Download The Nervous System Organization of the Nervous System

The Nervous System
Reading: Chapters 4,5.
Coordinates (with endocrine system) organ system activities in response
to varying external conditions.
Composed of the central nervous system (CNS) consisting of the brain
and spinal cord, and the peripheral nervous system consisting of
communication pathways from CNS to rest of the body.
Communication occurs via electrical impulses (high speed pathway).
Functions of the nervous system.
Monitors the internal and external environments.
Integrates sensory information.
Coordinates voluntary and involuntary activities of other organ systems.
These functions are performed by neurons, which are supported by
neuroglia (external support cells).
Anatomy and Physiology for Engineers
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Organization of the Nervous System
Within peripheral nervous
system, there are the afferent
and efferent divisions.
Afferent brings sensory
information to CNS.
Efferent carries motor
commands from CNS to
muscles, glands, etc.
Within efferent division, there
are 2 sub-divisions:
Somatic nervous system (SNS)
provides voluntary control over
skeletal muscles.
Autonomic nervous system (ANS)
provides involuntary control of
cardiac and smooth muscles, and
glands.
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Cellular Organization of Neural Tissue
Neural system consists of all neural tissue in the
body.
Neural tissue consists of two types of cells: neurons
and neuroglia (also called glial cells).
Neurons are the basic units of the nervous system,
and all neural functions involve the communication
of one neuron with another, and with other cells.
Neuroglia regulate environment around the neurons,
provide structural support for neurons, and act to
recycle debris (phagocytes).
Neuroglia cells outnumber neurons.
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Neurons
Classified into 3 functional groups:
Sensory neurons (Number approximately 106).
Make up the afferent division of the peripheral nervous system.
Responsible for providing information regarding the external
environment (touch, pressure, temperature, smell, hearing), the position
of the skeletal muscles and joints, and the activities of the other organ
systems (digestive, cardiovascular, etc).
Motor neurons (Number approximately 500,000).
Part of the efferent system (carries info. from CNS to tissues, organs and
organ systems).
Somatic motor nuerons target voluntary tissues (skeletal muscles) while
visceral (autonomic) motor neurons target involuntary tissues (cardiac
and smooth muscles, glands).
Interneurons (Number approximately 20 X 109).
Located entirely within brain and spinal cord (CNS).
Function to interconnect other neurons.
Responsible for the analysis of sensory inputs.
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General Structure of Neurons
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General Structure of Neurons
Cell body (soma).
Soma of the neuron contains large round nucleus with
prominent nucleolus.
No centrioles ‡ neurons cannot be replaced once they
differentiate.
Numerous other cellular organelles (mitochondria, free and
fixed ribosomes, rough endoplasmic reticulum, etc).
Rough ER and ribosomes also found in clusters called Nissl
bodies (gray color ‡ gray matter).
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General Structure of Neurons
Cell body (soma).
Dendrites to receive information.
Axon to carry information.
Synaptic knobs to transmit
information to other cell.
Projecting from soma are variable number of dendrites and a single long
axon.
Stimulation of dendrite or cell body (mechanical, electrical, chemical)
produces action potential that travels along axon.
Base of axon connected to soma at axon hillock, where action potential
begins.
Axon may branch along its length producing collaterals.
Each collateral ends at a synaptic terminal (synaptic knob).
Synaptic terminal is part of the synapse ‡ where communication with
another cell takes place.
Anatomy and Physiology for Engineers
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Structural Classification of Neurons
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Neuroglia (glial cells)
Found in both the CNS and PNS, but CNS has greatest diversity.
4 types of glial cells found in CNS:
Astrocytes
Largest and most numerous neuroglia.
Responsible for maintaining the blood-brain barrier, structural stability of CNS,
and repair of damaged neural tissue.
Oligodendrocytes
Responsible for coating axons with cytoplasmic extensions
(myelin sheath).
Microglia
Smallest and rarest of neuroglia in CNS.
Phagocytic cells responsible for immune protection.
Ependymal cells
Line central canal of spinal cord and ventricles of the brain.
In certain locations, ependymal cells produce cerebrospinal fluid.
Anatomy and Physiology for Engineers
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Neuroglia in the CNS
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Neuroglia in the Peripheral Nervous System
Schwann cells are
most important glial
cells in peripheral
nervous system.
Cover every axon
outside CNS (whether
myelinated or
unmyelinated).
This myelin sheath
increases impulse
conduction rate.
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Neurophysiology
Cell membrane (for generic cell) has excess positive
charges (ions) on outside versus inside.
This membrane potential for neurons is around –70
mV.
Resting membrane potential
is due to relative
concentrations of ions.
Larger concentration of Na+,
Cl- in extracellular fluid.
Higher concentrations of K+
and negatively charged
proteins within cell.
Since cell membrane is more permeable to K+ ions than Na+ ions
in resting state, rate of K+ efflux is greater than Na+ influx ‡ cell
membrane is slightly negative.
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Changes in Membrane Potential
Changes in membrane potential can occur due to:
Alteration of permeability of cell membrane.
Alteration in the activity of an exchange pump.
This can occur due to exposure to chemical,
electrical, temperature, ionic, or mechanical
stimulations.
Changes can occur almost instantaneously (ex:
muscle cell contraction).
Usually stimulation opens closed channels within
membrane, thereby changing permeability of cell
membrane to specific ions.
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Polarization and Potential
Depolarization: making the membrane potential more
positive.
Ex: opening of selective Na+ channels, allowing extracellular Na+ to
diffuse faster into the cell, causing membrane potential to rise toward
0 mV.
Hyperpolarization: making the membrane potential more
negative:
Ex: opening of selective K+ channels, causing intracellular K+ to
diffuse out of cell.
Graded Potential: Variation in cell membrane permeability
and polarization at a local site.
Action Potential: Variation in polarization across entire cell
membrane.
Graded potential acts as the stimulus that produces the action
potential
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Generation of the Action Potential
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Conduction of an Action Potential
Continuous (unmyelinated) and saltatory (myelinated) conduction.
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Synaptic Communication
Action potential carries information from one point to another via the
nervous system.
Information transfer at the end of an axon occurs via the release of
chemicals known as neurotransmitters.
The location at which this information transfer takes place is the synapse.
Synaptic cleft: space between
the two cell membranes.
Presynaptic neuron ‡ transmits
information to postsynaptic
neuron, muscle or gland cell.
Presynaptic neuron contains
specific neurotransmitters
enclosed in vesicles.
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Events in the Synaptic Connection
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The Reflex Arc
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Anatomy of the Nervous System
Introduction to terminology.
In peripheral nervous system:
Ganglia: groups of neuron cell bodies.
Nerves: bundles of axons (spinal nerves connect to spinal cord; cranial nerves
connect to brain).
In central nervous system:
Centers: collection of neuron cell bodies that share a particular function.
Nucleus: center that has a distinct anatomical boundary.
Neural Cortex: gray matter on the surface of the brain.
Tracts: bundles of axons within CNS that share common origin, destination
and function.
Columns: groups of tracts.
Pathways: avenues that link brain centers to rest of the body (sensory
pathways ‡ bring information to brain; motor pathways ‡ transmit
information from brain to muscles).
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Anatomy of the Nervous System
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Anatomy of the Nervous System
Protection of central nervous system is provided by glial
cells and by the meninges and the blood-brain barrier.
Meninges:
Series of three specialized membranes.
Dura Mater
Tough fibrous outermost covering of the CNS.
Epidural space: between dura mater of spinal cord and walls of
vertebral column.
Arachnoid
Intervening space containing a layer of squamous cells.
Subarachnoid space: deeper layer within meninges that contains
the cerebrospinal fluid.
Pia Mater
Innermost layer, in direct contact with underlying neural tissue.
Highly vascular ‡ blood to supply CNS function.
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Anatomy of the Nervous System
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Blood-Brain Barrier
Neural tissue is isolated from rest of
circulation by this barrier.
Maintained by astrocytes, which cause
capillary membranes in the CNS to be
impermeable to various compounds.
Lipid soluble substances can get through.
Water soluble substances get through only via
facilitated diffusion.
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The Spinal Cord
Central canal filled with
cerebrospinal fluid (CSF).
Consists of 31 segments, each
identified by letter (vertebral
section) and number (individual
nerve).
Internal diameter decreases as it
extends inferiorly (with two
exceptions).
Cervical enlargement ‡ nerves to
pectoral girdle and upper limbs.
Lumbar enlargement ‡ nerves to
pelvis and lower limbs.
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The Spinal Cord
Each spinal segment
contains a pair of dorsal
root ganglia (group of
neuron cell bodies).
Each ganglion contains a
dorsal root (axons through
which sensory information
arrives) and a ventral root
(axons through which
information is sent out).
Dorsal and ventral roots unite into a single spinal nerve.
Spinal cord extends only to L1 or L2. After this, nerves
extend inferiorly without spinal cord protection.
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The Brain
Adult brain consists of 6 major regions:
Cerebrum
2 large paired hemispheres (left and right).
Home of conscious thought processes, sensations, intellectual functions, memory storage
and retrieval, and complex motor functions.
Diencephalon (thalmus and hypothalmus)
Hollow internal structure connected to the cerebrum.
Relaying and processing sensory information (thalmus), and processing emotions,
autonomic function and hormone production (hypothalmus).
Midbrain (also called the mesencephalon)
Part of the brain stem which connects brain to spinal cord.
Processes auditory and visual information; generates involuntary motor responses.
Pons (Also part of the brain stem)
Involved with somatic and visceral motor control.
Medulla Oblongata (Direct attachment to spinal cord)
Relays sensory information to various brain centers; also contains centers for autonomic
regulation of heart rate, digestion, blood pressure, respiration, etc.
Cerebellum
Found posterior and inferior (covers the brain stem).
Adjustment of voluntary and involuntary activities based on input and stored memories.
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The Brain
Cerebral Hemispheres and the Cerebellum
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The Brain
Brain Stem and the Cerebellum
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The Brain
Diencephalon, Brain Stem, and Cerebellum
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Ventricles and Cerebrospinal Fluid
Brain contains 4 ventricles (hollow chambers) filled with cerebrospinal fluid.
Two larger lateral ventricles located in each cerebral hemisphere.
Third ventricle located in diencephalon.
Fourth ventricle located within brain stem and is continuous with spinal canal.
Cerebrospinal fluid important for shock absorption, buoyancy, and transport of
nutrients and waste products.
CSF constantly produced by ependymal cells and circulated within the choroid
plexus (vascular network that extends into each ventricle).
CSF is in direct contact with interstitial fluid and free exchange takes place
between these fluids.
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Ventricles and Cerebrospinal Fluid
Cerebrospinal fluid
circulates within and
around brain and spinal
cord.
CSF forms at the
choroid plexus,
circulates within the
ventricles, down the
spinal canal, out into
the sub-arachnoid
space, and back up and
around the brain.
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The Cerebrum
Largest structure of the brain.
Two hemispheres: left hemisphere responsible for motor and
sensory control over right side of body, and vice versa.
Two hemispheres linked via region of white matter known as
the corpus callosum.
Responsible for conscious thought, intellectual function,
limited input and control of sensory information.
Includes gray and white matter,
Key structures:
Cerebral cortex: thick blanket of gray matter outer covering surface.
Gyri (gyrus): ridges on cerebral cortex.
Sulci (sulcus): depressions within cerebral cortex.
Longitudinal fissure: groove separating the hemispheres.
Lobes: well defined regions within each hemisphere.
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The Cerebrum
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The Cerebrum
The Cerebral Nuclei
Masses of gray matter that lie within the 2 larger ventricles
(lateral ventricles) and within the white matter of each
cerebral hemisphere.
Important in control of learned movement patterns.
Anatomy and Physiology for Engineers
The Cerebrum
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The Limbic System
Functional group that includes the olfactory cortex, various cerebral
nuclei, gyri and tracts along border between cerebrum and diencephalon.
Functions include:
Establishing
emotional states and
related behavioral
drives.
Linking conscious
and intellectual
functions of
cerebrum to
unconscious
autonomic functions
of brain stem.
Long term memory storage and retrieval.
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The Diencephalon
Provides control and relaying of conscious and unconscious sensory and motor
information.
Structures contained within diencephalon:
Third ventricle and pineal gland (endocrine structure that secretes hormone melatonin).
Thalmus
Final relay point for ascending sensory information (except smell).
Passes only a small portion of this information to active consciousness (primary
sensor cortex within cerebrum).
Remaining is relayed to cerebral nuclei and other centers within brain stem.
Also important for coordination of voluntary and involuntary motor commands.
Hypothalmus:
Located deep within diencephalon.
Contains centers associated with emotions (rage, pleasure, etc).
Adjusts and controls autonomic activities of pons and medulla oblongata.
Coordinates neural and endocrine activities.
Produces variety of hormones (Anti-diuretic hormone - ADH, oxytocin, etc).
Coordinates various voluntary and autonomic functions.
Maintains body temperature.
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The Diencephalon
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The Midbrain
Contains various nuclei and
bundles of ascending and
descending nerve fibers.
Home of cranial nerves
responsible for eye
movement.
Also home to the reticular
formation.
Network of interconnected
nuclei responsible for
waking and sleeping states.
Midbrain also responsible for
control of muscle tone.
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The Pons
Links cerebellum
with midbrain,
diencephalon,
cerebrum and medulla
oblongata.
Sensory and motor
control for cranial
nerves.
Also responsible for
generating depth and
pace of respiration.
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The Cerebellum
Two important functions:
Maintains posture and
balance by making
rapid adjustments in
muscle tone and
position.
Programs and finetunes various
involuntary and
voluntary movements.
These are performed by adjusting and regulating neural activity
between cerebrum and brain stem.
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The Medulla Oblongata
Provides physical connection
between brain and spinal cord.
All communication between
these two structures must pass
through medulla oblongata.
Many nerve pathways end in
synapses here, and information
is relayed onward through
connecting neurons.
Very important for
cardiovascular and respiratory
control.
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On to the Peripheral Nervous System
and Integrated Functions
PNS is the link between neurons in CNS and rest of the body.
Although PNS contains < 2% of all neural tissue, it is vital as
a pathway between brain and body.
Certain decisions may be made without or before entering
cerebral cortex and conscious awareness.
This is done via synaptic communication within brain stem
and spinal cord.
PNS is dominated by nerves (axons bundled together by
connective tissue) coming to or from the CNS.
PNS also contains sensory and motor neurons of the
autonomic nervous system (ANS).
PNS includes the cranial and spinal nerves.
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The Cranial Nerves
Components of the PNS that connect to the
brain rather than the spinal cord.
12 pairs of cranial nerves, numbered
according to their position along long-axis of
brain.
Responsible for olfactory, optic, oculomotor,
trochlear, facial muscles, acoustic, taste,
swallowing, speech, vocal cords, etc.
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The Cranial Nerves
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The Spinal Nerves
31 pairs of spinal nerves.
Grouped according to location
along vertebral column.
8 cranial pairs (C1-C8).
12 thoracic pairs (T1-T12)
5 lumbar pairs (L1-L5).
5 sacral pairs (S1-S5).
1 coccygeal pair (C1).
Several nerve axons bundled to
form a plexus with common
destination (ex: cervical plexus
innervates neck and diaphragm).
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