A 56-year-old woman is brought to the university eye center with a complaint of “loss of vision.” Because of visual impairment, she has lost her driver’s license and has fallen several times in her home. Examination reveals that her eyelids close involuntarily with a frequency and duration sufficient to prevent her from seeing her surroundings for more than brief moments at a time. When she holds her eyelids open with her fingers, she can see normally. She has no other muscle dysfunction. A diagnosis of blepharospasm is made. Using a fine needle, several injections of botulinum toxin type A are made in the orbicularis oculi muscle of each eyelid. After observation in the waiting area, she is sent home. Two days later, she reports by telephone that her vision has improved dramatically. How did botulinum toxin improve her vision? How long can her vision be expected to remain normal after this single treatment?
The nervous system is anatomically divided into the central nervous system (CNS; the brain and spinal cord) and the peripheral nervous system (PNS; neuronal tissues outside the CNS). Functionally, the nervous system can be divided into two major subdivisions: autonomic and somatic. The autonomic nervous system (ANS) is largely independent (autonomous) in that its activities are not under direct conscious control. It is concerned primarily with control and integration of visceral functions necessary for life such as cardiac output, blood flow distribution, and digestion. Evidence is accumulating that the ANS, especially the vagus nerve, also influences immune function and some CNS functions such as seizure discharge. Remarkably, some evidence indicates that autonomic nerves can also influence cancer development and progression. The motor portion of the somatic subdivision is largely concerned with consciously controlled functions such as movement, respiration, and posture. Both the autonomic and the somatic systems have important afferent (sensory) inputs that provide information regarding the internal and external environments and modify motor output through reflex arcs of varying complexity.
The nervous system has several properties in common with the endocrine system. These include high-level integration in the brain, the ability to influence processes in distant regions of the body, and extensive use of negative feedback. Both systems use chemicals for the transmission of information. In the nervous system, chemical transmission occurs between nerve cells and between nerve cells and their effector cells. Chemical transmission takes place through the release of small amounts of transmitter substances from the nerve terminals into the synaptic cleft. The transmitter crosses the cleft by diffusion and activates or inhibits the postsynaptic cell by binding to a specialized receptor molecule. In a few cases, retrograde transmission may occur from the postsynaptic cell to the presynaptic neuron terminal and modify its subsequent activity.
By using drugs that mimic or block the actions of chemical transmitters, we can selectively modify many autonomic functions. These functions involve a variety of effector tissues, including cardiac ...