2. peripheral nervous system

C. Functional Organization of the Nervous System

             sensory afferent -- information from PNS -> CNS

          central processing -- interpretation and decision-making in CNS

             motor efferent -- commands from CNS -> PNS to activate effectors

D. Cells of the Nervous System
1. neuroglia
a. several types, many glial cells in nervous system
b. several vital jobs




2. neurons
a. number

b. types

c. general structure
1) cell body
2) processes
a) dendrites
b) axon
1) initial segment of axon
2) axon terminal(s)

d. most important job of nerve cells


II. SIGNALS MADE BY NEURONS
A. Electrical and Chemical


B. Electrical Signals
1. changes in membrane potential
2. involve opening and closing ion channels
3. two categories
1) graded potentials
2) action potentials

B. Graded Potentials
1. graded potentials

2. action potentials (AP)

>> mechanism at resting membrane potential (such as -70mV)

• depolarization to threshold (about -55mV)
• causes opening of voltage-gated Na+ channels
• therefore, some more Na+ enters
• in turn, this causes further depolarization
• opens more Na+ channels, more Na+ enters, more depolarization, etc
• as a result, membrane potential approaches +40mV
• but, suddenly open voltage-gated Na+ channels inactivated and close
• also many more K+ channels open and more K+ leaves
• so ...... membrane potential falls back toward resting potential
• there may be a brief "afterhyperpolarization" before reaching resting potential
  
• ++ all this happens in 1-2 milliseconds ++
  

>> refractory periods

1) absolute refractory period


2) relative refractory period


Feel like jump-starting a neuron? Click here

C. Conduction (Propagation) of Action Potentials Along Axons
1. axon hillock to axon terminal
2. mechanism in non-myelinated axons
action potentials travel down entire length of axon

3. mechanism in myelinated axons
action potentials travel down entire length of axon; seem to jump from node to node ("saltatory conduction")

4. velocity of conduction
a. depends axon diameter and thickness of myelin sheath
b. examples
1) largest diameter, heavily myelinated axons -
2) smallest diameter, non-myelinated axons -

III. SYNAPSES
A. Introduction
1. sooner or later, signal comes to the end of the axon
2. a need to communicate with other cells
a. neuron contacts another cell (neuron, muscle or gland cell)
b. point of contact are synapses

B. Chemical Synapses: Structure and Function
1. major pats
a. presynaptic neuron (axon terminal, vesicles with neurotransmitter molecules)
b. synaptic cleft
c. postsynaptic cell (neurotransmitter receptors, ion channels)

2. how they function

• AP in presynaptic neuron
• depolarization of axon terminal
• exocytosis of neurotransmitter
• diffusion of neurotransmitter across cleft
• binding of neurotransmitter to receptor molecules
• initiates response in postsynaptic cell
• hyperpolarization = inhibitory
• depolarization = excitatory
• removal, destruction of released neurotransmitter

C. Neurotransmitters at Chemical Synapses
1. many different kinds of neurotransmitters
a. ACh

b. monoamines

c. amino acids and peptides

d. NO

2. action of neurotransmitters
a. binds to receptors
b. affect ion channels
1) directly - "ligand-operated"


2) indirectly - "G protein-operated"


3. effect on postsynaptic cell
a. depolarize at an excitatory chemical synapse


b. hyperpolarize at inhibitory chemical synapse



E. Neuronal Integration
1. multiple synapses on postsynaptic neurons
2. summation of synaptic potentials in neurons
adds up active excitatory and inhibitory inputs
3. key decision - is membrane potential at or above threshold
a. if yes, fire action potentials
b. if no, no AP
4. processing of signals as it passes through vast network of neurons

F. Electrical Synapses: Structure and Function
1. cells communicate; not the same as chemical synapse



2. direct connection of cells - structure
a. close point of contact
b. called "gap junctions"
c. protein channels in membranes link up
d. movement from one cell to the next



3. direct connection of cells - function
a. ions can flow between cells
b. change in membrane potential - same in both cells
c. either cell can be sender or receiver
4. locations
a. in nervous system
b. in smooth muscle and *** cardiac muscle***
c. fast communication
c. allows spread of signals - a group of cells act act together in sequence

OBJECTIVES: NEUROPHYSIOLOGY

1) Gain an understanding of the functional organization of the nervous system.
Broadly speaking, what does it do?
What are its major functional parts?
What neurons are associated with these functional parts?

2) Describe the primary parts of neurons and indicate their functional roles.
What happens at dendrites? What happens on axons? What is myelin?
Do all axons have myelin? What are nodes?

3) Explain how the membrane potential of neurons can change.
Describe graded potential and know their characteristics.
What is a depolarization? What is a hyperpolarization?
What is happening in a neuron to produce a change in membrane potential?
In what ways do graded potentials differ from action potentials?
Can graded potentials be used to communicate over long distances in the nervous system?
Why or why not?

4) Describe the processes involved in the production of an action potential.
(Concentrate on what happens to the movement of sodium and potassium ions.)
What are the major events that occur during an action potential?
What makes threshold such a special membrane potential?
What is happening to sodium and potassium ion channels during an action potential?
What happens following a subthreshold depolarization?
What happens following a suprathreshold depolarization?

5) Identify the types of refractory periods and explain what causes them.
When do they occur? How long do they last? How do they effect the
firing of subsequent action potentials?
Why is a larger-than-normal depolarization needed during one type of refractory period to start an action potential?

6) Describe how action potentials are conducted along an axon.
What factors influence the speed of action potential propagation?
Do action potentials change size as they travel along axons?
What is the directions that action potentials follow along axons?

7) Know the structure of electrical and chemical synapses and explain how they work.
For example, describe the components of a chemical synapse and follow the process of synaptic communication at such a synapse.

8) Explain how neurotransmitters influence the response of postsynaptic cells.
Gain an appreciation of how some drug actions are related to
the functioning of chemical synapses.

9) Describe what is meant by "neuronal integration" and how it comes about in the nervous system.
How does summation of excitatory and inhibitory synaptic potentials
influence the membrane potential of postsynaptic neurons?
Is the information that enters the nervous system changed as it passes through synapse after synapse?

10) Know the basics of electrical synapses.
What is the structure of an electrical synapse?
To what does the term "gap junction" refer?
Do ions like Na+ or K+ pass through the proteins linking cells at electrical synapses?
If one cell at an electrical synapses depolarizes, how does the membrane of the other cell change?
If one cell at an electrical synapses hyperpolarizes, how does the membrane of the other cell change?
Why are there no neurotransmitters at electrical synapses?