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section 1, chapter 10
Nervous System I
Basic Structure and Function
The nervous system is divided into 2 subdivisions
The Central Nervous System (CNS)
consists of the brain and spinal cord.
The Peripheral Nervous System (PNS)
• Consists of 12 pairs of cranial
nerves and 31 pairs of spinal nerves
• Nerves may be motor (efferent),
sensory (afferent), or both (mixed)
Divisions of the PNS
The Somatic Nervous System is under voluntary control
Somatic motor controls skeletal muscles
Somatic sensory relays info regarding touch, pressure, and
pain to the brain
The Autonomic Nervous System is under involuntary control
autonomic motor controls smooth muscles, cardiac muscles,
and glands
autonomic sensory relays visceral info regarding pH, blood
gasses, etc. to the brain
Figure 10.2. (a) overview of nervous system. CNS is grey, PNS is yellow. (b) CNS
receives sensory input from PNS, and sends motor output to PNS. Somatic division of
PNS is under voluntary control, while the autonomic division is under involuntary control.
The Autonomic Nervous System (ANS) is further
divided into two branches.
The Sympathetic branch
• prepares the body to respond to a stressful situation.
• “Fight or Flight” Response
The Parasympathetic branch
• Maintains normal body activities at rest
• “Resting and Digesting”
Cells of the Nervous System
Neurons
• Integrate, regulate, and coordinate body functions
• Functions
• Receive information - sensory
• Conduct impulses - motor
• Connect neurons - integrative
Neuroglia (glia = “glue”)
• Neuroglia provide neurons with
nutritional, structural, and functional
support
Neurons
Neurons vary in shape and size
3 Components of a neuron
1. Dendrites receive impulse
2. Call body (soma)
3. Axon – transmits the impulse away
from the cell body
Dendrites
Dendrites conduct information to the soma.
A cell may have a few dendrites, many
dendrites, or no dendrites.
Dendritic Spines are additional contact
points on some dendrites that increase the
number of synapses possible by a neuron
Cell Body – Soma
Contains organelles such as the nucleus
Mitochondria, Golgi Apparatus, etc.
The rough ER is often called
chromatophilic substance (Nissl Bodies)
Axon
Axon Hillock is a specialized part of the soma
that connects to the axon.
The axon hillock is often called the Trigger
Zone because action potentials begin here.
Each neuron has only 1 axon, but it may divide
into several branches, called collaterals
The end of the axon is called the axon
terminal and it enlarges into a synaptic
knob (bouton)
Axon
Microtubules called neurofibrils support long axons
and aid in axonal transport (transport of biochemicals
between the soma and the axon terminal)
Myelination of Axons
The myelin sheath is a thick fatty coating of insulation surrounding the
axon that greatly enhances the speed of impulses.
Myelination of axons occurs differently in the PNS than in the CNS.
Myelination of axons in the PNS.
Schwann Cells form the myelin sheath in the PNS.
They wrap around the axons in a jelly-roll fashion to
form a thick layer of fatty insulation.
The cytoplasm and the nucleus are pushed to the
outermost layer, forming the neurolemma
Schwann cells are separated by gaps, called Nodes
of Ranvier.
Schwann cells still surround the axons of
unmyelinated neurons, but they do not form the
myelin sheath.
Unmyelinated axons with a Schwann cell.
A myelinated axon with a Schwann Cell.
Myelination of Axons in CNS
Within the CNS the myelin sheath is formed
by Oligodendrocytes.
1 Oligodendrocyte may form the myelin
sheath of several axons.
Gray and White Matter
A mass of myelinated axons in
the CNS forms the white matter.
A mass of cell bodies (which are
unmyelinated) along with unmyelinated
axons form the gray matter.
end of section 1, chapter 10
ivyanatomy.com
Chapter 10, Section 2
Neurons and Neuroglia
Structural Classification of Neurons
A multipolar neuron contains many dendrites and 1 axon.
• Includes most neurons in the brain and motor neurons
A bipolar neuron contains 1 dendrite and 1 axon
• Includes some sensory neurons such as
photoreceptors and olfactory neurons
A unipolar neuron contains a single process extending from the soma
• Example includes the cells of the dorsal root ganglion
Peripheral Process – conducts information from PNS
Central Process – conducts information to CNS
structural classifications of neurons
Functional Classification of Neurons
An afferent or sensory neuron conducts information from the PNS to CNS
• Dendrites may act as receptors (eyes, ears, touch)
• Most afferent neurons are unipolar, and some are bipolar
An efferent or motor neuron conducts impulses from CNS to PNS
Voluntary Control – in somatic nervous system
Involuntary control – in autonomic nervous system
An interneuron or association neuron is located completely within the CNS.
Interneurons link neurons together in the CNS, and they also connect
sensory neurons to motor neurons.
Functional Classification of Neurons
Figure 10.7. Neurons classified by their functions. Sensory, Motor, and Interneurons.
Neuroglia in the CNS are different from those in the PNS
Neuroglia in CNS
Astrocytes “star-shaped” attach blood vessels
to neurons.
Astrocytes aid in metabolism, strengthen
synapses, and participate in the bloodbrain-barrier
Ependymal cells line the central canal of the
spinal cord and the ventricles of the brain.
Ependyma regulate the composition of
cerebral spinal fluid (CSF)
Neuroglia in CNS
Microglia are normally small cells, but they
enlarge into macrophages during an infection.
•Phagocytize bacteria and cell debris
Oligodendrocytes form the myelin sheath in
the CNS
Figure 10.8. Types of neuroglia in the CNS.
Neuroglia compose half of the brain’s volume
Neuroglia of the PNS
Schwann Cells form the myelin sheath in the PNS.
Satellite Cells support clusters of cell bodies, called ganglia in PNS.
Disorders of Neuroglia
Multiple Sclerosis (MS)
The immune system attacks neurons in the CNS, destroying the
myelin sheath of neurons.
The damaged myelin sheath is replaced with Connective tissue,
leaving behind scars (scleroses)
Scars block the transmission of underlying neurons, so muscles no
longer receive stimulation and begin to whither (atrophy).
End Section 2, Chapter 10
ivyanatomy.com
section 3, chapter 10
The Synapse
And
Membrane Potential
Synaptic Transmission
Synaptic transmission is the mechanism that
transmits a signal from the pre-synaptic
neuron to the post-synaptic neuron.
An action potential causes the release of
neurotransmitters from the presynaptic cell
that diffuse across the synapse and bind to
the postsynaptic cell.
Steps involved in Synaptic Transmission
1. A nerve impulse (action potential) travels
down the axon to the axon terminal.
2. The action potential opens calcium channels
causing calcium to diffuse into the synaptic
knob.
3. The calcium influx triggers the release of
neurotransmitters from synaptic vesicles
into the synapse.
4. The neurotransmitters diffuse across the
synapse and bind to receptors on the postsynaptic cell
Some neurotransmitters are inhibitory
whereas others are excitatory, so the postsynaptic cell may be stimulated or it may be
inhibited depending on the neurotransmitter.
Cell Membrane Potential
The cell membrane is usually polarized (charged)
• Inside the membrane is negatively charged relative to outside the membrane
• Polarization is due to unequal distribution of ions across the membrane
•Polarization is maintained by a series of ion pumps and channels
Factors that maintain the cell membrane potential
1. Sodium/Potassium (Na+/K+) pump
The sodium/potassium pump actively transports
3Na+ out of the cell, and 2K+ into the cell.
• It creates a high extracellular [Na+] and a
high intracellular [K+]
• requires ATP
• The Na+/K+ pump only contributes a small
amount (-5mV) to the membrane potential.
Factors that maintain the cell membrane potential
2. Non-gated potassium channels “K+ leak channels”
• The cell membrane has many K+ leak channels, but
only a few Na+ leak channels
• K+ continually leaks out of the cell, making the inside
of the cell more negative.
Factors that maintain the cell membrane potential
Figure A. The sodium-potassium pumps
transports sodium out of the cell, while
transporting potassium into the cell.
Figure B. Leak channels allow some of the
potassium to leak out of the cell, contributing
to the positively charged extracellular fluid.
Factors that maintain the cell membrane potential
The distribution of ions across the membrane
creates a membrane potential (electrical gradient).
For a neuron at rest the membrane potential is -70mV inside the
cell. This is the Resting Membrane Potential
RMP = -70mV inside the cell.
RMP = -70 mV inside the cell.
Factors that change the cell membrane potential
Gated ion channels open and close in response
to a stimulus.
Gated Ion Channels
1. Mechanically-Gated Channels
• Open or close in response to physical stress.
• Touch, hearing, vibrations, ect.
2. Ligand-Gated Ion Channels
• Open or close in response to a ligand
(neurotransmitter, hormone, or other molecule)
• Includes ACh receptors on motor endplates
3. Voltage-Gated Ion Channels
• Open or close in response to small changes in the
membrane potential (millivolts = mV)
• Voltage-gated Na+ channels open when membrane
potential reaches -55mV.
Gated Ion Channels
Figure 10.15b. Ligand-gated Na+ channels (blue) open in
response to neurotransmitters. Voltage-gated Na+ channels
(pink) open in response to changes in membrane potential.
End of section 3, chapter 10
Chapter 10,
Section 4
Graded and Action
Potentials
Changes in Membrane Potential
Resting Membrane Potential (RMP) for a neuron = -70mV
• Membrane potential of a cell at rest
Environmental stimuli cause changes in membrane potential
by opening gated ion channels
• Ligand-gated ion channels
• Voltage-gated ion channels
• Other-gated ion channels
(respond to mechanical, temperature, or other stimulus)
If membrane potential becomes more negative, it has hyperpolarized
e.g. A membrane potential of -100mV is hyperpolarized
If membrane potential becomes less negative, it has depolarized
e.g. A membrane potential of -60mV is depolarized
Local Potential Changes
Graded Potentials
• Local changes in membrane potential (usually occurs at dendrites)
• Magnitude of response is proportional to stimulus
• Graded potentials summate (add together)
• Graded potentials generate action potentials
If a graded potential reaches threshold stimulus (-55mV),
it results in an action potential
Summation of Graded Potentials
Summation of graded potentials my occur by:
1. Spatial Summation – stimulating multiple dendrites
2. Temporal Summation – Stimulating a dendrite at a high frequency
3. Combined – stimulating multiple dendrites at a high frequency
Graded Potentials are summed together at the Axon Hillock “Trigger Zone”
• If summation of graded potentials reaches threshold stimulus (-55mV), an action
potential is initiated at the axon hillock.
Figure 10.15. (a) a subthreshold depolarization will not result in an action potential. (b)
Summation of graded potentials may reach threshold stimulus, initiating an action potential at the
trigger zone. The action potential begins when voltage-gated Na+ channels open at the trigger
zone.
3 Phases of an Action Potential
1. Depolarization Phase
• Voltage-gated Na+ channels open at
-55mV (threshold stimulus)
• Na+ diffuses into cell
2. Repolarization Phase
• Voltage-gated K+ channels open at
+30mV
• K+ rushes out of the cell repolarizing
the membrane
• Na+ channels close
3. Hyperpolarization Phase
• The slower voltage-gated K+ channels
remain open briefly, resulting in a slight
hyperpolarization (-90mV).
Figure 10.17. An oscilloscope
records and action potential
1
2
3
Action Potential
Figure 10.16(a) At rest, the membrane is polarized (RMP = -70mV).
Sodium is mostly outside the cell and potassium is within the cell.
Figure 10.16(b) When a stimulus reaches threshold stimulus (-55mV),
voltage-gated Na+ channels open. With Na+ channels open, sodium
rapidly diffuses into the cell, depolarizing the membrane up to +30mV.
Action Potential
Figure 10.16(c) When the membrane reaches +30mV, voltage-gated K+ channels
open an quickly repolarize the membrane. Sodium channels also close at this point.
Following an action potential, Na+/K+ pumps work to
actively reestablish the Na+ and K+ concentration gradients.
Action Potential Propagation
Once initiated an action potential is
propagated along the entire axon at
full strength. It does not weaken.
Figure 10.18
An action potential in one region,
depolarizes the adjacent region to
threshold stimulus (-55mV).
Once the adjacent region reaches
threshold stimulus, it triggers another
action potential.
The second action potential causes
depolarization in its adjacent region,
triggering yet another action potential.
This sequence continues all the way to
the end of the axon at full strength.
All-Or-None Response
All-or-none response
• Action potentials occur completely, or they do not occur at all.
• An action potential occurs whenever a stimulus of threshold intensity
or above is applied to a neuron.
• Greater stimulation does not produce a stronger impulse
(although a greater stimulation will produce more impulses per second)
Refractory Period
Refractory Period: For a brief period following an action potential,
a threshold stimulus will not trigger another action potential.
Absolute Refractory Period
• no new action potentials can be produced
• Occurs while the membrane is changing in sodium permeability
• Between the depolarization and repolarization phases
Relative Refractory Period
• Action potential can be generated with a high intensity stimulus
• Occurs while membrane is reestablishing its resting membrane potential
• Lasts from the hyperpolarization phase, until RMP is reestablished
End of Chapter 10, Section 4
ivyanatomy.com
section 5, chapter 10
Impulse Conduction
&
Neurotransmitters
Impulse Conduction
Myelinated axons conduct impulses differently than unmyelinated axons.
Unmeylinated Axon generates a series of action potentials along the
entire axon
The impulse is slow (travels at 1 mile/hour)
Myelinated Axon
Myelin is an electrical insulator and prevents action potentials along
myelinated portions of the axon.
Action potentials are generated only at the Nodes of Ranvier
The impulse travels through the myelinated portions by electrical
conduction.
The impulse is fast (travels at 285 miles/hour!)
Saltatory Conduction – Action potentials appear to jump from node to node
on myelinated axons
Figure 10.19. On a myelinated axon, a nerve
impulse appears to jump from node to node.
Myelinated Vs. Unmeylinated neurons
Myelinated neurons transmit impulses rapidly whereas
unmyelinated neurons transmit impulses slowly.
Example: Think when you cut yourself with a knife.
The sharp instant pain travels on myelinated neurons. Shortly
after, the slow throbbing pain travels on unmyelinated neurons.
Synaptic Transmission
The summary of events leading to the release of neurotransmitters. These
events are also outlined in chapter 10, section 3
Synaptic Transmission
Neurotransmitters diffuse across the synapse and bind to receptors
(ligand-gated ion channels) on postsynaptic dendrites.
The neurotransmitters cause changes in local (graded) membrane
potential on postsynaptic neuron = synaptic potentials
The neurotransmitters may either excite the post-synaptic cell or it may
inhibit the post-synaptic cell.
Synaptic Potentials
• EPSP = Excitatory postsynaptic potential
EPSPs depolarize the local membrane of the postsynaptic neuron
EPSPs increase the likelihood of generating an action potential.
• IPSP = Inhibitory postsynaptic potential
IPSPs hyperpolarize the local membrane of the postsynaptic neuron
IPSPs decrease the likelihood of generating an action potential
Summation of EPSPs and IPSPs
EPSPs and IPSPs are added together in a
process called summation
Summation occurs at axon hillock
The integrated sum of EPSPs and IPSPs
determines if an action potential occurs
If threshold stimulus is reached an action
potential is triggered.
Figure 10.20 The synaptic knobs of many axons
may communicate with the cell body of a neuron.
Neurotransmitters
The nervous system produces at least thirty different types of neurotransmitters.
Examples:
1. Acetylcholine – skeletal muscle contractions
2. Monoamines
• Norepinephrine
- in CNS it creates a sense of well-being
- in PNS it may stimulate or inhibit autonomic nervous system
•
Dopamine
- in CNS it creates a sense of well-being
- Amphetamines increase the levels of norepinephrine and dopamine
3. Amino Acids
• GABA – inhibitory neurotransmitter of the CNS
• Many sedatives and anesthesia enhances GABA secretions
• Schizophrenia is associated with a deficiency of GABA
4. Gases
• Nitric Oxide
• Vasodilation in PNS
Impulse Processing
Nerve impulses are processed by the CNS in a way that reflects
the organization of neurons in the brain and spinal cord.
Neuronal Pools – organized group of interneurons within the CNS.
Pools are organized as neural circuits that perform a common
function, even though they may be in different parts of the CNS
May have either excitatory or inhibitory effects on effectors or other
pools
Neuronal Pools
Convergence – several neurons synapse onto one
post-synaptic neuron
•A neuron may sum impulses from different sources
e.g. Information from various sensory receptors
may converge onto a single processing center
Divergence – impulse spreads from one axon
to several post-synaptic neurons.
• A single neuron may ultimately stimulate
many neurons - Amplifies an impulse
End of chapter 10