In: Anatomy and Physiology
DQ1 Describe the events that must occur to generate an AP. Relate the sequence of changes in permeability to the changes in ion Channels, and explain why the AP is an all or none phenomenon .
DQ 2 What constitutes the blood brain barrier?
DQ3. List four ways in which the CNS is protected?
1. Action potential is the mechanism of transmission of impulse through an excitable cell by rapid changes in the resting membrane potential and finally restore it. Phases in action potential generation include depolarization phase and repolarization phase.
Events in action potential
Resting state; At resting membrane potential, all voltage gated ion channels like Na+ and K+ channels are closed. This makes the axon at resting membrane potential of -70 mV.
Depolarization phase; Stimulus cause the axon membrane potential to reach to the threshold potential, -55mV. Then the voltage gated Na+ channels open and influx of Na+ ions into the neuron occur resulting in build up of positive charges inside surface of the membrane and depolarization of the membrane takes place. The membrane potential reach + 30 mV.
Repolarization phase; When the depolarization reach the peak, the Na+ channel closes and voltage gated K+ channels opens leads to outflux of K+ ions ( K+ ions leaves the neurones) causes build up of more negative charge inside neuron and lead to repolarization.
Hyperpolarization phase; K+ channels remains open and more ions will leave the neuron, The neuron become more negatively charged and the membrane potential become -90mV. This is hyperpolarization. The voltage gated K+ channels closes and the resting membrane potential return back to - 70 mV.
All or none phenomenon
The action potential generate in response to threshold stimulus or it does not occur if the stimulus is in sub threshold level. This effect of action potential is all or none phenomenon.
2. Blood brain barrier is the physiological mechanism which limit the passage of substance between circulation and brain. It is a physical barrier between Interstitial fluid and blood around the brain which consist of tight junctions and astrocytes. The tight junction limit the movement of substances. It is permeable to water, respiratory gases, fat soluble molecules like O2, CO2, glucose, alcohol, anesthetic agents and impermeable to Water soluble and potentially harmful substances like proteins and toxins.
3. The central nervous system consist of brain and spinal cord . the CNS is protected by bone, meninges, cerebrospinal fluid and blood brain barrier.
Bone ; Skull and vertebral column protect from external forces and injuries.
Meninges ; It is a three layered connective tissue membrane which cover the CNS structures.The outer most layer is the dura mater which connects with the periosteum of skull. The inner layer is the arachnoid matter and the inner layer is the pia mater which is close to the brain and Spinal cord. It prevent the bacteria and viral infections of CNS.
Cerebrospinal fluid : It is a color less fluid protect the brain from external injury by acting as a shock absorbing medium. It has a function of maintenance of chemical environment of the CNS and exchange of nutrients and waste from the CNS.
Blood brain barrier; BBB protect the CNS from harm full toxins and pathogens with its limited permeability.