Nervous Tissue

Pathways of sensation and action

Found in the brain, spinal cord and nerves, nervous tissue (or neural tissue) is of enormous importance to the control and coordination of many body systems and functions including:

• Gathering sensory information
• Creating environmental awareness
• Stimulating voluntary and involuntary muscle contraction
• Creating and storing memory
• Regulating emotion
• Driving processes of higher cognition and reasoning

Each of these abilities is made possible by the generation and conducting of electrical nerve impulses which are actively and passively supported by two basic cellular constituents of nervous tissue.

Neurons

Electrically active neurons are primarily responsible for communication that occurs by way of electrochemical signaling among each other and with non-neural target cells. Each elongate neuron is structurally adapted to do this by coordinating three distinct parts of its anatomy:

• The cell body, or soma, forms the control center of the neuron and contains the nucleus where cellular DNA is stored. This is also the part of the neuron where proteins are translated and transported throughout the axonal and dendritic regions of the cell.
• The dendrite is a branched, cytoplasmic extension responsible for receiving impulses from other cells and conveying them to the cell body. Dendritic branching can be highly extensive, sometimes allowing for more than 100,000 input pathways to the cell body of a single neuron.
• The axon is an output structure of the neuron that can range in length from 1mm to a full meter or more (as in the case of the sciatic nerve). The ends of axons can also be highly branched into separate telodendria and synaptic endpoints, allowing for transmission of impulses to the receiving dendrites of many different target neurons at once.

Glial cells

The second basic cellular constituents of nervous tissue are the glial (or neuroglial) cells, primarily responsible for providing neurons with support in several known ways:

• Binding neurons together and insulating them
• Facilitating nutrient delivery to neurons from blood vessels
• Protecting against pathogenic attack via phagocytosis
• Maintaining the extracellular environment around neurons
• Optimizing signal conduction between neurons

Although glial cells were long believed to outnumber neurons by roughly ten to one, more recent research has suggested something much closer to a 1:1 ratio. The emerging role of neuroglia as signal-processing entities in their own right, similar to neurons, is another area of ongoing research.