NEURONS
The nervous system is built upon a scaffold of neurons. Neurons are extremely specialized cells responsible for signal transduction and communication of the nervous system. They are unique in having long extensions called axons that transmit electrical signals and branching dendrites that receive signals from synapses that were established from adjacent neurons.
They vary in shape, size, and function. The complex structure of the neurons allows complex communications within the nervous system. Information transfer occurs at specialized junctions referred to as synapses, where the neurotransmitters play a role in bridging the gap between the neurons enabling vast possibilities of signaling (Figure 8.2).The neuronal cell body, also known as the soma or perikaryon, is the metabolic and structural centre of the neuron involved in integrating incoming signals from dendrites, processing the information, and generating outgoing signals along the axon. It consists of a nucleus that houses the cell’s genetic material (DNA) and controls cellular activities by regulating gene expression. Organelles such as the endoplasmic reticulum, Golgi apparatus, mitochondria, and ribosomes, which are required for protein synthesis, metabolism, and cellular maintenance, are found in the cytoplasm. Along with them, Nissl bodies, also known as Nissl substances or Nissl granules, which are clusters of rough endoplasmic reticulum and ribosomes, are also found in the neuronal cell body. They are involved in protein synthesis, particularly the production of neurotransmitters and other proteins needed for neuronal function. A continuation of the body is a specialized region called the axon hillock, where the axon originates. It serves as the initial segment of the axon (a long, slender projection that conducts electrical impulses away from the cell body and toward other neurons, muscles, or glands) and features a high density of voltage-gated ion channels, which are essential for generating action potentials. These channels allow sodium ions to enter the neuron in response to depolarization, triggering the initiation of an action potential traveling at a speed of 0.5-120 m/s along the axon.
Unlike the neuronal cell body, the axon hillock lacks Nissl bodies, contributing to the rapid conduction of action potentials along the axon. The axon hillock is also responsible for the integration of excitatory and inhibitory signals from dendrites and other synaptic inputs to initiate an action potential if the threshold for excitation is reached. Furthermore, the cell body has branching extensions that receive incoming signals from other neurons or sensory receptors, called dendrites. Dendrites have receptive surfaces that are covered with numerous small protrusions called dendritic spines, which provide surface area for synaptic connections with other neurons. These spines contain receptors for neurotransmitters released by axon terminals of neighbouring neurons. These chemical signals can be either excitatory, depolarizing the dendrite, or inhibitory, hyperpolarizing the dendrite. However, the dendrites integrate these signals from multiple synaptic inputs, summing up excitatory and inhibitory signals to determine whether the neuron will generate an action potential at the axon hillock. Though dendrites do not typically generate action potentials themselves, they propagate electrical signals, called graded potentials, toward the cell body.8.3