Cells of the Nervous System
The nervous system is made up of two types of cells: neurons and glia.
Neurons are the fundamental cellular unit of the central nervous system (CNS) and peripheral nervous system (PNS)
Essential features of neurons:
- The purpose of neurons is to receive, process, and transfer information. This information is made up of chemical or electrical signals
- They are unidirectional
- Itís an Ďall-in-oneí transfer of information (see Neuronal Signaling section)
- Mature neurons do not regenerate in the CNS if their soma (cell body) is damaged. However, the dendrites can regenerate, and sometimes the axons can generate also. (This allows other neurons to assume the function of damaged ones.) Regeneration in the PNS is more probable and successful.
- Mature neurons do not proliferate after birth, however, progenitor (stem) cells are found in the brain and can divide after birth. Progenitor cells can become neurons or glial cells.
- Histology is the method by which neurons are studied:
- Analysis of the brain is done by first hardening it (e.g., in a formaldehyde solution), then cutting extremely thin slices
- Nissl Stains color the cell body (soma) purple. This makes it possible to separate the cell bodies of neurons from glial cells, count the number of neurons in a particular area, etc.
- Myelin Stains color myelin sheaths, and therefore make it easy to view axons or axonal connections
- Golgi Stains, or Silver Stains, color the entire neurons silver. However, only 2% of the neurons present will be highlighted in this way
There are 3 main types of neurons:
- Sensory neurons (afferent neurons) transfer information from the external environment to the CNS
- Motor neurons (efferent neurons) transfer information from the CNS to external environment
- Interneurons, or association neurons, process information in the CNS and transfers the information from one neuron to the other within the CNS
Neurons are divided into three parts: dendrites, soma, and axon:
The Ďoutputí of the neuron:
Transfer information to other neurons
Begin at the axon hillock, which is a swelling at the junction of the axon and soma where there are many Na+ channels and the action potential starts (see Neuronal Signaling section)
Are usually long (some reaching several feet)
Axon means axis in Greek
Have terminal boutons at the end of the axon where the synapse is located
- The Ďinputí of the neuron:
- Receive information from other neurons or the external environment and transfer to the cell body (soma) or axons
- Are numerous, relatively short, and branch extensively in a tree-like fashion
- Dendrite means tree in Greek
- Dendrites have numerous spines on them, and provide a greater surface area for other neurons to synapse on, i.e., attach to.
- Dendrites receive information from other cells at these synapses. This makes dendrites postsynaptic.
- The connection between axons that synapse on dendrites is called axodendritic
- The connection between dendrites that synapse on other dendrites is called dendrodendritic
- The cellís body:
- Has typical cell components used for cell maintenance:
- the membrane (a lipid bilayer), which separates and protects the cell from its environment;
- the nucleus (with a large nucleolus), which contains the genetic information of the cell;
- endoplasmic reticulum and ribosomes, where proteins are produced;
- mitochondria, the energy power houses of the cell;
- golgi apparatus, where proteins are packaged in vesicles for secretion outside the cell;
- and other miscellaneous organelles.
- The soma is usually large
- Soma means body in Greek
The space between the terminal boutons and the next cell is known as the synaptic cleft, and is approximately 20 nm thick.
Most are myelinated:
- This makes axons presynaptic
- This swelling at the terminal bouton is where the neuron synapses with another neuron
- Contains numerous vesicles which hold neurotransmitter
- Has many Ca2+ channels in the membrane
- Forms presynaptic membrane in any kind of axonal synapse
Some are not myelinated
Axons synapse on other cells in various forms:
- Have myelin sheaths that are made by Schwann cells or oligodendrocytes (see glia below)
- Myelin acts as insulator to help conduction of action potential
- There are openings between the Schwann cells called Nodes of Ranvier. These help with the conduction of action potentials (see Neuronal Signaling below).
- Axoaxonal: Axon is connected to another neuronís axon
- Axodendritic: Axon is connected to another neuronís dendrites
- Axosomatic: Axon is connected directly to another neuronís soma
- In neuromuscular junctions, axons synapse directly on muscles.
Neurons have different shapes:
- Unipolar: Has only one process extending from the soma, branching into dendrites or axon terminals (typical of invertebrate animals)
- Bipolar: The neuron, it has one input process from dendrites and one output process to dendrites (typical of sensory neurons: visual, auditory, olfactory)
- Multipolar: One axon but many dendrites extending directly from the soma (used for motor and sensory processing). This is the prototypical neuron
- Pseudounipolar: Were originally bipolar, but the dendrites and axon extensions have fused (typical of the dorsal root ganglia in the spinal cord)
Glia, short for neuroglial cells, provide support to the nervous system:
- Their purpose is to provide metabolic support, insulate, protect, reinforce, repair, and cleanup damaged areas
- They probably do not conduct information, though they perhaps offer some integration function of their own, but currently this is in question. As of today we do not yet completely understand their function
- Are much more numerous than neurons. In some places they outnumber neurons 3 to 1. In other places, they are 10 to 50 times more numerous. In total, neurons only form 10% of the nervous system
- They are found in both the CNS and the PNS, but there are different types in each
- Neuroglia means "nerve glue"
- Unlike neurons, glia proliferate (overproduction of glia is what causes brain tumor)
The different types of glia:
- Are star-shaped (hence, "astro")
- Are large
- They may hold the neurons together, since the brain does not have any connective tissue
- They may play a role in maintaining the ion concentration of neurons. An astrocyteís membrane has sodium channels, which may take up sodium when there is too much of it, thereby maintaining a neuronís resting potential
- Make contact with blood vessels
- Contact point is called end-feet
- Transport ions across the vascular wall. This maintains ion concentration around the neurons
- Prevents certain chemicals from leaving the blood and affecting the neurons. This is known as the blood-brain barrier (BBB).
- Are irregular-shaped
- Are small (hence, micro)
- Are mobile
- Are phagocytic, which means they engulf and consume foreign microorganisms and damaged or dead neurons.
- Wraps its membrane around the axons in concentric circles to form myelin, a white and shiny fatty substance, in the CNS. This squeezes out all the cytoplasm from that region, leaving only the lipid-bilayer. This vastly improves electrical conduction in the axon (see Neuronal Signaling section)
- Offer nourishment to the neurons
- Wraps itself around several axons at once. See cross section cutting vertically though three axons and an oligodendrocyte
- Many are needed to cover the whole length of an axon
- Schwann Cells
- Like oligodendrocytes, they form myelin sheaths around axons, but in the PNS.
- A Schwann cell wraps itself around only one axon