physiology/pathophysiology of the nervous system system 49.728 physiology/pathophysiology for nurses
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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
Types of Cells:
• Neurons:– Multipolar
– Unipolar
– Bipolar
• Glial Cells:– Schwann cells
– Oligodendrocytes
– Astrocytes
– Fibrocytes
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?
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?
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 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
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?