Human Nervous System Lecture 3 PDF

Human Nervous System Lecture 3 Types of Neuroglia (supporting cells): 1.Schwann Cells: Create myelin sheaths around nerves in the peripheral nervous system (PNS). 2.Oligodendrocytes: Create myelin sheaths around nerves in the central nervous system (CNS). 3.Microglia: Help protect nerve ce...

Human Nervous System Lecture 3 Types of Neuroglia (supporting cells): 1.Schwann Cells: Create myelin sheaths around nerves in the peripheral nervous system (PNS). 2.Oligodendrocytes: Create myelin sheaths around nerves in the central nervous system (CNS). 3.Microglia: Help protect nerve cells by removing harmful substances. 4.Astrocytes: These are found all over the brain and do several important functions: Connect to blood vessels and help form the blood-brain barrier, which protects the brain. Surround and support nerve cell connections (synapses) and nerve cell surfaces. Produce substances that nourish nerve cells and help control the levels of ions and neurotransmitters. Have receptors that respond to various neurotransmitters. Glial Cells Neural excitation and conduction: Neural stimulation may be electrical, chemical, or mechanical. Neural impulses are transmitted throughout the nerve (i.e., axon) electrically via ions transport across the cell membrane. The electrical events in neurons are rapid, being measured in milliseconds (ms); and the potential changes are small, being measured in millivolts (mV). Conduction of nerve impulses, although rapid, is much slower than that of electricity. The main moving ions involved in the propagation of nerve impulse (i.e., action potential) are: Sodium (Na+), potassium (K+). Resting membrane potential: The cellular membrane of the nerve cell (and other excitatory cells) is polarized in the resting state due to the energy-based ion transport by Na+/K+ ATPase. Na+/K+ATPase is an active ions transport pump (Need energy in terms of ATP). It transfers 3 Na+ outside the cell and at the same time 2K+ inside the cell. The net result is a continuous accumulation of positive charges outside the cell membrane leaving the inside of the membrane negatively charged. Concentrations of specific ions on the two sides of the axon plasma membrane of a resting neuron (not conducting an impulse) are very different. Concentration of K+ ions is greater inside the cell than outside (Why?). Concentration of Na+ is times greater outside the cell than inside (Why?). The concentrations of chloride ions (Cl-) and calcium ions (Ca2+) are also maintained at greater levels outside the cell. This separation of positive and negative charge is called the resting membrane potential and measured in millivolts (= -70 mV).  Mechanism of membrane action potential: Starting a neuronal electrical signal throughout the nerve cell in response to a stimulus occurs by opening the Na+ channels which allows the rapid influx of Na+ inside the cell (i.e., starting the action potential). This rapid influx of positive charges (in terms of Na+) causes depolarization of the membrane where the potential becomes near zero. The rapid influx of Na+ makes a current that shifts the membrane potential to the opposite sign (+30 mV) where, the Na+ becomes accumulated inside the cell (more positive inside). The rapid depolarization and membrane potential change triggers two events: 1- opening of K+ channels and 2- closing the Na+ channels. Opening K+ channels causes rapid outflux of K+ outside the cell making the membrane becomes negative again (the repolarization process). This rapid outflux makes a current that makes the membrane potential becomes hyperpolarized ( about -100 mV). After that gradually (slower process) the membrane potential becomes -70 mV (resting potential) again by the action of Na+/K+ ATPase.  Na+ channels close and K+ channels open Na+ channels open Na+/K+ ATPase Refractory Period: Time when cell membrane is insensitive to additional stimulation (refractory). Once an action potential has begun, for about 1msec, another action potential cannot be triggered, no matter how large the stimulus. Action potentials cannot overlap because of refractory periods. There are two types of refractory periods: 1. Absolute refractory period: corresponding to the period from the time the firing level is reached until repolarization is about one-third complete (-40 mV). No stimulus no matter how strong can excite the membrane and initiate an action potential. 2. Relative refractory period: lasting from this point to the start of after depolarization. the membrane is able to fire second action potential but with stronger stimulus than normally is required. -40 mV Impulse conduction (propagation of action potentials): Electrical impulses typically travel along neurons at a speed of anywhere from 1 to 120 meters per second The speed of conduction can be influenced by: 1- The diameter of a nerve, 2.Temperature, 3- The presence or absence of myelin. Neurons with myelin conduct impulses much faster than those without myelin. Conduction velocity for non-myelinated nerve = ~1meter/sec (depends upon diameter) Conduction velocity for myelinated nerve = ~100 meters/sec.  The all or none law: (occur as a maximal depolarization if stimulus reaches threshold. or do not occur at all if stimulus is below threshold); ✓ If the stimulus is too low there is no action potential (this is the "none" part). ✓ If the stimulus is above a threshold the action potential is always the same size- it does not get larger for stronger stimuli.