Lesson 9A The Nervous Sys 23 PP.pdf

2023-10-17 THE NERVOUS SYSTEM CHAPTER 7 1 THE NERVOUS SYSTEM • Master control and communication system • Three Functions: • Sensory input - gathered • Integration – Process, interpretation, decision • Motor output – effects a response 2 1 2023-10-17 3 • CENTRAL NERVOUS SYSTEM • PERIPHERAL...

2023-10-17 THE NERVOUS SYSTEM CHAPTER 7 1 THE NERVOUS SYSTEM • Master control and communication system • Three Functions: • Sensory input - gathered • Integration – Process, interpretation, decision • Motor output – effects a response 2 1 2023-10-17 3 • CENTRAL NERVOUS SYSTEM • PERIPHERAL NERVOUS SYSTEM 4 2 2023-10-17 ORGANIZATION OF THE NERVOUS SYSTEM Figure 7.2 5 PERIPHERAL NERVOUS SYSTEM • AFFERENT (SENSORY) • Somatic • Visceral • EFFERENT (MOTOR) • Somatic • Autonomic • Sympathetic • Parasympathetic 6 3 2023-10-17 STRUCTURE AND FUNCTION NEUROGLIA: Cells of the nervous system that support the nerve cells via insulation, attachment and protection 7 CNS GLIA CELLS • ASTROCYTES: • Star shaped, abundant • Numerous projections • Cling to neurons, brace and anchor to capillaries • Role in exchanges between neurons and • capillaries – protect from harmful substances in the blood Absorb excess ions and recapture neurotransmitters in the brain 8 4 2023-10-17 9 CNS GLIA CELLS MICROGLIA: spider-like phagocytes 10 5 2023-10-17 11 CNS GLIA CELLS EPENDYMAL CELLS: Line the brain and spinal column cavities Have cilia to assist movement of Cerebral Spinal Fluid 12 6 2023-10-17 13 CNS GLIA CELLS OLIGODENDROCYTES: Wrap flat extensions around nerve fibers Act as insulation coverings Called Myelin sheath. 14 7 2023-10-17 15 PNS SUPPORTING CELLS • SCHWANN CELLS • Form myelin sheath around nerve fibers in the peripheral system • SATELLITE CELLS • Protective cushioning cells 16 8 2023-10-17 17 NEURON ANATOMY • Cell body • Dendrites • Axons • Axon hillock • Axon terminals • Synaptic Cleft 18 9 2023-10-17 19 20 10 2023-10-17 • MYELIN: • White, fatty material • Protects and insulates fibers • Increases the transmission rate of impulses • Schwann cells myelinate the PNS fibers • Oligodendrocytes are the myelin sheath of the CNS 21 MYELIN SHEATH • Schwann cells wrap tightly around the axon of PNS neurons • Tight coil encloses the axon • External to the myelin sheath is called the neurilemma • Made of many Schwann cells therefore there are gaps between cells – Nodes of Ranvier 22 11 2023-10-17 23 MYELIN SHEATH • Oligodendrocytes in CNS • Multiple flat extensions • Coil around up to 60 different fibers by one glia cell • No neurolemma in CNS 24 12 2023-10-17 MORE ANATOMY TERMS • Nuclei: • Cluster of cell bodies in the CNS • Ganglia: • Small bundle of nerve cell bodies in PNS • Tracts: • Bundles of nerves running through the CNS • Nerves: • Bundles of nerves running through the PNS 25 • White Matter • Myelinated tracts • In CNS • Gray Matter • Unmyelinated tracts and cell bodies • In CNS 26 13 2023-10-17 FUNCTIONAL CLASSIFICATION OF NEURONS • SENSORY (Afferent) • Carry impulses from the sensory receptor TOWARDS the CNS • These keep us informed of what is happening inside and outside the body • Cell bodies of these nerves are found in ganglia outside the CNS 27 FUNCTIONAL CLASSIFICATION OF NEURONS • MOTOR (EFFERENT): • Carry impulses AWAY from the CNS to viscera and/or muscles, or glands • Cell bodies are always located in the CNS 28 14 2023-10-17 FUNCTIONAL CLASSIFICATION OF NEURONS • INTERNEURONS (ASSOCIATION): • Connect motor and sensory neurons • Cell bodies are always in the CNS 29 30 15 2023-10-17 STRUCTURAL CLASSIFICATION OF NEURONS • MULTIPOLAR • All motor and association neurons • Most common • BIPOLAR • Axon and dendrite only • Rare, only in some special sense organs • UNIPOLAR • short process that conducts impulses towards and away from the cell body • Found in PNS ganglia 31 32 16 2023-10-17 ABOUT NEURONS • Born & die with the same neurons • Neurons have longevity • Amitotic – do not replicate . • High metabolic rate! Require a continuous supply of nutrients • Also have 2 major functions • Irritability – the ability to respond to stimulus converting to an electrical charge (nerve impulse) • Conductivity – pass the impulse to another neuron/source 33 NERVE IMPULSES • Generated in response to a stimulus (irritability) • Involve converting the stimulus into an electrical charge • Sending this electrical message to the next source (conductivity) 34 17 2023-10-17 NERVE IMPULSES • Also called ACTION POTENTIAL • If the action potential (nerve impulse) starts, it is propagated over the entire axon • Can only travel in ONE direction (away from the stimulus source) • Involves changing electrical states of the cell membranes by moving ions 35 NERVE CELLS AT REST • Polarization (-70mV) Resting membrane (polarized) has positive charge on the extracellular surface & a negative charge on intracellular surface with lots of Sodium (Na) outside and lots of Potassium (K) and other particles inside 36 18 2023-10-17 37 REMEMBER: • The cell wall (plasma membrane) is semipermeable thus controls what enters and exits: Na+ and K+ cannot freely move in or out of the cell through the membrane. • Laws of diffusion state that when there is an imbalance in concentration the constituents will attempt to balance the concentration when possible 38 19 2023-10-17 ACTION POTENTIAL • The action potential is a large change in membrane potential (rest to peak to rest) • At rest the membrane potential value is -70 milli-volts (mV) • At the peak of an action potential it reaches +30 mV • The end of an action potential returns it to -70 mV • The action potential results from a rapid change in the permeability of the neuronal cell membrane to sodium and potassium. The permeability changes as voltagegated ion channels open and close. 39 40 20 2023-10-17 STARTING A NERVE IMPULSE • The arrival of a stimulus opens Sodium gates on neuron’s membrane • This allows sodium (Na+) to flow inside the membrane • This exchange of ions is depolarization and initiates an action potential in the neuron Figure 7.9a–c 41 • Depolarization • Neurotransmitter Chemicals released in response to stimulus cause cell membranes to become temporarily permeable to Na+ • Sodium gates open and Na+ rushes into cell • If enough gates open the Intracellular space becomes positive & extracellular space negative (opposite to resting!) • This event is called action potential 42 21 2023-10-17 43 • A weak stimulus causes a few sodium gates on the cell membrane to open. This is not enough to cause a large change in the membrane potential. • The stronger the signal the more gates will open until the MAGIC number (Threshold) is reached causing most sodium gates to open allowing a huge rush of Na+ into the cell! 44 22 2023-10-17 • If the signal is strong enough and enough Na+ gates open allowing the membrane potential to change from -70 mV to – 55 mV (Threshold) this triggers the sodium channel gates of the cell membrane to open and allows Na+ to rush INTO the cell • This is an all or nothing event 45 • Almost immediately after the sodium channel gates open (a set time every time!) they are closed and locked. No more sodium can enter (or exit) • BUT potassium channel gates then open – allowing for K+ to exit this “too positive” cell environment • This occurs when the membrane potential meets +30 mV 46 23 2023-10-17 47 • Repolarization: • When the Na+ gates lock closed the membrane potential is now +30 mV • At this time K+ channel gates open and allow K+ to move OUT of the cell • The movement of positive ions out of the cell help move the membrane potential back towards its resting state (-70 mV) • Until this occurs this neuron cannot conduct another impulse! (absolute refractory) 48 24 2023-10-17 49 50 25 2023-10-17 • At this time membrane potential is being restored BUT • Na+ is inside the cell and K+ is outside the cell!!! • This is opposite to what we need/want • The Sodium-Potassium pump uses energy to restore the correct configuration to Na+ outside and K+ inside with a resting potential of -70 mV 51 • The sodium-potassium pump restores the original configuration • This action requires ATP • 2 Potassium ions are moved in for every 3 Sodium ions moved out of the cell 52 26 2023-10-17 53 54 27 2023-10-17 WHERE DOES THIS OCCUR? • Stimulus is sensed and gathered by nerve cell dendrites • This stimulation causes messages to be sent to the cell body and focused towards the axon hillock (funnel) • These “messages” from the dendrites need to be strong enough to reach the axon hillock. They are considered GRADED POTENTIALS as they fade in strength as they travel 55 • At the axon hillock the sum of the graded potentials leads to the sodium gates to be opened. • If the sum of the messages is strong enough to open enough Sodium gates to change the action potential to at least -55mV (Threshold) – An action potential will be generated 56 28 2023-10-17 • After an action potential is generated at the axon hillock, it is propagated down the axon. • Positive charge flows along the axon, depolarizing adjacent areas of membrane, which reach threshold and generate an action potential. • The action potential thus moves along the axon as a wave of depolarization traveling away from the cell body. 57 NOTES ABOUT ACTION POTENTIALS • At threshold (-55mV), an action potential is generated. • This is an all-or-nothing event. • Action potentials always have the same amplitude and the same duration. 58 29 2023-10-17 59 60 30 2023-10-17 61 • Just after the neuron has generated an action potential, it cannot generate another one. Many sodium channels are inactive and will not open, no matter what voltage is applied to the membrane. Most potassium channels are open. This period is called the absolute refractory period 62 31 2023-10-17 63 • Immediately after the absolute refractory period, the cell can generate an action potential, but only if it is depolarized to a value more positive than normal threshold. This is true because some sodium channels are still inactive and some potassium channels are still open. This is called the relative refractory period 64 32 2023-10-17 65 NERVE IMPULSE PROPAGATION • The action potential is generated at the axon hillock, • These local signals travel for only a short distance and are very • different from action potentials. (Graded potentials) The action potential begins when signals from the dendrites and cell body reach the axon hillock and cause the membrane potential there to become more positive (depolarization). 66 33 2023-10-17 NERVE IMPULSE PROPAGATION • The impulse continues to move toward the axon terminal • Impulses travel faster when fibers have a myelin sheath Figure 7.9d–f 67 NERVE IMPULSE PROPAGATION • The presence of myelin sheaths allow for faster conduction of impulses • Impulses “leap” from Node to Node • Currents can not flow across the axon membrane where there is myelin insulation 68 34 2023-10-17 Conductivity • (2nd function of neurons) • For a neuron to send an impulse to another neuron, this action potential runs along the entire membrane down to the axonal terminal • This causes the axonal terminal to release neurotransmitter (NT) • NT floats across synaptic cleft & binds to sites at the next neuron 69 • If enough NT is released, action potential begins again in the next neuron!! • The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter • An graded potential or action potential (depending on the destination cell) is started in the dendrite 70 35 2023-10-17 SYNAPTIC COMMUNICATION Figure 7.10 71 36