Terminal buttons are structures on the end of the axon, the trailing part of a neuron, that carry signals to neighboring neurons, glands, or muscles. When electrical signals enter a neuron, they travel down the length of the axon, which branches out to create a number of terminal buttons. Small sacs known as vesicles at each button fill with neurotransmitters and burst open when triggered by a signal from the neuron. This releases chemicals that can leap to a neighboring cell to excite or inhibit it, depending on the neurotransmitter involved.
Also known as end bulbs, terminal buttons are a key component of the anatomy of the neuron. In cases where they communicate with other neurons and gland cells, a small space known as the synapse provides room for the chemical signal to travel. At the neuromuscular junction, a neuron fires neurotransmitters across a synapse to a muscle cell, which can trigger a movement. The number of terminal buttons on any given axon can vary, and they may contain large numbers of vesicles to provide ample supplies of neurotransmitters.
Neurons have different functions in the body that can lead to variations in structure. A motor neuron, for example, is involved in the regulation of movement, and thus works differently than a sensory neuron. In all cases, these specialized cells rely on the ability to communicate with extreme rapidity across a synapse. Neurons can fire so quickly that a response appears almost instantaneous, when in fact it may involve a long relay of signals from cell to cell.
For example, when a sensory neuron is stimulated by the experience of pain, it can send a signal along the nerve pathways to the spinal cord, relaying a signal to the brain. The brain can signal motor neurons to tell the body to change position, thereby avoiding the sensation. Time elapsed between an experience like touching a hot pan and jerking back in pain can seem instant, illustrating how quickly the brain can respond to a situation. Large supplies of vesicles at the terminal buttons allow neurons to fire, relax, and fire again very quickly.
Research on the structure of neurons provides important information about what occurs when there are problems with the brain and nervous system. People with demyelinating diseases, for instance, experience a gradual erosion of the protective sheath which covers nerves. This degrades efficiency, making it harder for nerves to send and control signals. As a result, the patient can develop symptoms like weakness and tremors.