Download Unit One: Introduction to Physiology: The Cell and General Physiology

Chapter 45: Organization of the
Nervous System, Basic Functions of
Synapses, and Neurotransmitters
Guyton and Hall, Textbook of Medical Physiology, 12th edition
General Design of the Nervous System
• CNS Neuron: The Basic Functional Unit
Fig. 45.1
General Design of the Nervous System
• Sensory Part of the Nervous System- Sensory
Receptors
Fig. 45.2 Somatosensory axis of
the nervous system
General Design of the Nervous System
• Sensory Part of the Nervous System- Sensory
Receptors
a. Information enters the CNS through peripheral
nerves and is conducted immediately to sensory
areas in
1. The spinal cord at all levels
2. The reticular substance of the medulla, pons,
and mesencephalon
3. Cerebellum
4. Thalamus
5. Areas of the cerebral cortex
General Design of the Nervous System
• Motor Part of the Nervous System- Effectors- most
important role of the nervous system is to control
various bodily activities. This is achieved by
controlling:
a. Contraction of appropriate skeletal muscles
b. Contraction of smooth muscles in internal organs
c. Secretion of chemical substances by exocrine and
endocrine glands
General Design of the Nervous System
• Skeletal Motor Axis
Fig. 45.3 Skeletal motor nerve axis of the nervous system
General Design of the Nervous System
• Skeletal Motor Axis- skeletal muscles can be
controlled from many levels of the CNS
a. The spinal cord
b. The reticular substance of the medulla, pons,
and mesencephalon
c. The basal ganglia
d. Cerebellum
e. Motor cortex
General Design of the Nervous System
• Processing of Information- “Integrative Function
of the Nervous System
a. Channeling and processing of information
b. Approximately 99% of sensory information is
filtered out and considered irrelevant and
unimportant by the nervous system
General Design of the Nervous System
• Role of Synapses in Processing Information
a. Some synapses transmit info from one neuron to
another with ease, and others with difficulty
b. Facilitatory and inhibitory signals from other areas
of the nervous system can control synaptic
transmission
c. Synapses perform a selective action, often blocking
weak signals and allowing strong signals to pass
but sometimes select and amplify certain weak
signals
General Design of the Nervous System
• Storage of Information (Memory)
a. Information stored for future control of motor
activities and for use in the thinking process is
stored in the cerebral cortex
b. Facilitation-each time a synapse transfer info, the
synapses become more and more capable
Major Levels of CNS Function
• Spinal Cord Level
a. A conduit for information to travel from the
periphery of the body to the brain and vice versa
b. Can cause walking movements
c. Withdrawal reflexes
d. Reflexes that stiffen the legs to support the body
against gravity
e. Reflexes that control local blood vessels, G.I.
movements, and urinary excretion
Major Levels of CNS Function
• Lower Brain or Subcortical Level
a. Control of most of the “subconscious” activities
b. Arterial pressure and respiration
c. Control of equilibrium
d. Feeding reflexes
e. Many emotional patterns (anger, excitement,
sexual response, reaction to pain and pleasure)
Major Levels of CNS Function
• Higher Brain or Cortical Level
a. Cerebral cortex is an extremely large memory
storehouse
b. Never functions alone but in association with
lower centers of the nervous system
c. Essential for most thought processes
CNS Synapses
• Types of Synapses
a. Chemical
1. Almost all of the synapses in the CNS
2. First neuron secretes a neurotransmitter
3. Neurotransmitter binds to receptors on the
second neuron (excites, inhibits, or modifies
its sensitivity
CNS Synapses (cont.)
• Types of Synapses
b. Electrical
1. Have direct open fluid channels that conduct
electricity from one cell to the next
2. Have gap junctions which allow the movement
of ions
3. Very few in the CNS but are the predominant
type in the periphery of the body (i.e. skeletal
muscle and smooth muscle contraction)
CNS Synapses (cont.)
• “One-Way Conduction at Chemical Synapses
a. Always transmit signals in one direction (from the
pre-synaptic neuron (releases neurotransmitter) to
the post-synaptic neuron
b. Called the principle of one-way conduction
c. Allows signals to be directed toward specific goals
CNS Synapses (cont.)
• Physiologic Anatomy of the Synapse
Fig. 45.5 Typical anterior motor neuron, showing pre-synaptic
terminals on the neuronal soma and dendrites
CNS Synapses (cont.)
• Physiologic Anatomy of the Synapse
a. Presynaptic terminals may be either stimulatory or
inhibitory
b. (Fig. 45.5) Neurons in other parts of the spinal cord
and brain differ from the anterior motor neuron in:
1.
2.
3.
4.
Size of the cell body
Length, number, and size of the dendrites
Length and size of the axon
The number of presynaptic terminals
CNS Synapses (cont.)
• Presynaptic Terminals
Fig. 45.6 Physiologic anatomy of the synapse
CNS Synapses (cont.)
• Neurotransmitter Release From the Presynaptic Terminal
a. The membrane of the presynaptic terminal contains
large numbers of voltage gated Ca channels
b. When the membrane depolarizes, the channels open and
Ca ions flow into the terminal
c. Quantity of transmitter released is directly related to the
amount of Ca that enters
d. Ca binds with special proteins called release sites which
open and allow the transmitter to diffuse into the
synaptic cleft
CNS Synapses (cont.)
• Action of the Neurotransmitter
a. The postsynaptic membrane contains receptor proteins
that have two components:
1. A binding part that protrudes outward and binds the
neurotransmitter, and
2. An ionophore part that passes through to the interior
of the postsynaptic neuron
3. The ionophore is either an ion channel or a second
messenger activator
CNS Synapses (cont.)
• Ion Channels- two types
a. Cation- most often allow Na ions to pass, but sometimes
K, and Ca also; lined with negative charges which attract
cations but repel anions; opened by excitatory transmitters
b. Anion- when channels are large enough, Cl ions pass
through (cations are hydrated and too large); opened
by inhibitory transmitters
CNS Synapses (cont.)
• Second Messenger Systems
Fig. 45.7 Second messenger systems
CNS Synapses (cont.)
• Second Messenger Systems- the alpha component of the
G protein performs one of four functions:
a. Opening specific ion channels through the postsynaptic membrane
b. Activation of cAMP or cGMP
c. Activation of one or more cellular enzymes
d. Activation of gene transcription
CNS Synapses (cont.)
• Excitatory Receptors in the Postsynaptic Membrane
a. In excitation: the opening of Na channels to allow
large numbers of + electrical charges to flow to the
interior. This raises the membrane potential toward
threshold (most widely used method of excitation)
b. In excitation: depressed conduction through chloride
or potassium channels or both; decreases the diffusion
of Cl to the inside or K to the outside which makes the
membrane potential more positive
c. Metabolic changes to excite cell activity, increase
excitatory receptors or decrease inhibitory receptors
CNS Synapses (cont.)
• Inhibitory Receptors in the Postsynaptic Membrane
a. Opening of chloride channels allowing the rapid influx
of ions which causes the membrane potential to become
more negative, and therefore inhibitory
b. Increase in conductance of potassium ions out of the
neuron allowing positive ions to diffuse to the outside
causing increased negativitiy, and therefore inhibitory
c. Activation of receptor enzymes that inhibit metabolic
functions or increase the number of inhibitory receptors
or decrease the number of excitatory receptors
Types of Neurotransmitters
• Small Molecule, Rapidly Acting Transmitters
Table 45.1
Class I
Acetylcholine
Class II:
The Amines
Class III:
Amino Acids
Norepinephrine
GABA
Epinephrine
Glycine
Dopamine
Glutamate
Serotonin
Aspartate
Histamine
Class IV
Nitric Oxide
Types of Neurotransmitters
• Neuropeptide, Slow Acting Transmitters or Growth Factors
Hypothalamic
Releasing
Hormones
Table 45.2
Pituitary
Peptides
Peptides-Act on
Gut and Brain
Peptides- Act
on Gut and
Brain
From Other
Tissues
Thyrotropin RH
ACTH
Leucine
enkephalin
Insulin
Angiotensin II
Leutinizing
HRH
Betaendorphin
Methionine
enkephalin
Glucagon
Bradykinin
Somatostatin
Alpha-MSH
Substance P
Carnosine
Prolactin
Gastrin
Sleep peptides
LH
CCK
Calcitonin
Thyrotropin
VIP
GH
Nerve growth
factor
Vasopressin
Brain derived
neurotropic
factor
Oxytocin
Neurotensin
Electrical Events During Excitation
• Resting Membrane Potential (-65 mV for a
spinal motor neuron)
Fig. 45.8
Electrical Events During Excitation
• Concentration Difference of Ions
Fig. 45.8
Electrical Events During Excitation
• Uniform Distribution of Electrical Potential
Inside the Soma
• Effect of Synaptic Excitation on the Postsynaptic
Membrane—Excitatory Postsynaptic
Potential
Electrical Events During Excitation
Fig. 45.9 Three states of a
neuron
Electrical Events During Excitation
• Generation of APs in the Initial Segment
a. Axon hillock
b. The membrane has 7x the voltage gated Na
channels as does the membrane of the soma
c. Threshold is about -45 mv (Fig. 45.9)
Electrical Events During Inhibition
• Effect of Inhibitory Synapses on the Postsynaptic
Membrane—Inhibitory Postsynaptic Potential
a. Inhibitory synapses open mostly Cl channels
b. As the chloride ions enter, the membrane
potential becomes more negative (toward -70 mV)
c. Opening K channels allows the positive ions to move
out; with the Cl, this causes a hyperpolarization
d. Causes an IPSP (inhibitory postsynaptic potential)
Electrical Events During Inhibition
• Presynaptic Inhibition
a. Release of an inhibitory substance onto the outside
of the presynaptic nerve fibrils (usually GABA)
b. Opens anion channels, allows Cl to diffuse inward
c. Negative charges cancel much of the excitatory effect
d. Occurs in many sensory pathways
Electrical Events During Inhibition
• Time Course of Postsynaptic Potentials
Fig. 45.10 EPSPs
Electrical Events During Inhibition
• Spatial Summation- stimulation of many
presynaptic terminals; the effects can summate
until neuronal excitation occurs (Fig. 45.10)
• Temporal Summation- successive discharges from
a single presynaptic terminal; if they occur
rapidly enough, they also summate
Electrical Events During Inhibition
• Simultaneous Summation of IPSPs and EPSPsthe two effects either completely or partially
nullify each other
• Facilitation of Neurons
a. Occurs when the summated postsynaptic potential is
excitatory but has not reached the threshold
b. Another excitatory signal can then excite the
membrane quite easily
Electrical Events During Inhibition
• Special Functions of Dendrites for Exciting Neurons
a. Large spatial field of excitation of the dendrites- 80-95%
of all presynaptic terminals of the anterior motor neuron
terminate on dendrites
b. Most dendrites cannot transmit APs but they can
transmit signals by ion conduction of the fluids in
cytoplasm
Electrical Events During Inhibition
• Decrement of Electrotonic Conduction in the DendritesGreater Excitatory or Inhibitory Effect by Synapses
Located Near the Soma
Fig. 45.11
Electrical Events During Inhibition
• Summation of Excitation and Inhibition in Dendrites
Fig. 45.11
Electrical Events During Inhibition
•Relation of State of Excitation of the Neuron to Rate
of Firing- excitatory state is the summated degree
of excitatory drive to the neuron
Fig. 45.12 Response characteristics of different
types of neurons to different levels
of excitatory state
Special Characteristics of Synaptic Transmission
• Fatigue of Synaptic Transmission
• Effect of Acidosis or Alkalosis
• Effect of Hypoxia
• Effects of Drugs
• Synaptic Delay