chapter-11-functional-organization-of-nervous
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Transcript 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