Nerve 1 PDF

Dr. Neven Makram Aziz  Neuron  Neuron is the structural unit of the nervous system which is composed of billions of nerve cells or neurons. It is one of the excitable tissue that can generate and transmit an action potential.  Neurons are similar to other...

Dr. Neven Makram Aziz  Neuron  Neuron is the structural unit of the nervous system which is composed of billions of nerve cells or neurons. It is one of the excitable tissue that can generate and transmit an action potential.  Neurons are similar to other cells in the body because they contain cytoplasm, nucleus, mitochondria and other organelles that surrounded by a cell membrane.  In addition, neurons carry out basic cellular processes such as protein synthesis (by Nissl granules) and energy production (by mitochondria).  The cell bodies of neurons formed the gray matter of the brain and spinal cord and also grouped together in separate areas in the CNS named nuclei.  However, neurons differ from other cells in the body because they have specialized cell parts called dendrites and axons.  Dendrites and axons are usually called nerve fibers that is formed the white matter of the brain and spinal cord.  Neurons also contain some specialized structures (synapses) and chemicals (neurotransmitters) through which there communicate with each other. Synapse: It is a place of contact (no continuity) between presynaptic and postsynaptic neurons at which information is transmitted from one cell to another. A small space separates the pre and postsynaptic neurons called the synaptic cleft in which a chemical transmitter is released.  Types of synapse: I- According to site: Synapse may be between 1.Two neurons. 2.Neuron and a muscle. 3.Neuron and gland. II- According to site of contact: 1- Axo-dendritic (most common & least excitable). 2- Axo-somatic. 3- Axo-axonic (least common & most excitable).  Electrical synapse Chemical synapse - Rare. - Most CNS synapse are of chemical - Contain gap junctions. type. - Impulse transmission may be bi- - No gap junction. directional. - Transmit information by the released chemical transmitter in one direction.  Properties of Neurons: 1. Excitability (the most excitable tissue). 2. Conductivity: it is the ability of nerve to conduct the nerve impulse which is an active self-propagation process. 3. The nerve never gets fatigued.  Excitability: The ability of any living tissue to respond to a stimulus. It is a property of life.  A stimulus: It is any change in the environment surrounding living tissue that causes it to react.  Types of stimuli: 1- Electrical. 2- chemical. 3- mechanical. 4- thermal.  Factors that affect the degree of response to a particular stimulus: 1- The rate of application: A sudden change is more effective than a slowly occurring one of the same intensity. 2- The strength of the stimulus and duration of application.  This is best illustrated by the strength- duration curve shown in the figure. From the figure we can conclude that: 1- The stronger is the stimulus, the shorter is the time needed to excite and vice versa within limits (minimal for intensity and duration). 2- There is minimal or threshold intensity or rheobase which can stimulate. If the intensity of the stimulus is reduced below (R), it cannot stimulate even if the duration of application is prolonged. The threshold (rheobase) stimulus: it is the minimal intensity of a current of a very long duration (as galvanic current) which can stimulate. Current of lower intensity than the threshold is called subthreshold (subminimal) stimuli.  There are two types of current stimulation:  Galvanic current: of low intensity and long duration.  Faradic current: of high intensity and short duration. 3- The utilization time (U.T): it is the (longest) time needed by the rheobase (threshold) to stimulate. 4-There is a minimal duration (t) required for stimulation. If the duration of application of the stimulus is reduced below (it) it cannot stimulate however high the intensity is.  Significance: it is useful for diathermy i.e. internal heating of tissue by the use of high voltage alternating current (faradic current) for a very short period of time less than (t) during each phase. They cause raid oscillation of ions leading to heating of tissue without stimulation. 5- The chronaxia: it is the time needed by a current of double the rheobase intensity to stimulate.  Significance: it is used to compare excitability of different tissue. The shorter the chronaxia the greater is the excitability. The excitability of the nerve (shortest chronaxia and highest excitability) > skeletal muscle > cardiac muscle > smooth muscle (longest chronaxia and lowest excitability).  The Resting Membrane Potential (RMP):  Under normal conditions, it is the potential difference between inner and outer surface of the membrane. It is equal -70 mV (the negative sign indicates the type of the charge inside the nerve that predominates).  This is measured using microelectrodes and oscilloscope. Causes of RMP; Unequal ions distribution that is caused by: 1- Active Na+/K+ pump: It is an active pump transmit 3 Na+ outside the cell coupled with 2 K+ inside leading to increase the negative charge inside the membrane (positive outside). 2- Selective permeability of the membrane: A- The resting membrane of the nerve is 50- 100 time more permeable to K+ than to Na+, Why?  The membrane Na+ channels are closed under resting conditions, while the K+ channels are open.  The Na+ channels are guarded by Ca+2 from outside that form gates, which repel Na+ and prevent its entry under resting conditions while K+ have no guard.  Na+ moves through the membrane with more difficulty than K+ because the hydration energy for Na+ is greater than that for K+ (The hydration energy of the ion is inversely proportional to its size, and since Na+ ion (atomic number 11) is smaller than K+ ion (atomic number 19) so, the hydrated Na+ ion is thicker than the hydrated K+ ion). B- Non diffusible ions (proteins, sulfate and phosphate) remain inside being greater than the positive ions (they are not neutralized by the +ve ions) so the inside become negative while the outside become positive developing RMP.