21st Century Biology









 

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The Neuron

Function of a Neuron

Neurons, or nerve cells, are the functional unit of the nervous system for transmitting signals through the body. They regulate the contraction of muscles and secretion of hormones.

Organization of a Neuron

Neurons have three main parts.
  1. The cell body (yellow) contains the nucleus, endoplasmic reticulum, mitochondria, nissel bodies etc.
  2. Dendrites (purple) are short extentions of the cell body. They receive information, and transmit it to the cell body. Dendrites can be compared to the hearing part of a telephone-- the dendrite hears a message that is being sent from sensory receptors or another neuron.
  3. The Axon (orange) is a thin tube that can be up to 3 feet long. This part of the neuron takes impulses away from the cell body. Following the same metaphor, the axon is like the telephone wire that transmits the message.
Axons are insulated by the myelin sheath (blue) that are the stacked membranes of Schwann cells. The myelin sheath creates a layer around the axon and helps to maintain the ionic charge of nerve impulses during electrical transmission of information (a good metaphor for this is a coat that keeps you insulated in the winter). The gap that separates the portions of the myelin sheath formed by two adjacent Schwann cells is called the node of ranvier. These nodes are the only regions along the axon where the axonal membrane is in direct contact with extracellular liquid.

Types of Neurons

Neurons can be different in size, shape, and function. There are three major classes of nerve cells, each pertaining to the three functions of the nervous system.

  1. Sensory neurons communicate information about the internal and external environment from sensory receptors to the central nervous system via interneurons. They are the only type of neuron whose dendrites do not receive information from other neurons.
  2. Interneurons combine sensory input with motor output, and can only make connections with other neurons.
  3. Motor neurons bring impulses from the central nervous system to effector cells. Muscle or gland cells that perform the body?s responses to stimuli. They are the only type of neuron that does not communicate with other neurons, and their dendrites are therefore shorter than that of the other two types of neurons.
 

 
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Last Updated: 12/20/2004 HEC