9.1 INTRODUCTION
• Neurons: masses of nerve cells.
• Structural and functional units of the nervous
system.
• Specialized to react to physical and chemical
changes in their surroundings.
• Nerve impulses: electrochemical changes
neurons transmit.
NEURONS
• Cell body: rounded area
• Extensions: dendrites and axons
• Dendrites: receive electrochemical messages.
• Axons: extensions that send information in the
form of nerve impulses.
• Neurons usually have only one axon.
NERVES
• Bundles of axons.
• Neuroglial cells: provide physical support,
insulation, and nutrients for neurons.
ORGANS OF NERVOUS SYSTEM
• Central nervous system: brain, spinal cord.
• Peripheral nervous system: nerves that connect
the central nervous system to other body parts.
3 FUNCTIONS
1. Sensory
2. Integrative
3. motor
9.2 GENERAL FUNCTIONS OF THE NERVOUS
SYSTEM
• Sensory receptors: ends of peripheral
neurons.
• Gather information by detecting changes
inside and outside the body.
• Monitor external environmental factors, such
as light and sound intensities, and conditions
of the body’s internal environment, such as
temp. and oxygen level.
• Convert environmental information into nerve
impulses.
• Transmitted over peripheral nerves to the
CNS.
• Signals are integrated; brought together,
creating sensations, adding to memory, or
helping to produce thoughts that translate
sensations into perceptions.
• Integrative function: we make conscious or
subconscious decisions.
• Motor functions: to act on them.
• Effectors: carry impulses from the CNS to
responsive structures.
• Outside the nervous system, include muscles that
contract and glands that secrete when stimulated
by nerve impulses.
MOTOR FUNCTIONS OF THE PERIPHERAL
NERVOUS SYSTEM
2 categories
• Somatic NS: consciously controlled, controls
skeletal muscle.
• Autonomic NS: controls effectors that are
involuntary, hear, smooth muscle in blood
vessels, and various glands.
9.3 NEUROGLIAL CELLS
• Fill spaces between neurons, provide
structural frameworks, produce the fatty
lipoprotein myelin, and carry on phagocytosis.
• Neuroglial cells greatly outnumber neurons.
NEUROGLIAL CELLS
1. Microglial cells: scattered throughout the CNS.
They support neurons and phagocytize bacterial
cells and cellular debris
2. Oligodendrocytes: align along nerve fibers.
They provide insulating layers of myelin, called a
myelin sheath around axons within the brain
and spinal cord.
3. Astrocytes: commonly found between neurons
and blood vessels, provide structural support,
join parts by their abundant cellular processes,
and help regulate the concentrations of
nutrients and ions within the tissue. Also form
scar tissue.
4. Ependymal cells: form an ephithelia-like
membrane that covers specialized brain parts
(choroid plexuses) and forms the inner linings
that enclose spaces within the brain
(ventricles) and spinal cord (central canal).
Schwann cells: neuroglial cells that form a
myelin sheath around axons.
9.4 NEURONS
• Neurofibrils: fine threads, extend into the axons
(in the cell body).
• Neuron cell body: granular cytoplasm, cell
membrane, organelles.
• Chromatophilic substance: membranous sacs,
similar to rough endoplasmic reticulum.
• Axon: conducts nerve impulses away from the
cell body.
• Schwann cells: contain myelin, neurilemma
sheath that surrounds the myelin sheath.
• Nodes of Ranvier: gaps between Schwann cells
• Axons with myelin sheaths are called
myelinated.
• Unmyelinated: lack sheaths.
• Peripheral nerve Axons can regenerate
• Schwann cells help do this.
• CNS axons usually cannot regenerate
themselves.
CLASSIFICATION OF NEURONS
• Neurons differ in structure, size, and shape of
their cell bodies.
• Trigger zone: sensitive region of the axon,
send a nerve impulse.
THREE MAJOR GROUPS OF NEURONS
1. Multipolar neurons: many processes arising
from their cell bodies. Only one process of each
neuron is an axon; the rest are dendrites.
2. Bipolar neurons: only two proceses, one an axon
and the other a dendrite. Ex: eyes, nose, and
ears
3. Unipolar neurons: single process, divides into
two branches, single axon. One branch is
associated w/ dendrites near a peripheral body
part. The other branch enters the brain or spinal
cord.
• Ganglia: masses of nervous tissue.
• Located outside the brain and spinal cord
• Different functions of neurons:
• Carry impulses into the brain or spinal cord
• Transmit impulses out of the brain or spinal
cord
• Conduct impulses from neuron to neuron
within the brain or spinal cord
GROUPS OF DIFFERENT NEURONS
1. Sensory neurons: (afferent neurons) carry
nerve impulses from peripheral body parts
into the brain or spinal cord. Either have
specialized receptor ends at the tips of their
dendrites, or they have dendrites that are
closely associated with receptor cells in the
skin or in sensory organs. (most unipolar,
some bipolar)
2. Interneurons: (association or internuncial
neurons) lie within the brain or spinal cord.
Multipolar, link other neurons.
3. Motor neurons: (efferent neurons) are
multipolar and carry nerve impulses out of
the brain or spinal cord to effectors.
Stimulate muscles to contract and glands to
release secretions.
9.5 CELL MEMBRANE POTENTIAL
• Polarized: surface of the cell membrane,
electrically charged, unequal distribution of
positive and negative ions
• Action potential: change in charge of the
membrane, forms a nerve impulse
DISTRIBUTION OF IONS
• Active transport of sodium and potassium
ions.
• More sodium ions are outside and more
potassium ions inside.
• Cytoplasm: PO4-3, SO4-2, protein, K+
• Potassium pass through cell membrane easier
than sodium ions.
• K+ a major contributor to membrane
polarization.
RESTING POTENTIAL
• Resting cell membrane is more permeable to K+
that to Na+, K+ diffuse out of the cell more
rapidly.
• Result, outside of cell gains a slight surplus of +
charge.
• Inside – charge
• Potential difference: difference in electrical
charge between two regions.
• Resting potential: potential difference between
the region inside the membrane and the region
outside the membrane.
• As long as a nerve cell membrane is
undisturbed, the membrane remains in this
polarized state.
POTENTIAL CHANGES
• Nerve cells are excitable, they can respond to
changes in their surroundings.
• Some detect changes in: temp, light, pressure
• Many neurons respond to other neurons.
• depolarizing: inside membrane becomes less
negative compared to outside.
• Change in potential is proportional to the
intensity of stimulation.
• Summation: change in potential is increases as
more stimulation comes to cell
Threshold potential: action potential occurs
ACTION POTENTIAL
• When threshold potential occurs:
permeability changes at the trigger zone of
the neuron being stimulated.
• Channels highly selective for Na+ open and
allow Na+ to diffuse freely inward.
• Aided by the negative electrical condition on
the inside of the membrane, which attracts
the positively charged sodium ions.
• As sodium ions diffuse inward, the membrane
loses its negative electrical charge and
becomes depolarized.
• Membrane channels open that allow
potasssium ions to pass through, inside of
membrane becomes negatively charged once
more.
• Membrane returns to the resting potential.
• Rapid sequence of depolarization and
repolarization, 1/1000 of a second, called
action potential.
• http://www.youtube.com/watch?v=U0NpTdge
3aw
9.6 NERVE IMPULSES
• Wave of action potentials along a nerve axon –nerve
impulse.
• Pg. 213
IMPULSE CONDUCTION
• Unmyelinated axon: conducts an impulse over its
entire surface.
• Myelinated axon prevents almost all ion flow through
the membrane it encloses.
• Though, nodes of Ranvier between schwann cells have
sodium and potassium channels, nerve impulse
traveling along a myelinated axon appears to jump
from node to node.
• Many times faster than conduction on an unmyelinated
axon.
• Speed of nerve impulse conduction is
proportional to the diameter of the axon.
• 120m/s nuerons of skeletal muscle
• 0.5m/s unmyelinated axon neuron in skin
• http://www.youtube.com/watch?v=xx-f9Y8wjg
EVENTS LEADING TO THE CONDUCTION OF A
NERVE IMPULSE
1. Neuron membrane maintains resting potential
2. Threshold stimulus is received
3. Sodium channels in the trigger zone of the
neuron open
4. Sodium ions diffuse inward, depolarizing the
membrane
5. Potassium channels in the membrane open
6. Potassium ions diffuse outward, repolarizing the
membrane
7. The resulting action potential causes a local
bioelectric current that stimulates adjacent
portions of the membrane.
8. A wave of action potentials travels the length
of the axon as a nerve impulse.
ALL OR NONE RESPONSE
• If a neuron responds at all, it responds
completely.
• A greater intensity of stimulation does not
produce a stronger impulse, but rather, more
impulses per second.
• Refractory period: limits the frequency of
impulses in a neuron, a threshold stimulus will
not trigger another impulse on an axon.
• Common 100 impulses/second, 700 could
happen
Homework
Pg. 246
Ques. 1-14
Quiz Friday on previous slides
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/ch
apter14/animation__the_nerve_impulse.html
9.7 THE SYNAPSE
• Nerve pathways: pathway of nerve impulses.
• Synapse: junction between any two
communicating neurons
• Gap, synaptic cleft
SYNAPTIC TRANSMISSION
• Neuron carrying the impulse: presynaptic
neuron
• Neuron that receives this input, pstsynaptic
neuron
• Synaptic transmission: process of crossing the
synaptic cleft with this message.
• Neurotransmitters: biochemicals that aid the
synaptic transmission
• Distal end of axon have extensions called
synaptic knobs.
• In dendrites they contain synaptic vesicles
• When a nerve impulse reaches a synaptic
knob, some of the synaptic vesicles release
neurotransmitters.
• Neurotransmitters diffuses across the synaptic
cleft and reacts with specific receptors ont eh
postsynaptic neuron membrane.
EXCITATORY AND INHIBITORY ACTIONS
• Neurotransmitters that increase postsynaptic
membrane permeability to sodium ions will
bring the postsynaptic membrane closer to
threshold and may trigger nerve impulses.
• Excitatory: these neurotransmitters
• Inhibitory: neurotransmitters that make it less
likely that threshold will be reached. Lessens
the chance that a nerve impulse will occur.
• If more excitatory than inhibitory
neurotransmitters are released, the
postsynaptic neuron’s threshold may be
reached, and a nerve impulse will be
triggered.
NEUROTRANSMITTERS
• Fifty types of neurotransmitters have been
identified in the nervous system.
• Neurotransmitters: acetylcholine,
monoamines (epinephrine, norepinephrine,
dopamine, serotonin)
• Amino acids (glycine, glutamic acid, aspartic
acid, gamma-aminobutyric acid)
• Neuropeptides (short chains of amino acids)
• Acetylcholine and norepinephrine are
excitatory
• Dopamine, GABA and glycine are inhibitory
• Neurotransmitters are made in the cytoplasm
of the synaptic knobs, stored in the synaptic
vesicles
• When an action potential reaches the
membrane of a synaptic knob, it increases the
membrane’s permeability to calcium ions by
opening the membrane’s calcium ion
channels.
• Calcium ions diffuse inward.
• Some neurotransmitters are decomposed by
enzymes in the synaptic cleft
(acetylcholinesterase)
• Decomposition or removal of
neurotransmitters prevents continuous
stimulation of postsynaptic neurons.
EVENTS LEADING TO THE RELEASE OF A
NEUROTRANSMITTER
1. Action potential passes along an axon and over
the surface of its synaptic knob
2. Synaptic knob membrane becomes more
permeable to calcium ions, and they diffuse
inward.
3. In the presence of calcium ions, synaptic vesicles
fuse to synaptic knob membrane
4. Synaptic vesicles release their neurotransmitter
into synaptic cleft
5. Synaptic vesicles reenter cytoplasm of axon and
pick up more neurotransmitter
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/ch
apter14/animation__the_nerve_impulse.html
9.8 IMPULSE PROCESSING
• How nervous system process and respond to
nerve impulses.
NEURONAL POOLS
• How nuerons are organized.
• Work together to perform a common
function.
• Recives input from neurons
• May have exitatory or inhibitory effects on
other pools
FACILITATION
• May receive excitatory and inhibitory input.
• If the net effect of the input is excitatory,
threshold may be reached, and an outgoing
impulse triggered.
• Facilitation: if it was sub-threshold, the neuron
is more excitable to incoming stimulation than
before.
CONVERGENCE
• Single neuron may receive impulses from two or
more incoming axons.
• Convergence: axons originating from different
parts of the nervous system and leading to the
same neuron.
• Makes it possible for impulses arriving from
different sources to have an additive effect on a
neuron.
• May reach threshold from two neurons from two
different places.
• Allows the nervous system to collect a variety of
kinds of information, process it, and respond to it
in a special way.
DIVERGENCE
• Impulses leaving a neuron of a neuronal pool
often pass into several other output neurons.
• An impulse from one neuron might stimulate
two others.
• Then those two stimulate several others and
so forth.
• Can amplify an impulse, spread it to more
neurons within a pool.
• Neuron ---reaching many muscles
9.9 TYPES OF NERVES
• Sensory fibers (Afferent)
• Motor fibers (efferent)
• Nerve: cordlike bundle of nerve fibers within
layers of connective tissue.
• Sensory nerves: conduct impulses to the brain
or spinal cord.
• Motor nerves: conduct impulses to muscles or
glands.
• Mixed nerves, nerves that include both
9.10 NERVE PATHWAYS
• Routes nerve impulses follow.
• Reflex arc: few neurons, structural and
functional basis for involuntary actions.
REFLEX ARCS
• Receptor at end of a sensory fiber
• Interneurons within CNS
• Reflex center