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The Nervous System

The Nervous System

Functions of the Nervous System

Sensory input—gathering information

To monitor changes occurring inside and outside the body
Changes = stimuli
Integration
To process and interpret sensory input and decide if action is needed

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

Structural Classification of the Nervous System

Central nervous system (CNS)
Brain
Spinal cord

Function

Integration; command center
Interpret incoming sensory information
Issues outgoing instructions

Peripheral nervous system (PNS)
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

Functions

Serve as communication lines among sensory organs, the brain and spinal cord, and glands or muscles

Functional Classification of the Peripheral Nervous System

Sensory (afferent) division

Nerve fibers that carry information to the central nervous system

Motor (efferent) division
Nerve fibers that carry impulses away from the central nervous system

Two subdivisions
Somatic nervous system = voluntary
Consciously controls skeletal muscles

Autonomic nervous system = involuntary

Automatically controls smooth and cardiac muscles and glands
Further divided into the sympathetic and parasympathetic nervous systems

Nervous Tissue: Support Cells
Support cells in the CNS are grouped together as “neuroglia”

General functions

Support
Insulate
Protect neurons

Astrocytes
Abundant, star-shaped cells
Brace neurons
Form barrier between capillaries and neurons
Control the chemical environment of the brain

Microglia
Spiderlike phagocytes
Dispose of debris

Ependymal cells
Line cavities of the brain and spinal cord
Cilia assist with circulation of cerebrospinal fluid

Oligodendrocytes
Wrap around nerve fibers in the central nervous system
Produce myelin sheaths 

Satellite cells
Protect neuron cell bodies

Schwann cells
Form myelin sheath in the peripheral nervous system

Nervous Tissue: Neurons

Neurons = nerve cells
Cells specialized to transmit messages

Major regions of neurons

Cell body—nucleus and metabolic center of the cell
Processes—fibers that extend from the cell body 

Nissl bodies
Specialized rough endoplasmic reticulum

Neurofibrils 
Intermediate cytoskeleton 
Maintains cell shape
Nucleus with large nucleolus

Processes outside the cell body

Dendrites—conduct impulses toward the cell body
Neurons may have hundreds of dendrites

Axons—conduct impulses away from the cell body
Neurons have only one axon arising from the cell body at the axon hillock
End in axon terminals
Axon terminals contain vesicles with neurotransmitters
Axon terminals are separated from the next neuron by a gap
Synaptic cleft—gap between adjacent neurons
Synapse—junction between nerves

Myelin sheath—whitish, fatty material covering axons
Schwann cells—produce myelin sheaths in jelly roll-like fashion around axons (PNS)
Nodes of Ranvier—gaps in myelin sheath along the axon
Oligodendrocytes—produce myelin sheaths around axons of the CNS

Neuron Cell Body Location
Most neuron cell bodies are found in the central nervous system
Gray matter—cell bodies and unmyelinated fibers
Nuclei—clusters of cell bodies within the white matter of the central nervous system
Ganglia—collections of cell bodies outside the central nervous system

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


Sensory (afferent) neurons
Carry impulses from the sensory receptors to the CNS
Cutaneous sense organs
Proprioceptors—detect stretch or tension

Motor (efferent) neurons
Carry impulses from the central nervous system to viscera, muscles, or glands

Interneurons (association neurons)
Found in neural pathways in the central nervous system
Connect sensory and motor neurons

Structural Classification of Neurons

Multipolar neurons—many extensions from the cell body

All motor and interneurons are multipolar
Most common structure

Bipolar neurons—one axon and one dendrite
Located in special sense organs such as nose and eye
Rare in adults

Unipolar neurons—have a short single process leaving the cell body
Sensory neurons found in PNS ganglia

Functional Properties of Neurons
Irritability
Ability to respond to stimuli
Conductivity
Ability to transmit an impulse

Nerve Impulses
Resting neuron
The plasma membrane at rest is polarized
Fewer positive ions are inside the cell than outside the cell

Depolarization 
A stimulus depolarizes the neuron’s membrane
The membrane is now permeable to sodium as sodium channels open
A depolarized membrane allows sodium (Na+) to flow inside the membrane

Action potential
The movement of ions initiates an action potential in the neuron due to a stimulus
A graded potential (localized depolarization) exists where the inside of the membrane is more positive and the outside is less positive

Propagation of the action potential
If enough sodium enters the cell, the action potential (nerve impulse) starts and is propagated over the entire axon
Impulses travel faster when fibers have a myelin sheath

Repolarization
Potassium ions rush out of the neuron after sodium ions rush in, which repolarizes the membrane
Repolarization involves restoring the inside of the membrane to a negative charge and the outer surface to a positive charge
Initial ionic conditions are restored using the sodium-potassium pump.
This pump, using ATP, restores the original configuration
Three sodium ions are ejected from the cell while two potassium ions are returned to the cell

Transmission of a Signal at Synapses
When the action potential reaches the axon terminal, the electrical charge opens calcium channels
Calcium, in turn, causes the tiny vesicles containing the neurotransmitter chemical to fuse with the axonal membrane
The entry of calcium into the axon terminal causes porelike openings to form, releasing the transmitter
The neurotransmitter molecules diffuse across the synapse and bind to receptors on the membrane of the next neuron
If enough neurotransmitter is released, graded potential will be generated
Eventually an action potential (nerve impulse) will occur in the neuron beyond the synapse
The electrical changes prompted by neurotransmitter binding are brief 
The neurotransmitter is quickly removed from the synapse

The Reflex Arc
Reflex—rapid, predictable, and involuntary response to a stimulus
Occurs over pathways called reflex arcs
Reflex arc—direct route from a sensory neuron, to an interneuron, to an effector

Somatic reflexes
Reflexes that stimulate the skeletal muscles
Example: pull your hand away from a hot object

Autonomic reflexes
Regulate the activity of smooth muscles, the heart, and glands
Example: Regulation of smooth muscles, heart and blood pressure, glands, digestive system

Five elements of a reflex:
Sensory receptor–reacts to a stimulus
Sensory neuron–carries message to the integration center
Integration center (CNS)–processes information and directs motor output
Motor neuron–carries message to an effector 
Effector organ–is the muscle or gland to be stimulated

Central Nervous System (CNS)
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
Cerebral hemispheres (cerebrum)
Diencephalon
Brain stem
Cerebellum

Regions of the Brain: Cerebrum
Cerebral Hemispheres (Cerebrum) 
Paired (left and right) superior parts of the brain
Includes more than half of the brain mass
The surface is made of ridges (gyri) and grooves (sulci)

Lobes of the cerebrum
Fissures (deep grooves) divide the cerebrum into lobes
Surface lobes of the cerebrum
Frontal lobe
Parietal lobe
Occipital lobe
Temporal lobe

Specialized areas of the cerebrum 
Primary somatic sensory area 
Receives impulses from the body’s sensory receptors
Located in parietal lobe
Primary motor area 
Sends impulses to skeletal muscles
Located in frontal lobe
Broca’s area
Involved in our ability to speak
Regions of the Brain: Cerebrum
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 in the cerebral cortex composed mostly of neuron cell bodies
White matter—fiber tracts deep to the gray matter
Corpus callosum connects hemispheres
Basal nuclei—islands of gray matter buried within the white matter

Regions of the Brain: 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
Regions of the Brain: Diencephalon

Hypothalamus 
Under the thalamus
Important autonomic nervous system center
Helps regulate body temperature
Controls water balance
Regulates metabolism
Houses the limbic center for emotions
Regulates the nearby pituitary gland
Produces two hormones of its own
Regions of the Brain: Diencephalon

Epithalamus 
Forms the roof of the third ventricle
Houses the pineal body (an endocrine gland)
Includes the choroid plexus—forms cerebrospinal fluid

Regions of the Brain: Brain Stem
Attaches to the spinal cord

Parts of the brain stem
Midbrain
Pons
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 (RAS) plays a role in awake/sleep cycles and consciousness

Regions of the Brain: Cerebellum
Two hemispheres with convoluted surfaces
Provides involuntary coordination of body movements

Protection of the Central Nervous System
Scalp and skin
Skull and vertebral column
Meninges
Cerebrospinal fluid (CSF)
Blood-brain barrier

Meninges
Arachnoid layer
Middle layer
Web-like extensions span the subarachnoid space
Arachnoid villi reabsorb cerebrospinal fluid
Pia mater
Internal layer
Clings to the surface of the brain

Cerebrospinal Fluid (CSF)
Similar to blood plasma composition
Formed by the choroid plexus
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
CSF is produced by the choroid plexus of each ventricle.
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.
CSF flows through the subarachnoid space.
CSF is absorbed into the dural venous sinuses via the arachnoid villi.

Hydrocephalus in a Newborn
CSF accumulates and exerts pressure on the brain if not allowed to drain
Possible in an infant because the skull bones have not yet fused
In adults, this situation results in brain damage 

Blood-Brain Barrier
Includes the least permeable capillaries of the body
Excludes many potentially harmful substances
Useless as a barrier 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) or Stroke
Result 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
Hemiplegia–One-sided paralysis
Aphasis–Damage to speech center in left hemisphere
Transischemia-attack (TIA)–temporary brain ischemia (restriction of blood flow) 
Warning signs for more serious CVAs

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 foramen magnum of the skull to the first or second lumbar vertebra
Provides a two-way conduction pathway from the brain to and from the brain
31 pairs of spinal nerves arise from the spinal cord
Cauda equina is a collection of spinal nerves at the inferior end

Spinal Cord Anatomy
Internal gray matter is mostly cell bodies
Dorsal (posterior) horns
Anterior (ventral) horns 
Gray matter surrounds the central canal
Central canal is filled with cerebrospinal fluid
Exterior white mater—conduction tracts
Dorsal, lateral, ventral columns
Meninges cover the spinal cord
Spinal 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
Contains axons

Peripheral Nervous System (PNS)
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
Endoneurium surrounds each fiber
Groups of fibers are bound into fascicles by perineurium
Fascicles are bound together by epineurium

PNS: Classification of Nerves
Mixed nerves
Both sensory and motor fibers
Sensory (afferent) nerves
Carry impulses toward the CNS
Motor (efferent) nerves
Carry impulses away from the CNS

PNS: Cranial Nerves
Twelve pairs of nerves that mostly serve the head and neck
Only the pair of vagus nerves extend to thoracic and abdominal cavities
Most are mixed nerves, but three are sensory only

PNS: Cranial Nerves Device
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: Cranial Nerves
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 one eye muscle
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

PNS: Spinal Nerves
There is a pair of spinal nerves at the level of each vertebrae 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
Spinal nerves divide soon after leaving the spinal cord
Ramus—branch of a spinal nerve; contains both motor and sensory fibers
Dorsal rami—serve the skin and muscles of the posterior trunk
Ventral rami—form a complex of networks (plexus) for the anterior

PNS: Spinal Nerve Plexuses
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:
Cervical
Brachial
Lumbar
Sacral

PNS: Autonomic Nervous System
Motor subdivision of the PNS
Consists only of motor nerves
Also known as the involuntary nervous system
Regulates activities of cardiac and smooth muscles and glands
Two subdivisions--Sympathetic division and Parasympathetic division

PNS: Anatomy of the Parasympathetic Division
Preganglionic neurons originate from the craniosacral regions:
The cranial nerves III, VII, IX, and X 
S2 through S4 regions of the spinal cord
Due to 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
Preganglionic neurons originate from T1 through L2
Ganglia are at the sympathetic trunk (near the spinal cord)
Short pre-ganglionic neuron and long post-ganglionic neuron transmit impulse from CNS to the effector
Neurotransmitters:  norepinephrine and epinephrine (effector organs)

PNS: 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” activites
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