Human Anatomy & Physiology I Dr. Sullivan Unit X Nervous Tissue Chapter 12

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1 Human Anatomy & Physiology I Dr. Sullivan Unit X Nervous Tissue Chapter 12 I. Two main functions of the Nervous System a) Maintain homeostasis (keep controlled conditions within limits that maintain life) b) Perceptions, behaviors, memories, and all voluntary movement c) Innervate: to give a nerve supply to i) i.e. the tibial nerve innervates the gastrocnemius muscle, the femoral cutaneous nerve innervates the skin of the anterior thigh. II. Divisions of the Nervous System a) Central Nervous System (CNS) Brain and Spinal cord i) Responsible for thoughts, emotions, and memories ii) Origin of signals that stimulate muscles to contract and glands to secrete. b) Peripheral Nervous System (PNS) All nervous tissue outside the brain and spinal cord (spinal nerves, cranial nerves, named nerves, ganglia, and sensory receptors. Three divisions: i) Somatic Nervous System (SNS) brings sensory information from special sensory receptors in head, body wall, and limbs to the CNS and brings voluntary motor commands from the CNS to skeletal muscle. ii) Autonomic Nervous System (ANS) brings sensory information from the organs (viscera) to the CNS and brings involuntary motor commands from the CNS to the smooth (involuntary) muscle of the organs. iii) Enteric Nervous System (ENS) Sensory neurons monitor chemical changes within the GI Tract and the stretching of it s walls via enteric plexus. Motor neurons govern contraction of the smooth muscle of the GI Tract, secretions of the digestive organs, and activity of endocrine cells (hormone production). III. Histology of Nervous Tissue a) Neuron: Single nerve cell responsible for sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions. i) Sensory Neurons (afferent): receive information from stimulus in the PNS and bring it to the CNS. ii) Motor Neurons (efferent): Carry motor commands from CNS to PNS to effector cell. iii) Interneurons (association neurons): Analyzes and stores sensory information and makes decisions on how to react to it. These are the vast majority of the body s neurons. b) Neuroglia: In between neurons. Support, nourish, and protect neurons. About half the volume of the CNS. Neuroglia fills space left by injured neurons that do not regenerate. i) Neuroglia cells of the CNS: (1) Astrocytes: structural support for neurons, wrap around capillaries to form blood-brain barrier, divide rapidly, form glial scars to replace damaged neurons, can form astrocytoma (common and aggressive form of cancerous brain tumor) (2) Oligodendrocytes: secrete the myelin sheath (see definition below) of CNS neurons helping to protect axons and increase the velocity of nerve signal conduction. (3) Microglia: respond to infections in the CNS by becoming phagocytes to remove cellular debris and release immunoresponsive chemicals to resist infection. (4) Ependymal cells: epithelium that lines the spaces within the CNS (brain ventricles & the spinal cord s central canal). Secrete cerebrospinal fluid that helps nourish & protect the CNS. ii) Neuroglia of the PNS: (1) Schwann Cells: myelinate the axons of the PNS (2) Sattelite cells: small, flat cells providing physical & chemical support for neurons located in ganglia (bundles) iii) Glioma: tumor of neuroglia. Usually malignant (cancerous) and grow rapidly (very aggressive). c) Nerve Fiber: any process coming off the cell body (dendrites and axons) i) Neuron Structure

2 (1) Cell Body (soma): control center of the neuron. Contains a nucleus and cytoplasm with typical cell organelles such as mitochondria, lysosomes, and golgi complexes and Nissl Bodies. (a) Nissl bodies are responsible for protein synthesis, growth of the neuron and (b) aids in regeneration of the axon in the PNS. (2) Dendrites: each cell can have many dendrites. They are responsible for receiving information (input). They are short, tapering, and highly branched. (3) Axon: Each neuron has no more than one axon. The axon is responsible for sending information to another neuron (output). Long, thin, and cylindrical. (4) Axon Hillock: cone shaped elevation that connects the axon to the cell body. (5) Initial segment: 1 st part of the axon (6) Trigger zone: The junction of the axon hillock and the initial segment. This is where signals are initiated in most neurons and are then sent along the axon. (7) Axon Collateral: Side branch of an axon (8) Axon Terminal (terminal arborization): a fine process at the end of an axon collateral or axon (9) Synaptic end bulb (knob): a swelling at the end of the axon terminal. Contributes to the synapse. (a) Synapse: Site of communication between a neuron and another neuron or a neuron and an effector cell (the cell that is being targeted by the nerve signal). (10) Synaptic Vesicles: Small sacs (containing neurotransmitter) found inside the synaptic end bulb. (a) Neurotransmitter: A chemical that carries an signal from one neuron to another or from one neuron to an effector cell. (11) Myelin Sheath: a fat and protein sheath covering the axon produced by neuroglia. Insulates the axon and speeds signal conduction. Myelinated vs. Unmyelinated. (a) Schwann Cell: One unit of neuroglia that produces myelin in the PNS. (b) Node of Ranvier: Area of exposed axon in between Schwann Cells. (c) Oligodendrocyte: One unit of neuroglia that produces myelin in the CNS. (d) Neurolemma: a coating of Schwann that covers and protects the myelin sheath of neurons in the PNS. Helps regenerate an injury in the myelin sheath. Neurons of the CNS have no neurolemma. (e) White Matter: area of spinal cord or brain that is comprised of myelinated processes of neurons. (f) Gray Matter: area of brain or spinal cord that is comprised of unmyelinated processes of neurons. d) Structural Diversity in Neurons i) Multipolar Neuron: Many processes branching from the cell body ii) Bipolar Neuron: Two processes branching from the cell body iii) Unipolar Neuron: One process branches off the cell body and gives way to the axon and the dendrites. iv) Anaxonic neurons: neuron with no axon IV. Regeneration and repair of Nervous Tissue a) Plasticity: ability to change based on experience. i) i.e.growing new dendrites when needed, synthesis of new proteins, and changes in synapses with other neurons. b) Mammals have very limited powers of nervous regeneration. Conditions have to be perfect. i) PNS: Damage to dendrites and myelinated axons can only be repaired if cell body remains in tact, if the myelin-producing Schwann cells remain active, and if scar tissue does not grow too rapidly. ii) CNS: Little or no repair to nervous tissue occurs despite the condition of the cell body due to the absence of a neurolemma. c) Neurogenesis in the CNS i) Neurogenesis: creation or birth of new neurons

3 (1) Epidermal Growth Factor (EGF): stimulates the formation and regeneration of CNS neurons and astrocytes (neuroglia) in the hippocampus (learning center of the brain). (2) Reasons for poor nervous regeneration in the CNS: (a) Oligodendrocytes create no neurolemma when forming the CNS myelin sheath (b) Absence of schwann cells make it impossible to create a regeneration tube (c) After an axon is damaged, astrocytes proliferate very rapidly forming scar tissue and creating a physical barrier to new neurons. V. Damage and Repair in the PNS a) Most nerves in the PNS have a neurolemma. Therefore, a damaged axon in the PNS has a good chance of repairing itself. b) 1 st, Nissl Bodies break down and spread throughout the cell body. c) 2 nd, the part of the axon distal to the injury degenerates and breaks down leaving the neurolemma intact. This is called Wallerian Degeneration. d) 3 rd, The segment of the axon immediately proximal to the injury and up to the next Node of Ranvier breaks down called retrograde degeneration. e) 4 th, Schwann cells begin to multiply and grow together creating a regeneration tube. f) Finally, the regeneration tube guides the axon through re-growth across the injured area and into the space previously occupied by the distal axon. g) *** If the injury creates a gap too large for the Schwann cells to grow back together quickly, scar tissue forms and prevents regeneration VI. The Nerve Signal: Electrical Signals in Neurons a) Neurons are electrically excitable b) Two types of electrical signal i) Action Potentials: communication over short and long distances. ii) Graded Potentials: communication over short distances only. iii) Production of each depends on the presence of a resting membrane potential and the presence of certain ion channels. c) Membrane Potential: the difference in charge on the inside of the membrane (cytosol) versus the charge on the outside of the membrane (ECF). This creates a voltage difference across the membrane that acts as potential energy. i) Graded and Action Potentials take place because of Ion Channels in the membrane that open and close allowing charged ions to pass through. d) Ion Channels i) Ions move from higher concentration to lower concentration when allowed to by open ion channels. ii) Anions (negative ions) and Cations (positive ions) also move toward concentrations of opposite charge. iii) Two main types of Ion Channels (1) Leakage Channels: always open and allow ions to flow freely. (2) Gated Channels: open and close in response to a stimulus. (a) Voltage-gated ion channels: open in response to a change in membrane potential (voltage). Used in action potentials. (b) Ligand-gated ion channels: open and close in response to specific chemical stimulus. Ligands are neurotransmitters, hormones, or ions. (c) Mechanically-gated ion channels: open and close in response to mechanical stimulus (vibration, pressure, tissue stretch). Found in auditory receptors of the ears, receptors that monitor stomach stretching, and touch receptors in the skin. e) Resting Membrane Potential i) Created by an equal buildup of Anions inside the cell membrane and Cations outside the cell membrane. ii) Potential energy is measured in millivolts and averages at 70mV. Negative indicating that inside is negative relative to outside. iii) Polarized: The cell maintains the normal membrane potential. iv) Depolarized: due to opening of ion channels in the cell s membrane, positive ions are allowed to enter the cytoplasm and make the potential less negative (closer to 0)

4 v) Hyperpolarized: due to opening of ion channels in the cell s membrane, negative ions are allowed to enter the cytoplasm and make the potential more negative (farther from 0). vi) Two factors maintain the resting membrane potential: (1) Unequal distribution of ions across the cell membrane. Na+ and Cl- on outside of cell and K+ on inside. (2) Membrane x more permeable to K+ than to Na+. f) Local (graded) potentials i) If a stimulus causes ligand-gated or mechanically-gated ion channels to open or close in an excitable cell s plasma membrane. ii) The response may cause the potential to be more negative (hyperpolarizing graded potential) or to be less negative (depolarizing graded potential). iii) Graded potential means that the response can vary in amplitude depending on how many ion channels open or close. iv) Graded potentials make a membrane more or less excitable by bringing the membrane s potential close to the threshold voltage that opens multiple voltage-gated ion channels firing an action potential. (a) Example: being touched by a feather will stimulate certain touch receptors, therefore opening mechanically gated ion channels. However, unless enough stimulus is present, it will not open enough ion channels to depolarize the membrane to threshold. So an action potential is never fired and you do not actually perception the touching of the feather. (b) Conversely, if enough channels are opened by the feather that enough ions can enter the cytoplasm, the membrane s potential will depolarize to threshold, opening up a large number of voltage-gated ion channels and allowing an action potential to fire along the axon. g) Action Potentials i) Sequence of rapidly occurring events that decrease and reverse the membrane potential. ii) Two types of channels open and close for Action Potentials: (1) Na+ channels allow Na+ to rush into a cell causing depolarization (2) K+ channels allow K+ to rush out of a cell causing repolarization. iii) The process takes approximately 1 msec in most neurons. iv) All-or-none Principal: if the depolarization reaches a certain level, or threshold (approx. 55mV), many voltage gated ion channels open and allow an action potential to fire. The amplitude is always the same. (1) Like pushing a domino. v) Threshold: the voltage that stimulates voltage-gated sodium channels to open. vi) Action Potentials function in both short and long distance communication. h) Step by Step Action Potential: i) Resting State: Voltage-gated Na+ and K+ are closed. Potential = -70mV. ii) Depolarization: When a graded potential results in threshold (-55mV) being reached, voltage-gated Na+ channels open rapidly and allow Na+ to rush into the cell bringing the potential up passed 0mV and to about +30mV. At the same time, K+ channels open slowly allowing some K+ to flow out, but not enough to make up the difference for Na+ flowing out. iii) Repolarization: Na+ channels begin to close and K+ stay open. K+ continues to flow out, bringing the potential back down to 70mV. iv) Refractory Period: K+ channels close and resting membrane potential is restored. However, this is a period of time when the cell cannot be stimulated. Graded potentials do not have a refractory period, only action potentials do. (1) Large diameter axons have shorter refractory periods than smaller diameter axons. (2) Larger diameter axons are capable of up to 1000 signals per second. (3) Smaller diameter axons are capable of up to 250 signals per second. i) Propagation of Nerve Signals (signal conduction) i) Propagation is how the signal travels from one body part to another. ii) Continuous Conduction: Unmyelinated axons conduct signals by depolarizing each adjacent portion of the axon.

5 iii) Saltatory Conduction: In myelinated axons, only the nodes of ranvier need to be depolarized and the signal appears to leap across the membrane from node to node very quickly. iv) Speed of Nerve Signal Conduction (1) Two determinants (a) Diameter of Axon (b) Myelination (c) *Cooling will also slow down conduction. Icing an injury decreases pain sensation by slowing the conduction. (2) A Fibers: (a) Largest diameter axons (b) Myelinated (c) Small refractory period (d) m/s conduction (3) B Fibers (a) 2 nd largest diamter axons (b) myelinated (c) somewhat longer refractory period than A fibers (d) 15 m/s conduction (4) C Fibers (a) Smallest axon diamter (b) unmyelinated (c) long refractory period (d).5-2 m/s conduction (5) Intensity of a stimulus is determined by frequency of signals and number of neurons stimulated, not by strength of stimulus. VII. Signal Transmission at Synapses a) Electrical Synapse i) Gap junctions carry the electrical signal across the synapse from the neuron to the second neuron or effector cell. (1) Seen in visceral smooth muscle, cardiac muscle, and developing embryo (2) Advantages (a) Faster communication than chemical synapse (b) Synchronizationof group of fibers (c) Two-way transmission can go back and forth b) Chemical Synapse: the method by which one neuron transfers its action potential to another neuron or its target cell. i) Action potential arrives at synaptic end bulb of presynaptic axon ii) Depolarization opens Ca++ channels allowing Ca++ to flow in as well as Na+ channels allowing Na+ into the cleft. iii) Ca++ triggers the synaptic vesicles to break open (exocytosis and release neurotransmitter into the synaptic cleft. iv) Neurotransmitter crosses cleft and binds to the neurotransmitter receptor sites on the postsynaptic membrane. v) Neurotransmitter opens ligand-gated channels and allows ions to flow into the postsynaptic membrane. vi) Depolarization or hyperpolarization occur depending on whether Na+ or Cl- ion channels are opened. vii) If depolarization occurs enough for a threshold to be reached, an action potential is fired. viii) Chemical Synapses are ONE-WAY only because the neurotransmitter receptors are only on the postsynaptic side. c) Excitatory or Inhibitory Postsynaptic Potentials i) When a chemical synapse occurs, there are two possible results in the postsynaptic membrane (the receiving neuron or muscle cell) (1) Excitatory Postsynaptic Potential (EPSP) (a) the synapse depolarizes the effector cell but less than threshold

6 (b) Causes the post-synaptic membrane to become more excitable, since it is closer to threshold than normal. (c) When an EPSP is enough to reach threshold it causes an action potential in the postsynaptic membrane, which may result in a nerve signal or a muscle contraction. ii) Inhibitory Postsynaptic Potential (IPSP) (1) the synapse hyperpolarizes the effector cell (2) Causes the post-synaptic membrane to become less excitable, since it is farther to threshold than normal. iii) Spatial Summation: when an action potential is fired by neurotransmitter from several presynaptic endbulbs. iv) Temporal Summation: When an action potential is fired by a build up of neurotransmitter released from one presynaptic end bulb. d) If the neurotransmitter is not removed from the synaptic cleft when the action potentials stop the will continue to stimulate the receptors on the postsynaptic membrane. This results in over stimulation of the membrane (if it s an excitatory neurotransmitter) or over inhibition of the membrane (if it s an inhibitory neurotransmitter). i) Three Methods of removal (1) Diffusion into the ECF (2) Enzymatic degradation (3) Re-uptake by presynaptic membrane e) Neurotransmitters f) Neurotransmitters are chemicals used to carry messages or signals within the nervous system. i) Can result in excitation or inhibition of the postsynaptic membrane. ii) Many neurotransmitters also act as hormones released into the bloodstream and not just used in the nervous system. g) Within the brain, neurons that secrete neurotransmitter as well as hormones are called neurosecretory cells. h) The effects of neurotransmitter can be modified in the following ways: i) Synthesis can be stimulated or inhibited ii) Release can be blocked or enhanced iii) Removal can be inhibited or stimulated iv) Receptor cites can be blocked or activated i) Agonist: An agent that enhances synaptic transmission or mimics the natural effect of a neurotransmitter i) i.e. cocaine is an agonist of dopamine because it blocks the transporters for dopamine reuptake allowing it to linger in the synapse longer and create a feeling of euphoria. j) Antagonist: An agent that blocks the action of a neurotransmitter. i) i.e. Curare is a poison that used to be put on the tip of blow darts in certain indigenous tribes. Curare blocks the receptor sites for Acetlycholine (ACh) paralyzing the skeletal muscles and eventually suffocating the victim because the diaphragm cannot contract. k) Neurotransmitters can be divided into two classes i) Small-molecule neurotransmitters (1) Acetylcholine (ACh): released by many PNS neurons and some CNS neurons (a) Sometimes excitatory and sometimes inhibitory (b) Excitatory: neuromuscular junctions to create muscular contraction (c) Inhibitory: slows the heart rate via the Vagus nerve (CN X) (d) Inactivate by the enzyme acetylcholine esterase, which breaks it down into acetate and choline. (2) Amino Acids: mostly used as the building blocks of proteins, but some are used as neurotransmitters. (a) Can also be excitatory or inhibitory (b) Exictatory: Glutamate and Aspartate (i) Nearly all excitatory synapses in the CNS and about half of all synapses in the brain communicate via glutamate. (c) Inhibitory: Gamma aminobutyric acid (GABA) and glycine (i) GABA is the most common inhibitory neurotransmitter in the brain

7 1. Valium is a GABA agonist. It mimics the effects of GABA causing inhibition of neurons reducing anxiety. (3) Biogenic Amines: excitation or inhibition (a) Norepinepherine and epinepherine: used for awakening from sleep, dreaming and regulating mood. (i) released by adrenal gland and also known as adrenaline. (ii) both also serve as hormones (b) Dopamine: involved in emotional responses and skeletal muscle tone. (i) degeneration of dopamine-containing axons occurs in Parkinson s disease. (c) Seratonin: found in the brain and is thought to be involved in sensory perception, temperature regulation, mood control, and induction of sleep. (i) a lot of anti-depressants are seratonin re-uptake inhibitors that allow seratonin to linger in the synapse longer regulating mood. ii) Neuropeptides (1) Enkaphalin, Endorphin, and Dynorphin: all are neuropeptides with very strong analgesic properties (a) Pain-killing effects 200x stronger than morphine. (2) Substance P: neuropeptides that transmit pain-related signals from the PNS to the CNS. (a) Enkaphalin suppresses the release of substance P

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