Nervous System.docx

Nervous system can be split into CNS and PNS CNS is spinal cord and brain. PNS is the nerves and ganglia. Somatic Autonomic Enteric Cells of the nervous system Astrocytes- helps with regulating synapses but don’t send signals themselves. Ependymal cells- production and movement of cerebrospinal f...

Nervous system can be split into CNS and PNS CNS is spinal cord and brain. PNS is the nerves and ganglia. Somatic Autonomic Enteric Cells of the nervous system Astrocytes- helps with regulating synapses but don’t send signals themselves. Ependymal cells- production and movement of cerebrospinal fluid (CSF) Oligodendrocytes- myelination of axons Microglia- ‘brain macrophages’, microbes. Protects brain from pathogens. Neurons- messenger cells, allows communication with rest of the body. Signal comes in via dendrites, travels through the axon and signal goes out via axon terminals. At rest, there is a high concentration of sodium ions outside and high concentration of potassium ions inside therefore a net negative charge inside (-70mV) The different distribution of ions inside and outside the membrane is what causes electrical potential across the membrane. Driving force is determined by BOTH concentration gradient and electrical gradient. Sodium-potassium pumps Maintains concentration gradient. Requires ATP Leak channels Always open (passive movement) Can establish electrical gradient. In neurons- sodium and potassium. Voltage-gated channel Triggered by change in membrane potential. Important for action potential Has different states Sodium voltage gated ion channels have 3 states whereas potassium voltage gated ion channels have 2 states. Trigger event such as voltage, chemical or mechanical. Sodium ions enter the cell. Na+ enters the cell making the membrane more positive (depolarisation). Threshold value (-50mV) has to be reached to trigger a action potential. K+ channels are also activated but slower to open. All or nothing response Nearby voltage gated sodium channels open and the membrane becomes 600x more permeable to Na+. More Na+ enters and make it more positive. Na+ channels quickly close and inactivate. K+ channels open. K+ rapidly leave the cell so the membrane potential then returns to net negative value. K+ channels are too slow to close which causes a ‘overshoot’ therefore the membrane potential goes more negative than usual. Na+ channels reset and ready for next AP. K+ channels closed, resting membrane potential restored by K+ leak and the sodium-potassium pump. Refractory periods 2 forms of refractory period Absolute Relative Repolarisation stage Depolarisation stage No more AP’s possible AP can be possible with a larger stimulus. From threshold until resting potential Sodium channels are completely inactivated. Contiguous conduction Axonal propagation During action potential, there is a certain area where its positive inside compared to the rest. The AP travels towards area of opposite charge and depolarises that portion of the membrane therefore triggering voltage gated sodium channels and influx of sodium ions inside the neurone. It can’t go backwards as the area has just been depolarised and its in refractory period. What effects speed of propagation? Axon diameter- larger diameter reduces resistance so conducts action potentials quicker. Myelination- insulating layer made of Schwann cells around the axon found in patches- helps the AP to ‘jump’ from node to node where there is no myelin sheath- process called saltatory conduction.