4. Overview of Nerve Structure and Function; Measurement Techniques

The human nervous system consists of some 10¹¹ nerve cells, or neurons, each one making an average of over 1000 interconnections. On an individual level we have a reasonable understanding of the functioning of a single nerve cell, but we have precious little knowledge of the larger-scale, or more global functioning, of our nervous system. Three main ways to categorize nerve cells include whether they are part of the central (brain + spinal cord) or peripheral (all else) nervous systems, part of the autonomic (connections with involuntary muscles and internal organs) or somatic (peripheral connections to voluntary muscles and surface sensors) nervous systems, or whether they are afferent (so-called sensory neurons, carrying information from the peripheral to the central nervous system) or efferent (so-called motor neurons, carrying information in the opposite direction). While there are many different types of neurons they all have common features and are believed to function in a very similar manner.

Neurons are single cells with a cell body containing a nucleus and usually a single long thin structure, the axon, which may be more than 1 m in length, with several shorter processes, known as the dendrites, radiating away from the cell body (Figure 18.16). Cell bodies tend to be clustered together in regions connected by bundles of axons. At the far end of the axon are the terminal endings.

Figure 18.16. Structure of the neuron.

Nerve cells conduct an electrical signal called the action potential, or nerve impulse, discussed in detail in the next section. These signals are very similar in all nerves, traveling from the dendritic end to the terminal bundle end at speeds of up to 100 m/s. Usually each neuron is electrically isolated from the next and signals are passed on to the next cell chemically. This occurs through the release of a neurotransmitter from synaptic vesicles at the terminal endings. These chemicals diffuse across the synapse, a small cleft between the terminal endings of one neuron and the dendrites of the next, and are detected by membrane receptors on the dendrites to provoke an electrical response. Receptors are membrane bound proteins that, on binding neurotransmitters either directly (through so-called ligand-gated channels) or indirectly through open ion channels, cause a membrane depolarization and a continuation of the action potential. In certain neurons direct electrical connections between neighboring cells occur via “gap junctions,” pores connecting two neighboring cells that allow the direct passage of very small molecules. These are commonly found in embryo tissue and are believed to provide a means for cell-cell communication in undeveloped tissue. In nerve cells, however, gap junctions do not allow as great a variety of control mechanisms as chemical synapses do, and are therefore relatively rare.

It is useful to describe the overall circuitry involved in a simple reflex response. At a minimum such a response requires 4 cells. The knee jerk reflex is well known as a simple reflex involving a muscle fiber, a receptor transducer cell, a sensory neuron, and a motor neuron. When a doctor taps the patellar tendon near the knee, the attached muscle is stretched. A stretch receptor senses this and produces an electrical response that is carried by an action potential along a sensory neuron to the spinal cord. There a reflex response is generated as an action potential in a motor neuron returning to the same muscle fiber. Arrival of this action potential generates a sequence of chemical steps that result in the contraction of the muscle, and the knee jerk response. A similar sequence of events occurs when you respond to a pin prick on your finger (Figure 18.17). Of course this is a simplistic view, and there are other neural connections that allow control over the sensory and motor signals from the central nervous system as well, but it serves to give a picture of the overall circuitry in a simple reflex.

Figure 18.17 A simple reflex circuit.

Electrical properties of individual neurons can be studied in living tissue using inserted microelectrodes. Most of the early work was done using the giant axon from a squid, a particularly large cell with an axon of about 1 mm in diameter. The electrode is a glass capillary tube containing a conducting salt solution and a metal wire electrode. Electrodes are used both to measure membrane voltages (with the wire inside the tube connected to a sensitive voltmeter) or to inject small amounts of current (with the wire attached to a power supply). Usually the microelectrode is set to zero potential in the extracellular medium and, when inserted through the membrane into the cell, reads the resting membrane potential as a small (0.1 V) negative voltage with respect to the outside. When used to study a nerve impulse, often current is applied through a second electrode as a stimulus and subsequent changes in potential are measured. Alternatively, a constant voltage step change could be applied, fixing the membrane potential, and the changes in current flow across the membrane measured. This method is known as the voltage-clamp technique.

On first thought, one might guess that the membrane could be voltage-clamped by connecting an ideal battery across its thickness. The battery would supply whatever current was needed to offset the membrane currents in order to maintain a fixed membrane potential. This is, however, not quite true since the battery terminals cannot be ‘attached’ to the membrane and there are unpredictable junction potentials at the metal-solution boundary due to the variable contact resistance that would vary with the current flow. Only the metal electrodes would be voltage-clamped, not the membrane itself. Instead, voltage-clamping involves using an electric feedback loop to continually inject small currents in order to maintain a fixed potential.

Figure 18.18 Three types of voltage-clamps. From left to right: gap method with insulating dividers, double electrode method for cells, patch-clamp method for pieces (patches) of membrane.

Figure 18.18 shows three examples of voltage-clamp circuitry using feedback loops. In each method, the membrane potentials are ‘space-clamped’ in such a way as to have no spatial variation of potential. In two of these methods two electrodes are used, with one measuring the potential relative to a reference voltage set at the desired level. This voltage difference signal is then used to inject a current through the second electrode to reduce the difference signal and maintain the voltage clamp. Such a procedure is an example of negative feedback, in which an ‘error signal’ is sent back to the source and used to make small corrections so as to restore a desired value of a variable. The space-clamping is achieved by either using long intracellular electrodes, or by using a small isolated membrane area by either applying insulators in gaps dividing the membrane or by a patch-clamp arrangement. Patch-clamping, developed in 1976, uses a micron diameter pipet tip pressed against an intact cell with some suction applied to form a very tight seal on a microscopic area of membrane so that the resistance between the inside and outside solutions is many GW (1GW = 10⁹ W). Patch clamping has led to a 100-fold increase in the sensitivity of membrane current measurements (as we will see later).