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Nerve Cell Physiology by Dr.Rafea Al-Fayyadh Al-fallujah Medicine College 2016 رافع عاوي الفياض.د 1 The Cells of the Nervous System • • The human nervous system is comprised of two kinds of cells: • Neurons • Glia The human brain contains approximately 100 billion individual neurons. • Behavior depends upon the communication between neurons. رافع عاوي الفياض.د 2 The Cells of the Nervous System • Like other cells in the body, neurons contain the following structures: 1. Membrane 2. Nucleus 3. Mitochondria 4. Ribosomes 5. Endoplasmic reticulum رافع عاوي الفياض.د 3 The Cells of the Nervous System • The membrane refers to the structure that separates the inside of the cell from the outside environment. • The nucleus refers to the structure that contains the chromosomes. • The mitochondria are the structures that perform metabolic activities and provides energy that the cells requires. • Ribosomes are the sites at which the cell synthesizes new protein molecules رافع عاوي الفياض.د 4 Embedded in the membrane are protein channels that permit certain ions to cross through the membrane at a controlled rate. رافع عاوي الفياض.د 5 The Cells of the Nervous System • Neuron cells are similar to other cells of the body but have a distinctive shape. • A motor neuron has its soma in the spinal cord and receives excitation from other neurons and conducts impulses along it axon to a muscle. • A sensory neuron is specialized at one end to be highly sensitive to a particular type of stimulation (touch, temperature, odor etc.) رافع عاوي الفياض.د 6 The components of motor neuron. رافع عاوي الفياض.د 7 The components of sensory neuron Note that the soma is located الفياض on a stalk off the main trunk of the axon رافع عاوي.د 8 The Cells of the Nervous System • All neurons have the following major components: 1. Dendrites. 2. Soma/ cell body. 3. Axon. 4. Presynaptic terminals. رافع عاوي الفياض.د 9 The Cells of the Nervous System • Dendrites: branching fibers with a surface lined with synaptic receptors responsible for bringing in information from other neurons. رافع عاوي الفياض.د 10 The Cells of the Nervous System • Soma: contains the nucleus, mitochondria, ribosomes, and other structures found in other cells. responsible for the metabolic work of the neuron. رافع عاوي الفياض.د 11 The Cells of the Nervous System • Axon: thin fiber of a neuron responsible for transmitting nerve impulses away to other neurons, glands, or muscles. • Some neurons are covered with an insulating material called the myelin sheath with interruptions in the sheath known as nodes of Ranvier. رافع عاوي الفياض.د 12 The Cells of the Nervous System رافع عاوي الفياض.د 13 Nodes of Ranvier 14 د.رافع عاوي الفياض Nodes of Ranvier 15 د.رافع عاوي الفياض The Cells of the Nervous System • Presynaptic terminals refer to the end points of an axon responsible for releasing chemicals to communicate with other neurons. رافع عاوي الفياض.د 16 The Cells of the Nervous System • Terms used to describe the neuron include the following: • Afferent axon - refers to bringing information into a structure. • Efferent axon - refers to carrying information away from a structure. رافع عاوي الفياض.د 17 18 د.رافع عاوي الفياض The Cells of the Nervous System • Glia are the other major component of the nervous system and include the following: • Astrocytes helps synchronize the activity of the axon by wrapping around the presynaptic terminal and taking up chemicals released by the axon. • Microglia - remove waste material and other microorganisms that could prove harmful to the neuron. رافع عاوي الفياض.د 19 Shapes of some glia cells رافع عاوي الفياض.د 20 21 د.رافع عاوي الفياض 22 د.رافع عاوي الفياض 23 د.رافع عاوي الفياض 24 د.رافع عاوي الفياض The Cells of the Nervous System • (Types of glia: • Oligodendrocytes & Schwann cells- build the myelin sheath that surrounds the axon of some neurons. • Radial glia- guide the migration of neurons and the growth of their axons and dendrites during embryonic development. رافع عاوي الفياض.د 25 blood-brain barrier • The blood-brain barrier is a mechanism that surrounds the brain and blocks most chemicals from entering. • Because neurons in the brain generally do not regenerate, it is vitally important for the blood brain barrier to block incoming viruses, bacteria or other harmful material from entering. رافع عاوي الفياض.د 26 The blood-brain barrier رافع عاوي الفياض.د 27 The Cells of the Nervous System • Active transport is the protein mediated process by which useful chemicals are brought into the brain. • Glucose, hormones, amino acids, and vitamins are brought into the brain via active transport. • Glucose is a simple sugar that is the primary source of nutrition for neurons. • Thiamine is a chemical that is necessary for the use of glucose. رافع عاوي الفياض.د 28 Basic schematic of excitable cell رافع عاوي الفياض.د 29 Types of ion channels رافع عاوي الفياض.د 30 Types of ion channels رافع عاوي الفياض.د 31 Important ions equilibrium potential رافع عاوي الفياض.د 32 Resting membrane potential رافع عاوي الفياض.د 33 Resting membrane potential رافع عاوي الفياض.د 34 Sodium-Potassium Exchange Pump What is the purpose of pumping sodium and potassium across a membrane? رافع عاوي الفياض.د 35 Who win the battle? رافع عاوي الفياض.د 36 The Na+/K+ ATPase pump • The sodium-potassium pump is a protein complex that continually pumps three sodium ions out of the cells while drawing two potassium ions into the cell. • Function: it helps to maintain the electrical gradient and restore the cell from action potential to resting potential state(maintaining a low intracellular concentration of Na+, keeping the inside of the cell negative). رافع عاوي الفياض.د 37 The sodium and potassium gradients for a resting membrane رافع عاوي الفياض.د 38 Resting membrane potential رافع عاوي الفياض.د 39 Resting membrane potential رافع عاوي الفياض.د 40 Resting membrane potential رافع عاوي الفياض.د 41 The action potential The action potential is a rapid depolarization followed by a repolarization (return of membrane potential to rest). The function is: • Nerves: conduct neuronal signals • Muscle: initiate a contraction رافع عاوي الفياض.د 42 The action potential رافع عاوي الفياض.د 43 The action potential phases رافع عاوي الفياض.د 44 The Nerve Impulse • A nerve impulse is the electrical message that is transmitted down the axon of a neuron. • The impulse does not travel directly down the axon but is regenerated at points along the axon. • The speed of nerve impulses ranges from approximately 1 m/s to 100 m/s. رافع عاوي الفياض.د 45 The Nerve Impulse • The resting potential of a neuron refers to the state of the neuron prior to the sending of a nerve impulse. • The membrane of a neuron maintains an electrical gradient which is a difference in the electrical charge inside and outside of the cell. رافع عاوي الفياض.د 46 The Nerve Impulse • The membrane is selectively permeable, allowing some chemicals to pass more freely than others. • Sodium, potassium, calcium, and chloride pass through channels in the membrane. • When the membrane is at rest: • Sodium channels are closed. • Potassium channels are partially closed. رافع عاوي الفياض.د 47 Ion channels in the membrane of a neuron رافع عاوي الفياض.د 48 Voltage-gated Ion Channels Voltage-gated (fast) Na+ Channels The opening of these channels is responsible for the rapid depolarization phase (upstroke) of the action potential. It has 2 gates and 3 conformational states. رافع عاوي الفياض.د 49 Voltage-gated Ion Channels رافع عاوي الفياض.د 50 Voltage-activated channels • Voltage-activated channels are membrane channels whose permeability depends upon the voltage difference across the membrane. • Sodium channels are voltage activated channels. • When sodium channels are opened, positively charged sodium ions rush in and a subsequent nerve impulse occurs. رافع عاوي الفياض.د 51 Voltage-activated channels • After an action potential occurs, sodium channels are quickly closed. • The neuron is returned to its resting state by the opening of potassium channels. • potassium ions flow out due to the concentration gradient and take with them their positive charge. • The sodium-potassium pump later restores the original distribution of ions. رافع عاوي الفياض.د 52 Voltage-gated Ion Channels رافع عاوي الفياض.د 53 Subthreshold Stimulus Important points regarding these stimuli are: The degree of depolarization is related to the magnitude of the stimulus. The membrane repolarizes (returns to rest). It can summate, which means if another stimulus is applied before repolarization is complete, the depolarization of the second stimulus adds onto the depolarization of the first (the 2 depolarizations sum together). رافع عاوي الفياض.د 54 Subthreshold Stimulus رافع عاوي الفياض.د 55 Threshold Stimulus If the initial stimulus is great enough to depolarize the neuron to threshold, then an action potential results. رافع عاوي الفياض.د 56 Steps of the action potential 1. At threshold, a critical mass of fast Na+ channels open, resulting in further depolarization and the opening of more fast Na+ channels. 2. Because Na+ conductance is high, the membrane potential rapidly approaches the equilibrium potential for Na+ (- +70 m V). 3. As membrane potential becomes positive, fast Na+ channels begin to inactivate, resulting in a rapid reduction in Na+ conductance. رافع عاوي الفياض.د 57 Steps of the action potential 4. Voltage-gated K+ channels open in response to the depolarization, but since their kinetics are much slower, the inward Na+ current (upstroke of the action potential) dominates initially. 5. K+ conductance begins to rise as more channels open. As the rise in membrane potential approaches its peak, fast Na+ channels are inactivating, and now the neuron has a high K+ conductance and a low Na+ conductance. رافع عاوي الفياض.د 58 Steps of the action potential 6. The high K+ conductance drives membrane potential toward K+ equilibrium ( -95 mV) resulting in a rapid repolarization. 7. As membrane potential becomes negative, K+ channels begin to close, and K+ conductance slowly returns to its original level. However, because of the slow kinetics, a period of hyperpolarization occurs. رافع عاوي الفياض.د 59 Steps of the action potential رافع عاوي الفياض.د 60 Steps of the action potential رافع عاوي الفياض.د 61 62 د.رافع عاوي الفياض the action potential phases رافع عاوي الفياض.د 63 the action potential phases رافع عاوي الفياض.د 64 Key points in action potential رافع عاوي الفياض.د 65 Properties of action potentials رافع عاوي الفياض.د 66 Refractory period • After an action potential, a neuron has a refractory period during which time the neuron resists another action potential. • The absolute refractory period is the first part of the period in which the membrane can not produce an action potential. • The relative refractory period is the second part in which it take a stronger than usual stimulus to trigger an action potential. رافع عاوي الفياض.د 67 Properties of action potentials رافع عاوي الفياض.د 68 Neuromuscular Junction (NMJ) The synapse between the axons of an alpha-motor neuron and a skeletal muscle fiber is called the neuromuscular junction (NMJ). The terminals of alpha-motor neurons contain acetylcholine (Ach). رافع عاوي الفياض.د 69 Neuromuscular Junction (NMJ) رافع عاوي الفياض.د 70 Sequence of events in neuromuscular junction 1) The action potential travelling down the motor neuron depolarizes the presynaptic membrane. 2) This depolarization opens voltage-gated Ca2+ channels in the presynaptic membrane, resulting in Ca+ influx into the presynaptic terminal. 3) The rise in Ca+ causes synaptic vesicles to release their contents(Ach). رافع عاوي الفياض.د 71 Sequence of events in neuromuscular junction 4) Ach binds to a nicotinic receptor located on the muscle membrane lead to depolarization. This depolarization is called an end-plate potential (EPP). 5) The resulting depolarization opens fast Na+ channels on the muscle membrane (sarcolemma) causing an action potential in the sarcolemma. 6) The actions of Ach are terminated by acetylcholinesterase (AchE), an enzyme located on the postsynaptic membrane that breaks down Ach into choline and acetate. رافع عاوي الفياض.د 72 Synapses Between Neurons رافع عاوي الفياض.د 73 Postsynaptic potential types رافع عاوي الفياض.د 74 Postsynaptic potential types رافع عاوي الفياض.د 75 Chapter summery 76 د.رافع عاوي الفياض Chapter summery 77 د.رافع عاوي الفياض