Glia Cells PDF

Glia Cells Neurons are supported by glial cells ( " glue " ) surround the neurons and hold them in place ◦ manufacture nutrient chemicals that neurons need ◦ absorb toxins and waste materials that would damage or kill neurons ◦ outnumber neurons: 10:1 ◦ guide developing neurons to their destin...

Glia Cells Neurons are supported by glial cells ( " glue " ) surround the neurons and hold them in place ◦ manufacture nutrient chemicals that neurons need ◦ absorb toxins and waste materials that would damage or kill neurons ◦ outnumber neurons: 10:1 ◦ guide developing neurons to their destinations ◦ provide myelin sheaths around axons. ◦ also play a role in responding to nerve activity ◦ modulating communication between nerve cells. ◦ most brain tumors are caused by mutations in glia. Glial Cells ᵒ astrocytes ◦ satellite ᵒ microglia ◦ ependymal ᵒ oligodendrocytes ◦ schwann cell ◦ radial - make contact with both capillaries and neurons in the CNS. - provide nutrients and other substances to neurons, - regulate the concentrations of ions and chemicals in the extracellular fluid - provide structural support for synapses. - form the blood-brain barrier: a structure that blocks entrance of toxic substances into the brain. - become active in response to nerve activity, - transmit calcium waves between astrocytes - modulate the activity of surrounding synapses.  - scavenge and degrade dead cells, protecting the brain from invading microorganisms. ◦ Form myelin sheaths around axons in the CNS. ◦ One axon can be myelinated by several oligodendrocytes; one oligodendrocyte can provide myelin for multiple neurons.  ◦ line fluid-filled ventricles of the brain and the central canal of the spinal cord. ◦ are involved in the production of cerebrospinal fluid (serves as a cushion for the brain), moves the fluid between the spinal cord and the brain ◦ is a component for the choroid plexus. ◦ Radial glia serve as bridges for developing neurons as they migrate to their end destinations.  ◦ provide nutrients and structural support for neurons in the PNS. ◦provides myelin for only one axon as the entire Schwann cell surrounds the axon. **◦ Neurogenesis** -- process by which new neurons are formed in the brain. ◦Discovered in the 1990's ◦ Adult neurogenesis is now accepted to be a normal process that occurs in the healthy brain. **◦ Neuroplasticity**- ability of the brain to change and adapt to new information. Ability of the brain to form new connections and build new pathways between neurons. "Rewiring" of the brain. **The electrical activity of neurons** Neurons do two important things ᵒ generate electricity that creates nerve impulse Nerve activation involves three basic steps 1\. at rest, the neuron has an electrical *resting potential* due to the distribution of positively and negatively charged chemical ions inside and outside the neuron.   2\. when stimulated, a flow of ions in and out through the cell membrane reverses the electrical charge of the resting potential, producing an *action potential,* or nerve impulse. 3\. the original ionic balance is restored, and the neuron is again at rest. - In the resting stage : ᵒ the neuron's sodium and potassium channels are closed ᵒ the concentration of Na^+^ ions is 10X higher outside the neuron than inside - When a neuron is stimulated sufficiently... - The sodium channels open up allowing the Na^+^ inside the cell, creating a state of *deporalization* - The inside becomes positive (+40 mV) in relation to the outside, producing the action potential or nerve impulse, which lasts for a millisecond (1/1,000 of a second). - In a reflex action to restore the resting potential, the cell closes the sodium channels and opens up the k^+^ channel, allowing the ions to flow out of the cell. - Eventually, the excess Na^+^ ions flow out and the K^+^ ions flow in reestablishing the resting potential. - Once an action potential occurs at any point on the membrane, its effects spread to adjacent Na^+^ channels, and the action potential flows down the axon to the axon terminals. - After an impulse pass along a point along the axon, there is a recovery period called the refractory period. - During this period, the membrane is not excitable and cannot discharge another impulse. **The All or None Principle** - Action potentials occur at a uniform and maximum intensity, or they do not occur at all. - The negative potential inside the axon has to be changed from -- 70mV to about + 50 mV (action potential threshold by the influx of Na^+^ into the axon before the nerve impulse can be triggered. - Changes in the negative resting potential that do not reach the \+ 50mV action potential threshold are called ***graded potential.*** **Myelin Sheath** - Whitish fatty insulation layer derived from glial cells during development. - Nerve impulse travels down the axon from node to node when it is myelinated ( *saltatory conduction* ) and in a point-to-point fashion in non-myelinated fibers. - Damage to the myelin coating can lead to jerky and uncoordinated movements and eventually paralysis as what happens with individuals with *multiple sclerosis.* - The person's own immune system attacks the myelin sheath, disrupting the delicate timing of nerve impulses to the muscles.