Nervous and Endocrine Systems
Neuron - nerve cell specialized for conducting nerve signals and has long cell extensions called fibres (which consist of cytoplasm covered with cell membrane)
Fibres are either: a) Dendrites - which carry signals toward the cell body. Or b) Axons - which carry signals away from the cell body.
Sensory Neurons - carry signals from sensory organs to the central nervous system (spinal cord and brain)
Motor Neurons - carry signals from the central nervous system to the muscles
Interneurons - neurons which transmit signals within the CNS
Some fibres are covered in a nuerilemma, which is a membrane that helps the fibre to repair itself if there is an injury
Many fibres are also covered with a white myelin sheath composed of Schwann Cells (between Schwann Cells are small spaces where the fibre is exposed, called nodes of Ranvier)
When the fibre is not transmitting a signal, it maintains a resting potential of -70 mV; There are more K+ channels and NA+ channels in the membrane which allows more potassium to diffuse out of the cell than sodium to diffuse in. And as potassium diffuses out, a positive charge is transferred to the outside of the cell. The negatively charged particles are trapped inside of the cell and the interior of the cell becomes negatively charged. To prevent the concentrations of NA+ and K+ from equalizing across the membrane, sodium/potassium pumps use active transport to move sodium out and potassium into the cell
If a fibre is stimulated strongly enough to reach a stimulus threshold, an action potent ional occurs in which sodium gates open and sodium ions rush in. Potassium channels close, preventing K+ from diffusing out. Increase in NA+ and K+ ions makes the inside of the fibre more positive - referred to as depolarization. When the cell voltage reaches approx +40 mV, the sodium channels close and the potassium channels open - repolarization. After repolarization, the potassium channels close slowly...