Download O rganization of the nervous system To go toward

Ch. 7 The Nervous System NOTES
Functions of the nervous system
1. Sensory input – gathering information
o
To monitor changes occurring inside and
outside the body
o
Changes = stimuli
2. Integration
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To process and interpret sensory input and
decide if action is needed
3. Motor output
o
A response to integrated stimuli
o
The response activates muscles or glands
Structural Classification of the Nervous
CNS: Central nervous system
o
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Brain
Spinal cord
PNS: Peripheral nervous system – nerves outside the brain and spinal cord
Functional Classification of the PNS
To go toward
Sensory (afferent) division
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nerve fibers that carry information toward the central nervous system
Motor (efferent) division
Nerve fibers that carry impulses away from the central nervous system
o
Two subdivisions

Somatic nervous system = voluntary

Autonomic nervous system = involuntary
Organization of the nervous system
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Nervous Tissue:
Support Cells (Neuroglia) – cannot transmit nerve impulses
CNS glia:
a) Astrocytes
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Abundant, star-shaped cells
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Brace neurons
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Form barrier between capillaries and neurons
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Control the chemical environment of the brain
b) Microglia
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Spider-like phagocytes
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Dispose of debris
c) Ependymal cells
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Line cavities of the brain and spinal cord
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Circulate cerebrospinal fluid with cilia
d) Oligodendrocytes
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Produce myelin sheath around
nerve fibers in the central
nervous system
PNS glia:
e) Satellite cells
o
f)
Protect neuron cell bodies
Schwann cells
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Form myelin sheath in the
peripheral nervous system
Neurons = nerve cells (do not routinely undergo cell division after birth)
Cells specialized to transmit messages
Structure:
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Cell body– nucleus (& large nucleolus) and metabolic center
of the cell

Nissl substance – specialized rough endoplasmic
reticulum

Neurofibrils – intermediate cytoskeleton that
maintains cell shape

Neuron Cell Body Location:
Most are found in the CNS
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Gray matter – cell bodies and unmylenated fibers
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Nuclei – clusters of cell bodies within the white matter of the central
nervous system
Ganglia – collections of cell bodies outside the CNS
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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
o
Synaptic cleft – gap between adjacent neurons
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Synapse – junction between nerves
Nerve Fiber Coverings
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Schwann cells – produce myelin sheaths in jelly-roll like fashion
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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
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Sensory (afferent) neurons

Carry impulses from the sensory receptors
Cutaneous sense organs
Proprioceptors (in muscles
& tendons) – detect
stretch or tension
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Motor (efferent) neurons

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Carry impulses from the CNS
Interneurons (association neurons)

Found in neural pathways in the CNS

Connect sensory and motor neurons
Structural Classification of Neurons
o Multipolar neurons –
o Bipolar neurons – one
o Unipolar neurons – have a
many extensions from
axon and one dendrite
short single process leaving
the cell body
the cell body
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Functional Properties of Neurons
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Irritability – ability to respond to stimuli
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Conductivity – ability to transmit an impulse
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The plasma membrane at rest is polarized

Fewer positive ions are inside the cell than outside the
cell
Nerve Impulses
Starting a Nerve Impulse
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Depolarization – a stimulus depolarizes the neuron’s membrane
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A deploarized membrane allows sodium (Na+) to flow inside the
membrane
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The exchange of ions initiates an action potential in the neuron
The Action Potential
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If the action potential (nerve impulse) starts, it is propagated
over the entire axon
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Potassium ions rush out of the neuron after sodium ions rush in,
which repolarizes the membrane
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The sodium-potassium pump restores the original configuration

This action requires ATP
Nerve Impulse Propagation
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The impulse continues to move toward the cell body
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Impulses travel faster when fibers have a myelin sheath
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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
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.
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Reflex Arc
Reflex – rapid, predictable, and involuntary responses to stimuli (always in the same direction)
o
Reflex arc – direct route from a sensory neuron, to an interneuron, to an effector
Simple reflex arc
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Types of Reflexes & Regulation
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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
All reflex arcs have a minimum of five elements
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sensory receptor – reacts to a stimulus
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effector organ – the muscle or gland
eventually stimulated
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afferent & efferent neurons – connect
the previous two
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integration center – the synapse
between the afferent & efferent
neurons
two-neuron reflex arc
(knee-jerk reflex)
three-neuron reflex arc
(withdrawal reflex)
Central Nervous System
CNS develops from the embryonic neural tube
o
The neural tube becomes the brain and spinal cord
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The opening of the neural tube becomes the ventricles

Four chambers within the brain

Filled with cerebrospinal fluid
Regions of the Brain
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Cerebral hemispheres
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Diencephalon
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Brain stem
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Cerebellum
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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
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Fissures divide the cerebrum into lobes
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Surface lobes of the cerebrum
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Frontal lobe
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Parietal lobe
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Occipital lobe
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Temporal lobe
Sensory and Motor Areas of the Cerebral Cortex
Specialized Areas of the Cerebrum
o Somatic sensory area – receives impulses from
the body’s sensory receptors

pain, coldness, light touch
o Primary motor area – sends impulses to skeletal
muscles

face, mouth, hands
o Broca’s area – involved in our ability to speak
Cerebral areas involved in special senses
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Gustatory area (taste)
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Visual area
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Auditory area
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Olfactory area
Interpretation areas of the cerebrum
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Speech/language region
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Language comprehension region
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General interpretation area
Layers of the Cerebrum
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Gray matter
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White matter
 Outer layer
 Fiber tracts inside the gray matter
 Composed mostly of neuron cell bodies
 Example: corpus callosum connects
 Basal nuclei – internal islands of gray matter
hemispheres
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Diencephalon
Sits on top of the brain stem
Enclosed by the cerebral hemispheres
Made of three parts
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Thalamus
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Hypothalamus
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Epithalamus
Thalamus
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Surrounds the third ventricle
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The relay station for sensory impulses
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Transfers impulses to the correct part of the
cortex for localization and interpretation
Hypothalamus
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Under the thalamus
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Important autonomic nervous system center
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
Helps regulate body temperature

Controls water balance
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Regulates metabolism
An important part of the limbic system (emotions)

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thirst, appetite, sex, pain, & pleasure
The pituitary gland is attached to the hypothalamus
Epithalamus
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Forms the roof of the third ventricle
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Houses the pineal body (an endocrine gland)
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Includes the choroid plexus – forms cerebrospinal fluid
Brain Stem
Attaches to the spinal cord
Parts of the brain stem
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Midbrain
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Pons
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Medulla oblongata
Midbrain:
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Mostly composed of tracts of nerve fibers

Has two bulging fiber tracts –
cerebral peduncles
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Has four rounded protrusions –
corpora quadrigemina
Reflex centers for vision and hearing
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o
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Pons

The bulging center part of the brain stem
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Mostly composed of fiber tracts
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Includes nuclei involved in the control of breathing
Medulla Oblongata
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The lowest part of the brain stem
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Merges into the spinal cord
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Includes important fiber tracts
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Contains important control centers
Heart rate control
Blood pressure regulation
Breathing
Swallowing
Vomiting
Reticular Formation
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Diffuse mass of gray matter along the brain stem
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Involved in motor control of visceral organs
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Reticular activating system plays a role in
awake/sleep cycles and consciousness
Cerebellum
Two hemispheres with convoluted surfaces
Provides involuntary coordination of body movements
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like an automatic pilot
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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”
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Double-layered external covering
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Periosteum – attached to surface of the skull

Meningeal layer – outer covering of the brain
Folds inward in several areas
Arachnoid layer – “spider/cobweb”
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Middle layer
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Web-like
Pia mater – “gentle mother”
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Internal layer
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Clings to the surface of the brain
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
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Fats and fat soluble molecules
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Respiratory gases
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Alcohol
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Nicotine
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Anesthesia
Traumatic Brain Injuries
Concussion
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Slight brain injury
o
No permanent brain damage
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Contusion
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Nervous tissue destruction occurs
o
Nervous tissue does not regenerate
Cerebral edema
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Swelling from the inflammatory response
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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:
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Exterior white mater – conduction tracts
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Internal gray matter - mostly cell bodies
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
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
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Dorsal root

Associated with the dorsal root ganglia –
collections of cell bodies outside the central
nervous system
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Ventral root
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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
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.”
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I Olfactory nerve – sensory for smell
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II Optic nerve – sensory for vision
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III Oculomotor nerve – motor fibers to
eye muscles
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IV Trochlear – motor fiber to eye muscles
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V Trigeminal nerve – sensory for the
face; motor fibers to chewing muscles
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VI Abducens nerve – motor fibers to eye
muscles
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VII Facial nerve – sensory for taste;
motor fibers to the face
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VIII Vestibulocochlear nerve – sensory
for balance and hearing
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IX Glossopharyngeal nerve – sensory for
taste; motor fibers to the pharynx
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X Vagus nerves – sensory and motor
fibers for pharynx, larynx, and viscera
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XI Accessory nerve – motor fibers to
neck and upper back
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XII Hypoglossal nerve – motor fibers to tongue
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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:
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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
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Autonomic Nervous System
The involuntary branch of the nervous system
Consists of only motor nerves
Divided into two divisions
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Sympathetic division
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Parasympathetic division
Differences Between Somatic and Autonomic Nervous Systems
Somatic
Autonomic
Nerves
One motor neuron
preganglionic and postganglionic
Effector organs
skeletal muscle
smooth muscle, cardiac muscle,and glands
Nerurotransmitters
always use acetylcholine
use acetylcholine, epinephrine, or norepinephrine
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
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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”
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Response to unusual stimulus
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Takes over to increase activities
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Remember as the “E” division = exercise,
excitement, emergency, and
embarrassment
Parasympathetic – housekeeping activities
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Conserves energy
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Maintains daily necessary body functions
o
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
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