1. Unit Nine: The Nervous System: A
Unit Nine: The Nervous System: A. General Principles and Sensory Physiology
Chapter 45: Organization of the Nervous System, Basic Functions of Synapses, and Neurotransmitters
Guyton and Hall, Textbook of Medical Physiology, 12th edition

2. General Design of the Nervous System
CNS Neuron: The Basic Functional Unit
Fig. 45.1

3. General Design of the Nervous System
Sensory Part of the Nervous System- Sensory
Receptors
Fig Somatosensory axis of
the nervous system

4. General Design of the Nervous System
Sensory Part of the Nervous System- Sensory
Receptors
Information enters the CNS through peripheral
nerves and is conducted immediately to sensory
areas in
The spinal cord at all levels
The reticular substance of the medulla, pons,
and mesencephalon
Cerebellum
Thalamus
Areas of the cerebral cortex

5. 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:
Contraction of appropriate skeletal muscles
Contraction of smooth muscles in internal organs
Secretion of chemical substances by exocrine and
endocrine glands

6. General Design of the Nervous System
Skeletal Motor Axis
Fig Skeletal motor nerve axis of the nervous system

7. General Design of the Nervous System
Skeletal Motor Axis- skeletal muscles can be
controlled from many levels of the CNS
The spinal cord
The reticular substance of the medulla, pons,
and mesencephalon
The basal ganglia
Cerebellum
Motor cortex

8. General Design of the Nervous System
Processing of Information- “Integrative Function
of the Nervous System
Channeling and processing of information
Approximately 99% of sensory information is
filtered out and considered irrelevant and
unimportant by the nervous system

9. General Design of the Nervous System
Role of Synapses in Processing Information
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
Synapses perform a selective action, often blocking
weak signals and allowing strong signals to pass
but sometimes select and amplify certain weak
signals

10. General Design of the Nervous System
Storage of Information (Memory)
Information stored for future control of motor
activities and for use in the thinking process is
stored in the cerebral cortex
Facilitation-each time a synapse transfer info, the
synapses become more and more capable

11. Major Levels of CNS Function
Spinal Cord Level
A conduit for information to travel from the
periphery of the body to the brain and vice versa
Can cause walking movements
Withdrawal reflexes
Reflexes that stiffen the legs to support the body
against gravity
Reflexes that control local blood vessels, G.I.
movements, and urinary excretion

12. Major Levels of CNS Function
Lower Brain or Subcortical Level
Control of most of the “subconscious” activities
Arterial pressure and respiration
Control of equilibrium
Feeding reflexes
Many emotional patterns (anger, excitement,
sexual response, reaction to pain and pleasure)

13. Major Levels of CNS Function
Higher Brain or Cortical Level
Cerebral cortex is an extremely large memory
storehouse
Never functions alone but in association with
lower centers of the nervous system
c. Essential for most thought processes

14. CNS Synapses
Types of Synapses
Chemical
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

15. CNS Synapses (cont.)
Types of Synapses
b. Electrical
Have direct open fluid channels that conduct
electricity from one cell to the next
Have gap junctions which allow the movement
of ions
Very few in the CNS but are the predominant
type in the periphery of the body (i.e. skeletal
muscle and smooth muscle contraction)

16. CNS Synapses (cont.)
“One-Way Conduction at Chemical Synapses
Always transmit signals in one direction (from the
pre-synaptic neuron (releases neurotransmitter) to
the post-synaptic neuron
Called the principle of one-way conduction
Allows signals to be directed toward specific goals

17. Physiologic Anatomy of the Synapse
CNS Synapses (cont.)
Physiologic Anatomy of the Synapse
Fig Typical anterior motor neuron, showing pre-synaptic
terminals on the neuronal soma and dendrites

18. CNS Synapses (cont.)
Physiologic Anatomy of the Synapse
Presynaptic terminals may be either stimulatory or
inhibitory
(Fig. 45.5) Neurons in other parts of the spinal cord
and brain differ from the anterior motor neuron in:
Size of the cell body
Length, number, and size of the dendrites
Length and size of the axon
The number of presynaptic terminals

19. Presynaptic Terminals
CNS Synapses (cont.)
Presynaptic Terminals
Fig Physiologic anatomy of the synapse

20. CNS Synapses (cont.)
Neurotransmitter Release From the Presynaptic Terminal
The membrane of the presynaptic terminal contains
large numbers of voltage gated Ca channels
When the membrane depolarizes, the channels open and
Ca ions flow into the terminal
Quantity of transmitter released is directly related to the
amount of Ca that enters
Ca binds with special proteins called release sites which
open and allow the transmitter to diffuse into the
synaptic cleft

21. CNS Synapses (cont.)
Action of the Neurotransmitter
The postsynaptic membrane contains receptor proteins
that have two components:
A binding part that protrudes outward and binds the
neurotransmitter, and
An ionophore part that passes through to the interior
of the postsynaptic neuron
The ionophore is either an ion channel or a second
messenger activator

22. CNS Synapses (cont.)
Ion Channels- two types
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
Anion- when channels are large enough, Cl ions pass
through (cations are hydrated and too large); opened
by inhibitory transmitters

23. Second Messenger Systems
CNS Synapses (cont.)
Second Messenger Systems
Fig Second messenger systems

24. CNS Synapses (cont.)
Second Messenger Systems- the alpha component of the
G protein performs one of four functions:
Opening specific ion channels through the post-
synaptic membrane
Activation of cAMP or cGMP
Activation of one or more cellular enzymes
Activation of gene transcription

25. CNS Synapses (cont.)
Excitatory Receptors in the Postsynaptic Membrane
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)
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
Metabolic changes to excite cell activity, increase
excitatory receptors or decrease inhibitory receptors

26. CNS Synapses (cont.)
Inhibitory Receptors in the Postsynaptic Membrane
Opening of chloride channels allowing the rapid influx
of ions which causes the membrane potential to become
more negative, and therefore inhibitory
Increase in conductance of potassium ions out of the
neuron allowing positive ions to diffuse to the outside
causing increased negativitiy, and therefore inhibitory
Activation of receptor enzymes that inhibit metabolic
functions or increase the number of inhibitory receptors
or decrease the number of excitatory receptors

27. Types of Neurotransmitters
Small Molecule, Rapidly Acting Transmitters
Table 45.1
Class I
Class II:
The Amines
Class III:
Amino Acids
Class IV
Acetylcholine
Norepinephrine
GABA
Nitric Oxide
Epinephrine
Glycine
Dopamine
Glutamate
Serotonin
Aspartate
Histamine

28. Types of Neurotransmitters
Neuropeptide, Slow Acting Transmitters or Growth Factors
Hypothalamic
Releasing
Hormones
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
Beta-endorphin
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
Table 45.2

29. Electrical Events During Excitation
Resting Membrane Potential (-65 mV for a
spinal motor neuron)
Fig. 45.8

30. Electrical Events During Excitation
Concentration Difference of Ions
Fig. 45.8

31. Electrical Events During Excitation
Uniform Distribution of Electrical Potential
Inside the Soma
Effect of Synaptic Excitation on the Postsynaptic
Membrane—Excitatory Postsynaptic
Potential

32. Electrical Events During Excitation
Fig Three states of a
neuron

33. Electrical Events During Excitation
Generation of APs in the Initial Segment
Axon hillock
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)

34. Electrical Events During Inhibition
Effect of Inhibitory Synapses on the Postsynaptic
Membrane—Inhibitory Postsynaptic Potential
Inhibitory synapses open mostly Cl channels
As the chloride ions enter, the membrane
potential becomes more negative (toward -70 mV)
Opening K channels allows the positive ions to move
out; with the Cl, this causes a hyperpolarization
d. Causes an IPSP (inhibitory postsynaptic potential)

35. Electrical Events During Inhibition
Presynaptic Inhibition
Release of an inhibitory substance onto the outside
of the presynaptic nerve fibrils (usually GABA)
Opens anion channels, allows Cl to diffuse inward
Negative charges cancel much of the excitatory effect
Occurs in many sensory pathways

36. Electrical Events During Inhibition
Time Course of Postsynaptic Potentials
Fig EPSPs

37. Electrical Events During Inhibition
Spatial Summation- stimulation of many
presynaptic terminals; the effects can summate
until neuronal excitation occurs (Fig )
Temporal Summation- successive discharges from
a single presynaptic terminal; if they occur
rapidly enough, they also summate

38. Electrical Events During Inhibition
Simultaneous Summation of IPSPs and EPSPs-
the two effects either completely or partially
nullify each other
Facilitation of Neurons
Occurs when the summated postsynaptic potential is
excitatory but has not reached the threshold
Another excitatory signal can then excite the
membrane quite easily

39. Electrical Events During Inhibition
Special Functions of Dendrites for Exciting Neurons
Large spatial field of excitation of the dendrites %
of all presynaptic terminals of the anterior motor neuron
terminate on dendrites
Most dendrites cannot transmit APs but they can
transmit signals by ion conduction of the fluids in
cytoplasm

40. Electrical Events During Inhibition
Decrement of Electrotonic Conduction in the Dendrites-
Greater Excitatory or Inhibitory Effect by Synapses
Located Near the Soma
Fig

41. Electrical Events During Inhibition
Summation of Excitation and Inhibition in Dendrites
Fig

42. 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 Response characteristics of different
types of neurons to different levels
of excitatory state

43. Special Characteristics of Synaptic Transmission
Fatigue of Synaptic Transmission
Effect of Acidosis or Alkalosis
Effect of Hypoxia
Effects of Drugs
Synaptic Delay