Ganong's Review of Medical Physiology, 23rd Edition

120
SECTION II
Physiology of Nerve & Muscle Cells

The EPSP is produced by depolarization of the postsynaptic
cell membrane immediately under the presynaptic ending.
The excitatory transmitter opens Na

• or Ca
  2+
  ion channels in
  the postsynaptic membrane, producing an inward current.
  The area of current flow thus created is so small that it does
  not drain off enough positive charge to depolarize the whole
  membrane. Instead, an EPSP is inscribed. The EPSP due to
  activity in one synaptic knob is small, but the depolarizations
  produced by each of the active knobs summate.
  EPSPs are produced by stimulation of some inputs, but
  stimulation of other inputs produces hyperpolarizing
  responses. Like the EPSPs, they peak 11.5 ms after the stim-
  ulus and decrease exponentially. During this potential, the
  excitability of the neuron to other stimuli is decreased; con-
  sequently, it is called an
  inhibitory postsynaptic potential
  (IPSP)
  (Figure 6–6)
  .
  An IPSP can be produced by a localized increase in Cl

• 
  

transport. When an inhibitory synaptic knob becomes
active, the released transmitter triggers the opening of Cl

• 
  

channels in the area of the postsynaptic cell membrane
under the knob. Cl

• moves down its concentration gradient.
  The net effect is the transfer of negative charge into the cell,
  so that the membrane potential increases.
  The decreased excitability of the nerve cell during the
  IPSP is due to movement of the membrane potential away
  from the firing level. Consequently, more excitatory (depo-
  larizing) activity is necessary to reach the firing level. The
  fact that an IPSP is mediated by Cl


• 
  

  can be demonstrated by
  repeating the stimulus while varying the resting membrane
  potential of the postsynaptic cell. When the membrane
  potential is at E
  Cl
  , the potential disappears (Figure 6–7), and
  at more negative membrane potentials, it becomes positive
  (reversal potential).
  Because IPSPs are net hyperpolarizations, they can be pro-
  duced by alterations in other ion channels in the neuron. For
  example, they can be produced by opening of K

  
  

• channels,
  with movement of K


• 
  

  out of the postsynaptic cell, or by clo-
  sure of Na

  
  


• 
  

  or Ca
  2+
  channels.

  
  

TEMPORAL & SPATIAL SUMMATION

Summation may be temporal or spatial
(Figure 6–8).
Tempo-
ral summation
occurs if repeated afferent stimuli cause new
EPSPs before previous EPSPs have decayed. A longer time
constant for the EPSP allows for a greater opportunity for
summation. When activity is present in more than one synap-
tic knob at the same time,
spatial summation
occurs and ac-
tivity in one synaptic knob summates with activity in another
to approach the firing level. The EPSP is therefore not an all-
or-none response but is proportionate in size to the strength
of the afferent stimulus.
Spatial summation of IPSPs also occurs, as shown by the
increasing size of the response, as the strength of an inhibi-
tory afferent volley is increased. Temporal summation of
IPSPs also occurs.

FIGURE 6–7

IPSP is due to increased Cl influx during

stimulation.

This

can be demonstrated by repeating the stimulus

while varying the resting membrane potential (RMP) of the postsynap-

tic cell. When the membrane potential is at E

Cl

, the potential disap-

pears, and at more negative membrane potentials, it becomes positive

(reversal potential).

FIGURE 6–8

Central neurons integrate a variety of synaptic

inputs through temporal and spatial summation. A)

The time con-

stant of the postsynaptic neuron affects the amplitude of the depolar-

ization caused by consecutive EPSPs produced by a single presynaptic

neuron.

B)

The length constant of a postsynaptic cell affects the ampli-

tude of two EPSPs produced by two presynaptic neurons, A and B.

(From Kandel ER, Schwartz JH, Jessell TM [editors]:

Principles of Neural Science,

4th ed.

McGraw-Hill, 2000.)

5 mV

5 ms

–100 mV

–90 mV EK

–70 mV ECl

–60 mV RMP

–40 mV

Recording

ATemporal summation

Axon

BSpatial summation

Axon

A

Synaptic

current

AAB

Synaptic

potential

Long time

constant

(100 ms)

Short time

constant

(20 ms)

Vm

Vm

Long length

constant

(1 mm)

Short length

constant

(0.33 mm)

Vm

Vm

2 × 10–10 A

2 mV

2 mV

25 ms

A

Recording

A

B