Download Ch. 7: The Nervous System

June 04, 2013
Ch. 7: The Nervous System
I. Functions of the Nervous System
1. Sensory input = gathering information
- To monitor changes (stimuli) occurring inside and outside the body.
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.
Functions
II. Structural Classification of the Nervous System
A. Central nervous system (CNS)
- Organs:
1. Brain
2. Spinal cord
- Functions:
1. Integration; command center.
2. Interpret incoming sensory information.
3. Issues outgoing instructions.
CNS
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B. Peripheral nervous system (PNS)
- Includes:
- Nerves outside the brain and spinal cord.
- Functions:
- Bring impulses from sensory organs to the CNS.
- Brings impulses from the CNS to glands or muscles.
Note the two subdivisions
"Afferent" and "Efferent"...
PNS
III. Nervous Tissue
A. Support Cells
- Support cells in the CNS are grouped together as “neuroglia”:
- General functions:
1. Support
2. Insulate
3. Protect Neurons
- 6 Types:
Support Cells
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1. Astrocytes
- Star-shaped cells that brace neurons.
- Connect capillaries and neurons. Some control over what
substances can go between the two.
- Most abundant and versatile neuroglia (almost half of neural
tissue).
Astrocytes support
2. Microglia
- Spiderlike phagocytes.
- Dispose of debris.
Microglia support
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3. Ependymal cells
- Line cavities of the brain and spinal cord.
- Cilia assist with circulation of cerebrospinal fluid.
Ependymal cells
4. Oligodendrocytes
- Wrap around nerve fibers in the central nervous system.
- Produce myelin sheaths in CNS.
Oligodendrocytes
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5. Satellite cells
- Protective, cushioning cells.
6. Schwann cells
- Form myelin sheath in the PNS.
Schwann Cells
B. Neurons = nerve cells
- NOTE: A nerve is a bundle of neurons (or a bundle of nerve cells). For
example, the motor nerve to your bicepts brachii isn't a single nerve
cell but a bundle of nerve cells (a bundle of neurons). Each individual
neuron (nerve cell) controls a motor unit.
- Cells specialized to transmit messages.
Neurons
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Neuron Pictures
1. Anatomy:
a. Cell Body: nucleus and metabolic center of the cell.
b. Processes: fibers that extend from the cell body.
1. Dendrites:
- Conduct impulses toward the cell body.
- Neurons may have hundreds of dendrites.
2. 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 that contain vesicles with neurotransmitters.
- Axon terminals are separated from the next neuron by a gap called
a Synaptic Cleft.
- Synapse = junction between nerves.
Dendrites & Axons
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Neuron Picture
c. Myelin Sheath
- Whitish, fatty material covering axons.
- Protects, insulates, & increases the transmission rate.
- Produced by Schwann cells (PNS) and
Oligodendrocytes (CNS).
- Nodes of Ranvier = gaps in myelin
sheath along the axon.
Myelin Sheath
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d. Terminology
- Most neuron cell bodies are found in the central nervous system.
- Nuclei = clusters of cell bodies within the white matter of the
central nervous system.
- Ganglia = collections of cell bodies outside the central nervous
system.
Bundles of nerve fibers are called:
- Tracts in the CNS.
- Nerves in the PNS.
- White matter = collections of myelinated fibers.
- Gray matter = collections of mostly unmyelinated fibers and cell bodies.
White vs. Gray Matter
2. Classification
a. Functional Classification
1. Sensory (Afferent) Neurons:
- Carry impulses from the sensory receptors to the CNS:
- Cutaneous sense organs.
- Proprioceptors: detect stretch or tension (determines
position in space).
2. Motor (Efferent) Neurons:
- Carry impulses from the central nervous system to viscera
(organs), muscles, or glands.
Afferent & Efferent
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Types of Sensors
3. Interneurons (Association Neurons)
- Found in neural pathways in the central nervous system.
- Connect sensory and motor neurons.
Interneurons
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b. Structural Classification
1. Multipolar Neurons
- Most common type.
- Many extensions from the cell body.
- All motor and interneurons are multipolar.
Multipolar Neurons
2. Bipolar Neurons
- One axon and one dendrite.
- Located in special sense organs such as nose and eye.
Bipolar Neurons
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3. Unipolar Neurons
- Have a short single process leaving the cell body.
- Sensory neurons found in PNS.
Unipolar Neurons
3. Physiology: Nerve Impulses
a. Resting Neurons
- When no signal is passing through, the plasma membrane is polarized.
The inside of the cell is negative, the outside is positive.
Resting Neurons
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b. Nerve Triggering and Impulse
- When the nerve is being stimulated, the polarity starts to even out.
- If the stimulus is strong enough, the polarity flips and the reversed
polarity travels down the length of the neuron.
- Stimulants like caffeine, ginseng, guarana, and cocaine make the nerve
plasma membrane more permeable so it makes nerves more likely to
fire...this makes you jittery or helps you be more alert/feel more alive.
http://www.usada.org/overcounter/energy.aspx
Nerve Triggering and Impulse
Journey Through an Impulse
1. Aggressive thumb wrestling causes the muscles of the thumb to produce much heat.
2. Nerve cells sensitive to temp. changes “feel” the heat.
3. When enough heat is generated, it is enough stimuli to the nerve for it to reach its threshold
potential and start an action potential (a fire) across the nerve cell membrane.
4. This action potential (fire/impulse) is when the cell membrane becomes permeable to Na+ ions and
these ions diffuse across the cell membrane through open channels into the nerve (depolarization).
5. The impulse travels across the cell membrane in both directions. It travels across the entire cell
membrane in unmyelinated cells but jumps from Node of Ranvier to Node of Ranvier in myelinated
cells.
6. At the end of the axon, neurotransmitters are released into the synapse (gap between nerves) to
signal the adjoining nerves to continue the impulse.
7. If 2 or more nerves converge onto one, the addition of their impulses may be enough to trigger the
larger nerve to continue the impulse on toward the CNS.
8. The CNS receives the signal and interprets the information, then it makes a decision.
9. The CNS sends an impulse out through a motor nerve to stimulate sweat glands to secrete.
10. The action potential travels through the motor neuron and can stimulate 2 or more nerves
adjoining it (divergence).
11. This can happen over and over so that a large # of sweat glands are contacted and enough
water is evaporated to reduce the body temperature in that area.
12. The sensory neurons will then stop firing impulses.
Journey through an Impulse
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c. Transmission of the Signal at Synapses
- When the action potential reaches the axon terminal, the electrical
charge opens calcium channels.
- Calcium causes the tiny vesicles containing the neurotransmitter
chemical to fuse with the axonal membrane and release the
neurotransmitter.
Synapse Transmission 1
- The neurotransmitter molecules diffuse
across the synapse and bind to receptors
on the membrane of the next neuron.
- If enough neurotransmitter is released, 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 and the ion channel
closes.
Neurotransmitter
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4. Physiology: Reflexes
- Rapid, predictable, and involuntary response to a stimulus.
- Occurs over pathways called reflex arcs - a direct route from a sensory
neuron, to an interneuron, to an effector. (Gets to the CNS but not the
brain!)
- Somatic reflexes 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.
Reflexes
- 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.
Reflexes 2
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- Two-neuron reflex arcs
- Simplest type
- Example: Patellar (knee-jerk) reflex
Two Neuron Reflex
- Three-neuron reflex arcs
- Consists of five elements: receptor, sensory neuron, interneuron,
motor neuron, and effector.
- Example: Flexor (withdrawal) reflex with sharp or hot object.
Three Neuron Reflex
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A. Functional Anatomy of the Brain
1. Cerebral Hemispheres (Cerebrum)
- Paired (left and right) superior parts of the brain.
- Connected by the corpus callosum.
- Includes more than half of the brain mass.
- The surface is made of ridges (gyri) and grooves (sulci).
Cerebrum
- Fissures (deep grooves) divide the cerebrum into 4 lobes:
1. Frontal lobe
2. Parietal lobe
3. Occipital lobe
4. Temporal lobe
Cebrum Parts
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2. Diencephalon
- Sits on top of the brain stem.
- Enclosed by the cerebral hemispheres.
- Made of three parts:
1. Thalamus
2. Hypothalamus
3. Epithalamus
Diencephalon
a. Thalamus
- Relay station for sensory impulses.
- Transfers impulses to the correct part of the cortex for
localization and interpretation.
b. Hypothalamus
- Under the Thalamus.
- Important autonomic nervous system center:
1. Helps regulate body temperature.
2. Controls water balance.
3. Regulates metabolism.
- Houses the limbic center for emotions.
- Regulates the nearby pituitary gland.
Produces two hormones of its own.
Thalamus & Hypo...
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c. Epithalamus
Forms the roof of the third ventricle.
Houses the pineal body (an endocrine gland).
- Forms cerebrospinal fluid.
(In the choroid plexus.)
Epithalamus
3. Brain Stem
- Connects brain and the spinal cord.
- 3 Parts:
a. Midbrain
- Mostly composed of tracts
of nerve fibers.
- Reflex centers for vision
and hearing.
B Stem & Midbrain
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b. Pons
- The bulging center part of the brain stem.
- Includes nuclei involved in the control of breathing.
Pons
c. Medulla oblongata
- The lowest part of the brain stem.
- Merges into the spinal cord.
- Controls:
1. Heart rate
2. Blood pressure
3. Breathing
4. Swallowing
5. Vomiting
http://www.youtube.com/watch?v=UfC4u5GCy3I
The little research I did shows that the amygdala and the
hypothalamus is more involved in aggression than the Medulla
Oblongata...figured I should say something for those of you who've
seen Water Boy.
Medulla Oblongata
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d. Reticular Formation
- Diffuse mass of gray matter along the brain stem
- Reticular activating system (RAS) plays a role in awake/sleep
cycles and consciousness.
Reticular Formation
4. Cerebellum
- Two hemispheres.
- Provides involuntary coordination of body movements.
Cerebellum
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B. Protection of the Central Nervous System (p. 247 - 251)
- Scalp and skin.
- Skull and vertebral column.
CNS Protection
1. Meninges
FYI
- Dura mater
- Tough outermost layer
- Double-layered external covering:
i. Periosteum—attached to inner surface of the skull
ii. Meningeal layer—outer covering of the brain
- Folds inward in several areas
- Falx cerebri
- Tentorium cerebelli
FYI-Meninges
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2. Cerebrospinal Fluid (CSF)
- Similar to blood plasma composition.
- Formed by the choroid plexus (capillaries in the ventricles of the
brain).
- Forms a watery cushion to protect the brain.
- Circulated around the brain and in spinal cord.
Cerebrospinal Fluid
- Hydrocephalus
FYI
- 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.
FYI-Hydrocephalus
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3. Blood-Brain Barrier
FYI
- 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
Blood Brain Barrier
C. Brain Dysfunctions (p. 251 - 253)
1. 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.
Traumatic Brain Injuries
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2. 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)
Strokes
3. 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.
Altzheimer's
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D. Spinal Cord
- 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.
Cauda equina is a collection of
spinal nerves at the inferior end.
Spinal Cord
1. Spinal Cord Anatomy
Internal gray matter (mostly cell bodies) surrounds the central canal.
- Central canal is filled with cerebrospinal fluid.
Exterior white mater = conduction tracts.
Meninges cover the spinal cord (dura mater, arachnoid mater, and
pia mater).
- Spinal nerves leave at the level of each vertebrae.
Spinal Cord Anat.
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Dorsal root = in
Ventral root = out
Spinal Cord
IV.Peripheral Nervous System
(p. 255 - 269)
- Nerves and ganglia outside the central
nervous system.
- Nerve = bundle of neuron fibers (a
bundle of nerve cells). (A nerve cell is
a neuron. A nerve is different, it's a
bundle of neurons (or nerve cells).
http://en.wikipedia.org/wiki/Nerve
- Neuron fibers are bundled by connective
tissue.
FYI
A. Structure of a Nerve (p. 256 & 257)
- Endoneurium surrounds each fiber.
- Groups of fibers are bound into
fascicles by perineurium.
- Fascicles are bound together by
epineurium.
PNS
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B. 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.
C. Cranial Nerves - FYI only
- 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.
Afferent vs. Efferent
D. Autonomic Nervous System (p. 264 - 269)
- 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
- Parasympathetic division
Feb 12-8:28 AM
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1. Comparing Somatic & Autonomic
Somatic Nervous System
Autonomic Nervous System
Nerves
One-neuron; it originates in the CNS
and axons extend to the skeletal
muscles served
Two-neuron system consisting of
preganglionic and postganglionic
neurons
Effector organ
Skeletal muscle
Smooth muscle, cardiac muscle,
glands
Subdivisions
None
Sympathetic and parasympathetic
Neurotransmitter
Acetylcholine
Acetylcholine, epinephrine,
norepinephrine
Feb 12-8:28 AM
Feb 12-8:28 AM
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2. Anatomy of the Parasympathetic Division
- Leave CNS at brain stem or
sacral regions.
- Terminal ganglia are at the
effector organs
- Neurotransmitter: acetylcholine
Parasymp. Anat.
3. 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).
Symp. Anat.
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Feb 12-8:28 AM
4. Autonomic Functioning
- Sympathetic—“fight or flight”
- Response to unusual stimulus that 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
(producing urine).
4. Autonomic Functioning
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V. Developmental Aspects of the Nervous System (p. 269 - 273)
- 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.
V. Developmental
Feb 15-4:23 PM
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