General Physiology

Shaul Hochstein

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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).

ENaEK

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.

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.