Upload
kineks
View
16
Download
0
Tags:
Embed Size (px)
DESCRIPTION
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 - PowerPoint PPT Presentation
Citation preview
NERVOUS SYSTEMChapter 7
I. OVERVIEWA. Three overlapping functions
1. Monitor changes inside and outside of the body (sensory input)
2. Processes sensory input and makes decisions about what to do (integration)
3. Then effects a response called a motor outputB. Works with the endocrine system to
maintain homeostasis
II. ORGANIZATION OF THE NERVOUS SYSTEMA. Structural Classification
1. Central nervous system – brain and spinal corda. Interpret incoming sensory information and issues
instructions based on past experiences and current conditions
2. Peripheral nervous system – outside of CNSa. Spinal nerves – impulse to and from spinal cordb. Cranial nerves – impulse to and from brain
II. ORGANIZATION OF THE NERVOUS SYSTEMB. Functional Classification
1. Concerned only with PNS2. Sensory (afferent) division – convey impulses to
the CNS from sensory receptorsa. Somatic sensory fibers – from skin, skeletal, and
jointsb. Visceral sensory fibers – from visceral organs
3. Motor (efferent) division – carries impulses from the CNS to effector organs, muscles, and glands
a. Somatic nervous system (voluntary nervous system) – allows us to consciously, or voluntarily, control our skeletal muscles (except for those reflex muscles)
b. Autonomic nervous system (involuntary nervous system) – regulates events that are automatic or involuntary
i. sympathetic and parasympathetic
III. NERVOUS TISSUE: STRUCTURE AND FUNCTIONA. Supporting Cells
1. Neuroglia (glia) – literally “nerve glue”, generally support, insulate, and protect
a. Resemble neurons, but not able to transmit nerve impulses
b. Never lose ability to divide, thus most brain tumors are gliomas or formed from neuroglia
III. NERVOUS TISSUE: STRUCTURE AND FUNCTIONc. Astrocytes – abundant, star-shaped, nearly half of neural
tissuei. Numerous swollen projections that cling to neurons and
anchor them to capillaries for nutrientsii. Form a barrier between capillaries to protect the
neurons from substances in the bloodiii. Control chemical environment by picking up excess ions
and released neurotransmitters
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION
d. Microglia – spider-like phagocytes that dispose of debris, including dead brain cells and bacteria
e. 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 FUNCTIONf. Oligodendrocytes – wrap
flat extensions tightly around nerve fibers, producing fatty insulating coverings called myelin sheath
g. PNS supporting cells e. Schwann cells – form
myelin sheathsf. Satellite cells – act as
protective, cushioning cells
III. NERVOUS TISSUE: STRUCTURE AND FUNCTIONB. Neurons
1. Anatomya. Neurons (nerve cells) – highly specialized to transmit
messages from one part of the body to anotheri. All include a cell body and one or more processes
b. Cell body is metabolic center of neuron containing organelles except for centriolesi. Particularly abundant are
rough ER (Nissl substance), neurofibrils, and intermediate filaments for cell shape
III. NERVOUS TISSUE: STRUCTURE AND FUNCTIONc. Arm-like processes vary from microscopic to 3-4 feet
i. Longest from lumbar region to big toeii. Processes conveying incoming messages are dendrites,
contains hundreds per neuroniii. Processes generating nerve impulses are axons, only
one per neuron, arises from axon hillockiv. 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 FUNCTIONv. 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
vi. There are gaps between the Schwann cells called nodes of Ranvier at regular intervals
III. NERVOUS TISSUE: STRUCTURE AND FUNCTIONvii. 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
vii. 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 FUNCTIONd. Clustered neuron cell bodies in the CNS are called nuclei, a few
small clusters outside the CNS in the PNS are called gangliae. Bundles of nerve fibers running through CNS are called tracts
and in PNS are called nervesf. Cell body carries out most of the metabolic functions, so if it is
damaged or dies it is not replacedg. Regions of the CNS
i. White matter is dense collections of myelinated fibers
ii. Gray matter is mostly unmyelinated fibers and cell bodies
III. NERVOUS TISSUE: STRUCTURE AND FUNCTION2. Homeostatic Imbalance
a. Importance of myelin sheaths is seen in people with multiple sclerosis (MS)
i. 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
3. Classificationa. Functional Classification
i. Groups neurons according to the direction the nerve impulse is traveling
III. NERVOUS TISSUE: STRUCTURE AND FUNCTIONii. Sensory neurons – cell body found outside the CNS in a
ganglion, dendrite endings have specialized receptors that are activated by changes nearby
a) Complex receptors of special sense organs (vision, hearing, equilibrium, taste, and smell)
b) Skin – cutaneous sense organs, extreme heat, cold or excessive pressure can be interpreted as pain
c) Muscles and tendons – proprioceptors, detect amount of stretch or tension so proper adjustments can be made to maintain balance and normal posture
d) Pain – bare dendrite endings, most numerousiii. Motor neurons – carry nerve impulses from CNS to viscera,
muscles, and/or glands, cell bodies are always located in CNS
iv. Associations neurons (interneurons) – connect motor and sensory neurons, cell bodies always located in CNS
III. NERVOUS TISSUE: STRUCTURE AND FUNCTIONb. Structural classification – based on number of
processes extending from the cell bodyi. Multipolar neuron – several processes, most common
structural type, include all motor and association neurons
ii. Bipolar neurons – two processes (axon and dendrite), rare in adult, only in special sense organs (eye, ear)
III. NERVOUS TISSUE: STRUCTURE AND FUNCTIONiii. 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
a) Conduct nerve impulses toward and away from the cell body, included are sensory neurons in the PNS ganglia
4. Physiologya. Nerve impulses – two major functions
i. Irritability - the ability to respond to a stimulus and convert it into a nerve impulse
a) 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
b) 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 FUNCTION
III. NERVOUS TISSUE: STRUCTURE AND FUNCTIONc) 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
d) Nerve impulses are an all-or-none response and either happens over the whole axon or not at all
e) 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 FUNCTIONf) 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)
g) 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 FUNCTIONh) Until repolarization occurs there cannot be another nerve
impulse conductedi) The initial concentrations must still be restored, so the Na-
K pump is activated, which uses ATPj) This nerve impulse is along unmyelinated fibersk) 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 FUNCTIONii. Conductivity – the ability
to transmit the nerve impulse to other neurons, muscles, or glands
a) Impulses from one neuron travel across the synapse to another by means of a neurotransmitter
b) Dendrite of the next neuron receives the neurotransmitter and an action potential is then started
III. NERVOUS TISSUE: STRUCTURE AND FUNCTIONiii. Homeostatic Imbalance – impact on conduction of
impulsesa) alcohol, sedatives, and anesthetics all reduce membrane
permeabilityb) 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 FUNCTIONb. Reflex arc – direct route from a sensory neuron, to
an interneuron, to an effectori. Reflexes are rapid, predictable, and involuntary
responses to stimuli that always travel in the same direction
ii. Autonomic reflexes regulate the activity of smooth muscles, the heart, and glands, saliva, eye pupils, digestion, elimination, blood pressure, sweating
iii. Somatic reflexes are all reflexes that stimulate skeletal muscles
IV. CENTRAL NERVOUS SYSTEMA. Composed of brain
and spinal cord1. 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 SYSTEMB. Functional Anatomy of the Brain
1. Opening of the neural tube becomes the four chambers of the brain called ventricles, filled with cerebrospinal fluid (CSF)
2. Cerebral hemispheres – paired, superior part of the brain, larger than all other regions combined
IV. CENTRAL NERVOUS SYSTEMa. Surface has ridges called
gyri seperated by shallow grooves called sulci, and few deeper grooves called fissures that separate large regions of the brain
i. these all serve as landmarks
ii. Single, deep fissure (longitudinal fissure) separates hemispheres
iii. Other fissures or sulci divide hemispheres into lobes named for cranial bones that lie over them
IV. CENTRAL NERVOUS SYSTEM
IV. CENTRAL NERVOUS SYSTEMb. Parietal lobe lies
posterior to the central sulcus and is home to the somatic sensory area
i. 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 SYSTEMc. Frontal lobe lies anterior
to the central sulcus and is home to the primary motor area
i. Axons of these motor neurons form major voluntary motor tract – the pyramidal, or corticospinal tract, which descends to the cord
ii. Like the somatic sensory cortex, all of the pathways are upside-down and backwards
IV. CENTRAL NERVOUS SYSTEMiii. Broca’s area is involved in the ability to speak, damage
causes inability to say words properlyiv. Higher intellectual reasoning located in anterior of frontal
lobe, and language comprehensiond. Speech area is at the
junction of temporal, parietal, and occipital lobes, it allows you to sound out words
e. Cell bodies of neurons involved in cerebral hemisphere functions are found only in the outermost gray matter – the cerebral cortex
IV. CENTRAL NERVOUS SYSTEMf. Most of the remaining
cerebral hemisphere – cerebral white matter – is composed of fiber tracts carrying impulses to/from the cortex
i. Corpus callosum – large fiber tract connecting cerebral hemispheres
g. 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 SYSTEM
1. Cerebral hemisphere 2.Corpus callosum 3.Thalamus 4.Midbrain 5.Pons 6.Cerebellum 7.Medulla oblongata
IV. CENTRAL NERVOUS SYSTEM3. Diencephalon
(interbrain) – sits atop brain stem and enclosed by cerebral hemispheres
a. 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 SYSTEMb. Hypothalamus makes up
floor of diencephalon, plays role in regulation of body temperature, water balance, and metabolism
i. Also center for many drives and emotions, called limbic system – thirst, appetite, sex, pain, and pleasure centers
ii. Regulates pituitary gland or hypophysis (endocrine organ)
iii. Mammillary bodies involved in olfaction (smell) bulge from floor
IV. CENTRAL NERVOUS SYSTEMc. Epithalamus – roof of third
ventricle, includes pineal body (endocrine) and choroid plexus (knots of capillaries in each ventricle that form the CSF)
4. Brain Stem – about the size of a thumb in diameter and about 3 inches long
a. Pathway for tracts, many small gray matter areas, control vital activities such as breathing and blood pressure
IV. CENTRAL NERVOUS SYSTEMb. Midbrain – small part of
brain stemi. Cerebral aqueduct is a tiny
canal that travels through the midbrain and connects the third and fourth ventricle
ii. Cerebral peduncles – two anteriorly bulging fiber tracts which convey ascending and descending impulses
iii. Corpora quadrigemina – four rounded dorsal protrusions, these are reflex centers involved with vision and hearing
IV. CENTRAL NERVOUS SYSTEMc. Pons – means bridge, rounded
structure protruding just below midbrain, mostly fiber tracts, nuclei for breathing
d. 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
e. Reticular formation – extends length of brain stem, diffuses mass of gray matter, involved in motor control of visceral organsi. 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 SYSTEM5. Cerebellum – large, cauliflower-
like projection dorsally from under occipital lobe of cerebrum
a. Much like cerebrum – two hemispheres, convoluted surface, outer cortex of gray matter and inner white matter
b. Provides precise timing for skeletal muscle activity and controls balance and equilibrium
c. Fibers come here from the equilibrium apparatus of the inner ear, the eye, the proprioceptors, and other areas
IV. CENTRAL NERVOUS SYSTEMC. Protection of the Central Nervous System
1. Meninges – three connective tissue membranes covering and protecting the CNSa. Dura mater – leathery, outermost layer, double-
layered membrane surrounding the braini. One layer is attached to the inner surface of the
skull forming the periosteumii. Other layer is outermost covering of the brain
that continues as the dura mater of the spinal cord, called meningeal layer
iii. Fused together except where they separate to enclose dural sinuses (collects venous blood)
iv. Dural sinuses fold inward in several places to attach brain to cranial cavity – falx cerebri and tentorium cerebelli
IV. CENTRAL NERVOUS SYSTEM
IV. CENTRAL NERVOUS SYSTEMb. Arachnoid mater – web-like middle meningeal layer
i. Thread-like extensions span subarachnoid space, which is filled with CSF, and attach to pia mater
ii. Arachnoid villi – protrude through dura mater, CSF absorbed into venous blood in dural sinuses
c. Pia mater – delicate, innermost membrane, clings tightly to surface of brain following every fold
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 SYSTEM2. Cerebrospinal Fluid – similar to blood plasma, but
less protein, more vitamin C, and different ion composition
a. Continually formed from blood by choroid plexuses, which are capillaries hanging from the top in each ventricle
b. Continually moving by circulating through lateral ventricles in cerebral hemispheres, third ventricle (diencephalon), and fourth ventricle (brain stem)
c. 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
d. Fluid is recycled through arachnoid villi into blood in the dural sinuses
e. Normal volume is about 150 mL or ½ cup
IV. CENTRAL NERVOUS SYSTEMf. 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
g. Since the fluid is decreased with this test, patient must remain horizontal for 6 – 12 hours to prevent a headache
IV. CENTRAL NERVOUS SYSTEM
IV. CENTRAL NERVOUS SYSTEM3. Hydrocephalus
a. If the CSF cannot drain it accumulates and causes pressure on the brain, this is called hydrocephalus, which literally means water on the brain
b. In babies the head enlarges to allow for the extra spinal fluid because of the soft bones
c. In adults though the condition can lead to brain damage because of hardened skull
d. A shunt to drain excess fluid is then placed
IV. CENTRAL NERVOUS SYSTEM4. The Blood-Brain Barrier
a. 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
b. Only water, glucose, and essential amino acids pass easily, but urea, toxins, proteins, and most drugs are prevented from entering
c. Useless against fats, respiratory gases, and other fat-soluble molecule (alcohol, nicotine, anesthetics)
IV. CENTRAL NERVOUS SYSTEMD. Brain Dysfunctions
1. Traumatic Brain Injuriesa. Leading cause of accidental
death in USb. Concussion – slight brain
injury, cause dizziness and brief loss of consciousness but no permanent brain damage
c. Contusion – result of marked tissue destruction
d. Hemorrhage or cerebral edema – individuals usually alert then deteriorate neurologically, caused from compression of vital brain tissue
IV. CENTRAL NERVOUS SYSTEM2. Cerebrovascular Accident (CVAs) – strokes
a. Third leading cause of death in the USb. Occur when blood circulation to a brain area is blocked by
clot, rupture, or tissue diesc. Area of brain damage can be determined by patient’s
symptomsd. Fewer than 1/3 of those that survive are alive three years
later, but not hopeless, undamaged neurons take over some of the lost functione. 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 SYSTEME. Spinal Cord
1. Approximately 17 inches long, glistening white continuation of brain stem
2. Provides two-way conduction pathway to/from brain3. Only extends to the 1st/2nd lumbar vertebrae because it
grows slower than vertebral column, spinal nerves at inferior end are called cauda equina
4. Cushioned and protected by meninges, which continue past end of spinal cord
5. 31 pairs of spinal nerves arise from cord
IV. CENTRAL NERVOUS SYSTEM6. Gray Matter of the
Spinal Cord and Spinal Roots
a. Gray matter looks like a butterfly or “H” in cross section, posterior projections are posterior/dorsal horns, anterior projections are anterior/ventral horns
b. Surrounds the central canal of the cord
IV. CENTRAL NERVOUS SYSTEMc. 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
i. If damaged then sensation from the body area will be lostd. Ventral horns of gray matter
contain cell bodies of motor neurons of the somatic system extend their axons out of the ventral root
i. If damaged then flaccid paralysis results and voluntary movement is impossible and will begin to atrophy
e. Dorsal and ventral roots fuse to form spinal nerves
IV. CENTRAL NERVOUS SYSTEM7. White Matter of the Spinal Cord
a. Composed of myelinated fiber tractsb. Because of irregular shape of gray matter, white matter
divided into three regions i. Posterior column – ascending tracts that carry
sensory input to the brainii. Lateral and anterior columns – contain ascending and
descending motor tracts8. Homeostatic Imbalance
a. Transection or crushing of the cord results in spastic paralysis where affected muscles stay healthy because of reflex arcs, meaning movements are involuntary
b. Quadriplegic – all 4 limbs affectedc. Paraplegic – only legs are paralyzed
V. PERIPHERAL NERVOUS SYSTEMA. Structure of a Nerve
1. Nerve – bundle of nerve fibers outside the CNSa. Each process is wrapped in a
connective tissue sheath called the endoneurium
b. Groups of fibers are bound by coarser connective tissue called the perineurium to form fiber bundles or fascicles
c. Fascicles are bound together by a tough fibrous sheath called the epineurium for form a cord-like nerve
V. PERIPHERAL NERVOUS SYSTEM
V. PERIPHERAL NERVOUS SYSTEM2. Like neurons, nerves are classified by direction
in which impulses are transmitteda. Mixed nerves carry both sensory and motor fibersb. Afferent or sensory nerves carry impulses toward the
CNSc. Efferent or motor nerves carry impulses away from
CNSB. Autonomic Nervous System
1. Motor subdivision of the PNS that controls body activities automatically, thus also called the involuntary nervous system
V. PERIPHERAL NERVOUS SYSTEM2. Somatic and Autonomic Nervous Systems
Compareda. Neurons
i. Somatic – cell bodies are inside the CNS, axons extend all the way to the skeletal muscle they serve
ii. 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
b. Effector organsi. Somatic – skeletal muscleii. Autonomic – smooth muscle, cardiac muscle, glands
c. Neurotransmittersi. Somatic – acetylcholineii. Autonomic – acetylcholine, epinephrine,
norepinephrine
V. PERIPHERAL NERVOUS SYSTEMd. Autonomic system is broken into sympathetic and
parasympathetic divisions, but they both serve the same organs, but counterbalance each other’s activities
i. Sympathetic mobilizes the body during extreme situations (fear, exercise, or rage)
ii. Parasympathetic allows us to unwind and conserve energy
V. PERIPHERAL NERVOUS SYSTEM3. Anatomy of the Sympathetic Division
a. Originates from T1 through L2b. Ganglia are at the sympathetic trunk (near the spinal
cord)c. Short preganglionic neuron and long postganglionic
neuron transmit impulse from CNS to the effectord. Norepinephrine and epinephrine are neurotransmitters
to the effector organs4. Anatomy of the Parasympathetic Division
a. Originates from the brain stem and S1 through S4b. Terminal ganglia are at the effector organsc. Always uses acetylcholine as a neurotransmitter
V. PERIPHERAL NERVOUS SYSTEM5. Autonomic Functioning
a. Sympathetic – “fight-or-flight”i. Response to unusual stimulusii. Takes over to increase activitiesiii. Remember as the “E” division – exercise, excitement,
emergency, and embarrassmentb. Parasympathetic – housekeeping activities
i. Conserves energyii. Maintains daily necessary body functionsiii. Remember as the “D” division – digestion,
defecation, diuresis
VI. DEVELOPMENTAL ASPECTS OF THE NERVOUS SYSTEMA. Embryonic development
1. Nervous system is formed during the first month of embryonic development
2. Any maternal infection can have extremely harmful effects
3. The hypothalamus is one of the last areas of the brain to develop, which regulates body temperature, thus preemies have trouble with this
4. No more neurons are formed after birth, but growth and maturation continues for several years
5. The brain reaches maximum weight as a young adult
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.