Every neuron is made of a cell body (also called soma), dendrites and an axon. Dendrites and axons are nerve fibers. There are about 86 billion neurons in the human brain, which comprises roughly 10% of all brain cells. Human brain has roughly 16 billion neurons in the cerebral cortex. The neurons are supported by glial cells and astrocytes.
Neurons are connected to one another and tissues. They do not touch each other, instead they form tiny gaps called synapses. These gaps can be chemical synapses or electrical synapses and pass the signal from one neuron to the next.
Types of neurons[change | change source]
By connection[change | change source]
There are three classes of neurons: afferent neurons, efferent neurons, and interneurons.
- Afferent neurons carry information from tissues and organs into the central nervous system.
- Efferent neurons transport signals from the central nervous system to the effector cells.
- Interneurons connect neurons within the central nervous system.
By function[change | change source]
- Sensory neurons carry signals from sense organs to the spinal cord and brain.
- Relay neurons carry messages between sensory or motor neurons and the central nervous system
- Motor neurons carry signals from the CNS to muscles, motor neurons are connected to the relay neurons. The signal passes between the neurons via synapses. Synapses are microscopic voids between cells where chemicals are released from the axon terminal of one cell to specialized chemical receptors on the dendrite of the receiving cell.
Cell division[change | change source]
Mature neurons never divide: that is the general rule. They do not undergo cell division. In most cases, neurons are generated by special types of stem cells. A type of glial cell, called astrocytes, have also been seen to turn into neurons. In humans, neurogenesis (the origin of new nerve cells) largely ceases during adulthood – but in two brain areas, the hippocampus and the olfactory bulb, there is strong evidence for substantial numbers of new neurons.
References[change | change source]
- Fullick, Ann. Edexcel ICSE Biology Revisions Guide1. p. 40. ISBN 9780435046767.
- Wade, Nicholas (1999-10-15). "Brain may grown new cells daily". The New York Times.
- Nowakowski, R.S. (2006). "Stable neuron numbers from cradle to grave". Proceedings of the National Academy of Sciences 103 (33): 12219. doi:10.1073/pnas.0605605103.
- Blinkov S.M. & Glezer I.I. 1968. The human brain in figures and tables: a quantitative handbook. New York, Plenum.
Other websites[change | change source]
- Fibrinogen found to inhibit EGFR in neuronal cells Blood clotting protein may inhibit spinal cord regeneration
- Cell Centered Database UC San Diego images of neurons.
- High Resolution Neuroanatomical Images of Primate and Non-Primate Brains.