Types of Neurons (Nerve Cells) Cells of the nervous system, called neurons, are specialized to carry electrochemical impulses called action potentials or nerve impulses. The human brain alone has about 100 billion neurons. Neurons vary in size and shape – The smallest have cell bodies only 4 microns wide, – The biggest have cell bodies 100 microns wide

How Neurons are similar to other cells: Surrounded by a cell membrane. Have a nucleus that contains genes. Contain cytoplasm, mitochondria and other "organelles". Carry out basic cellular processes like protein synthesis and energy production.

How Neurons Differ From Other Cells: Neurons have specialized extensions called dendrites and axons. – Dendrites bring information to the cell body and axons take information away from the cell body. Neurons communicate with each other through an electrochemical process. Neurons contain some specialized structures (for example, synapses) and chemicals (for example, neurotransmitters).

Neurons are the oldest and longest cells in the body. You have many of the same neurons for your whole life.

Structure of Neurons Structurally consist of 4 parts: – Dendrites – Cell body (soma) – Axon – Axon terminal

Dendrites Project from soma Pick up information from surrounding environment and transmit it (carry it) to soma.

Soma Contains the following organelles: Nucleus Nucleolus Ribosomes Endoplasmic reticulum (SER and RER) Golgi Apparatus Microfilaments/Neurotubules Mitochondria Cytoplasm

Axons Extension of soma Transmit impulses away from soma towards axon terminal

Axon Terminals Receive impulse from axon and transmits it to surrounding neurons Terminals are called synaptic knobs

Differences Between Axons and Dendrites Axons – Take information away from the cell body – Smooth Surface – Generally only 1 axon per cell – No ribosomes – Can have myelin – Branch further from the cell body

Axons grow out of the cell body – There is a steady transport of cell components (e.g., vesicles, mitochondria) from the cell body along the entire length of the axon. – This flow is driven by specialized molecular motor proteins moving along the microtubules in the cytoplasm within the axon. – It may take 2 weeks or longer for material synthesized in the cell body to reach the axon terminals in your big toe.

Dendrites – Bring information to the cell body – Rough Surface (dendritic spines) – Usually many dendrites per cell – Have ribosomes – No myelin insulation – Branch near the cell body

The Neuron

Classification of neurons by the function Sensory neurons Motor neurons Interneurons

Sensory (afferent) neurons Send information to the CNS In skin, muscles, joints, and organs – Sensitive to pressure, temperature, and pain. In the nose and tongue – sensitive to the molecular shapes we perceive as tastes and smells. In the inner ear – sensitive to vibration In the eyes – rods and cones of the retina are sensitive to light

Sensory Neurons

Motor or efferent neurons Carries information from spinal cord Stimulate muscle cells throughout the body

Interneurons Provide connections between sensory and motor neurons, as well as between themselves. The neurons of the central nervous system, including the brain, are all interneurons.

Interneuron/Association Neuron Interneurons: send information between sensory neurons and motor neurons. Most interneurons are located in the central nervous system.

Interneuron

Glial or Neuroglial Cells Special types of connective tissue cells that help support and protect neurons. Types of glial cells in CNS 1.Astroycytes : Form the blood-brain barrier 2.Oligodendrites: Form the myelin sheath around axons of the CNS. Help to hold nerve fibers together

3.Microglia: Phagocyte cells that migrate through the CNS removing foreign matter and degenerated brain tissue 4.Ependymal cells: Epithelial cells that line the brain and central canal of the spinal cord and form cerebrospinal fluid and aid in its circulation.

Types of glial cells in PNS 1.Schwann cells: Form myelin sheaths around peripheral axons and are composed of: 1.Myelin: A white fatty substance that insulates axons. Forms myelin sheath around some axons in the PNS 2.Nodes of Ranvier: Nonmyelinated gaps on axons that lie between adjacent Schwann cells. 3.Neurilemma: Outer membrane of a Schwann cell. Plays an important role in the regeneration of cut or injured axons.

2.Satellite cells – Support neurons in the PNS ganglia.

A clump of neuron cell bodies, is called a ganglion (plural: ganglia) or a nucleus (plural: nuclei). A fiber made up of many axons is called a nerve. In the brain and spinal cord – areas that are mostly axons are called white matter – Areas that include large number of cell bodies are called gray matter.