Electrical Properties Of Nerve Membrane

The basic structure of the unit membrane (UM) covering all nerve cells is the ubiquitous lipid bilayer, found on cells everywhere in the animal kingdom. The lipid bilayer is about 7.5 to 10 nm thick (how thick it appears under the transmission electron microscope often depends on the fixation techniques used to prepare the specimen). The lipid bilayer is composed of facing ordered arrays of phospholipid + cholesterol molecules. The fatty acid radicals on the molecules are hydrophobic and face each other to form the center of the membrane. The outer surfaces of the membrane are hydrophilic (attract water molecules) and contain the phosphate radicals of the phospholipid molecules. The lipid bilayer of the UM is a fluid, not a solid, because the UM can flow, much like the surface of a soap bubble, to adjust to internal and external forces on it. It can even reseal itself if slightly damaged.

Diagram The Myotatic Spinal Reflex

FIGURE 1.1-4 A schematic circuit diagram of a very simplified spinal myotatic reflex system. There are many parallel motoneurons, spindles and GTOs. Inhibition is represented by black ball synapses, excitation by arrows. aF is an a-motoneuron going to the flexor muscle. Rf is a Renshaw inhibitory interneuron providing a local negative feedback loop around the motoneuron. Renshaw cells themselves have excitatory and inhibitory "gain control" inputs. Neurons labeled IbF and I^ are inhibitory interneurons. When the flexor muscle is stretched passively, the spindle output is active, causing a spinal reflex motor activation of the flexor, resisting the stretch, and a reflex inhibition of the a-motoneurons serving the extensor muscle. Increasing force on the flexor is sensed by the GTO, which inhibits the flexor a-motoneuron.

FIGURE 1.1-4 A schematic circuit diagram of a very simplified spinal myotatic reflex system. There are many parallel motoneurons, spindles and GTOs. Inhibition is represented by black ball synapses, excitation by arrows. aF is an a-motoneuron going to the flexor muscle. Rf is a Renshaw inhibitory interneuron providing a local negative feedback loop around the motoneuron. Renshaw cells themselves have excitatory and inhibitory "gain control" inputs. Neurons labeled IbF and I^ are inhibitory interneurons. When the flexor muscle is stretched passively, the spindle output is active, causing a spinal reflex motor activation of the flexor, resisting the stretch, and a reflex inhibition of the a-motoneurons serving the extensor muscle. Increasing force on the flexor is sensed by the GTO, which inhibits the flexor a-motoneuron.

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