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General Physiology
Shaul Hochstein
2
Image of the Brain
The cell membrane:
conductance and capacitance,
with selective ion channels
determining membrane potential
Channels lead to conductance (g = 1/R i.e. 1/Resistance).
Membrane thinness allows a large capacitance (C).
Channel conductance can be changed by applied voltage (e.g. nerve fiber Action
Potentials) or by stretch (e.g. mechanical sense stretch receptors) or by ligands
extracellularly (e.g. chemical sense receptors) or intracellularly (e.g. synapses).
All nervous system – nerve cell – signals are changes in membrane channel
conductance to a specific ion. Membrane potential is set as a compromise
between the Potassium and the Sodium Equilibrium Potentials. Relative ion
conductance determines the relative contributions to the membrane potential.
250
IK
gk=1
INa
I-V Curves
Im
200
gNa=0.3
IK+INa
IK
150
INa
IK+INa
100
EK
ENa
50
0
-150
-100
-50
0
-50
-100
50
100
150
Em
The Stretch Receptor System:
example of sensory neuron,
motor neuron,
and synapse between them
An
invertebrate
stretch
receptor
neuron
An invertebrate stretch receptor neuron.
Besides the receptor neuron fiber (yellow) entering the central nervous system,
there are motor neuron fibers (red) activating the muscle and inhibitory fibers
(blue) determining the “set point” of the muscle stretch receptor.
Sensory nerve cell body is located in dorsal root ganglion (near spinal cord) with its
distal (far) end, located at the muscle spindle, sensitive to stretch. Sensory signal
enters spinal cord where sensory nerve terminals synapse onto dendrites of motor
nerve. Motor nerve fiber sends signal to muscle where its terminals synapse onto –
and activate – the muscle fibers.
The same sensory nerve terminal
also branches to activate inhibitory
interneurons which inhibit the activity
of motor neurons that in turn reduce
the activation of antagonistic
muscles.
A parallel system of Golgi stretch receptors have nerve endings in the tendons.
Their sensory nerve cell bodies are also located in the dorsal root ganglion.
Sensory signals enter the spinal cord and the sensory nerve terminals synapse
onto interneurons that inhibit the dendrites of the agonist motor nerve and activate
the antagonist motor neurons.
A vertebrate stretch receptor system at the specialized intra-fusal muscle fibers.
Besides the receptor neuron Group I and II fibers (green) entering the central
nervous system (“afferent”), there are alpha motor neuron fibers (grey)
activating the (extra-fusal) muscle fibers and gamma fibers (blue) determining
the “set point” of the muscle stretch receptor.
Responses in sensory neurons to stretch – following the generator potential
transduction of the graded receptor potential to a burst of “all-or-none” Action
Potentials. These can extend throughout the extended stretch or be curtailed in
rapidly adapting receptors.