OVERVIEW
Maintaining homeostasis
1. Nervous system more rapid and specific; electrical
signals
2. Endocrine system effects are much slower; hormones
released to blood
Functions of the nervous system
1. Sensory input changes monitored; information sent to
central nervous system
2. Integration processes, integrates information from
sensory input decides
3. Motor output response of integration center; efferent
signal to muscle or gland
Organization of the nervous system
1. CNS integration, control center; receives stimuli;
decides response to be taken
-a. Brain located in the cranium
-b. Spinal cord located in the vertebral column
2. PNS cranial and spinal nerves; communication system
between CNS and body
-a. Sensory (afferent) division sensory fibers conduct
impulse; receptors to CNS
-b. Motor (efferent) division motor fibers conduct
impulses; CNS to effectors
3. Somatic nervous system somatic, special senses; skeletal
muscles; voluntary
4. Autonomic nervous system autonomic sensory receptors; viscera;
involuntary
-a. Sympathetic division mobilizes body systems during
emergency situations
-b. Parasympathetic division conserves energy; promotes nonemergency functions
5. Enteric nervous system gut brain; enteric sensory;
smooth muscle, glands
HISTOLOGY OF NERVOUS TISSUE
Supporting cells
1. Neuroglia (glial cells) another name for supporting
cells
2. CNS include
-a. Astrocytes most numerous; projections anchor neurons
to capillaries
-b. Microglia spiny; monitor health; macrophage,
phagocytize debris, microbes
-c. Epidymal cells squamous to columnar, many ciliated;
line cavities
-d. Oligodendrocytes myelin sheath around larger
cytoplasmic extensions
3. PNS include
-a. Satellite cells surround neuron body; may control
chemical environment
-b. Schwann cells myelin sheath in the PNS; peripheral
nerve fiber regeneration
Neuron: functional characteristics
1. Longevity can live and function for a life time
2. Amitotic lose ability to divide; cant regenerate
3. High metabolic rate need a continual supply of oxygen
and nutrients
Neuron: cytology
1. Cell body (perikaryon or soma)
protein making machinery
-a. Golgi apparatus form arc around nucleus
-b. Nissl bodies rough
endoplasmic reticulum (stain darkly)
2. Dendrites shorter more numerous processes; contact with
many other neurons
-a. Receptive (input) region conducts impulses toward the
cell body
3. Axons the singular usually longer cell process
-a. Axonal hillock - cone shaped region exiting from cell
body; narrows to the axon
-b. Trigger zone axon hillock and first segment of axon;
nerve impulse starts
-b. Conduction zone conducts electrical impulse away from
cell body to terminals
-c. Nerve fibers when very long they are referred to as
nerve fibers
-d. Axonal collaterals rare branches off the axon usually
at right angles
-e. Terminal branches terminus of the axon branches into
10,000 or more
-f. Axonal terminals (synaptic knobs) the bulbous end of
each terminal branch
-g. Secretory zone at axonal terminals; vesicles with
neurotransmitters; exocytosis
-h. Microtubules move substances and organelles from body
to terminals
Myelin sheath
1. Myelinated fibers white fatty
layers; good insulator; impulses 150 time faster
2. Schwann cells wind the around axon; cytoplasm is
squeezed out
-a. Myelin sheath actual layers of plasma membrane which
cover the nerve fiber
-b. Neurolemma contains the
cytoplasm and nucleus of the Schwann cell
-c. Node of Ranvier gaps in the
myelin sheath occurring adjacent Schwann cells
2. Oligodendrocytes multiple flat processes can coil
around as many as 60 axons
3. Unmyelinated fibers no myelin
sheath; embedded but not surrounded
Classification of neurons
1. Structural classification based upon processes
-a. Multipolar neurons 0 -1
axons; many dendrites; brain, most, motor neurons
-b. Bipolar neurons rare; 1 axon and 1 dendrite; opposite
sides cell; senses
-c. Unipolar neurons 1 process
or axon; sensory neurons mostly
2. Functional classification can be correlated to
structural classification
-a. Sensory (afferent) neurons sensory receptors to the
CNS; usually unipolar
-b. Motor (efferent) neurons CNS to the effector organ;
are multipolar neurons
-c. Association neurons (interneurons) in the CNS; about
99%; diverse multipolar
NEUROPHYSIOLOGY
Basic electrical principles
1. Resting membrane potential an electrical voltage difference across the membrane
2. Current represents the flow of electrically charge
particles; flow of ions
Ion channels
1. Passive ion channels always open; passive movement of
ions through membrane
2. Active (gated) channels are not always open
-a. Chemically gated channels specific neurotransmitters
bind to them
-b. Voltage gated channels membrane potential (voltage)
Resting membrane potential
1. Resting membrane potential more negative in its
interior; - 70 mV (millivolts)
2. Charged particles includes anions and cations
-a. Sodium (Na+) most plentiful extracellular
cation
-b. Potassium (K+) most plentiful intracellular
cation
-c. Chloride ions (Cl−) most plentiful
extracellular anion
-d. Negatively charged proteins (A−) most
plentiful intracellular anion
3. Passive diffusion potassium diffuses out faster then
sodium diffuses
4. ATPase (Na+,K+ pump) prevents equilibrium, which
would occur over time
Graded potential
1. Graded potentials short lived depolarization or
hyperpolarization
2. Receptor potential receptor of a sensory neuron (heat,
light, touch, est.)
3. Postsynaptic potential excited by a neurotransmitter
released by another neuron
4. Depolarization sodium channel opens; sodium flows in;
excitatory
5. Hyperpolarization ligand
gated Cl channel; K channel less common; inhibitory
6. Capacitance current changes in charge distribution
along 2 sides of membrane
7. Decremental get weaker further from stimulation
Action potential generation
1. Resting state
all sodium and potassium voltage gated channels are closed
2. Depolarization
increase in sodium permeability; reversal membrane potential
-a. Local potential reaches trigger area
-b. Trigger area large number of voltage gated Na channels
-c. Voltage gated Na+ channels open
-d. Threshold depolarized to critical level, - 50 mV,
depolarization self generating
-e. Positive feedback sodium in, opens more voltage gated
sodium channels, more
3. Repolarization K+ moves out; return to
negative resting membrane potential
-a. Voltage gated K+ channels open
-b. Voltage gated Na+ channels closed; Na+
not entering cell
-c. Potassium leaves cells
4. Undershoot K+ channels slow to close; little
less negative resting potential
-a. Hyperpolarization what undershoot results in;
inhibitory
-b. Resting membrane potential quickly returns as K
channels close
-c. Na+,K+ATPase
repolarization restores resting potential; this restores ion levels
Action potential propagation
1. All-or-none law once threshold is reached; entire axon
fires
2. Nondecremental signal stays strong
even to axon terminals
3. Irreversible can not be stopped
4. Chain reaction action potential causes action potential
causes
5. Nerve signal chain reaction of action potentials
6. Refractory period area behind action potential will not
depolarize
7. Stimulus intensity greater frequency; greater strength
Refractory period
1. Absolute refractory period can not be excited again no
matter how strong
-a. Voltage gated Na+ channels are inactivated; can not be
open no matter what
2. Relative refractory period sodium gates have closed;
returned to resting state
Conduction velocities
1. Axon diameter increases the velocity as it offers less
resistance to the current
2. Myelin sheath flow of sodium only at the nodes of Ranvier
-a. Saltatory conduction
electrical signal jumps from node to; much faster
-b. Multiple sclerosis autoimmune disease in which myelin
sheath is destroyed
3. Speed from 1 m/s (2mph) to 150 m/s (300 mph); diameter
and myelin sheath
Synapse
1. Location where
-a. Axodendritic between
presynaptic axons and postsynaptic dendrites
-b. Axosomatic between
presynaptic axons and postsynaptic cell bodies
-c. Less common and less
understood; axoaxonic, dendrodendritic,
dendrosomatic
2. Varieties two
-a. Electrical synapse cytoplasm connected; embryo; adult,
rare, jerky eye move
-b. Chemical most common; neurotransmitter
Chemical synapse
1. Presynaptic neurons axonal terminal knob like
structure
-a. Neurotransmitters contained in membrane bound vesicles
2. Synaptic cleft - fluid filled space between axonal
terminal the postsynaptic neuron
3. Postsynaptic neuron receptors - bind to neurotransmitter
4. Neurotransmitter release involves
-a. Calcium gates open axonal terminal, depolarization
also opens these
-b. Exocytosis calcium causes neurotransmitter vesicles to
spill their contents
5. Receptor binding across the synaptic cleft; bind
reversible to receptors
6. Ion channels open membrane proteins of the receptor to
shape change, opens
7. Neurotransmitters fate after it does its job; can not
remain
-a. Enzyme degradation neurotransmitter is broken down
(acetylcholine)
-b. Reuptake presynaptic terminal, astrocyte;
destroyed, stored (norepinephrine)
-c. Diffusion of the transmitter from the synapse
8. Synaptic delay time for all this to occur; less neurons
better
Postsynaptic potential
1. Excitatory postsynaptic potentials EPSP graded
depolarization; less negative
2. Inhibitory postsynaptic potential IPSP graded
hyperpolarization; more negative
-a. Cl channels open, chloride ion rushes in
-b. K+ channels open, potassium leaves cell
Summation
1. Temporal summation presynaptic neurons fire in rapid
order
2. Spatial summation stimulated by many axonal terminals
Modification
1. Synaptic potentiation repeated use enhances ability pre- to excite postsynaptic
-a. Presynaptic increase Ca
increases release of neurotransmitters
-b. Postsynaptic calcium channel activated; Ca activates
enzymes; more responsive
-c. Memory possible important in this
2. Presynaptic inhibition neurotransmitter of one axon inhibited by other neuron
-a. Axoaxonic synapse type that is usually involved in
this
3. Neuronal modulation neurotransmitter or other chemical
changes metabolism
Neurotransmitters: chemical classification
1. Acetylcholine neuromuscular junctions; CNS and
autonomic nervous system
2. Biogenic amines all synthesized from some amino acid
-a. Catecholamines from tyrosine
(dopamine, norepinephrine, epinephrine)
-b. Indolamines serotonin (from trytopphan); histamine (from histidine)
3. Amino acids gamma amino butyric acid (GABA), glutamate,
glycine, aspartate
4. Peptides substance P (pain); endorphins and enkephalins (opiates)
5. Novel messengers ATP; nitric oxide (NO)
Neurotransmitters: functional classification
1. Effects excitatory, inhibitory, or both
2. Mechanism direct, indirect, or both
-a. Directly opening ion channels
-b. Indirectly using a second messenger intracellularly
Neurotransmitter receptors
1. Channel linked receptors membrane proteins open a
central canal; ions pass
2. G protein linked receptors neurotransmitter activates G
protein
-a. Second messenger cAMP, cGMP; channel open, enzyme,
gene activation
MISCELLANEOUS
Neuronal pools
1. Neuronal pools functional groups, 1000s neurons,
integrate incoming messages
2. Discharge zone closer to, more connections to incoming axon more likely to
fire
3. Facilitated zone periphery, less connections; not as
likely to; acted on by other
Circuits
1. Circuits the pattern of neuronal connections in
neuronal pools;
function
2. Diverging circuits one incoming fiber triggers the firing of many subsequent
-a. Amplification a single signal sent to many different
places
3. Converging circuits many different inputs coming into the pool and the signal is
-a. Concentration how different stimuli can bring about
the same feeling
4. Reverberating circuits collateral axons stimulate the
previous axon
-b. Rhythmic activities such as breathing
5. Parallel after discharge circuits input; parallel neurons; common output neuron
-a. Precise mental functions like doing math
Neural processing
1. Serial processing one neuron stimulates the next in the sequence, stimulates next
-a. Reflex arc good example; predictable outcome
2. Parallel processing inputs are to many pathways; dealt
with simultaneously
-a. Higher brain functions important for this
Nerve fiber regeneration
1. Distal end seals off
2. Wallerian degeneration axon distal to site of injury
3. Macrophages cleanup debris and release a mitosis stimulating substance
4. Schwann cells proliferate, release growth factors
-a. Growth factors cause axonal filaments to grow distally
-b. Guide the filaments into place
5. CNS doesnt happen for several reasons; most importantly no Schwann cells