Download chapter-11-functional-organization-of-nervous



The master controlling and
communicating system of the
body
Functions
 Sensory input – monitor
internal and external
stimuli

Integration – interpretation
of sensory input

Motor output – response
to stimuli by activating
effector organs
 Central nervous system (CNS)
 Consists of Brain and spinal cord
 Controls entire organism
 Integrates incoming information and responses
 Peripheral nervous system (PNS)
 Link between CNS, body and environment
 Consists of Spinal and cranial nerves
 Carries messages to and from the spinal cord and brain
 Sensory (afferent) division
 Sensory afferent fibers –
carry impulses from skin,
skeletal muscles, and joints
(sensory receptors) to the
brain
 Visceral afferent fibers –
transmit impulses from
visceral organs to the brain
 Motor (efferent) division
 Transmits impulses from the
CNS to effector organs
(muscles, glands)
 Two Divisions:
 Somatic nervous system
(=Voluntary)
 Conscious control of skeletal
muscles
 Conducts impulses from the
CNS to skeletal muscles
 Autonomic nervous system
(ANS= involuntary)
 Regulates smooth muscle,
cardiac muscle, and glands
 Subconscious or involuntary
control
 Sympathetic Nervous System (Thoraco-lumbar outflow)
 “Flight or fright system”
 Most active during physical activity
 Parasympathetic Nervous System (Cranial-sacral outflow)
 Regulates resting or vegetative functions such as
digesting food or emptying of the urinary bladder
 The two principal cell types of the nervous
system are:
Neurons – excitable cells that transmit
electrical signals
Supporting (glial) cells – cells that surround
and wrap neurons
 Structural units of the nervous
system
 Receive stimuli and transmit
action potentials
 Long-life
 Amitotic
 Have a high metabolic rate
 Each neuron consists of:
 Body
 Axon
 dendrites
 Cell Body (soma or perikaryon)
 Contains the nucleus and a nucleolus &
usual organelles
 Has no centrioles (amitotic nature)
 Has well-developed Nissl bodies (rough
ER)
 Nissl bodies -primary site of protein
synthesis
 Contains an axon hillock – cone-shaped
area from which axons arise
 Short, branched
cytoplasmic extensions
 They are the receptive, or
input, regions of the
neuron
 Electrical signals are
conveyed toward the cell
body
 Slender processes of uniform
diameter arising from the axon
hillock
 Initial segment: beginning of
axon
 Axoplasm : Cytoplasm of axon
 Axolemma : Plasma membrane
of axon
 Long axons are called nerve
fibers
 Usually there is only one
unbranched axon per neuron
 Rare branches, if present, are
called axon collaterals
 Presynaptic (Axon) terminal –
branched terminus of an axon
 Trigger zone: site where action
potentials are generated; axon
hillock and part of axon nearest to
cell body
 AP are conducted along the axons
to axonal terminals and release
neurotransmitters
 AP conduction away from cell
body
 Neurons can be classified by structure:
 Multipolar
 Most common in both CNS & PNS
 Single axon, many dendrites (motor
neurons and interneurons of CNS)
 Bipolar
 two processes (one axon and one
dendrite)
 Are sensory neurons found in the
retina, olfactory nerve
 Unipolar
 single short process extending from
cell body
 Divides into two branches and
functions as both dendrite and axon
(sensory neurons , dorsal root
ganglia)
 Neurons can be classified by function:
 Sensory (afferent) — transmit impulses from receptors
toward the CNS
 Motor (efferent) — carry impulses from CNS to muscles and
glands
 Interneurons (association neurons) Link sensory and motor
neurons within CNS
 Make up 99% of neurons in body
NERVOUS SYSTEM CELL TYPES
NEUROGLIA (Glial cells)
 Supporting cells
 Surround neurons
 Non-conducting
 6 types
Astrocyte
 2 in PNS
Oligodendrocytes
 4 in CNS
Microglia
Ependymal Cells
Satellite Cells
Schwann Cells
 1) Astrocytes
 In CNS only
 Anchor neurons to capillaries
 Regulate what substances reach the
CNS from the blood (blood-brain
barrier)
 Regulate extracellular brain fluid
composition
 Pick up excess K+
 Recapture released (recycle)
neurotransmitters
 2) Ependymal Cells
 CNS only
 Line the cavities of the
brain and spinal cord
 Ciliated
 Circulate the
cerebrospinal fluid
(CSF)
 3) Microglia
 CNS only
 Migrate toward injured
neurons
 Specialized macrophages
 Phagocytize necrotic
tissue, microorganisms,
and foreign substances
that invade the CNS
 4) Oligodendrocytes
 CNS only
 Wrap extensions
around neuron fibers
(axons)
 Form myelin sheath
 1) Schwann Cells or
Neurolemmocytes
 PNS only
 Wrap around axons of
neurons in the PNS
 Forms myelin sheath
 2) Satellite Cells
 PNS only
 Surround neuron cell
bodies
 Provide support and
nutrients to neuronal cell
bodies
 Protect neurons from
heavy metal poisons (lead,
mercury) by absorbing
them
 Myelinated Axons: Whitish, fatty (proteinlipid), segmented sheath around most long
axons
 Functions:
 Protect the axon
 Electrically insulate fibers from one another
 Increase the speed of nerve impulse
transmission
 Formed by Schwann cells in the PNS
 In CNS formed by oligodendrocytes
 Nodes of Ranvier : Gaps in the myelin sheath
between adjacent Schwann cells
 Unmyelinated Axons : Schwann cell
surrounds nerve fibers but coiling does not
take place
 White matter – dense collections of myelinated fibers
 Gray matter – mostly nerve cell bodies and unmyelinated
fibers
 In brain: gray is outer cortex as well as inner nuclei; white
is deeper
 In spinal cord: white is outer, gray is deeper
 Synapse
 Junction between one neuron and
another
 Where two cells communicate with
each other
 Presynaptic neuron – conducts
impulses toward the synapse
 Postsynaptic neuron – Cell that
receive the impulse
 Most are axo-dendritic or
axo-somatic
 Electrical Synapses:
 Are gap junctions that allow
ion flow between adjacent
cells by protein channels
called Connexons
 Not common in CNS
 Found in cardiac muscle and
many types of smooth muscle
Action potential of one cell
causes action potential in next
cell
Chemical Synapses
 Most are this type
 Neurotransmitter released from synaptic vesicles
of presynaptic neuron
 Neurotransmitter binds to receptors on
postsynaptic membrane
 Binding of neurotransmitter to receptor 
permeability change in postsynaptic membrane
 Released at chemical synapses
 In response to AP Voltageregulated calcium channels open
 Ca2+ diffuse into presynaptic
terminal
 And causes synaptic vesicles to
fuse with presynaptic membrane
 This fusion releases
neurotransmitter into the synaptic
cleft via exocytosis
When bound to receptors on
postsynaptic neuron, the
neurotransmitter can either
excite or inhibit the
postsynaptic neuron
 Resting neurons maintain a difference in
electrical charge inside and outside cell
membrane = RESTING MEMBRANE
POTENTIAL (RMP)
 The inside of the resting neuron is
negatively charged, the outside is
positively charged.
 Concentration of K+ higher inside than
outside cell
 Na+ higher outside than inside
 RMPs vary from -40 to
-90mV in different neuron types
 When bound to receptors on the
postsynpatic neuron membrane:
 Causes the opening of positive
ion channels
 Sodium ions enter rapidly
 RMP becomes more positive
 This positive change in the RMP
is called depolarization
 This brings the neuron closer to
firing
• A positive change in the
RMP
– Caused by influx of
positive ions
– Causes the inside of
the cell membrane to
become less negative
– Depolarization spreads
to adjacent areas
 When bound to receptors on the
postsynaptic membrane:
 Make the membrane more
permeable to negative ions (usually
Cl-)
 As negative ions rush into the
neuron, the RMP becomes more
negative
 The negative change in the RMP =
hyperpolarization
 Brings the neuron farther from firing
• A negative change in
RMP
• Usually caused by
influx of chloride ions
• Decreases the
likelihood of the
neuron firing
• Short changes in the RMP in
small regions of the membrane
• Can be positive or negative
(depolarize or hyperpolarize the
membrane)
• Alone, not strong enough to
initiate an impulse
• summate or add onto each other
• Together, can trigger a nerve
impulse (action potential)
 EPSP (Excitatory Postsynaptic
Potential)
 When depolarization occurs, response is
stimulatory
 & graded potential is called EPSP
 Binding of a neurotransmitter on the
postsynaptic membrane more positive
RMP, reaches threshold (depolarization
occurs)
 producing an action potential and cell
response
 IPSP (Inhibitory Postsynaptic Potential)
 When hyperpolarization occurs,
response is inhibitory
 & graded potential is called IPSP
 Binding of the neurotransmitter on the
postsynaptic membrane more negative
RMP (hyperpolarization)
 Decrease action potentials by moving
membrane potential farther from
threshold
 40 to 50 Known Neurotransmitters
 Acetylcholine (ACh)
 Norepinephrine (NE)
 GABA
 Dopamine
 Serotonin
Action Potential = Nerve Impulse
Consists of:
 Depolarization
 Propagation
 Repolarization
 If depolarization reaches threshold (usually a positive
change of 15 to 20 mV or more), an action potential is
triggered
 The positive RMP change causes electrical gates in the
axon hillock to open
 Sudden large influx of sodium ions causes a reversal in the
membrane potential (becomes approx. 100mV more
positive)
 Begins at the axon hillock and travels down the axon
 Chemically gated channels – open with binding of
a specific neurotransmitter
 Voltage-gated channels – open and close in
response to membrane potential
Chemically Gated
Voltage Gated
(on dendrite or soma)
(on axon hillock and axon)
Movement of the action
potential down the
axolemma
voltage-gated sodium
channels open in
response to positive
RMP change
 Restoration of the RMP back to it’s
negative state
 A repolarization wave follows the
depolarization wave
 3 factors contribute to restoring the
negative RMP:
 Sodium (Na+) gates close (it no longer
enters)
 Potassium (K+) gates open, potassium
rushes out
 Sodium/potassium pump kicks in
 An active process: requires
cellular energy
 Actively pumps 3 sodium (Na+)
ions out of the cell and 2
potassium (K+) ions in
 Potassium leaks back out
 Period of time when electrical
sodium gates are open
 From beginning of action
potential until near end of
repolarization
 No matter how large the
stimulus, a second action
potential cannot be produced
 The interval following the
absolute refractory period
when:
 Sodium gates are closed
 Potassium gates are open
 Repolarization is occurring
 A stronger-than-threshold
stimulus can initiate another
action potential
 A single EPSP cannot induce
an action potential
 EPSP’s can add together or
SUMMATE to initiate an
action potential
 Spatial Summation
 Large numbers of axon
terminals stimulate the
postsynaptic neurons
simultaneously
Temporal Summation
One or more
presynaptic
neurons transmit
impulses in rapid
fire succession
 An action potential is an “all or none” phenomenon
 When threshold is reached, the action potential will occur completely
 If threshold is not reached, the action potential will not occur at all
 Occurs only in myelinated axons
 Depolarization wave jumps from one node of Ranvier to the next
 Results in faster nerve impulse transmission
 A nerve impulse in the presynaptic neuron causes release of
neurotransmitter into synaptic cleft
 Neurotransmitter binding to receptors on postsynaptic neuron
dendrite or soma cause certain chemically gated ion channels to
open
 If Na+ channels open:
 Rapid influx of Na+ ions (depolarization)
 A small positive graded potential occurs (EPSP)
 If RMP changes in a positive direction by 20mV (or reaches the
threshold), voltage gated sodium channels in the axon hillock open
 Sodium rushes in at the axon hillock resulting in an action potential
 As the positive ions get pushed down the axon, more voltage
gated sodium channels open and the depolarization continues
down the axon (propagation)
 The process of restoring the negative RMP begins immediately
following the depolarization wave (repolarization)
 The larger the axon diameter, the faster the
impulse travels
 Myelinated axons conduct impulses more
rapidly
 Fiber Types:
 Type A fibers
 Large diameter axon with thick myelin
sheath
 Impulse travels at 15 to 150 m/sec.
 Sensory and motor fibers serving skin,
muscles, joints
 Type B fibers
 Intermediate diameter axon, lightly
myelinated
 Impulse travels at 3 to 15 m/sec, Part of
ANS
 Type C fibers
 Small axon diameter, unmyelinated
 Slow impulse conduction (1 m/sec. or less)
 Part of ANS
 Organization of neurons in CNS
varies in complexity
 Convergent pathways: many
converge and synapse with smaller
number of neurons. E.g., synthesis of
data in brain
 Divergent pathways: small number of
presynaptic neurons synapse with
large number of postsynaptic
neurons. E.g., important information
can be transmitted to many parts of
the brain
 Oscillating circuit: outputs cause
reciprocal activation