1. The Nervous System: General Principles and Sensory Physiology
Organization of the Nervous System
Basic Functions of Synapses, and Neurotransmitters

2. General design of the nervous system
The central nervous system
Receives information from different sensory nerves and sensory organs then integrates all these to determine response to be made by the body

3. NEURON
Is the basic functional unit
Incoming signals enters neuron through synapses mainly located on the neuronal dendrites & also on the cell body

4. 1.SENSORY PART OF THE CNS Sensory receptors
Most activities of the CNS are initiated by sensory experiences that excite sensory receptors
visual receptors in the eyes
Auditory receptors in the ears
Tactile receptors on the surface of the body
Etc
These sensory experiences can either cause
immediate reactions from the brain
memories of the experiences can be stored in the brain for
minutes, weeks, months, years
determine bodily reactions at some future date

5. Somatosensory axis of the nervous system
Information enters the nervous system through the peripheral nervous
And is conducted immediately to multiple sensory areas in the
Spinal cord at all levels
Reticular substance of the brain
Medulla
pons and
mesencephalon
Cerebellum
Thalamus
Areas of the cerebral cortex

6. 2. Motor part of the central nervous system
Effectors
These are actual anatomical structures that perform the function dictated by nerve signals
Muscles & glands
The most eventual role of the CNS is to control various body activities
This is achieved by controlling
Contraction of appropriate skeletal muscles thru out the body
Contraction of smooth muscles in the internal organs
Secretion of active chemical substances by both exocrine & endocrine glands
These activities are collectively called the motor actions of the nervous system

7. Skeletal motor Axis of the NS
For controlling skeletal muscle contraction
Levels of control includes
The spinal cord
The reticular substance of the medulla,pons and mesencephalon
The basal ganglia
The cerebellum
The motor cortex

8. Skeletal motor Axis of the NS
The lower regions
concerned primarily with automatic instantaneous responses to sensory stimuli
The higher regions
concerned with deliberate complex muscle movements controlled by the thought process of the brain

9. 3. Processing of Information
Integrative Function of the Nervous System
The incoming information is processed in such a way that appropriate mental and motor responses will occur
More than 99% of all sensory information is discarded by the brain as irrelevant and unimportant
Important sensory information that excites the mind is channeled into proper integrative and motor regions of the brain to cause a desired response
Integration is the channeling and processing of information

10. Role of synapses in processing of the information
Is a junction point from one neuron to the next
Determine the directions that the nervous signals will spread through the nervous system
Synaptic transmission
Can also be controlled from other areas of the CNS
Facilitatory- open up synapses for transmission
Inhibitory-closes up the synapses
In addition some postsynaptic neurons respond with
Large numbers of out put impulses
Few numbers of output impulses

11. Roles of synapses Thus synapses can Perform a selective function
Often blocking weak signals while allowing strong signals to pass
Select and amplify weak signals
Channel signals in many directions rather than in only one direction

12. 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

13. Major Levels of CNS Function
1. 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

14. Major Levels of CNS Function
2. 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)

15. Major Levels of CNS Function
3. 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

16. 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

17. 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. smooth
muscle and cardiac muscle contraction)

18. CNS Synapses (cont.) characteristics
“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

19. 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

20. 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

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

22. 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

23. 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

24. 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

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

26. 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

27. 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

28. 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

29. 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

30. 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

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

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

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

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

35. 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)

36. 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)

37. 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

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

39. 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

40. 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

41. 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

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

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

44. 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

45. Special Characteristics of Synaptic Transmission
Fatigue of Synaptic Transmission
Decrease response with increased stimulation
Protection against over-excitation
Exhaustion of neurotransmitter stores
Progressive inactivation of postsynaptic membrane receptors
Slow development of abnormal conc. Of ions inside the post synaptic neuronal cell
Effect of Acidosis or Alkalosis
Alkalosis-increase neuronal excitability
Acidosis –depresses neuronal activity

46. Special Characteristics of Synaptic Transmission
Effect of Hypoxia
Decrease excitability
Brain -unconscious
Effects of Drugs
Increases excitability
Caffeine, theophylline, theobromine
Reduce threshold of excitation
Anaesthetics
Increase neuronal membrane threshold for excitation

47. Special Characteristics of Synaptic Transmission
Synaptic Delay
Time consumed during transmission from presynaptic to post synaptic neuron
Discharge of transmitter substance by the presynaptic terminal
Diffusion of transmitter to the postsynaptic membrane
Action of the NT on the membrane receptor
Action of the receptor to increase the membrane permeability
Inward diffusion of sodium to raise the excitatory postsynaptic potential to high enough level to elicit an action potential