Transcript Nerve Physiology

Nerve physiology
Physiology of Nerves

There are two major regulatory systems in the
body, the nervous system and the endocrine
system.

The endocrine system regulates relatively slow,
long-lived responses

The nervous system regulates fast, short-term
responses
Neuron structure

Neurons all have same basic structure, a cell body
with a number of dendrites and one long axon.
Types of neurons
Divisions of the nervous system
Non-excitable cells of the nervous
system
Structure of gray matter
Signal transmission in neurons
Resting potential
Ionic basis of Em


NaK-ATPase
pumps 3Na+
out for 2 K+
pumped in.
Some of the
K+ leaks back
out, making
the interior of
the cell
negative
Electrochemical Gradients
Figure 12.12
Ion channels




Remember Ohm’s
Law: I=E/R
When a channel
opens, it has a
fixed resistance.
Thus, each channel
has a fixed current.
Using the patchclamp technique,
we can measure
the current
through individual
channels
Gated channels: ligand-gated
Gated channels: voltage-gated
Gated channels: mechanically-gated
Graded potential

A change in potential that decreases with distance

Localized depolarization or hyperpolarization
Graded Potentials
Graded Potentials
Action Potential


Appears when region of excitable membrane
depolarizes to threshold
Steps involved
Membrane depolarization and sodium channel
activation
 Sodium channel inactivation
 Potassium channel activation
 Return to normal permeability

The Generation of an Action
Potential
Figure 2.16.1
Graded potentials
vs
Action Potential
Characteristics of action
potentials



Generation of action potential follows all-ornone principle
Refractory period lasts from time action
potential begins until normal resting
potential returns
Continuous propagation


spread of action potential across entire
membrane in series of small steps
salutatory propagation
The Generation of an Action Potential
Induction of an action potential I
Induction of an action potential II
Voltage-gated
+
Na
channels




These channels have
two voltage sensitive
gates.
At resting Em, one gate
is closed and the other
is open.
When the membrane
becomes depolarized
enough, the second
gate will open.
After a short time, the
second gate will then
shut.
Voltage-gated



Voltage-gated K+
channels have only
one gate.
This gate is also
activated by
depolarization.
However, this gate is
much slower to
respond to the
depolarization.
+
K
channels
Cycling of V-G channels
Action potential propagation


When the V-G Na+
channels open, they
cause a depolarization
of the neighboring
membrane.
This causes the Na+
and K+ channels in
that piece of
membrane to be
activated
AP propagation cont.



The V_G chanels in
the neighboring
membrane then open,
causing that membrane
to depolarize.
That depolarizes the
next piece of
membrane, etc.
It takes a while for the
Na+ channels to
return to their voltagesensitive state. Until
then, they won’t
respond to a second
depolarization.
Propagation of an Action Potential
along an Unmyelinated Axon
Saltatory Propagation along a
Myelinated Axon
Saltatory Propagation along a
Myelinated Axon
Schwann cells cont.

In unmyelinated nerves,
each Schwann cell can
associate with several
axons.

These axons become
embedded in the
Schwann cell, which
provides structural
support and nutrients.
Synaptic transmission
g Aminobutyric Acid

Also know as GABA

Two know receptors for GABA

Both initiate hyperpolarization in the post-synaptic
membrane
GABAA receptor allows an influx of Cl- ions
 GABAB receptors allow an efflux of K+ ions

Transmitter effects on Em

Most chemical stimuli result in an influx of cations


This causes a depolarization of the membrane potential
At least one transmitter opens an anion influx

This results in a hyperpolarization.
EPSPs and IPSPs




If the transmitter opens a cation influx, the
resulting depolarization is called an Excitatory
Post Synaptic Potential (EPSP).
These individual potentials are sub-threshold.
If the transmitter opens an anion influx, the
resulting hyperpolarization is called an Inhibitory
Post Synaptic Potential (IPSP
All these potentials are additive.
Post-synaptic integration
Signal integration
Signal integration cont.
Presynaptic inhibition
Presynaptic facillitation
Neural circuits I
Neural circuits II
Myelination I

In the central nervous
system, myelin is formed
by the oligodendrocytes.

One oligodendrocyte can
contribute to the myelin
sheath of several axons.
Myelination II

In the peripheral nervous
system, myelin is formed
by Schwann cells.

Each Schwann cell
associates with only one
axon, when forming a
myelinated internode.
White and gray matter in the nervous
system
Structure of the spinal cord I

The CNS is made
up not only of the
brain, but also the
spinal cord.

The spinal cord is
a thick, hollow
tube of nerves
that runs down
the back, through
the spine.
Structure of the spinal cord II
Structure of the spinal cord III
Structure of the spinal cord IV