Download Functions of the Nervous System Functions of the

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
o To process and interpret sensory input and decide whether action is needed
Functions of the Nervous System
3. Motor output
o A response to integrated stimuli
o The response activates muscles or glands
Organization of the Nervous System
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Nervous system is classified based on:
o Structures (structural classification)
o Activities (functional classification)
Structural Classification of the Nervous System
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Central nervous system (CNS)
o Organs
! Brain
! Spinal cord
o Function
! Integration; command center
! Interpret incoming sensory information
! Issues outgoing instructions
Structural Classification of the Nervous System
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Peripheral nervous system (PNS)
o Nerves extending from the brain and spinal cord
! Spinal nerves—carry impulses to and from the spinal cord
! Cranial nerves—carry impulses to and from the brain
o Functions
! Serve as communication lines among sensory organs, the brain and
spinal cord, and glands or muscles
Functional Classification of the Peripheral Nervous System
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Sensory (afferent) division
o Nerve fibers that carry information to the central nervous system
! Somatic sensory fibers carry information from the skin, skeletal
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muscles, and joints
! Visceral sensory fibers carry information from visceral organs
Motor (efferent) division
o Nerve fibers that carry impulses away from the central nervous system organs
Functional Classification of the Peripheral Nervous System
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Motor (efferent) division (continued)
o Two subdivisions
! Somatic nervous system = voluntary
o Consciously controls skeletal muscles
! Autonomic nervous system = involuntary
o Automatically controls smooth and cardiac muscles and glands
o Further divided into the sympathetic and parasympathetic
nervous systems
Nervous Tissue: Support Cells
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Support cells in the CNS are grouped together as neuroglia
General functions
o Support
o Insulate
o Protect neurons
Nervous Tissue: Support Cells
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CNS glial cells: astrocytes
o Abundant, star-shaped cells
o Brace neurons
o Form barrier between capillaries and neurons
o Control the chemical environment of
the brain
Nervous Tissue: Support Cells
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CNS glial cells: microglia
o Spiderlike phagocytes
o Dispose of debris
Nervous Tissue: Support Cells
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CNS glial cells: ependymal cells
o Line cavities of the brain and spinal cord
o Cilia assist with circulation of cerebrospinal fluid
Nervous Tissue: Support Cells
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CNS glial cells: oligodendrocytes
o Wrap around nerve fibers in the central nervous system
o Produce myelin sheaths
Nervous Tissue: Support Cells
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PNS glial cells
o Satellite cells
! Protect neuron cell bodies
o Schwann cells
! Form myelin sheath in the peripheral nervous system
Nervous Tissue: Neurons
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Neurons = nerve cells
o Cells specialized to transmit messages
o Major regions of neurons
! Cell body—nucleus and metabolic center of the cell
! Processes—fibers that extend from the cell body
Nervous Tissue: Neurons
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Cell body
o Nissl bodies
! Specialized rough endoplasmic reticulum
o Neurofibrils
! Intermediate cytoskeleton
! Maintains cell shape
o Nucleus with large nucleolus
Nervous Tissue: Neurons
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Processes outside the cell body
o Dendrites—conduct impulses toward the cell body
! Neurons may have hundreds of dendrites
o Axons—conduct impulses away from the cell body
! Neurons have only one axon arising from the cell body at the axon
hillock
Nervous Tissue: Neurons
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Axons
o End in axon terminals
o Axon terminals contain vesicles with neurotransmitters
o Axon terminals are separated from the next neuron by a gap
! Synaptic cleft—gap between adjacent neurons
! Synapse—junction between nerves
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Nervous Tissue: Neurons
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Myelin sheath—whitish, fatty material covering axons
o Schwann cells—produce myelin sheaths in jelly roll–like fashion around axons
(PNS)
! Nodes of Ranvier—gaps in myelin sheath along the axon
o Oligodendrocytes—produce myelin sheaths around axons of the CNS
Myelin sheaths speed the nerve impulse transmission
Neuron Cell Body Location
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Most neuron cell bodies are found in the central nervous system
o Gray matter—cell bodies and unmyelinated fibers
o Nuclei—clusters of cell bodies within the white matter of the central nervous
system
Ganglia—collections of cell bodies outside the central nervous system
Neuron Cell Body Location
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Tracts—bundles of nerve fibers in the CNS
Nerves—bundles of nerve fibers in the PNS
White matter—collections of myelinated fibers (tracts)
Gray matter—collections of mostly unmyelinated fibers and cell bodies
Functional Classification of Neurons
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Sensory (afferent) neurons
o Carry impulses from the sensory receptors to the CNS
! Cutaneous sense organs
! Proprioceptors—detect stretch or tension
Functional Classification of Neurons
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Motor (efferent) neurons
o Carry impulses from the central nervous system to viscera, muscles, or glands
Interneurons (association neurons)
o Found in neural pathways in the central nervous system
o Connect sensory and motor neurons
Structural Classification of Neurons
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Structural classification is based on number of processes extending from the cell
body
Multipolar neurons—many extensions from the cell body
o All motor and interneurons are multipolar
o Most common structure
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Structural Classification of Neurons
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Bipolar neurons—one axon and one dendrite
o Located in special sense organs, such as nose and eye
o Rare in adults
Structural Classification of Neurons
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Unipolar neurons—have a short single process leaving the cell body
o Sensory neurons found in PNS ganglia
o Conduct impulses both toward and away from the cell body
Functional Properties of Neurons
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Irritability
o Ability to respond to a stimulus and convert it to a nerve impulse
Conductivity
o Ability to transmit the impulse to other neurons, muscles, or glands
Nerve Impulses
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Resting neuron
o The plasma membrane at rest is polarized
o As long as inside is more negative than outside, the cell stays at rest
o Fewer positive ions are inside the cell than outside the cell
! K is the major positive ion inside the cell
! Na is the major positive ion outside the cell
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Nerve Impulses
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Action potential initiation and generation
o A stimulus depolarizes the neuron’s membrane
o The membrane is now permeable to sodium as sodium channels open
o A depolarized membrane allows sodium (Na ) to flow inside the membrane
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Nerve Impulses
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Action potential initiation and generation
o A stimulus leads to the movement of ions, which initiates an action potential in
the neuron
o A graded potential (localized depolarization) exists where the inside of the
membrane is more positive and the outside is less positive
o If the stimulus is strong enough and sodium influx great enough, local
depolarization activates the neuron to conduct an action potential (nerve
impulse)
Nerve Impulses
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Propagation of the action potential
o If enough sodium enters the cell, the action potential (nerve impulse) starts
and is propagated over the entire axon
o All-or-none response means the nerve impulse either is propagated or is not
o Fibers with myelin sheaths conduct nerve impulses more quickly
Nerve Impulses
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Repolarization
o Potassium ions rush out of the neuron after sodium ions rush in, repolarizing
the membrane
o Repolarization involves restoring the inside of the membrane to a negative
charge and the outer surface to a positive charge
o Until repolarization is complete, a neuron cannot conduct another nerve
impulse
Nerve Impulses
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Repolarization
o Initial ionic conditions are restored using the sodium-potassium pump
o This pump, using ATP, restores the original configuration
o Three sodium ions are ejected from the cell while two potassium ions are
returned to the cell
Transmission of a Signal at Synapses
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When the action potential reaches the axon terminal, the electrical charge opens
calcium channels
Transmission of a Signal at Synapses
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Calcium, in turn, causes the tiny vesicles containing the neurotransmitter chemical to
fuse with the axonal membrane
Transmission of a Signal at Synapses
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The entry of calcium into the axon terminal causes porelike openings to form,
releasing the transmitter
Transmission of a Signal at Synapses
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The neurotransmitter molecules diffuse across the synapse and bind to receptors on
the membrane of the next neuron
Transmission of a Signal at Synapses
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If enough neurotransmitter is released, graded potential will be generated
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Eventually an action potential (nerve impulse) will occur in the neuron beyond the
synapse
Transmission of a Signal at Synapses
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The electrical changes prompted by neurotransmitter binding are brief
The neurotransmitter is quickly removed from the synapse by either:
o Reuptake
o Enzymatic activity
Transmission of an impulse is electrochemical
o Transmission down neuron is electrical
o Transmission to next neuron is chemical
The Reflex Arc
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Reflexes are:
o Rapid
o Predictable
o Involuntary responses to a stimulus
Reflexes occur over neural pathways called reflex arcs
The Reflex Arc
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Somatic reflexes
o Reflexes that stimulate the skeletal muscles
o Example: pulling your hand away from a hot object
Autonomic reflexes
o Regulate the activity of smooth muscles, the heart, and glands
o Example: regulation of smooth muscles, heart and blood pressure, glands,
digestive system
The Reflex Arc
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Five elements of a reflex:
1. Sensory receptor—reacts to a stimulus
2. Sensory neuron—carries message to the integration center
3. Integration center (CNS)—processes information and directs motor output
4. Motor neuron—carries message to an effector
5. Effector organ—is the muscle or gland to be stimulated
Two-Neuron Reflex Arc
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Two-neuron reflex arcs
o Simplest type
o Example: patellar (knee-jerk) reflex
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Three-Neuron Reflex Arc
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Three-neuron reflex arcs
o Consists of five elements: receptor, sensory neuron, interneuron, motor
neuron, and effector
o Example: flexor (withdrawal) reflex
Central Nervous System (CNS)
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CNS develops from the embryonic neural tube
o The neural tube becomes the brain and spinal cord
o 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 (cerebrum)
Diencephalon
Brain stem
Cerebellum
Regions of the Brain: Cerebrum
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Cerebral hemispheres are paired (left & right) superior parts of the brain
o Includes more than half of the brain mass
o The surface is made of ridges (gyri) and grooves (sulci)
Three main regions of cerebral hemisphere
1. Cortex (gray matter)
2. White matter
3. Basal nuclei (deep pockets of gray matter)
Regions of the Brain: Cerebrum
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Lobes of the cerebrum
o Fissures (deep grooves) divide the cerebrum into lobes
o Surface lobes of the cerebrum
! Frontal lobe
! Parietal lobe
! Occipital lobe
! Temporal lobe
Regions of the Brain: Cerebrum
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Specialized areas of the cerebrum
o Primary somatic sensory area
! Receives impulses from the body’s sensory receptors
o Pain, temperature, light touch
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Located in parietal lobe posterior to central sulcus
Sensory homunculus is a spatial map
Left side of the primary somatic sensory area receives impulses from
right side (and vice versa)
Regions of the Brain: Cerebrum
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Cerebral areas involved in special senses
o Visual area (occipital lobe)
o Auditory area (temporal lobe)
o Olfactory area (temporal lobe)
Regions of the Brain: Cerebrum
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Specialized areas of the cerebrum
o Primary motor area
! Sends impulses to skeletal muscles
! Located in frontal lobe
! Motor neurons form corticospinal (pyramidal) tract, which descends to
spinal cord
! Motor homunculus is a spatial map
Regions of the Brain: Cerebrum
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Broca’s area
o Involved in our ability to speak
o Usually in left hemisphere
Other specialized areas
o Anterior and posterior association areas
o Speech area
Regions of the Brain: Cerebrum
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Layers of the cerebrum
o Gray matter—outer layer in the cerebral cortex; composed mostly of neuron
cell bodies
o White matter—fiber tracts deep to the gray matter
! Corpus callosum connects hemispheres
Basal nuclei (ganglia)—islands of gray matter buried within the white matter
Regions of the Brain: Diencephalon
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Sits on top of the brain stem
Enclosed by the cerebral hemispheres
Made of three parts:
1. Thalamus
2. Hypothalamus
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3. Epithalamus
Regions of the Brain: Diencephalon
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Thalamus
o Surrounds the third ventricle
o The relay station for sensory impulses
o Transfers impulses to the correct part of the cortex for localization and
interpretation
Regions of the Brain: Diencephalon
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Hypothalamus
o Under the thalamus
o Important autonomic nervous system center
! Helps regulate body temperature
! Controls water balance
! Regulates metabolism
o Houses the limbic center for emotions
o Regulates the nearby pituitary gland
o Houses mammillary bodies for olfaction (smell)
Regions of the Brain: Diencephalon
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Epithalamus
o Forms the roof of the third ventricle
o Houses the pineal body (an endocrine gland)
o Includes the choroid plexus—forms cerebrospinal fluid
Regions of the Brain: Brain Stem
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Attaches to the spinal cord
Parts of the brain stem
o Midbrain
o Pons
o Medulla oblongata
Regions of the Brain: Brain Stem
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Midbrain
o Composed mostly of tracts of nerve fibers
o Two bulging fiber tracts, cerebral peduncles, convey ascending and
descending impulses
o Four rounded protrusions, corpora quadrigemina, visual and auditory reflex
centers
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Regions of the Brain: Brain Stem
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Pons
o The bulging center part of the brain stem
o Mostly composed of fiber tracts
o Includes nuclei involved in the control of breathing
Regions of the Brain: Brain Stem
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Medulla oblongata
o The lowest part of the brain stem
o Merges into the spinal cord
o Includes important fiber tracts
o Contains important control centers
! Heart rate control
! Blood pressure regulation
! Breathing
! Swallowing
! Vomiting
Regions of the Brain: Brain Stem
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Reticular formation
o Diffuse mass of gray matter along the brain stem
o Involved in motor control of visceral organs
o Reticular activating system (RAS)
! Plays a role in awake/sleep cycles and consciousness
! Filter for incoming sensory information
Regions of the Brain: Cerebellum
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Two hemispheres with convoluted surfaces
Controls balance and equilibrium
Provides precise timing for skeletal muscle activity and coordination of body
movements
Protection of the Central Nervous System
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Scalp and skin
Skull and vertebral column
Meninges
Cerebrospinal fluid (CSF)
Blood-brain barrier
Meninges
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Dura mater
o Tough outermost layer
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o Double-layered external covering
! Periosteum—attached to inner surface of the skull
! Meningeal layer—outer covering of the brain
o Folds inward in several areas
! Falx cerebri
! Tentorium cerebelli
Meninges
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Arachnoid layer
o Middle layer
o Weblike extensions span the subarachnoid space
o Arachnoid villi reabsorb cerebrospinal fluid
Pia mater
o Internal layer
o Clings to the surface of the brain
Cerebrospinal Fluid (CSF)
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Similar to blood plasma composition
Formed by the choroid plexus
o Choroid plexuses–capillaries in the ventricles of the brain
Forms a watery cushion to protect the brain
Circulated in arachnoid space, ventricles, and central canal of the spinal cord
Cerebrospinal Fluid (CSF) Pathway of Flow
1. CSF is produced by the choroid plexus of each ventricle.
2. CSF flows through the ventricles and into the subarachnoid space via the median and
lateral apertures. Some CSF flows through the central canal of the spinal cord.
3. CSF flows through the subarachnoid space.
4. CSF is absorbed into the dural venous sinuses via the arachnoid villi.
Hydrocephalus in a Newborn
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Hydrocephalus
o CSF accumulates and exerts pressure on the brain if not allowed to drain
o Possible in an infant because the skull bones have not yet fused
o In adults, this situation results in brain damage
Blood-Brain Barrier
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Includes the least permeable capillaries of the body
Excludes many potentially harmful substances
Useless as a barrier against some substances
o Fats and fat-soluble molecules
o Respiratory gases
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o Alcohol
o Nicotine
o Anesthesia
Blood-Brain Barrier
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Water-soluble items that can travel through barrier:
o Water
o Glucose
o Essential amino acids
Items prevented from passing through:
o Metabolic wastes
o Most drugs
o Nonessential amino acids
o Potassium ions
Traumatic Brain Injuries
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Concussion
o Slight brain injury
o No permanent brain damage
Contusion
o Nervous tissue destruction occurs
o Nervous tissue does not regenerate
Cerebral edema
o Swelling from the inflammatory response
o May compress and kill brain tissue
Cerebrovascular Accident (CVA), or Stroke
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Results from 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
o Hemiplegia—one-sided paralysis
o Aphasia—damage to speech center in left hemisphere
Transient ischemic attack (TIA)—temporary brain ischemia (restriction of blood flow)
o Warning signs for more serious CVAs
Alzheimer’s Disease
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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
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Spinal Cord
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Extends from the foramen magnum of the skull to the first or second lumbar vertebra
Provides a two-way conduction pathway to and from the brain
31 pairs of spinal nerves arise from the spinal cord
Ends around vertebra L1 or L2
Cauda equina is a collection of spinal nerves at the inferior end
Spinal Cord Anatomy
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Internal gray matter is mostly cell bodies
o Dorsal (posterior) horns house interneurons
! Receive information from sensory neurons in the dorsal root
o Anterior (ventral) horns house motor neurons of the somatic (voluntary)
nervous system
! Send information out ventral root
o Gray matter surrounds the central canal, which is filled with cerebrospinal fluid
Spinal Cord Anatomy
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Exterior white mater—conduction tracts
o Dorsal, lateral, ventral columns
o Sensory (afferent) tracts conduct impulses toward brain
o Motor (efferent) tracts carry impulses from brain to skeletal muscles
Spinal Cord Anatomy
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Meninges cover the spinal cord
Spinal nerves leave at the level of each vertebra
o Dorsal root
! Associated with the dorsal root ganglia—collections of cell bodies
outside the central nervous system
o Ventral root
! Contains axons
Peripheral Nervous System (PNS)
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PNS consists of nerves and ganglia outside the central nervous system
Nerve = bundle of neuron fibers
Neuron fibers are bundled by connective tissue
PNS: Structure of a Nerve
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Endoneurium surrounds each fiber
Groups of fibers are bound into fascicles by perineurium
Fascicles are bound together by epineurium
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PNS: Classification of Nerves
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Mixed nerves
o Both sensory and motor fibers
Sensory (afferent) nerves
o Carry impulses toward the CNS
Motor (efferent) nerves
o Carry impulses away from the CNS
PNS: Cranial Nerves
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12 pairs of nerves that mostly serve the head and neck
Only the pair of vagus nerves extends to thoracic and abdominal cavities
Most are mixed nerves, but three are sensory only:
1. Optic
2. Olfactory
3. Vestibulocochlear
PNS: Cranial Nerves Mnemonic Device
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Oh – Olfactory
Oh – Optic
Oh – Oculomotor
To – Trochlear
Touch – Trigeminal
And – Abducens
Feel – Facial
Very – Vestibulocochlear
Green – Glossopharyngeal
Vegetables – Vagus
A – Accessory
H – Hypoglossal
PNS: Spinal Nerves
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There is a pair of spinal nerves at the level of each vertebra, for a total of 31 pairs
Formed by the combination of the ventral and dorsal roots of the spinal cord
Named for the region from which they arise
PNS: Anatomy of Spinal Nerves
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Spinal nerves divide soon after leaving the spinal cord
Ramus—branch of a spinal nerve; contains both motor and sensory fibers
o Dorsal rami—serve the skin and muscles of the posterior trunk
o Ventral rami—form a complex of networks (plexus) for the anterior
PNS: Spinal Nerve Plexuses
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Plexus–networks of nerves serving motor and sensory needs of the limbs
Form from ventral rami of spinal nerves in the cervical, lumbar, and sacral regions
Four plexuses:
1. Cervical
2. Brachial
3. Lumbar
4. Sacral
PNS: Autonomic Nervous System
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Motor subdivision of the PNS
o Consists only of motor nerves
Also known as the involuntary nervous system
o Regulates activities of cardiac and smooth muscles and glands
Two subdivisions:
1. Sympathetic division
2. Parasympathetic division
PNS: Anatomy of the Parasympathetic Division
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Preganglionic neurons originate from the craniosacral regions:
o The cranial nerves III, VII, IX, and X
o S2 through S4 regions of the spinal cord
Because it is the site of preganglionic neuron origination, the parasympathetic
division is also known as the craniosacral division
Terminal ganglia are at the effector organs
Neurotransmitter: acetylcholine
PNS: Anatomy of the Sympathetic Division
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Preganglionic neurons originate 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
Neurotransmitters: norepinephrine and epinephrine (effector organs)
PNS: Autonomic Functioning
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Sympathetic—“fight or flight” division
o Response to unusual stimulus
o Takes over to increase activities
o Remember as the “E” division:
! Exercise
! Excitement
! Emergency
! Embarrassment
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PNS: Autonomic Functioning
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Parasympathetic—“housekeeping” activites
o Conserves energy
o Maintains daily necessary body functions
o Remember as the “D” division
! Digestion
! Defecation
! Diuresis
Developmental Aspects of the Nervous System
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The nervous system is formed during the first month of embryonic development
Any maternal infection can have extremely harmful effects
Oxygen deprivation destroys brain cells
The hypothalamus is one of the last areas of the brain to develop
Developmental Aspects of the Nervous System
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Severe congenital brain diseases include:
o Cerebral palsy
o Anencephaly
o Hydrocephalus
o Spina bifida
Developmental Aspects of the Nervous System
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Premature babies have trouble regulating body temperature because the
hypothalamus is one of the last brain areas to mature prenatally.
Development of motor control indicates the progressive myelination and maturation
of a child’s nervous system.
Developmental Aspects of the Nervous System
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Brain growth ends in young adulthood. Neurons die throughout life and are not
replaced; thus, brain mass declines with age.
Healthy aged people maintain nearly optimal intellectual function.
Disease—particularly cardiovascular disease—is the major cause of declining mental
function with age.
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