Lloyd D. Partridge

Alteration of Membrane Potential

We began our investigation of cellular electrical properties, in the last chapter, with a look at the resting membrane potential, which is the ultimate power source for nervous system signaling. We will begin this chapter with a discussion of ways in which the membrane potential is altered so that signaling occurs over short distances (i.e., 10s or 100s of µm). In the second part of the chapter, we will again return to signaling based on changes in the membrane potential, but there we will emphasize long-distance signaling (i.e., up to meters).

Convergence of Information

When the magnitude of an external variable is signaled through photons, sound waves, or diffused chemicals, that magnitude must be transduced into a neural signal before the nervous system can act on the information that it represents. Information concerning remote objects is delivered by light as a distribution in space and time of photons that are individually insignificant to the recipient nervous system. Likewise, information delivered as sound is a temporally and spatially distributed pattern of individual molecular impacts that have significance only in the aggregate. Chemical communication is a distribution of specific molecules diffusing in air or water. In these and other inputs, the information delivered to the sensory impulse generators only statistically represents the sensed variable. Thus, for example, the average impact of individual air molecules, which constitutes sound pressure, accumulates at the tympanic membrane before there is any mechanical wave analysis and receptor stimulation. The external quantal elements must be combined as physical signals and then transduced into an equivalent neural impulse signal before they can report the remote condition.

Selectivity by Accessory Structures

We have already considered some examples of the ability of sensory receptors to transduce specific forms of energy. Further differences in receptor responses are determined by accessory structures that selectively affect the energy before it reaches the receptive cells where the transduction takes place (figure 3.1). Accessory structures, such as the optics of the eye and the mechanical elements of the middle ear, attenuate, amplify, or in other ways modify selected parts of the energy before it reaches a specific receptor. This process is often the first step in determining the modality of a sensory system. This chapter will be concerned with these accessory structures.

Chemical Transmitters and Effectors

Until the latter half of the 19th century, it was assumed that neurons formed an interconnected network that was continuous with muscles. The extensive histological work of Santiago Ramon y Cajal before the beginning of the 20th century established that individual neurons were separate, distinct entities. The work that led to this important conceptual shift is summarized in Cajal’s book Neuron Theory or Reticular Theory. If communication did not occur through a syncytium, though, it was apparent that there must be some specialized structures that allow signals to pass between cells. Sir Charles Sherrington, in 1897, called this structure a synapse, deriving the term from the Greek words συv – together and απτeιv – to touch. The basis of synaptic communication forms the major topic of this chapter.

Information Important to Metazoans

The word information is commonly used simply to mean acquired knowledge. A more rigorous use of the term in formal information theory, as was described by Claude Shannon in 1948, includes additional factors such as error-correcting codes. Information relevant to the nervous system falls somewhere between these two and is a representation of objects, events, and conditions in the world at a level comparable to a verbal description.

From Reception to Pattern Recognition and Perception

The reception, encoding, and transmission of information about the internal and external environment lack value unless they leads to a different outcome from the one that would have otherwise occurred. Practically, the utilization of neural information always involves some modifications, even if no more than delay and relocation. In this chapter we will consider the relatively complex combination and processing of input signals that result in new patterns of neuronal activity.

Neural Network Operations

Neuronal networks in the brain produce very complex, multi-component pathways, and only the simplest of these are understood in any detail. Two examples of well-studied networks are the monosynaptic spinal stretch reflex and the tri-synaptic circuit of the hippocampus. These two circuits are sketched in figure 16.1. It is very misleading to define a neuronal network by simply counting synapses. The circuit of a spinal stretch reflex (figure 16.1A) includes afferent fibers from muscle spindle proprioceptors and motor neurons that innervate the muscle fibers of the same muscle. The basic reflex is modulated by inputs from spindle proprioceptors of antagonistic muscles and from tendon proprioceptors of the same muscle. To add to the complexity of the simple circuit shown in this figure, a muscle such as the quadriceps may contain several hundred spindle proprioceptors, each of which makes contact with 100 to 150 motor neurons. The afferent axons from each proprioceptor branch and make only some of the 10,000 individual synapses found on a typical motor neuron. Each of these synapses is capable of producing only a small response in the motor neuron, and 50 to 100 afferent fibers must fire reasonably simultaneously in order to produce an action potential in a motor neuron.

Internal Transmission of Information

Most biological processes require the coordination of activities at multiple locations. Thus, the activity of an individual cell usually occurs in coordination with the activity of other cells and larger assemblages of cells generally require compensation or support from other multicellular structures. This type of interrelated activity requires either direct communication among the participating units or common receipt of related signals from a controlling structure. This chapter is concerned with the variety of means of communication among separate parts of animals.

Generation of the Membrane Potential

Knowledge of the existence of bioelectric potentials must certainly go back to the first unfortunate encounters that people had with electric fish, but an understanding of the origin of these potentials awaited the advances in physical chemistry that occurred near the end of the 19th century.

Sensory Receptor Transduction

Isaac Newton wrote that “We no other way know the extension of bodies than by our senses” (Principles Book III, rule III). This chapter will consider the ways we use our senses to know the extension of bodies and to further determine many other relevant dimensions of the world that we inhabit.