Assafa College Neurophysiology Lecture Notes PDF

Assafa College Department of Physiology NERUROPHYSIOLOGY Dr. Rayan Khalid, MBBS, M.Sc, MD Lecture No (2)  HOMEOSTASIS ▪ Cells, tissues and organs of body are all working for organisms survival. ▪ Need good communication and control. ▪ Neuroendocrin...

Assafa College Department of Physiology NERUROPHYSIOLOGY Dr. Rayan Khalid, MBBS, M.Sc, MD Lecture No (2)  HOMEOSTASIS ▪ Cells, tissues and organs of body are all working for organisms survival. ▪ Need good communication and control. ▪ Neuroendocrine = Nervous System + Endocrine System ▪ Reflex= Receptor > Integration > Effector ▪ Nervous system controls body activities , quicker than endocrine system.  GENERAL FUNCTIONS OF THE NERVOUS SYSTEM Maintain homeostasis by receiving sensory information + coordinating +transmitting the appropriate responses through muscles and glands. Working with the endocrine system to integrate rapid reflex responses with slower hormonal responses Generate complex neural pathways of all higher brain functions: Self awareness Thinking, learning Speech, communication Emotions  NEURON o The structural and functional unit of nervous system. o Generally don’t divide after birth, live up to 100 years. o very high metabolic rate: require glucose, can’t use alternate fuels require lots of O2 – only aerobic metabolism o Can’t survive more than a few minutes without O2 Parts Of Neuron 1. Cell body : does not have centrosome, it cannot undergo division. 2. Dendrite: is the branched process of neuron, transmits impulses towards the nerve cell body. 3. Axon: the longest process of nerve cell, transmits impulses away from the nerve cell body. Up to 3-4 feet long. Classification Of Neuron Depending upon the number of poles: 1. Unipolar neurons 2. Bipolar neurons 3. Multipolar neurons. Depending upon the length of axon: Pyramidal cells: cerebral cortex, apical dendrites, pear shaped. Golgi type I: long axon , run in a major fiber tract. Golgi type II: shorter axons, Found in the interneurons  Depending upon the function: Motor ( efferent) : carry impulses from CNS to peripheral effector organs Sensory (afferent) : carry sensory impulses from periphery to CNS. Interneurons :Link neurons – multipolar - in CNS NEUROGLIA nerve glue, neuron support  NERVES Each nerve is an organ composed mainly of nervous tissue (neurons and neuroglia) & fibrous connective tissue with rich supply of blood vessels. Sheath arranged in pattern similar to that of muscle organs: Endoneurium: around each individual neuron Perineurium : around bundles of neurons (fascicles) Epineurium :around entire nerve  MYELIN SHEATH Thick lipoprotein sheath , insulates nerve fiber, formed by Schwann cells , absent at regular intervals (node of Ranvier). Functions of Myelin Sheath: Faster conduction: salutatory conduction. Insulating capacity: prevents the stimulation of neighboring nerve fibers.  CLASSIFICATION OF NERVE FIBERS ❖Depending upon structure: Myelinated nerve fibers Non-myelinated nerve fibers ❖Depending upon distribution: Somatic : supply the skeletal muscles Visceral (autonomic) nerve fibers: supply the various internal organs of the body.  ❖Depending upon ❖Depending upon diameter and conduction function of impulse (erlanger- gasser classification) Sensory nerve fibers Type A nerve fibers: Motor nerve fibers Type B nerve fibers ❖Depending upon Type C nerve fibers. secretion of neurotransmitter Adrenergic nerve fibers ❖Depending upon origin Cranial nerve fibers Cholinergic nerve fibers Spinal nerve fibers  PROPERTIES OF NERVE FIBERS Excitability: Local potential: subliminal strength stimulus > electronic potential develops (AP does not develop = Non- propagated). Action potential or nerve impulse: stimulus with adequate strength is necessary for producing threshold.  ▪ Conductivity: ▪ Ability of nerve fibers to ▪ In-fatigability transmit the impulse from ▪ Nerve fiber can the area of stimulation to conduct only one the other areas. action potential at a ▪ Adaptation time. At that time, it is completely ▪ When a nerve fiber is refractory and does stimulated continuously, not conduct another depolarization occurs action potential. continuously > inactivates the sodium pump and increases the efflux of potassium ions.  SYNAPSE ▪ Synapse is the junction between two neurons. ▪ A physiological continuity between two nerve cells ▪ Classification of synapse : ▪ A. Anatomical classification: ▪ Depending upon ending of axon : ▪ axoaxonic ▪ axodendritic ▪ axosomatic  B. Functional classification: based on mode of impulse transmission: 1. Electrical synapse: direct exchange of ions between the two neurons through the gap junction. Less synaptic delay , e.g cardiac muscle fibers. 2. Chemical synapse: signals are transmitted by the release of chemical transmitter in synaptic cleft between the two neurons.  NEUROTRANSMITRERS Neurotransmitter is a chemical substance that acts as a mediator for the transmission of nerve impulse from one neuron to another neuron through a synapse. Criteria for neurotransmitter It must be found in a neuron It must be produced by a neuron It must be released by a neuron After release, it must act on a target area and produce some biological effect After the action, it must be inactivated. TRANSPORT AND RELEASE OF NEUROTRANSMITTER  Synapses are divided into two types: Excitatory synapses: which transmit the impulses (excitatory function) Inhibitory synapses: which inhibit the transmission of impulses (inhibitory function). NT is gamma-aminobutyric acid( GABA)  PROPERTIES OF SYNAPSE One way conduction: From presynaptic neuron to postsynaptic neuron. Synaptic delay: time taken for: Release , Passage & Action of neurotransmitter. Normal duration of synaptic delay is 0.3 to 0.5 millisecond. Fatigue: due to the depletion of neurotransmitter substance, acetylcholine. Summation: many presynaptic excitatory terminals are stimulated simultaneously or when single presynaptic terminal is stimulated repeatedly. Convergence: the process by which many presynaptic neurons terminate on a single postsynaptic neuron Divergence: the process by which one presynaptic neuron terminates on many postsynaptic neurons. ▪ Summation Multiple excitatory or inhibitory signals are added to generate a cumulative effect on the postsynaptic neuron at a chemical synapse. The excitatory postsynaptic potential (EPSP) is a potential change in the positive direction due to the opening of the sodium channels.  DEGENERATION OF NERVE FIBERS When a nerve fiber is injured, various changes occur in the nerve fiber and nerve cell body. Causes for Injury Obstruction of blood flow Local injection of toxic substances Crushing of nerve fiber Transection of nerve fiber. ❖Degrees of injury: First degree (neuropraxia) Applying pressure over a nerve for a short period → occlusion of blood flow and hypoxia → Mild demyelination. The function returns within few hours to few weeks Second degree (axonotmesis) Prolonged severe pressure, the endoneurium is intact. Repair and restoration of function take about 18 months. Third degree (neurotmesis) The endoneurium is interrupted, recovery is slow and poor or incomplete.