Download Nervous System

Nervous System
Structure and Function
Functions

Sensory input

Integration

Motor output
Classification

Central Nervous System


CNS
Peripheral Nervous System

PNS
Divisions of Peripheral Nervous System

Sensory Division

Motor Division
 muscles and
glands

Divisions of the
Motor
 Somatic
 Autonomic
Peripheral nervous system (PNS)
Central nervous system (CNS)
Cranial nerves and spinal nerves
Communication lines between the
CNS and the rest of the body
Brain and spinal cord
Integrative and control centers
Sensory (afferent) division
Somatic and visceral sensory
nerve fibers
Conducts impulses from
receptors to the CNS
Somatic sensory
fiber
Motor (efferent) division
Motor nerve fibers
Conducts impulses from the CNS
to effectors (muscles and glands)
Somatic nervous
system
Somatic motor
(voluntary)
Conducts impulses
from the CNS to
skeletal muscles
Skin
Visceral sensory fiber
Stomach
Skeletal
muscle
Motor fiber of somatic nervous system
Sympathetic division
Mobilizes body
systems during activity
Sympathetic motor fiber of ANS
Structure
Function
Sensory (afferent)
division of PNS
Motor (efferent)
division of PNS
Parasympathetic motor fiber of ANS
Autonomic nervous
system (ANS)
Visceral motor
(involuntary)
Conducts impulses
from the CNS to
cardiac muscles,
smooth muscles,
and glands
Parasympathetic
division
Conserves energy
Promotes housekeeping functions
during rest
Heart
Bladder
Figure 11.2
How it works
Neurons = nerve cells
Long lived, no mitosis,
 Cell body- developed Golgi
 Extensions outside the cell body
Dendrites
Axons
Axonal terminals contain vesicles with
neurotransmitters
 Axonal terminals are separated from by a gap
Synapse

Nerve Coverings

Myelin- Lipid/Protein

Schwann cells

Nodes of Ranvier
Schwann cell
plasma membrane
Schwann cell
cytoplasm
Axon
1
A Schwann cell
envelopes an axon.
Schwann cell
nucleus
2
The Schwann cell then
rotates around the axon,
wrapping its plasma
membrane loosely around
it in successive layers.
Neurilemma
Myelin sheath
(a) Myelination of a nerve
fiber (axon)
3
The Schwann cell
cytoplasm is forced from
between the membranes.
The tight membrane
wrappings surrounding
the axon form the myelin
sheath.
Figure 11.5a
Classification of Neurons

Multipolar neurons

Bipolar

Unipolar
Classification Cont..



Sensory Neurons
 afferent
 most are unipolar
 some are bipolar
Interneurons
 multipolar
 in CNS
Motor Neurons
 multipolar
 carry impulses to effectors, muscle
Table 11.1 (2 of 3)
Neuroglial Cells: Support Cells

Schwann Cells-PNS

Oligodendrocytes-CNS

Microglia-CNS

Astrocytes- CNS

Ependyma-CNS
Regeneration of Injury (if possible)
Principles of Electricity
Opposite charges attract each other
 Energy is required to separate opposite
charges across a membrane
 Energy is liberated when the charges
move toward one another
 If opposite charges are separated, the
system has potential energy

Definitions
Voltage (V): measure of potential energy
generated by separated charge
 Potential difference: voltage measured
between two points
 Current (I): the flow of electrical charge
(ions) between two points

Definitions
Resistance (R): hindrance to charge flow
(provided by the plasma membrane)
 Insulator: substance with high electrical
resistance
 Conductor: substance with low electrical
resistance

Role of Membrane Ion Channels
1. Leakage (nongated) channels—always
open
2. Gated channels (three types):



Chemically gated (ligand-gated)
Voltage-gated channels
Mechanically gated channels
Generating a Nerve Impulse
 polarized
membrane:
inside is
negative
relative to
the outside
under
resting
conditions
 -70 mV
Voltmeter
Plasma
membrane
Ground electrode
outside cell
Microelectrode
inside cell
Axon
Neuron
Figure 11.7
Action Potential (AP)
Brief reversal of membrane potential with
a total amplitude of ~100 mV
 Occurs in muscle cells and axons of
neurons
 Does not decrease in magnitude over
distance
 Principal means of long-distance neural
communication

The big picture
1 Resting state
3 Repolarization
Membrane potential (mV)
2 Depolarization
3
4 Hyperpolarization
2
Action
potential
Threshold
1
4
1
Time (ms)
Figure 11.11 (1 of 5)
Generation of an Action
Potential

Resting state
 Only
leakage channels for Na+ and K+ are
open
 All gated Na+ and K+ channels are closed
Depolarizing Phase
Na+ influx causes more depolarization
 At threshold (–55 to –50 mV) positive
feedback leads to opening of all Na+
channels, and a reversal of membrane
polarity to +30mV (spike of action
potential)

Repolarizing Phase

Repolarizing phase
 Na+
channel slow inactivation gates close
 Membrane permeability to Na+ declines to
resting levels
 Slow voltage-sensitive K+ gates open
 K+ exits the cell and internal negativity is
restored
Hyperpolarization

Hyperpolarization
K+ channels remain open, allowing
excessive K+ efflux
 This causes after-hyperpolarization of the
membrane (undershoot)
 Some
3
2
Action
potential
Na+ permeability
K+ permeability
1
4
1
Relative membrane permeability
Membrane potential (mV)
The AP is caused by permeability changes in
the plasma membrane
Time (ms)
Figure 11.11 (2 of 5)
Voltage
at 0 ms
Recording
electrode
(a) Time = 0 ms. Action
potential has not yet
reached the recording
electrode.
Resting potential
Peak of action potential
Hyperpolarization
Figure 11.12a
Voltage
at 2 ms
(b) Time = 2 ms. Action
potential peak is at the
recording electrode.
Figure 11.12b
Voltage
at 4 ms
(c) Time = 4 ms. Action
potential peak is past
the recording electrode.
Membrane at the
recording electrode is
still hyperpolarized.
PLAY
A&P Flix™: Propagation of an Action Potential
Figure 11.12c
Impulse Conduction
Coding for Stimulus Intensity

All action potentials are alike and are
independent of stimulus intensity
 How
does the CNS tell the difference between
a weak stimulus and a strong one?
Strong stimuli can generate action
potentials more often than weaker stimuli
 The CNS determines stimulus intensity by
the frequency of impulses



Saltatory Conduction
Appear the
jump from
node to node.
Speed of
impulses is
much faster on
myelinated
nerves then
unmyelinated
ones. Speed
also increases
with increase
in diameter.
Ex.) 120m/s
skeletal
muscle .5m/s
skin.
Conduction Velocity
Conduction velocities of neurons vary widely
 Effect of axon diameter
 Effect of myelination

 Myelin
sheaths insulate and prevent leakage of
charge
 Saltatory conduction in myelinated axons is
about 30 times faster
Nerve Fiber Classification

Group A fibers
 Large
diameter, myelinated somatic sensory
and motor fibers

Group B fibers
 Intermediate
diameter, lightly myelinated ANS
fibers

Group C fibers
 Smallest
diameter, unmyelinated ANS fibers
The Synapse
Presynaptic neuron—conducts impulses
toward the synapse
 Postsynaptic neuron—transmits impulses
away from the synapse
 Axodendritic
 Axosomatic
 Some electrical, most chemical
 Cleft = gap

Axodendritic
synapses
Dendrites
Axosomatic
synapses
Cell body
Axoaxonic synapses
(a)
Axon
Axon
Axosomatic
synapses
(b)
Cell body (soma) of
postsynaptic neuron
Figure 11.16
Chemical synapses
transmit signals from
one neuron to another
using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon terminal.
2 Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Mitochondrion
Ca2+
Ca2+
Ca2+
3 Ca2+ entry causes
neurotransmittercontaining synaptic
vesicles to release their
contents by exocytosis.
Axon
terminal
Ca2+
Synaptic
cleft
Synaptic
vesicles
4 Neurotransmitter
diffuses across the synaptic
cleft and binds to specific
receptors on the
postsynaptic membrane.
Postsynaptic
neuron
Ion movement
Enzymatic
degradation
Graded potential
Reuptake
Diffusion away
from synapse
5 Binding of neurotransmitter
opens ion channels, resulting in
graded potentials.
6 Neurotransmitter effects are
terminated by reuptake through
transport proteins, enzymatic
degradation, or diffusion away
from the synapse.
Figure 11.17
Membrane potential (mV)
Threshold
An EPSP is a local
depolarization of the
postsynaptic membrane
that brings the neuron
closer to AP threshold.
Neurotransmitter binding
opens chemically gated
ion channels, allowing
the simultaneous passage of Na+ and K+.
Stimulus
Time (ms)
(a) Excitatory postsynaptic potential (EPSP)
Figure 11.18a
Membrane potential (mV)
Threshold
An IPSP is a local
hyperpolarization of the
postsynaptic membrane
and drives the neuron
away from AP threshold.
Neurotransmitter binding
opens K+ or Cl– channels.
Stimulus
Time (ms)
(b) Inhibitory postsynaptic potential (IPSP)
Figure 11.18b
Integration: Summation
A single EPSP cannot induce an action
potential
 EPSPs can summate to reach threshold
 IPSPs can also summate with EPSPs,
canceling each other out

Neurotransmitters
Most neurons make two or more
neurotransmitters, which are released at
different stimulation frequencies
 50 or more neurotransmitters have been
identified
 Classified by chemical structure and by
function
 Some excite and some inhibit
 Can be nucleotides, gas, protein, amino acid,
lipoprotein

Neurotransmitters
Ion flow blocked
Ions flow
Ligand
Closed ion channel
Open ion channel
(a) Channel-linked receptors open in response to binding
of ligand (ACh in this case).
Figure 11.20a
1 Neurotransmitter
Closed ion
channel
Adenylate cyclase
(1st messenger) binds
and activates receptor.
Open ion
channel
Receptor
G protein
5a
cAMP changes
membrane permeability
by opening or closing ion
channels.
5c cAMP activates
specific genes.
5b
GDP
2 Receptor
activates G
protein.
3 G protein
activates
adenylate
cyclase.
4 Adenylate
cAMP activates
enzymes.
cyclase converts
ATP to cAMP
(2nd messenger).
Nucleus
Active enzyme
(b) G-protein linked receptors cause formation of an intracellular second messenger (cyclic
AMP in this case) that brings about the cell’s response.
Figure 11.17b
Figure 11.22a
Figure 11.22b
Figure 11.22c, d
Clinical Application.
Multiple Sclerosis
Symptoms
• blurred vision
• numb legs or arms
• can lead to paralysis
Treatments
• no cure
• bone marrow transplant
• interferon (anti-viral drug)
• hormones
Causes
• myelin destroyed in
various parts of CNS
• hard scars
(scleroses) form
• nerve impulses
blocked
• muscles do not
receive innervation
• may be related to a
virus
10-29