Physiology/Pathophysiology of the Nervous System System

49.728

Physiology/Pathophysiology

for Nurses

Agenda• Organization of the Nervous System

– Central Nervous System Structure

– Peripheral Nervous System

• Cells of the Nervous System– Neurons

– Glial cells

• Signalling Mechanisms– Equilibrium potentials

– Graded potentials

– Action potentials

Organization of the Nervous System:CNS

• Three divisions of brain:– Forebrain

• cerebral hemispheres

– Midbrain• Corpora quadrigemini, tegmentum, cerebral

peduncles

– Hindbrain• Cerebellum, pons, medulla

• Brainstem:– Midbrain, medulla, pons– Connects cerebrum, cerebeluum, spinal

cord

Organization of the Nervous System:Reticular Activating System

• Key Regulatory Functions:– CV, respiratory systems

– Wakefulness

• Clinical Link:– Disturbances in the RAS are

linked to sleep-wake disturbances Reticular Formation

Ascending Sensory Tracts

Thalamus

Radiation Fibres

Visual Inputs

Peripheral Nervous System

Autonomic, Motor, Sensory Divisions

Sensory

Motor

Sympathetic

Parasympathetic

Parasympathetic

Some Basic Plumbing

Types of Cells:

• Neurons:– Multipolar

– Unipolar

– Bipolar

• Glial Cells:– Schwann cells

– Oligodendrocytes

– Astrocytes

– Fibrocytes

So, what are some roles of glial cells?

Some Neuron Basics:

Soma

Dendrites

Axon

Axon TerminalsInitial Segment

Functional Significance?

Equilibrium Potential

• Membrane Potential at which there is NON NET FLUX of a specific ion

• Equilibrium potential for K+ is close to that of the resting membrane potential

• Why?

Equilibrium PotentialTime = 0

K+

K+

K+

K+K+

K+

- -

---

- - - -

--

-

-

-+

Equilibrium PotentialTime = 1

K+

K+

K+

K+K+

K+

- -

---

- - - -

--

-

-

-+

Equilibrium PotentialTime = 2

K+

K+

K+

K+

K+

- -

---

- - - -

--

-

-

-+

K+

Equilibrium PotentialTime = 4

K+

K+

K+

K+

- -

---

- - - -

--

-

-

-+ K+

K+

At the Equilibrium PotentialNet Flux = 0

• Equilibrium Potential (Ev) is Different for each ion– Why?

• Ev for K+ is close to, but not the same as, the resting membrane potential (Em)

– Why?

We can describe this relationship mathematically via the Nernst Equation:

Ep = 0.058 log [K+]outside

[K+]inside

So what?

Neuronal Signaling:

Graded Potentials and

Action Potential

Graded Potentials

• Occur over most of neuron

• “Graded”• Can summate

+ +++

- -

--

-- -

-

--

- - ---+

-

-

-----

Excitatory Synapse

Inhibitory Synapse

Em

Time

Em

Time

Action Potentials

• Can start only at initial segment– Why?

• “All or None” – not graded

• Do not summate

Action Potentials Star With Current Flow to the Initial Segment

• Excitatory synapse (green) generates positive influx• Positive charge flow throughout soma• Some positive charge reaches initial segment, where

the membrane becomes depolarized

++

+++ +

- -

--

-- -

-

--

- - ---+

-

-

What Occurs at the Initial Segment When Membrane is at Rest?

• EFFLUX (K+) >> INFLUX

K+K+

What Happens at the Initial Segment When Threshold is NOT Reached?

• INFLUX (Na+) > EFFLUX (K+)

Na+

K+K+

What Happens at the Initial Segment When Threshold is Reached?

• INFLUX (Na+) >> EFFLUX (K+)

Na+ Na+ Na+ Na+

K+K+

Ionic Basis for the Action Potential

-70

+25

Mem

bra

ne

Po

ten

tial

(m

V)

Mem

bran

e Perm

eability

Time (ms)

PNa+

PK+

So why does permeability change?

Voltage Gate

Activation Gate

When the membrane is polarized,The voltage gate is closed, and the activation gate is open.

In

Out

So why does permeability change?

Voltage Gate

Activation Gate

When the membrane is depolarized,The voltage gate opens, and Na+ entersdown its chemical gradient.

In

Out

Na+

So why does permeability change?

Voltage Gate

Activation Gate

After a short interval, the activation gate Closes, preventing Na+ entry.

In

Out

So why does permeability change?

Voltage Gate

Activation Gate

When the membrane repolarizes,the volatge gate closes, resetting theactivation gate.

In

Out

Three States of the V-Gated Na+ Channel

Na+

Closed Open Inactivated

What’s Happening During the Action Potential

-70

+25

Mem

bra

ne

Po

ten

tial

(m

V)

Mem

bran

e Perm

eability

Time (ms)

PNa+

What About K+?Works like Na+, only slower

-70

+25

Mem

bra

ne

Po

ten

tial

(m

V)

Mem

bran

e Perm

eability

Time (ms)

PK+

Some Clinical Points Related to the Action Potential

• What is the effect of high extracellular K+?

• Tetrodotoxin is a blocker of v-gated Na+m channels. What is the effect on action potentials? On movement?

• What is the effect of ischemia on neouronal resting membrane potential? On neurotransmitter release?