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THE NERVOUS SYSTEM
Nervous system glossary
Major anatomical and functional divisions:
central nervous system (CNS)
which contain control centers responsible for processing and integrating planning
and responses to stimuli and providing short-term control over the activities of
other systems
peripheral nervous system (PNS)
neural tissue outside the CNS whose function is to link the CNS with sense organs
and other systems
autonomic nervous system (ANS)
components of the central nervous system and peripheral nervous system that are
connected with the control of visceral functions
Gross anatomy
nucleus:
a group of cell bodies with boundaries only in the CNS
Center:
a group of neuron cell bodies in the central nervous system that share a common
function
tract:
a bundle of axons within the CNS that share a common origin, destination, and
function
Column:
a group of tracks down within a specific region of the spinal cord
pathways:
centers and tracts that connect the brain with other organs and systems in the
body
ganglia:
and anatomically distinct collection of sensory or motor neuron cell bodies within
the peripheral nervous system
nerve:
a bundle of axons in the peripheral nervous system
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Histology
Grey matter:
neural tissue dominated by neuronal cell bodies
White matter:
neural tissue dominated by myelinated axons
Neural Cortex:
a layer of gray matter at the surface of the brain
neuron:
the basic functional unit of the nervous system; a highly specialized cell; a nerve
cell
sensory neuron:
a neuron whose acts on carries sensory information from the PNS towards the CNS
motor neuron:
a neuron whose acts on carries motor commands from the CNS towards the factors
in the PNS
Soma:
the cell body of a neuron
dendrite:
neuronal processes that are specialized to respond to specific stimuli in the
extracellular environment
axon:
a long slender cytoplasmic process of a neuron; axons are capable of conducting
nerve impulses called action potentials
myelin:
a membranous wrapping, produced by glial cells, that coats axons and increases
the speed of action potential propagation; axons coded with Mylan are said to be
myelinated
neuroglial or glial cells:
supporting cells that interact with neurons and regulate the extracellular
environment, provide defense against pathogens, and perform repairs with neural
tissues
FUNCTIONAL CATEGORIES:
Receptor:
effector:
reflex:
somatic:
visceral:
voluntary:
a specialized cell, dendrite, or organ that responds to specific stimuli in the
extracellular environment and who stimulation alters the level of activity in a
sensory neuron
a muscle, gland, or other specialized cell or organ that responds to neural
stimulation by altering its activity and producing a specific effect
a rapid stereotyped response to a specific stimulus
pertaining to the control of skeletal muscle (somatic motor) activity or sensory
information from skeletal muscles, tendons, and joints (somatic sensory)
pertaining to the control of functions, such as digestion, circulation, etc. (visceral
motor) or sensory information from visceral organs (visceral sensory)
under direct conscious control
involuntary:
subconscious:
action potential:
not under direct conscious control, operations performed by ANS
pertaining to centers in the brain that operate outside of persons conscious
awareness
sudden, transient changes in the membrane potential that are propagated along
the surface of an axon or sarcolemma
ARCHITECTURE OF THE NERVOUS SYSTEM
The diagram below is a representation of the architecture of the nervous system. If you'll notice at the very top is the
central nervous system. It's necessary to understand that the brain and spinal cord receive all sensory information from the
outside world and the inside world called visceral. The brain is completely isolated from everything. It is dependent upon
sensory receptors from our five senses and sensors in our organs and internal tissues.
The two main divisions of the nervous system are sensory and motor. The function of the sensory system is to transmit
information from the outside world and the viscera to the central nervous system. The function of the motor system is to
send commands from the brain to the peripheral nervous system.
The sensory system is comprised of special sensory receptors such as eyes ears nose and mouth and touch, as well as
somatic sensory receptors which monitors skeletal muscles and joints skin surface; provide position sense and touch,
pressure, pain and temperature sensations. Visceral sensory receptors monitor internal organs, including those of
cardiovascular, respiratory, digestive, urinary, and reproductive systems.
The motor commands also called efferent, include signals to somatic or skeletal muscles and the viscera via the autonomic
nervous system. The autonomic nervous system has two divisions called sympathetic and parasympathetic. Both of these
systems, sympathetic and parasympathetic have opposite actions on the viscera which include smooth muscle cardiac
muscle and glands. The sympathetic system increases activity and the parasympathetic system decreases activity. The
smooth muscle cardiac muscle and glands are called effect or organs. Skeletal muscles are also called effect or organs.
Functions of the Nervous System
1)
Sensory input
•
2)
Monitor/collecting stimuli occurring inside and outside the body – outside = somatic sensory (SS), inside = visceral
sensory (VS)
Integration (by interneurons)
•
To process and interpret sensory input and decide if action is needed (homeostasis)
3)
Motor output
•
A response to integrated stimuli which only sends an AP
•
The response activates muscles or glands
Structural Classification of the Nervous System
Central nervous system (CNS)
•
Brain
•
Spinal cord
Peripheral nervous system (PNS)
•
Nerves outside the brain and spinal cord including cranial nerves, nerves and ganglia
Functional Classification of the Peripheral Nervous System
•
Sensory (afferent) division
•
•
Nerve fibers that carry information to the central nervous system from the five senses and the internal viscera (guts)
Motor (efferent) division
•
Nerve fibers that carry impulses away from the central nervous system to muscles and glands
•
Somatic nervous system = voluntary
•
Autonomic nervous system = involuntary (sympathetic and parasympathetic)
Organization of the Nervous System

Neurons and neuroglial cells (glial)
Neuroglial cells:
1)
Astrocytes
•
The most abundant cell type, star-shaped cells in CNS
•
They regulate neurotransmitter levels, signal increased blood flow through capillaries in active portions of the brain,
control ionic environment of neurons and assist synaptic formation in developing neural tissue
2) Microglial Cells:
•
The smallest and least abundant glial cells with elongated cell bodies and cell processes with many ointed
projections. They are phagocytes, keeping the area clear of debris and foreign materials
3) Ependymal Cells:
These important cells line the ventricles and have cilia which circulate the cerebrospinal fluid (CSF)
4)
Oligodendrocytes
•
Produce myelin sheath around nerve fibers in the central nervous system (CNS) One cell can myelinate many dendrites
and axons (see figure)
Neuroglia of PNS
1)
Satellite cells
•
2)
Protect neuron cell bodies within ganglia (a group of cell bodies in the PNS)
Schwann cells
•
Form myelin sheath around axons in the peripheral nervous system which increases the speed of the AP and insulates
the impulse from neighboring axons
Neuroglia vs. Neurons
•
•
Neuroglia
•
Do not transmit electrical impulses
•
Undergo mitosis (brain tumors)
Neurons (nerve cells)
•
Cells that are highly specialized to transmit messages by way of Action Potentials (AP)
•
Neurons live as long as you do. They do not reproduce!
•
Neurons are very highly metabolic requiring constant glucose and oxygen without which they can only live minutes
Neuron Anatomy
1)
Cell body
•
Nissl substance is specialized rough endoplasmic reticulum
•
Neurofibrils are intermediate cytoskeleton that maintains cell shape
•
Nucleus
•
Large nucleolus
•
Axon hillock – the area at the junction of the cell body and the axon
2)
Dendrites are extensions that conduct impulses toward the cell body
3)
Axons are extensions that conduct impulses away from the cell body
4)
Neurons may have many shapes: multipolar – more than two processes, numerous dendrites and one axon, Bipolar: two
processes that extend from opposite sides of the cell body and appear only in the inner ear, olfactory epithelium of the nose and
the retina. Unipolar: one short process that emerges from the body and divides like an inverted T into two long branches.
Axons and Nerve Impulses
•
Axons end in axonal terminals, also called bulb, button or bouton
•
Axonal terminals contain vesicles with neurotransmitters
•
Axonal terminals are separated from the next neuron by a gap
•
Synaptic cleft is the gap between adjacent neurons
• Synapse is the junction between nerves
Nerve Fiber Coverings
•
Schwann cells produce myelin sheaths (insulation) in jelly-roll like fashion
•
Nodes of Ranvier are gaps in myelin sheath along the axon caused by separation of different schwann cells
•
Multiple sclerosis is an autoimmune disease where myelin sheaths are destroyed causing an electrical short-circuits
Functional Classification of Neurons
•
Sensory (afferent) neurons carry impulses towards the CNS
•
Cutaneous sense organs (pain/temperature, touch, pressure)
•
Proprioceptors – detect stretch or tension in the viscera
Interneurons: integrate information between sensor and motor neurons (very abundant in the brain, less so in the spinal cord and
only appear in the CNS)
•
Motor (efferent) neurons
•
Carry impulses from the central nervous system
Functional Properties of Neurons
1)
Irritability is the ability to respond to stimuli
2)
Conductivity is the ability to transmit an impulse
Continuation of the Nerve Impulse between Neurons
•
Transmission of a nerve impulse is an electrochemical event
•
Impulses are able to cross the synapse to another nerve (can be disrupted by analgesics or decreased blood flow)
•
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
The Reflex Arc
•
Reflexes are rapid, predictable, and involuntary responses to stimuli
•
A reflex arc is direct route from a sensory neuron, to an interneuron, to an effector involving both PNS and CNS
•
Reflexes contain 4-5 components
Types of Reflexes and Regulation
•
•
Autonomic reflexes
1)
Smooth muscle regulation
2)
Pupillary response to light
3)
Heart and blood pressure regulation
4)
Regulation of glands
5)
Digestive system regulation
Somatic reflexes
1) Activation of skeletal muscles
Gross Anatomy of Nerves
1)
2)
3)
4)
5)
6)
a nerve is a cablelike organ in the PNS containing many axons and dendrites.
Within a nerve, each axon is covered by Schwann cells (myelin)
Schwann cells are covered in a thin layer of loose connective tissue called Endoneurium
Groups of axons are bound into bundles called nerve fascicles (similar to muscles) with a covering call the Perineurium
The whole nerve is covered by Epineurium
A nerve fiber is a long axon