The Nervous System Reading: Chapters 4,5. Coordinates (with endocrine system) organ system activities in response to varying external conditions. Composed of the central nervous system (CNS) consisting of the brain and spinal cord, and the peripheral nervous system consisting of communication pathways from CNS to rest of the body. Communication occurs via electrical impulses (high speed pathway). Functions of the nervous system. Monitors the internal and external environments. Integrates sensory information. Coordinates voluntary and involuntary activities of other organ systems.

These functions are performed by neurons, which are supported by neuroglia (external support cells).

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Organization of the Nervous System Within peripheral nervous system, there are the afferent and efferent divisions. Afferent brings sensory information to CNS. Efferent carries motor commands from CNS to muscles, glands, etc. Within efferent division, there are 2 sub-divisions: Somatic nervous system (SNS) provides voluntary control over skeletal muscles. Autonomic nervous system (ANS) provides involuntary control of cardiac and smooth muscles, and glands.

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Cellular Organization of Neural Tissue Neural system consists of all neural tissue in the body. Neural tissue consists of two types of cells: neurons and neuroglia (also called glial cells). Neurons are the basic units of the nervous system, and all neural functions involve the communication of one neuron with another, and with other cells. Neuroglia regulate environment around the neurons, provide structural support for neurons, and act to recycle debris (phagocytes). Neuroglia cells outnumber neurons. Anatomy and Physiology for Engineers Slide 5-3

Neurons Classified into 3 functional groups: Sensory neurons (Number approximately 106). Make up the afferent division of the peripheral nervous system. Responsible for providing information regarding the external environment (touch, pressure, temperature, smell, hearing), the position of the skeletal muscles and joints, and the activities of the other organ systems (digestive, cardiovascular, etc).

Motor neurons (Number approximately 500,000). Part of the efferent system (carries info. from CNS to tissues, organs and organ systems). Somatic motor nuerons target voluntary tissues (skeletal muscles) while visceral (autonomic) motor neurons target involuntary tissues (cardiac and smooth muscles, glands).

Interneurons (Number approximately 20 X 109). Located entirely within brain and spinal cord (CNS). Function to interconnect other neurons. Responsible for the analysis of sensory inputs. Anatomy and Physiology for Engineers Slide 5-4

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General Structure of Neurons

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General Structure of Neurons Cell body (soma).

Soma of the neuron contains large round nucleus with prominent nucleolus. No centrioles ‡ neurons cannot be replaced once they differentiate. Numerous other cellular organelles (mitochondria, free and fixed ribosomes, rough endoplasmic reticulum, etc). Rough ER and ribosomes also found in clusters called Nissl bodies (gray color ‡ gray matter). Anatomy and Physiology for Engineers Slide 5-6

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General Structure of Neurons Cell body (soma). Dendrites to receive information. Axon to carry information. Synaptic knobs to transmit information to other cell. Projecting from soma are variable number of dendrites and a single long axon. Stimulation of dendrite or cell body (mechanical, electrical, chemical) produces action potential that travels along axon. Base of axon connected to soma at axon hillock, where action potential begins. Axon may branch along its length producing collaterals. Each collateral ends at a synaptic terminal (synaptic knob). Synaptic terminal is part of the synapse ‡ where communication with another cell takes place. Anatomy and Physiology for Engineers Slide 5-7

Structural Classification of Neurons

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Neuroglia (glial cells) Found in both the CNS and PNS, but CNS has greatest diversity. 4 types of glial cells found in CNS: Astrocytes Largest and most numerous neuroglia. Responsible for maintaining the blood-brain barrier, structural stability of CNS, and repair of damaged neural tissue.

Oligodendrocytes Responsible for coating axons with cytoplasmic extensions (myelin sheath). Microglia Smallest and rarest of neuroglia in CNS. Phagocytic cells responsible for immune protection.

Ependymal cells Line central canal of spinal cord and ventricles of the brain. In certain locations, ependymal cells produce cerebrospinal fluid.

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Neuroglia in the CNS

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Neuroglia in the Peripheral Nervous System Schwann cells are most important glial cells in peripheral nervous system. Cover every axon outside CNS (whether myelinated or unmyelinated). This myelin sheath increases impulse conduction rate. Anatomy and Physiology for Engineers Slide 5-11

Neurophysiology Cell membrane (for generic cell) has excess positive charges (ions) on outside versus inside. This membrane potential for neurons is around –70 mV. Resting membrane potential is due to relative concentrations of ions. Larger concentration of Na+, Cl- in extracellular fluid. Higher concentrations of K+ and negatively charged proteins within cell. Since cell membrane is more permeable to K+ ions than Na+ ions in resting state, rate of K+ efflux is greater than Na+ influx ‡ cell membrane is slightly negative. Anatomy and Physiology for Engineers Slide 5-12

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Changes in Membrane Potential Changes in membrane potential can occur due to: Alteration of permeability of cell membrane. Alteration in the activity of an exchange pump.

This can occur due to exposure to chemical, electrical, temperature, ionic, or mechanical stimulations. Changes can occur almost instantaneously (ex: muscle cell contraction). Usually stimulation opens closed channels within membrane, thereby changing permeability of cell membrane to specific ions. Anatomy and Physiology for Engineers Slide 5-13

Polarization and Potential Depolarization: making the membrane potential more positive. Ex: opening of selective Na+ channels, allowing extracellular Na+ to diffuse faster into the cell, causing membrane potential to rise toward 0 mV.

Hyperpolarization: making the membrane potential more negative: Ex: opening of selective K+ channels, causing intracellular K+ to diffuse out of cell.

Graded Potential: Variation in cell membrane permeability and polarization at a local site. Action Potential: Variation in polarization across entire cell membrane. Graded potential acts as the stimulus that produces the action potential Anatomy and Physiology for Engineers Slide 5-14

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Generation of the Action Potential

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Conduction of an Action Potential Continuous (unmyelinated) and saltatory (myelinated) conduction.

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Synaptic Communication Action potential carries information from one point to another via the nervous system. Information transfer at the end of an axon occurs via the release of chemicals known as neurotransmitters. The location at which this information transfer takes place is the synapse. Synaptic cleft: space between the two cell membranes. Presynaptic neuron ‡ transmits information to postsynaptic neuron, muscle or gland cell. Presynaptic neuron contains specific neurotransmitters enclosed in vesicles.

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Events in the Synaptic Connection

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The Reflex Arc

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Anatomy of the Nervous System Introduction to terminology. In peripheral nervous system: Ganglia: groups of neuron cell bodies. Nerves: bundles of axons (spinal nerves connect to spinal cord; cranial nerves connect to brain).

In central nervous system: Centers: collection of neuron cell bodies that share a particular function. Nucleus: center that has a distinct anatomical boundary. Neural Cortex: gray matter on the surface of the brain. Tracts: bundles of axons within CNS that share common origin, destination and function. Columns: groups of tracts. Pathways: avenues that link brain centers to rest of the body (sensory pathways ‡ bring information to brain; motor pathways ‡ transmit information from brain to muscles).

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Anatomy of the Nervous System

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Anatomy of the Nervous System Protection of central nervous system is provided by glial cells and by the meninges and the blood-brain barrier. Meninges: Series of three specialized membranes. Dura Mater Tough fibrous outermost covering of the CNS. Epidural space: between dura mater of spinal cord and walls of vertebral column.

Arachnoid Intervening space containing a layer of squamous cells. Subarachnoid space: deeper layer within meninges that contains the cerebrospinal fluid.

Pia Mater Innermost layer, in direct contact with underlying neural tissue. Highly vascular ‡ blood to supply CNS function. Anatomy and Physiology for Engineers Slide 5-22

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Anatomy of the Nervous System

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Blood-Brain Barrier Neural tissue is isolated from rest of circulation by this barrier. Maintained by astrocytes, which cause capillary membranes in the CNS to be impermeable to various compounds. Lipid soluble substances can get through. Water soluble substances get through only via facilitated diffusion.

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The Spinal Cord Central canal filled with cerebrospinal fluid (CSF). Consists of 31 segments, each identified by letter (vertebral section) and number (individual nerve). Internal diameter decreases as it extends inferiorly (with two exceptions). Cervical enlargement ‡ nerves to pectoral girdle and upper limbs. Lumbar enlargement ‡ nerves to pelvis and lower limbs. Anatomy and Physiology for Engineers Slide 5-25

The Spinal Cord Each spinal segment contains a pair of dorsal root ganglia (group of neuron cell bodies). Each ganglion contains a dorsal root (axons through which sensory information arrives) and a ventral root (axons through which information is sent out). Dorsal and ventral roots unite into a single spinal nerve. Spinal cord extends only to L1 or L2. After this, nerves extend inferiorly without spinal cord protection. Anatomy and Physiology for Engineers Slide 5-26

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The Brain Adult brain consists of 6 major regions: Cerebrum 2 large paired hemispheres (left and right). Home of conscious thought processes, sensations, intellectual functions, memory storage and retrieval, and complex motor functions.

Diencephalon (thalmus and hypothalmus) Hollow internal structure connected to the cerebrum. Relaying and processing sensory information (thalmus), and processing emotions, autonomic function and hormone production (hypothalmus).

Midbrain (also called the mesencephalon) Part of the brain stem which connects brain to spinal cord. Processes auditory and visual information; generates involuntary motor responses.

Pons (Also part of the brain stem) Involved with somatic and visceral motor control.

Medulla Oblongata (Direct attachment to spinal cord) Relays sensory information to various brain centers; also contains centers for autonomic regulation of heart rate, digestion, blood pressure, respiration, etc.

Cerebellum Found posterior and inferior (covers the brain stem). Adjustment of voluntary and involuntary activities based on input and stored memories. Anatomy and Physiology for Engineers Slide 5-27

The Brain Cerebral Hemispheres and the Cerebellum

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The Brain Brain Stem and the Cerebellum

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The Brain Diencephalon, Brain Stem, and Cerebellum

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Ventricles and Cerebrospinal Fluid Brain contains 4 ventricles (hollow chambers) filled with cerebrospinal fluid. Two larger lateral ventricles located in each cerebral hemisphere. Third ventricle located in diencephalon. Fourth ventricle located within brain stem and is continuous with spinal canal. Cerebrospinal fluid important for shock absorption, buoyancy, and transport of nutrients and waste products. CSF constantly produced by ependymal cells and circulated within the choroid plexus (vascular network that extends into each ventricle). CSF is in direct contact with interstitial fluid and free exchange takes place between these fluids.

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Ventricles and Cerebrospinal Fluid Cerebrospinal fluid circulates within and around brain and spinal cord. CSF forms at the choroid plexus, circulates within the ventricles, down the spinal canal, out into the sub-arachnoid space, and back up and around the brain.

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The Cerebrum Largest structure of the brain. Two hemispheres: left hemisphere responsible for motor and sensory control over right side of body, and vice versa. Two hemispheres linked via region of white matter known as the corpus callosum. Responsible for conscious thought, intellectual function, limited input and control of sensory information. Includes gray and white matter, Key structures: Cerebral cortex: thick blanket of gray matter outer covering surface. Gyri (gyrus): ridges on cerebral cortex. Sulci (sulcus): depressions within cerebral cortex. Longitudinal fissure: groove separating the hemispheres. Lobes: well defined regions within each hemisphere. Anatomy and Physiology for Engineers Slide 5-33

The Cerebrum

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The Cerebrum

The Cerebral Nuclei

Masses of gray matter that lie within the 2 larger ventricles (lateral ventricles) and within the white matter of each cerebral hemisphere. Important in control of learned movement patterns. Anatomy and Physiology for Engineers

The Cerebrum

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The Limbic System

Functional group that includes the olfactory cortex, various cerebral nuclei, gyri and tracts along border between cerebrum and diencephalon. Functions include: Establishing emotional states and related behavioral drives. Linking conscious and intellectual functions of cerebrum to unconscious autonomic functions of brain stem. Long term memory storage and retrieval. Anatomy and Physiology for Engineers Slide 5-36

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The Diencephalon Provides control and relaying of conscious and unconscious sensory and motor information. Structures contained within diencephalon: Third ventricle and pineal gland (endocrine structure that secretes hormone melatonin).

Thalmus Final relay point for ascending sensory information (except smell). Passes only a small portion of this information to active consciousness (primary sensor cortex within cerebrum). Remaining is relayed to cerebral nuclei and other centers within brain stem. Also important for coordination of voluntary and involuntary motor commands.

Hypothalmus: Located deep within diencephalon. Contains centers associated with emotions (rage, pleasure, etc). Adjusts and controls autonomic activities of pons and medulla oblongata. Coordinates neural and endocrine activities. Produces variety of hormones (Anti-diuretic hormone - ADH, oxytocin, etc). Coordinates various voluntary and autonomic functions. Maintains body temperature. Anatomy and Physiology for Engineers Slide 5-37

The Diencephalon

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The Midbrain Contains various nuclei and bundles of ascending and descending nerve fibers. Home of cranial nerves responsible for eye movement. Also home to the reticular formation. Network of interconnected nuclei responsible for waking and sleeping states.

Midbrain also responsible for control of muscle tone.

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The Pons Links cerebellum with midbrain, diencephalon, cerebrum and medulla oblongata. Sensory and motor control for cranial nerves. Also responsible for generating depth and pace of respiration. Anatomy and Physiology for Engineers Slide 5-40

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The Cerebellum Two important functions: Maintains posture and balance by making rapid adjustments in muscle tone and position. Programs and finetunes various involuntary and voluntary movements. These are performed by adjusting and regulating neural activity between cerebrum and brain stem.

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The Medulla Oblongata Provides physical connection between brain and spinal cord. All communication between these two structures must pass through medulla oblongata. Many nerve pathways end in synapses here, and information is relayed onward through connecting neurons. Very important for cardiovascular and respiratory control.

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On to the Peripheral Nervous System and Integrated Functions PNS is the link between neurons in CNS and rest of the body. Although PNS contains < 2% of all neural tissue, it is vital as a pathway between brain and body. Certain decisions may be made without or before entering cerebral cortex and conscious awareness. This is done via synaptic communication within brain stem and spinal cord. PNS is dominated by nerves (axons bundled together by connective tissue) coming to or from the CNS. PNS also contains sensory and motor neurons of the autonomic nervous system (ANS). PNS includes the cranial and spinal nerves. Anatomy and Physiology for Engineers Slide 5-43

The Cranial Nerves Components of the PNS that connect to the brain rather than the spinal cord. 12 pairs of cranial nerves, numbered according to their position along long-axis of brain. Responsible for olfactory, optic, oculomotor, trochlear, facial muscles, acoustic, taste, swallowing, speech, vocal cords, etc.

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The Cranial Nerves

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The Spinal Nerves 31 pairs of spinal nerves. Grouped according to location along vertebral column. 8 cranial pairs (C1-C8). 12 thoracic pairs (T1-T12) 5 lumbar pairs (L1-L5). 5 sacral pairs (S1-S5). 1 coccygeal pair (C1). Several nerve axons bundled to form a plexus with common destination (ex: cervical plexus innervates neck and diaphragm). Anatomy and Physiology for Engineers Slide 5-46

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