Transmission between Neurons

What happens when the action potential reaches the end of the neuron, and the signal must be transmitted to the next cell? In the patellar reflex, signals are relayed from one cell to another at two locations: from the sensory neuron to the motor neuron in the spinal cord, and from the motor neuron to the muscle cells in the quadriceps muscle. The point of contact where signals are transmitted from one neuron to another is called a synapse. In the patellar reflex, both the synapse between the sensory neuron and the motor neuron and the synapse between the motor neuron and the muscle cells are chemical synapses, in which

Sensory neuron

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Muscle cell

Sensory neuron

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Muscle cell

Figure 1-3 Recording the action potential in the nerve fiber of the sensory neuron in the patellar stretch reflex. (a) A diagram of the recording configuration. A tiny microelectrode is inserted into the sensory nerve fiber, and a voltmeter is connected to measure the voltage difference (E) between the inside (a) and the outside (fa) of the nerve fiber. (b) When the microelectrode penetrates the fiber, the resting membrane potential of the nerve fiber is measured. When the sensory neuron is activated by stretching the muscle, an action potential occurs and is recorded as a rapid shift in the recorded membrane potential of the sensory nerve fiber.

an action potential in the input cell (the presynaptic cell) causes it to release a chemical substance, called a neurotransmitter. The molecules of neuro-transmitter then diffuse through the extracellular space and change the membrane potential of the target cell (the postsynaptic cell). The change in membrane potential of the target then affects the firing of action potentials

Figure 1-4 Chemical transmission mediates synaptic communication between cells in the patellar reflex. The flow diagram shows the sequence of events involved in the release of chemical neurotransmitter from the synaptic terminal.

Presynaptic action potential

Depolarization of synaptic terminal

Release of chemical neurotransmitter

Neurotransmitter changes electrical potential of postsynaptic cell by the postsynaptic cell. This sequence of events during synaptic transmission is summarized in Figure 1-4.

Because signaling both within and between cells in the nervous system involves changes in the membrane potential, the brain is essentially an electrochemical organ. Therefore, to understand how the brain functions, we must first understand the electrochemical mechanisms that give rise to a transmembrane voltage in cells. The remaining chapters in Part I are devoted to the task of developing the basic chemical and physical principles required to comprehend how cells communicate in the nervous system. In Part II, we will then consider how these electrochemical principles are exploited in the nervous system for both long-distance communication via action potentials and local communication at synapses.

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