Chapter 12
Functional Organization of the Nervous System
I.
Two anatomic divisions: CNS and PNS
A. central nervous system (CNS)
1. consists of the brain and spinal cord and is encased in bone.
2. Surrounded / protected by meninges and CSF
3. Fxn: Highly sophisticated; Processes, integrates, stores, and responds to information from PNS.
Produces ideas and emotions, that are not the automatic consequences of information input.
B. peripheral nervous system (PNS)
1. PNS consists of nervous tissue outside of the CNS,
a. consists of nerves and ganglia.
(1) ganglia are collections of nerve bodies located outside the CNS
b. 43 pairs of nerves originating in the CNS make up the PNS
(1) 12 cranial nerves
(2) 31 pair of spinal nerves.
2. Fxn: The PNS detects stimuli and transmits to and receives information from the CNS.
3. Anatomic Divisions of the PNS (sensory and motor)
a. afferent or sensory division
(1) transmits action potentials to the from sensory organs to the CNS
(2) consists of single neurons with cell bodies in ganglia.
(a) ganglia are located outside the spinal cord or within cranial nerves.
b. efferent or motor division
(1) carries action potential away from the CNS
(2) effects muscle or glands.
4. Functional subdivisions of PNS (somatic motor and ANS)
a. somatic motor nervous system
(1) It consists of single neurons that have their cell bodies located within the CNS.
(2) innervates skeletal muscle
(3) under voluntary control.
(4) axon extends to neuromuscular junction on muscle
b. autonomic nervous system (ANS)
(1) innervates cardiac muscle, smooth muscle, and glands.
(2) called involuntary nervous system with two neurons between the CNS and effector
organs. The first neuron has its cell
body in CNS, and the second neuron
has its cell body within autonomic ganglia
and extends to effector organ.
Two (functional) divisions of ANS:
Sympathetic -T1-L2 - ganglian are near
spinal cord
Parasympathetic - Cranial nerves and S2S4 - ganglia are in or near organ affected.
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II. Cells of the nervous system
A. Cell types: Neurons and Neuroglia (nerve glue)
B. Neurons
1. Neurons receive stimuli and transmit action potentials.
2. Form complex networks.
3. three main components
a. cell body (soma)
(1) single large nucleus with prominent nucleolus
(2) is the primary site of protein synthesis and contains an extensive endoplasmic reticulum.
(3) contains large numbers of intermediate filaments / microtubules
b. dendrites (lat. tree)
(1) generally short, highly branched cytoplasmic extensions of the cell body
(2) contain dendritic spines and are tapered toward end
(3) usually conduct electric signals toward the cell body.
(4) respond to neurotransmitter
c. Axons (lat. axel)
(1) eminate form axon hillock
(2) cytoplasmic extensions of the cell body (axoplasm)
(3) that transmit action potentials to other cells.
(4) uniform diameter / straight
C. Types of Neurons
1. Multipolar neurons
a. several dendrites and a single axon.
b. Association and motor neurons are multipolar.
2. Bipolar neurons
a. a single axon and dendrite
b. found as components of sensory organs.
3. Unipolar neurons
a. have a single axon.
b. Most sensory neurons are unipolar.
D. Neuroglia
1. Neuroglia are nonneural cells that support and aid the neurons of the CNS and PNS.
2. CNS neuroglia
a. Astrocytes - provide structural support for neurons and blood vessels and forms the bloodbrain barrier that regulates the movements of substances between the blood and the CNS.
b. Microglia -macrophages that phagocytize microorganisms, foreign substances, or necrotic
tissue.
c. Ependymal cells - line the ventricles and the central canal of the spinal cord. Some are
specialized to produce cerebrospinal fluid.
d. Oligodendrocytes - form myelin sheaths around the axons of neurons of the CNS.
3. (PNS neuroglia)
a. Schwann cells, (neurolemmocytes) - form myelin sheaths around the axons of neurons of the
PNS.
b. Satellite cells support and nourish neuron cell bodies within ganglia.
E. Axon Sheaths
1. Unmyelinated axons rest in invaginations of oligodendrocytes (CNS) or Schwann cells (PNS).
They conduct action potentials slowly.
2. Myelinated axons are wrapped by several layers of cell membrane from oligodendrocytes (CNS)
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or Schwann cells (PNS). Spaces between the wrappings are the nodes of Ranvier, and action
potentials are conducted rapidly be saltatory conduction from one node of Ranvier to the next.
F. Nerve Fibers are classified by size
1. Type A - large diameter
a. (15-22 m/sec) motor and sensory nerves - rapid response
2. Type B - medium diameter
a. (3-15 m/sec) autonomic nervous system - heart lung
3. Type C- small diameter
a. (2 m/sec) autonomic NS - digestion.
III. Organization of Nervous Tissue
A. CNS Structure - Nervous tissue can be grouped into white an gray matter.
1. White matter is made up of myelinated axons and functions to propagate actions potentials.
a. White matter forms nerve tracts in the CNS and nerves in the PNS
(1) Association fibers
(2) commissural fibers
(3) projection fibers
2. Gray matter is collections of neuron cell bodies and unmyelinated axons.
a. Gray matter forms cortex and nuclei in the CNS and ganglia in the PNS.
b. Within gray matter axons synapse with neuron cell bodies,
(1) This is functionally the site of integration in the nervous system.
3. In the brain, gray matter is found on the outside of the cortex and white matter makes up the
nerve tracts within the brain.
4. In the spinal cord, gray matter if found internal (anterior/posterior horn) and white matter is
surrounds the gray matter on the outside.
B. PNS structure
1. Bundles of axons and their sheaths from nerves. The endoneurium surroundes individual axons.
Fascicles are groups of axons that are bound together by the perineurium. The epineurium holds
groups of fascicles together to form the nerve.
IV. The Synapse
A. Anatomy of the synapse.
1. presynaptic terminals - The enlarged ends of the axon that contain synaptic vesicles.
2. postsynaptic membranes - contain receptors for the neurotransmitter and are found on other
neurons, muscles or glands.
3. synaptic cleft - the space that separates the presynaptic and postsynaptic membranes.
B. Synaptic transmission
1. An action potential arriving at the presynaptic terminal causes Ca++ gates to open
2. Ca++ ion diffuse into the synaptic terminal
3. Ca++ causes synaptic vessicles containing the neurotransmitter to bind to the synaptic membrane
releasing the neurotransmitter,
4. The neurotransmitter diffuses across the synaptic cleft and binds to the receptors of the
postsynaptic membrane.
5. Binding of a ligand to the receptor invokes a response in the postsynaptic cell.
C. Neurotransmitter inactivation (three methods):
1. The neurotransmitter is broken down by an enzyme
a. Eg. acetylcholinesterase
2. The neurotransmitter is taken up by the presynaptic terminal.
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a. Epinerpherine is taken up repackaged in vessicles and reused or inactivated within the
presynaptic terminal by monoamine oxidase (MAO).
3. The neurotransmitter diffuses out of the synaptic cleft.
D. Receptor molecules in synapses
1. Receptors for neurotransmitters are specific.
2. A neurotransmitter can bind to several different receptor types
a. Therefore a neurotransmitter can be stimulatory (depolarize) in one synapse and inhibitory
(hyperpolarize) in another, depending on the type of receptor present.
3. Some presynaptic terminals have receptors.
a. Release of norepinepherine (NE) can bind to presynaptic receptors which decrease the
release of NE. (modifies its own release).
E. Neurotransmitter and Neuromodulators
1. Neurotransmitters are substances released at a synapse that affect another cell.
2. Once thought that each neuron contained only one neurotransmitter.
a. Some neurons can secrete more than one type of neurotransmitter.
b. A neuron makes use of the same combination of chemical messengers at all of its synapses.
c. Contrary to book, neurons can release one neurotransmitter in greater abundance compared
to another neurotransmitter.
(1) Size of versicle and frequency of action potential.
3. Neuromodulators influence the likelihood that an action potential in a presynaptic terminal will
result in an action potential in a postsynaptic cell.
a. Can influence the release of other neurotransmitters.
4. Types of Neurotransmitters.
Substance
Location
Effect
Example
Used by motor neurons in
spinal cord and at all nerve
skeletal muscle junctions.
Widespread use throughout
the brain and in ANS
Excite or inhibit
Alzheimer’s disease decrease in Ach release
Myasthenia gravis - decrease
in ACH receptors.
Cateholamines (DNE) from
tyrosine.
excite
Parkinson’s - dopamine
schizophrenia/vomiting.
Amphetamines / cocaine increase epi.
1. Small molecule transmitters substances
Acetylcholine
2. Biogenic Amines
Dopamine
Norepinephrine
exictatory / inhibitory
Epinephrine
Serotonin
Histamine
exictatory / inhibitory
Inhibitory
inhibitory
Mood anxiety sleep
arousal from sleep and
thermoregulation
3. Amino Acids
major inhibitory trans.
Inhibitory
exitatory
GABA
Glycine
Glutamate and
aspartate
4. Neuropeptides (Over 50 have been identifed.)
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Used to treat epilepsy
blocked by strychnine
drugs prevent seizures.
Opioids
regulate pain, reproduction,
thermoregulation and
maternal behaviors
Endorphins / enkephalins
Tachykinins
regulate sex,
Substance P,
Gastrins
regulate sex and maternal
behaviors
CCK, Galanin
Neurohypophyseal
vassopressin, oxytocin,
neurophysins
Secretins
glucagon, VIP, Growth
Hormone RH , histidine
Insulins
regulate blood sugar levels
and growth
insulin, insulin-like growth
factors
V. Excitatory and Inhibitory Postsynaptic Potentials
A. A combination of neurotransmitters causes either a depolarization (excitatory)or hyperpolarization
(inhibitory)of the postsynaptic membrane.
B. Depolarization
1. caused by an excitatory postsynaptic potential (EPSP) an increase in membrane permeability to
Na+ ions
2. Caused by.
3. If depolarization reaches threshold then an action potential is generated.
4. Novocain decreases permiability to Na+.
C. Hyperpolarization
1. caused by an increase in membrane permeability to chloride ions or potassium ions is an
inhibitory postsynaptic potential (IPSP).
a. Cl- moves into cell making inside more negative.
b. K+ moves out of cell making inside more negative.
D. Presynaptic inhibitions and facilitation
1. Axoaxonic synapses on presynaptic terminal can alter the amount of neurotransmitter released.
a. Presynaptic inhibition decreases neurotransmitter release.
b. Presynaptic facilitation increases neurotransmitter release.
2. The greater the amount of neurotransmitter the greater the IPSP or EPSP
E. Spatial and temporal summation
1. Presynaptic action potentials through neurotransmitters produce local potentials in postsynaptic
neurons.
a. Summation: A series of presynaptic action potentials causes a series of local potentials in
the postsynaptic neurons that act at the axon hillock to generate an action potential if the
local potentials are strong enough. These local potentials can summate to produce an action
potential at the axon hillock.
2. Two type of summation
a. Spatial summation occurs when two are more presynaptic terminals simultaneously stimulate
a postsynaptic neuron.
(1)
b. Temporal summation occurs when two are more action potentials arrive in succession at a
single presynaptic terminal.
3. Inhibitory and excitatory presynaptic neurons can converge an a postsynaptic neuron. The
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activity of the postsynaptic neuron is determined be the integration of the EPSPs and IPSPs
produced in the postsynaptic neuron.
VI. Reflexes
A. A reflex arc is the functional unit of the nervous system.
1. Sensory receptors respond to stimuli and produce action potentials in afferent neurons.
2. Afferent neurons propagate action potentials to the CNS.
3. Association neurons in the CNS synapse with afferent neurons and with efferent neurons.
4. Efferent neurons carry action potentials from the CNS to the effector organ.
5. Effector organs such as muscles or glands respond to the action potential.
B. Reflexes do not require conscious thought, and they produce a consistent and predictable result.
C. Reflexes are homeostatic. They remove the body from painful stimuli, keep the body from falling,
maintain blood pressure, pH, CO 2 levels, and water intake.
D. Reflexes are integrated within the brain and spinal cord. Higher brain centers can suppress or
exaggerate reflexes.
VII.
Neuronal pathways and Circuits.
A. Organization of the neurons varies from simple to extremely complex.
1. Branching / synaptology can be complex to simple.
2. There are three basic patterns of neuronal circuitry: convergent, divergent and oscilating
B. Convergent pathways have many neurons synapsing (converge) with only a few neurons.
1. Eg. Spinal cord convergence of CNS circuits and sensory neuron in reflex arc.
C. Divergent pathways have a few neurons synapsing with many neurons.
1. Information in one circuit can be spread to several other circuits.
2. Eg. Sensory nerve sends information to 1)an interneuron the goes out to the motor neurons and
2) another neuron that goes to the brain.
D. Oscillating circuits have collateral branches of postsynaptic neurons synapsing with presynaptic
neurons or themselves (fig. 12.22).
1. Lets citcuits produce action potentials more than once (after discharge)
a. Prolongs response (positive feedback circuit)
b. Stimulation continues until:
(1) fatigue is reached
(2) inhibited by another neuron.
c. CNS control of respiration (breathing in and out) is controlled by an oscillating circuit.
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