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Ch. 7 The Nervous System

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Amber Britton

on 7 December 2016

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Transcript of Ch. 7 The Nervous System

Ch. 7
The Nervous System
The Nervous System
Nervous Tissue: Structure & Function
State the function of neurons & neuroglia.
Describe the general structure of a neuron & name its important anatomical regions.
Describe the composition of gray matter & white matter.
List the two major functional properties of neurons.
Classify neurons according to structure & function
List the types of general sensory receptors & describe their functions
Describe the events that lead to the generation of a nerve impulse & its conduction from one neuron to another.
Define reflex arc & list its elements.
Central Nervous System
Identify & indicate the functions of the major regions of the cerebral hemispheres, diencephalon, brain stem, & cerebellum on a human brain model or diagram.
Name the three meningeal layers & state their functions.
Discuss the formation & function of cerebrospinal fluid & the blood-brain barrier.
Compare the signs of a CVA with those of Alzheimer's disease; of a contusion with those of a concussion.
Define EEG & explain how it evaluates neural functioning.
List two important functions of the spinal cord.
Describe spinal cord structure.
Peripheral Nervous System
Developmental Aspects of the Nervous System
List several factors that may have harmful effects on brain development.
Briefly describe the cause, signs, & consequences of the following congenital disorders: spina bifida, anencephaly, cerebral palsy.
Explain the decline in brain size & weight that occurs with age.
Define senility & list some possible causes.
Dissection
Organization of the Nervous System
List the general functions of the nervous system.
Explain the structural & functional classification of the nervous system.
Define central nervous system & peripheral nervous system & list the major parts of each.
1. Sensory input – gathering information
To monitor changes occurring inside and outside the body
Changes = stimuli

2. Integration
To process and interpret sensory input and decide if action is needed

3. Motor output
A response to integrated stimuli
The response activates muscles or glands

Structural Classification of the Nervous System
CNS: Central nervous system
Brain
Spinal cord
PNS: Peripheral nervous system – nerves outside the brain and spinal cord

Functional Classification of the Nervous System
Sensory (afferent) division
nerve fibers that carry information toward the central nervous system
Motor (efferent) division
Nerve fibers that carry impulses away from the central nervous system
Two subdivisions
Somatic nervous system = voluntary
Autonomic nervous system = involuntary

Support Cells (Neuroglia) – cannot transmit nerve impulses
Abundant, star-shaped cells
Brace neurons
Form barrier between capillaries and neurons
Control the chemical environment of the brain

Spider-like phagocytes
Dispose of debris
Line cavities of the brain and spinal cord
Circulate cerebrospinal fluid with cilia
Produce myelin sheath around nerve fibers in the central nervous system
Protect neuron cell bodies
Form myelin sheath in the peripheral nervous system
Neurons = nerve cells (do not routinely undergo cell division after birth)
Cells specialized to transmit messages
– nucleus (& large nucleolus) and metabolic center of the cell
– specialized rough endoplasmic reticulum
– intermediate cytoskeleton that maintains cell shape
Cell Body Location
Most are found in the CNS
Gray matter – cell bodies and unmylenated fibers
Nuclei – clusters of cell bodies within the white matter of the CNS
Ganglia – collections of cell bodies outside the CNS
Processes – fiber extensions outside the cell body
Dendrites – conduct impulses toward the cell body
Axons – conduct impulses away from the cell body

Axons & Nerve Impulses:
Axons end in axonal terminals
Axonal terminals contain vesicles with chemicals called neurotransmitters
Axonal terminals are separated from the next neuron by a gap
Synaptic cleft – gap between adjacent neurons
Synapse – junction between nerves

Nerve Fiber Coverings
Schwann cells – produce myelin sheaths in jelly-roll like fashion
Nodes of Ranvier – gaps in myelin sheath along the axon


A lack of or, deterioration of myelin sheaths causes the nervous current to short-circuit. This results in a loss of the ability to control muscles. It’s commonly referred to as multiple sclerosis (MS).
Functional Classification of Neurons
Sensory (afferent) neurons
Carry impulses from the sensory receptors
Cutaneous sense organs
Proprioceptors (in muscles & tendons) – detect stretch or tension
Motor (efferent) neurons
Carry impulses from the CNS
Interneurons (association neurons)
Found in neural pathways in the CNS
Connect sensory and motor neurons

Structure of Neurons
– many extensions from the cell body
– one axon and one dendrite
– have a short single process leaving the cell body
Functional Properties of Neurons
Irritability – ability to respond to stimuli
Conductivity – ability to transmit an impulse

The plasma membrane at rest is polarized
Fewer positive ions are inside the cell than outside the cell
• Starting a Nerve Impulse
Depolarization – a stimulus depolarizes the neuron’s membrane
A deploarized membrane allows sodium (Na+) to flow inside the membrane
The exchange of ions initiates an action potential in the neuron

The Action Potential
If the action potential (nerve impulse) starts, it is propagated over the entire axon
Potassium ions rush out of the neuron after sodium ions rush in, which repolarizes the membrane
The sodium-potassium pump restores the original configuration
This action requires ATP

Alcohol, sedatives, & anesthetics can block nerve impulses by reducing membrane permeability to Na+. Cold, & continuous pressure can hinder circulation (and hence delivery of nutrients) to neurons.
Reflex – rapid, predictable, and involuntary responses to stimuli (always in the same direction)
Reflex arc – direct route from a sensory neuron, to an interneuron, to an effector

Simple Reflex Arc
Types of Reflexes & Regulation
Autonomic reflexes: (involuntary) ex.: salivary reflex, pupillary reflex, etc…
Smooth muscle regulation
Heart and blood pressure regulation
Regulation of glands
Digestive system regulation
Somatic reflexes: (voluntary) ex.: pulling your hand away from a hot object
Activation of skeletal muscles
two-neuron reflex arc (knee-jerk reflex)
three-neuron reflex arc (withdrawal reflex)
All reflex arcs have a minimum of 5 elements
sensory receptor – reacts to a stimulus
effector organ – the muscle or gland eventually stimulated
afferent & efferent neurons – connect the previous two
integration center – the synapse between the afferent & efferent neurons
CNS develops from the embryonic neural tube
The neural tube becomes the brain and spinal cord
The opening of the neural tube becomes the ventricles
Four chambers within the brain
Filled with cerebrospinal fluid

Regions of the Brain
o Cerebral hemispheres
o Diencephalon
o Brain stem
o Cerebellum

(converts it into a nerve impulse)
(transmits impulses to other neurons, muscles, or glands)
Conductivity
Irritability
Nerve Impulse Propagation
The impulse continues to move toward the cell body
Impulses travel faster when fibers have a myelin sheath
Impulses are able to cross the synapse to another nerve
Neurotransmitter is released from a nerve’s axon terminal
The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter
An action potential is started in the dendrite

Until repolarization occurs, a neuron cannot conduct another impulse. This resting state is called a refractory period.
Cerebral Hemispheres (Cerebrum)
Paired (left and right) superior parts of the brain
Include more than half of the brain mass
The surface is made of ridges (gyri), shallow grooves (sulci), and deeper grooves (fissures)
Lobes of the Cerebrum
Fissures divide the cerebrum into lobes
Surface lobes of the cerebrum
Frontal lobe
Parietal lobe
Occipital lobe
Temporal lobe

Specialized Areas of the Cerebrum
Somatic sensory area – receives impulses from the body’s sensory receptors
pain, coldness, light touch
Primary motor area – sends impulses to skeletal muscles
face, mouth, hands
Broca’s area – involved in our ability to speak

Cerebral areas involved in special senses
Gustatory area (taste)
Visual area
Auditory area
Olfactory area
Interpretation areas of the cerebrum
Speech/language region
Language comprehension region
General interpretation area
Layers of the Cerebrum
Gray matter
Outer layer
Composed mostly of neuron cell bodies
Basal nuclei – internal islands of gray matter
White matter
Fiber tracts inside the gray matter
Example: corpus callosum connects hemispheres

Diencephalon
Sits on top of the brain stem
Enclosed by the cerebral hemispheres
Made of three parts
Thalamus
Hypothalamus
Epithalamus
Thalamus
Surrounds the third ventricle
The relay station for sensory impulses
Transfers impulses to the correct part of the cortex for localization and interpretation
Hypothalamus
Under the thalamus
Important autonomic nervous system center
Helps regulate body temperature
Controls water balance
Regulates metabolism
An important part of the limbic system (emotions)
thirst, appetite, sex, pain, & pleasure
The pituitary gland is attached to the hypothalamus
Epithalamus
Forms the roof of the third ventricle
Houses the pineal body (an endocrine gland)
Includes the choroid plexus – forms cerebrospinal fluid
Brain Stem
• Attaches to the spinal cord
• Parts of the brain stem
o Midbrain
o Pons
o Medulla oblongata

Midbrain:
Mostly composed of tracts of nerve fibers
Has two bulging fiber tracts –
cerebral peduncles
Has four rounded protrusions –
corpora quadrigemina
Reflex centers for vision and hearing
Pons
The bulging center part of the brain stem
Mostly composed of fiber tracts
Includes nuclei involved in the control of breathing
Medulla Oblongata
The lowest part of the brain stem
Merges into the spinal cord
Includes important fiber tracts
Contains important control centers
Heart rate control
Blood pressure regulation
Breathing
Swallowing
Vomiting
Reticular Formation
Diffuse mass of gray matter along the brain stem
Involved in motor control of visceral organs
Reticular activating system plays a role in awake/sleep cycles and consciousness
Cerebellum
• Two hemispheres with convoluted surfaces
• Provides involuntary coordination of body movements
o like an automatic pilot
Protection of Central Nervous System
Scalp and skin
Skull and vertebral column
Meninges
Cerebrospinal fluid
Blood brain barrier

Meninges
Dura mater – “tough or hard mother”
Double-layered external covering
Periosteum – attached to surface of the skull
Meningeal layer – outer covering of the brain
Folds inward in several areas
Arachnoid layer – “spider/cobweb”
Middle layer
Web-like
Pia mater – “gentle mother”
Internal layer
Clings to the surface of the brain

Meningitis is an inflammation of the meninges. It is a serious threat to the brain because bacterial or viral meningitis may spread into the nervous tissues of the CNS. This brain inflammation is called encephalitis. eningitis is usually diagnosed by taking a sample of CSF from the subarachnoid space.
Cerebrospinal fluid (CSF)
Similar to blood plasma composition
Formed by the choroid plexus
Forms a watery cushion to protect the brain
Circulated in arachnoid space, ventricles, and central canal of the spinal cord
Blood Brain Barrier
Includes the least permeable capillaries of the body
Excludes many potentially harmful substances
Useless against some substances
Fats and fat soluble molecules
Respiratory gases
Alcohol
Nicotine
Anesthesia

Traumatic Brain Injuries
Concussion
Slight brain injury
No permanent brain damage
Contusion
Nervous tissue destruction occurs
Nervous tissue does not regenerate
Cerebral edema
Swelling from the inflammatory response
May compress and kill brain tissue
Cerebrovascular Accident (CVA)
Commonly called a stroke
The result of a ruptured blood vessel supplying a region of the brain
Brain tissue supplied with oxygen from that blood source dies
Loss of some functions or death may result
Alzheimer’s Disease
Progressive degenerative brain disease
Mostly seen in the elderly, but may begin in middle age
Structural changes in the brain include abnormal protein deposits and twisted fibers within neurons
Victims experience memory loss, irritability, confusion and ultimately, hallucinations and death

Spinal Cord
Extends from the medulla oblongata to the region of T12
Below T12 is the cauda equina (a collection of spinal nerves)
Enlargements occur in the cervical and lumbar regions

Anatomy:
Exterior white mater – conduction tracts
Internal gray matter - mostly cell bodies
Dorsal (posterior) horns
Anterior (ventral) horns
Central canal filled with cerebrospinal fluid
Meninges cover the spinal cord
Nerves leave at the level of each vertebrae
Dorsal root
Associated with the dorsal root ganglia – collections of cell bodies outside the central nervous system
Ventral root

Describe the general structure of a nerve
Identify the cranial nerves by number & by name, & list the major functions of each.
Describe the origin & fibercomposition of (a) ventral & dorsal roots, (b) the spinal nerve proper, & (c) ventral & dorsal rami.
Discuss the distribution of the dorsal & ventral rami of spinal nerves.
Name the four major nerve plexuses, give the major nerves of each, & describe their distribution.
Identify the site of origin & explain the function of the sympathetic & parasympathetic divisions of the autonomic nervous system.
Contrast the effect of the parasympathetic & sympathetic divisions on the following organs: heart, lungs, digestive system, blood vessels.
Peripheral Nervous System
Nerves and ganglia outside the central nervous system
Nerve = bundle of neuron fibers
Neuron fibers are bundled by connective tissue
Structure of a Nerve
Endoneurium surrounds each fiber
Groups of fibers are bound into fascicles by perineurium
Fascicles are bound together by epineurium
Classifications of Nerves
Mixed nerves – both sensory and motor fibers
Afferent (sensory) nerves – carry impulses toward the CNS
Efferent (motor) nerves – carry impulses away from the CNS

I Olfactory nerve – sensory for smell II Optic nerve – sensory for vision
III Oculomotor nerve – motor fibers to eye muscles
IV Trochlear – motor fiber to eye muscles
V Trigeminal nerve – sensory for the face; motor fibers to chewing muscles
VI Abducens nerve – motor fibers to eye muscles
VII Facial nerve – sensory for taste; motor fibers to the face
VIII Vestibulocochlear nerve – sensory for balance and hearing
IX Glossopharyngeal nerve – sensory for taste; motor fibers to the pharynx
X Vagus nerves – sensory and motor fibers for pharynx, larynx, and viscera
XI Accessory nerve – motor fibers to neck and upper back
XII Hypoglossal nerve – motor fibers to tongue

Cranial Nerves
12 pairs of nerves that mostly serve the head and neck
Numbered in order, front to back
Most are mixed nerves, but three are sensory only
“Oh, Oh, Oh, to touch and feel very good velvet ah.”
Spinal Nerves
There is a pair of spinal nerves at the level of each vertebrae for a total of 31 pairs
Spinal nerves are formed by the combination of the ventral and dorsal roots of the spinal cord
Spinal nerves are named for the region from which they arise
Anatomy:
Spinal nerves divide soon after leaving the spinal cord
Dorsal rami – serve the skin and muscles of the posterior trunk
Ventral rami – forms a complex of networks (plexus) for the anterior

Autonomic Nervous System
The involuntary branch of the nervous system
Consists of only motor nerves
Divided into two divisions
Sympathetic division
Parasympathetic division
Anatomy of the Sympathetic Division
Originates from T1 through L2
Ganglia are at the sympathetic trunk (near the spinal cord)
Short pre-ganglionic 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

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, and diuresis

Development Aspects of the Nervous System
The 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
No more neurons are formed after birth, but growth and maturation continues for several years
The brain reaches maximum weight as a young adult


chicken embryo
mouse embryo
human embryo
Fisrt stage of development: the CNS
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