doorman to the cerebrum by conveying nerve impulses to & from the cerebrum. • Several visual reflexes located here, including those that turn the eyes in concert with the head and auditory reflexes such as those that turn the head in the direction of a sound. Human Anatomy & Physiology P. Wilson 78 Pons • Helps regulated rate & depth of breathing • Relays impulses to the cerebellum and from the periphery to the higher brain centers Human Anatomy & Physiology P. Wilson 79 Medulla Oblongata • Transmits all ascending and descending impulses to the spinal cord • Control center for vital reflexes that an individual alive (heart, breathing) • Also the center for reflexes like sneezing and coughing Human Anatomy & Physiology P. Wilson 80 Cerebellum • Cerebellum is the reflex control center – it integrates sensory information concerning the position of the body in space, and for coordinating complex skeletal movements • Damage to the cerebellum results in tremor, loss of precision in skeletal muscles, loss of muscle tone, abnormal gait, & loss of balance. Human Anatomy & Physiology P. Wilson 81 PNS – 2 parts: Somatic & Autonomic • The somatic system is under conscious control • The autonomic system controls effectors that are involuntary Human Anatomy & Physiology P. Wilson 82 Cranial Nerves There are 12 pair of cranial nerves. I Olfactory………………. On II Optic…………………… Old III Oculomotor……………. Olympus IV Trochlear………………. Towering V Trigeminal……………… Tops VI Abducens…………….... A VII Facial…………………… Friendly VIII Vestibulocochlear……... Viking IX Glossopharyngeal……... Grew X Vagus……………………. Vines XI Accessory……………….. And XII Hypoglossal…………...... Hops Human Anatomy & Physiology P. Wilson 83 Divisions of the Autonomic Nervous System • The sympathetic division – prepares you for energy-expending, stressful, or emergency situations (fight or flight) • The parasympathetic division – is most active under ordinary, restful conditions (rest / digest) Human Anatomy & Physiology P. Wilson 84