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Nervous System. Chapter 7. I. Overview. Three overlapping functions Monitor changes inside and outside of the body (sensory input) Processes sensory input and makes decisions about what to do (integration) Then effects a response called a motor output

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i overview
I. Overview
  • Three overlapping functions
    • Monitor changes inside and outside of the body (sensory input)
    • Processes sensory input and makes decisions about what to do (integration)
    • Then effects a response called a motor output
  • Works with the endocrine system to maintain homeostasis
ii organization of the nervous system
II. Organization of the Nervous System
  • Structural Classification
    • Central nervous system – brain and spinal cord
      • Interpret incoming sensory information and issues instructions based on past experiences and current conditions
    • Peripheral nervous system – outside of CNS
      • Spinal nerves – impulse to and from spinal cord
      • Cranial nerves – impulse to and from brain
ii organization of the nervous system1
II. Organization of the Nervous System
  • Functional Classification
    • Concerned only with PNS
    • Sensory (afferent) division – convey impulses to the CNS from sensory receptors
      • Somatic sensory fibers – from skin, skeletal, and joints
      • Visceral sensory fibers – from visceral organs
    • Motor (efferent) division – carries impulses from the CNS to effector organs, muscles, and glands
      • Somatic nervous system (voluntary nervous system) – allows us to consciously, or voluntarily, control our skeletal muscles (except for those reflex muscles)
      • Autonomic nervous system (involuntary nervous system) – regulates events that are automatic or involuntary
        • sympathetic and parasympathetic
iii nervous tissue structure and function
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION

Supporting Cells

Neuroglia (glia) – literally “nerve glue”, generally support, insulate, and protect

Resemble neurons, but not able to transmit nerve impulses

Never lose ability to divide, thus most brain tumors are gliomas or formed from neuroglia

iii nervous tissue structure and function1
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION

Astrocytes – abundant, star-shaped, nearly half of neural tissue

Numerous swollen projections that cling to neurons and anchor them to capillaries for nutrients

Form a barrier between capillaries to protect the neurons from substances in the blood

Control chemical environment by picking up excess ions and released neurotransmitters

iii nervous tissue structure and function2
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION

Microglia – spider-like phagocytes that dispose of debris, including dead brain cells and bacteria

Ependymal cells – line cavities of brain and spinal cord, cilia help to circulate cerebrospinal fluid and forms a protective cushion around the CNS

iii nervous tissue structure and function3
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION

Oligodendrocytes – wrap flat extensions tightly around nerve fibers, producing fatty insulating coverings called myelin sheath

PNS supporting cells

Schwann cells – form myelin sheaths

Satellite cells – act as protective, cushioning cells

iii nervous tissue structure and function4
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION

Neurons

Anatomy

Neurons (nerve cells) – highly specialized to transmit messages from one part of the body to another

All include a cell body and one or more processes

Cell body is metabolic center of neuron containing organelles except for centrioles

Particularly abundant are rough ER (Nissl substance), neurofibrils, and intermediate filaments for cell shape

iii nervous tissue structure and function5
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION

Arm-like processes vary from microscopic to 3-4 feet

Longest from lumbar region to big toe

Processes conveying incoming messages are dendrites, contains hundreds per neuron

Processes generating nerve impulses are axons, only one per neuron, arises from axon hillock

Most long nerve fibers are covered with whitish, fatty material, with a waxy material called myelin – protects, insulates, and increases transmission rates

iii nervous tissue structure and function6
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION

Axons outside CNS are surrounded by Schwann cells which wrap their membranes tightly around the axon, forming a myelin sheath; the cytoplasm ends up on the outside and the outer layer is called the neurilemma

There are gaps between the Schwann cells called nodes of Ranvier at regular intervals

iii nervous tissue structure and function7
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION

Myelinated fibers also found in CNS, but formed from oligodendrocytes, which can coil around as many as 60 fibers with their flat extensions, but lack a neurilemma which means they are not protected and cannot regenerate

Axons occasionally give off collateral branches along its length, but branch profusely at their terminal end to form axonal terminals which contain hundreds of tiny vesicles, or membranous sacs, containing neurotransmitters which are released when stimulated; they are separated from next neuron by a tiny gap called synaptic cleft (synapse)

iii nervous tissue structure and function8
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION

Clustered neuron cell bodies in the CNS are called nuclei, a few small clusters outside the CNS in the PNS are called ganglia

Bundles of nerve fibers running through CNS are called tracts and in PNS are called nerves

Cell body carries out most of the metabolic functions, so if it is damaged or dies it is not replaced

Regions of the CNS

  • White matter is dense collections of myelinated fibers
  • Gray matter is mostly unmyelinated fibers and cell bodies
iii nervous tissue structure and function9
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION

Homeostatic Imbalance

Importance of myelin sheaths is seen in people with multiple sclerosis (MS)

Myelin sheaths are gradually destroyed, converted to hardened sheaths (scleroses) and when this happen the electrical current is short-circuited, which causes loss of muscle control and they become increasingly disabled

Classification

Functional Classification

Groups neurons according to the direction the nerve impulse is traveling

iii nervous tissue structure and function10
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION

Sensory neurons – cell body found outside the CNS in a ganglion, dendrite endings have specialized receptors that are activated by changes nearby

Complex receptors of special sense organs (vision, hearing, equilibrium, taste, and smell)

Skin – cutaneous sense organs, extreme heat, cold or excessive pressure can be interpreted as pain

Muscles and tendons – proprioceptors, detect amount of stretch or tension so proper adjustments can be made to maintain balance and normal posture

Pain – bare dendrite endings, most numerous

Motor neurons – carry nerve impulses from CNS to viscera, muscles, and/or glands, cell bodies are always located in CNS

Associations neurons (interneurons) – connect motor and sensory neurons, cell bodies always located in CNS

iii nervous tissue structure and function11
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION

Structural classification – based on number of processes extending from the cell body

Multipolar neuron – several processes, most common structural type, include all motor and association neurons

Bipolar neurons – two processes (axon and dendrite), rare in adult, only in special sense organs (eye, ear)

iii nervous tissue structure and function12
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION

Unipolar neurons – single process, very short and divides almost immediately into proximal (central) and distal (peripheral) fibers; only the small branches at the end of the peripheral process are dendrites, remainder act as axons

Conduct nerve impulses toward and away from the cell body, included are sensory neurons in the PNS ganglia

iii nervous tissue structure and function13
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION
  • Physiology
    • Nerve impulses – two major functions
      • Irritability - the ability to respond to a stimulus and convert it into a nerve impulse
        • A neuron is polarized when it is resting, or inactive; this also means that there are fewer positive ions inside the plasma membrane than out
        • Ions inside are K+ and ions outside are Na+, and as long as the inside remains more negative the neuron stays inactive
iii nervous tissue structure and function14
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION
        • Nerves are excited by many different types of stimuli, but most of the neurons in the body are excited by neurotransmitters released by other neurons
  • Nerve impulses are an all-or-none response and either happens over the whole axon or not at all
  • When a neuron is adequately stimulated, the “sodium gates” open and Na+ quickly diffuses into the neuron, changing the polarity of the membrane; this process is called depolarization
iii nervous tissue structure and function15
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION
  • If this stimulus is strong enough, and there’s a large enough Na+ in-rush, then depolarization activates the neuron to initiate and transmit an action potential (nerve impulse)
  • Once Na+ ions rush in the membrane permeability changes again and is no longer permeable to Na+, but permeable to K+, which diffuse out and the electrical charge is restored; this is called repolarization
iii nervous tissue structure and function16
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION

Until repolarization occurs there cannot be another nerve impulse conducted

The initial concentrations must still be restored, so the Na-K pump is activated, which uses ATP

This nerve impulse is along unmyelinated fibers

Nerve impulses occur much faster though down myelinated fibers because it leaps from node to node since the current cannot flow across myelin insulation – this is called saltatory conduction

Nerve Impulses

iii nervous tissue structure and function17
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION
  • Conductivity – the ability to transmit the nerve impulse to other neurons, muscles, or glands
    • Impulses from one neuron travel across the synapse to another by means of a neurotransmitter
    • Dendrite of the next neuron receives the neurotransmitter and an action potential is then started
iii nervous tissue structure and function18
Homeostatic Imbalance – impact on conduction of impulses

alcohol, sedatives, and anesthetics all reduce membrane permeability

Cold and continuous pressure interrupt blood circulation, so after warming up or pressure removed then prickly feeling comes when impulses are transmitted again

III. NERVOUS TISSUE: STRUCTURE AND FUNCTION
iii nervous tissue structure and function19
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION

Reflex arc – direct route from a sensory neuron, to an interneuron, to an effector

Reflexes are rapid, predictable, and involuntary responses to stimuli that always travel in the same direction

Autonomic reflexes regulate the activity of smooth muscles, the heart, and glands, saliva, eye pupils, digestion, elimination, blood pressure, sweating

Somatic reflexes are all reflexes that stimulate skeletal muscles

iv central nervous system
IV. CENTRAL NERVOUS SYSTEM

Composed of brain and spinal cord

During embryonic development, appears as a simple tube (neural tube) that extends down the dorsal median plane, and by the fourth week the anterior end begins to expand and brain formation begins

iv central nervous system1
IV. CENTRAL NERVOUS SYSTEM

Functional Anatomy of the Brain

Opening of the neural tube becomes the four chambers of the brain called ventricles, filled with cerebrospinal fluid (CSF)

Cerebral hemispheres – paired, superior part of the brain, larger than all other regions combined

iv central nervous system2
IV. CENTRAL NERVOUS SYSTEM

Surface has ridges called gyri seperated by shallow grooves called sulci, and few deeper grooves called fissures that separate large regions of the brain

these all serve as landmarks

Single, deep fissure (longitudinal fissure) separates hemispheres

Other fissures or sulci divide hemispheres into lobes named for cranial bones that lie over them

iv central nervous system4
IV. CENTRAL NERVOUS SYSTEM

Parietal lobe lies posterior to the central sulcus and is home to the somatic sensory area

all of the body’s sensory receptors (except special senses) are localized and interpreted here, and all of the pathways are upside-down and backwards

iv central nervous system5
IV. CENTRAL NERVOUS SYSTEM

Frontal lobe lies anterior to the central sulcus and is home to the primary motor area

Axons of these motor neurons form major voluntary motor tract – the pyramidal, or corticospinal tract, which descends to the cord

Like the somatic sensory cortex, all of the pathways are upside-down and backwards

iv central nervous system6
IV. CENTRAL NERVOUS SYSTEM

Broca’s area is involved in the ability to speak, damage causes inability to say words properly

Higher intellectual reasoning located in anterior of frontal lobe, and language comprehension

Speech area is at the junction of temporal, parietal, and occipital lobes, it allows you to sound out words

Cell bodies of neurons involved in cerebral hemisphere functions are found only in the outermost gray matter – the cerebral cortex

iv central nervous system7
IV. CENTRAL NERVOUS SYSTEM

Most of the remaining cerebral hemisphere – cerebral white matter – is composed of fiber tracts carrying impulses to/from the cortex

Corpus callosum – large fiber tract connecting cerebral hemispheres

Basal nuclei – islands of gray matter deep in white matter, help regulate voluntary motor activities by modifying instructions sent to the skeletal muscles by the primary motor cortex

iv central nervous system9
IV. CENTRAL NERVOUS SYSTEM

Diencephalon (interbrain) – sits atop brain stem and enclosed by cerebral hemispheres

Thalamus – encloses shallow third ventricle, relay station for sensory impulses passing to sensory cortex and recognizes whether the impulse in pleasant or unpleasant

iv central nervous system10
IV. CENTRAL NERVOUS SYSTEM

Hypothalamus makes up floor of diencephalon, plays role in regulation of body temperature, water balance, and metabolism

Also center for many drives and emotions, called limbic system – thirst, appetite, sex, pain, and pleasure centers

Regulates pituitary gland or hypophysis (endocrine organ)

Mammillary bodies involved in olfaction (smell) bulge from floor

iv central nervous system11
IV. CENTRAL NERVOUS SYSTEM

Epithalamus – roof of third ventricle, includes pineal body (endocrine) and choroid plexus (knots of capillaries in each ventricle that form the CSF)

Brain Stem – about the size of a thumb in diameter and about 3 inches long

Pathway for tracts, many small gray matter areas, control vital activities such as breathing and blood pressure

iv central nervous system12
IV. CENTRAL NERVOUS SYSTEM

Midbrain – small part of brain stem

Cerebral aqueduct is a tiny canal that travels through the midbrain and connects the third and fourth ventricle

Cerebral peduncles – two anteriorly bulging fiber tracts which convey ascending and descending impulses

Corpora quadrigemina – four rounded dorsal protrusions, these are reflex centers involved with vision and hearing

iv central nervous system13
IV. CENTRAL NERVOUS SYSTEM

Pons – means bridge, rounded structure protruding just below midbrain, mostly fiber tracts, nuclei for breathing

Medulla oblongata – most inferior of brain stem, merges into spinal cord, fiber tract area, control center for heart rate, blood pressure, breathing, swallowing, vomiting, and others

Reticular formation – extends length of brain stem, diffuses mass of gray matter, involved in motor control of visceral organs

Reticular activating system (RAS) plays a role in consciousness and the awake/sleep cycles and damage to this area can result in coma

iv central nervous system14
IV. CENTRAL NERVOUS SYSTEM

Cerebellum – large, cauliflower-like projection dorsally from under occipital lobe of cerebrum

Much like cerebrum – two hemispheres, convoluted surface, outer cortex of gray matter and inner white matter

Provides precise timing for skeletal muscle activity and controls balance and equilibrium

Fibers come here from the equilibrium apparatus of the inner ear, the eye, the proprioceptors, and other areas

iv central nervous system15
IV. CENTRAL NERVOUS SYSTEM

Protection of the Central Nervous System

Meninges – three connective tissue membranes covering and protecting the CNS

Dura mater – leathery, outermost layer, double-layered membrane surrounding the brain

One layer is attached to the inner surface of the skull forming the periosteum

Other layer is outermost covering of the brain that continues as the dura mater of the spinal cord, called meningeal layer

Fused together except where they separate to enclose dural sinuses (collects venous blood)

Dural sinuses fold inward in several places to attach brain to cranial cavity – falx cerebri and tentorium cerebelli

iv central nervous system17
IV. CENTRAL NERVOUS SYSTEM

Arachnoid mater – web-like middle meningeal layer

Thread-like extensions span subarachnoid space, which is filled with CSF, and attach to pia mater

Arachnoid villi – protrude through dura mater, CSF absorbed into venous blood in dural sinuses

Pia mater – delicate, innermost membrane, clings tightly to surface of brain following every fold

iv central nervous system18
IV. CENTRAL NERVOUS SYSTEM

Pneumococcal meningitis in an alcoholic patient.

Head opened at autopsy revealing purulent inflammation of leptomeninges beneath reflected dura mater.

iv central nervous system19
IV. CENTRAL NERVOUS SYSTEM

Cerebrospinal Fluid – similar to blood plasma, but less protein, more vitamin C, and different ion composition

Continually formed from blood by choroid plexuses, which are capillaries hanging from the top in each ventricle

Continually moving by circulating through lateral ventricles in cerebral hemispheres, third ventricle (diencephalon), and fourth ventricle (brain stem)

Some fluid reaching fourth ventricle travels down central canal of spinal cord, but most stays in subarachnoid space which leaves through holes in fourth ventricle

Fluid is recycled through arachnoid villi into blood in the dural sinuses

Normal volume is about 150 mL or ½ cup

iv central nervous system20
IV. CENTRAL NERVOUS SYSTEM

Any changes in CSF can indicate a problem in brain or spinal cord, and a sample can be obtained through a lumbar (spinal) tap, and checked for blood cells

Since the fluid is decreased with this test, patient must remain horizontal for 6 – 12 hours to prevent a headache

iv central nervous system22
IV. CENTRAL NERVOUS SYSTEM
  • Hydrocephalus
    • If the CSF cannot drain it accumulates and causes pressure on the brain, this is called hydrocephalus, which literally means water on the brain
    • In babies the head enlarges to allow for the extra spinal fluid because of the soft bones
    • In adults though the condition can lead to brain damage because of hardened skull
    • A shunt to drain excess fluid is then placed
iv central nervous system23
IV. CENTRAL NERVOUS SYSTEM
  • The Blood-Brain Barrier
    • Separates neurons from blood-borne substances, composed of least permeable capillaries in the whole body which provide most of the protection and the astrocytes help with this
    • Only water, glucose, and essential amino acids pass easily, but urea, toxins, proteins, and most drugs are prevented from entering
    • Useless against fats, respiratory gases, and other fat-soluble molecule (alcohol, nicotine, anesthetics)
iv central nervous system24
Brain Dysfunctions

Traumatic Brain Injuries

Leading cause of accidental death in US

Concussion – slight brain injury, cause dizziness and brief loss of consciousness but no permanent brain damage

Contusion – result of marked tissue destruction

Hemorrhage or cerebral edema – individuals usually alert then deteriorate neurologically, caused from compression of vital brain tissue

IV. CENTRAL NERVOUS SYSTEM
iv central nervous system25
IV. CENTRAL NERVOUS SYSTEM

Cerebrovascular Accident (CVAs) – strokes

Third leading cause of death in the US

Occur when blood circulation to a brain area is blocked by clot, rupture, or tissue dies

Area of brain damage can be determined by patient’s symptoms

Fewer than 1/3 of those that survive are alive three years later, but not hopeless, undamaged neurons take over some of the lost function

  • Not all strokes are “completed”, result in temporary restriction of blood flow called a transient ischemic attack (TIA) that last from 5 to 50 minutes, these are red flags though for impending more serious CVAs
iv central nervous system26
Spinal Cord

Approximately 17 inches long, glistening white continuation of brain stem

Provides two-way conduction pathway to/from brain

Only extends to the 1st/2nd lumbar vertebrae because it grows slower than vertebral column, spinal nerves at inferior end are called caudaequina

IV. CENTRAL NERVOUS SYSTEM

Cushioned and protected by meninges, which continue past end of spinal cord

31 pairs of spinal nerves arise from cord

iv central nervous system27
IV. CENTRAL NERVOUS SYSTEM

Gray Matter of the Spinal Cord and Spinal Roots

Gray matter looks like a butterfly or “H” in cross section, posterior projections are posterior/dorsal horns, anterior projections are anterior/ventral horns

Surrounds the central canal of the cord

iv central nervous system28
IV. CENTRAL NERVOUS SYSTEM

Cell bodies of sensory neurons, whose fibers enter the cord by the dorsal root, are found in an enlarged area called the dorsal root ganglion

If damaged then sensation from the body area will be lost

  • Ventral horns of gray matter contain cell bodies of motor neurons of the somatic system extend their axons out of the ventral root
    • If damaged then flaccid paralysis results and voluntary movement is impossible and will begin to atrophy
  • Dorsal and ventral roots fuse to form spinal nerves
iv central nervous system29
IV. CENTRAL NERVOUS SYSTEM

White Matter of the Spinal Cord

Composed of myelinated fiber tracts

Because of irregular shape of gray matter, white matter divided into three regions

Posterior column – ascending tracts that carry sensory input to the brain

Lateral and anterior columns – contain ascending and descending motor tracts

Homeostatic Imbalance

Transection or crushing of the cord results in spastic paralysis where affected muscles stay healthy because of reflex arcs, meaning movements are involuntary

Quadriplegic – all 4 limbs affected

Paraplegic – only legs are paralyzed

v peripheral nervous system
V. PERIPHERAL NERVOUS SYSTEM

Structure of a Nerve

Nerve – bundle of nerve fibers outside the CNS

Each process is wrapped in a connective tissue sheath called the endoneurium

Groups of fibers are bound by coarser connective tissue called the perineurium to form fiber bundles or fascicles

Fascicles are bound together by a tough fibrous sheath called the epineurium for form a cord-like nerve

v peripheral nervous system2
V. PERIPHERAL NERVOUS SYSTEM

Like neurons, nerves are classified by direction in which impulses are transmitted

Mixed nerves carry both sensory and motor fibers

Afferent or sensory nerves carry impulses toward the CNS

Efferent or motor nerves carry impulses away from CNS

Autonomic Nervous System

Motor subdivision of the PNS that controls body activities automatically, thus also called the involuntary nervous system

v peripheral nervous system3
V. PERIPHERAL NERVOUS SYSTEM

Somatic and Autonomic Nervous Systems Compared

Neurons

Somatic – cell bodies are inside the CNS, axons extend all the way to the skeletal muscle they serve

Autonomic – chain of two motor neurons, first is in brain or spinal cord and its axon (preganglionic axon) leaves CNS to synapse with second motor neuron in a ganglion outside the CNS, the axon of this neuron is the postganglionic axon

Effector organs

Somatic – skeletal muscle

Autonomic – smooth muscle, cardiac muscle, glands

Neurotransmitters

Somatic – acetylcholine

Autonomic – acetylcholine, epinephrine, norepinephrine

v peripheral nervous system4
V. PERIPHERAL NERVOUS SYSTEM

Autonomic system is broken into sympathetic and parasympathetic divisions, but they both serve the same organs, but counterbalance each other’s activities

Sympathetic mobilizes the body during extreme situations (fear, exercise, or rage)

Parasympathetic allows us to unwind and conserve energy

v peripheral nervous system5
V. PERIPHERAL NERVOUS SYSTEM

Anatomy of the Sympathetic Division

Originates from T1 through L2

Ganglia are at the sympathetic trunk (near the spinal cord)

Short preganglionic neuron and long postganglionic neuron transmit impulse from CNS to the effector

Norepinephrine and epinephrine are neurotransmitters to the effector organs

Anatomy of the Parasympathetic Division

Originates from the brain stem and S1 through S4

Terminal ganglia are at the effector organs

Always uses acetylcholine as a neurotransmitter

v peripheral nervous system6
V. PERIPHERAL NERVOUS SYSTEM

Autonomic Functioning

Sympathetic – “fight-or-flight”

Response to unusual stimulus

Takes over to increase activities

Remember as the “E” division – exercise, excitement, emergency, and embarrassment

Parasympathetic – housekeeping activities

Conserves energy

Maintains daily necessary body functions

Remember as the “D” division – digestion, defecation, diuresis

vi developmental aspects of the nervous system
VI. DEVELOPMENTAL ASPECTS OF THE NERVOUS SYSTEM

Embryonic development

Nervous system is formed during the first month of embryonic development

Any maternal infection can have extremely harmful effects

The hypothalamus is one of the last areas of the brain to develop, which regulates body temperature, thus preemies have trouble with this

No more neurons are formed after birth, but growth and maturation continues for several years

The brain reaches maximum weight as a young adult

vi developmental aspects of the nervous system1
VI. DEVELOPMENTAL ASPECTS OF THE NERVOUS SYSTEM

The apparent enlargement of the ventricles seen here is due to atrophy of the head of the caudate from neuronal loss with Huntington's disease, an autosomal dominant condition characterized clinically by choreiform movements.