Chapter 5b Nerve Cells • Chris Rorden University of South Carolina Norman J. Arnold School of Public Health Department of Communication Sciences and Disorders University of South Carolina
MCQ • Visual problem after superficial damage to this region of left hemisphere… • Blind • Blind left of fixation • Blind right of fixation • These regions not responsible for vision.
MCQ • Movement problem after superficial damage to this region of left hemisphere… • Paralyzed on both sides • Weak on left • Weak on right • These regions not responsible for movement.
MCQ • Somatosensory problem after superficial damage to this region of left hemisphere… • Unable to feel on either side • Numb on left • Num on right • These regions not responsible for touch.
MCQ • Language problem after superficial damage to this region of left hemisphere… • Poor speech comprehension • Poor language comprehension • Poor speech production • Poor writen language production
Hierarchy of Organism Structures • Organism • Organ Systems • Organs • Tissues • Cells • Organelles • Organic Molecules
Cell components • Channels • Structural Proteins • Sodium-Potasium Pump (Na-K) • Extracellular fluid • Intracellular fluid • Membranes – lipids attached to proteins. • Lipids (fats) do not dissolve in water • Separates extra and intra-cellular fluids.
Cell membranes • Lipoproteins line up in double layer with protein (head) to outside and lipid tail to inside of membrane
Resting Potentials • All Cells have General Characteristic of Irritability. • Need Irritability to Respond to Outside Influences. • Well Developed in Neurons. • Intracellular Fluid is -70 mvolts as Compared to Extracellular Fluid.
Why? • Uneven distribution of • Positively charged sodium • Positively charged potassium • Negatively charged chloride ions • Other negatively charged proteins. • Channels Open to Selectively Allow Movement of Ions. • Na-K Pump Helps to Keep Resting Potential.
MCQ • What is hyperkalemia • Not enough potassium • Not enough sodium • Too much patassium • Too much sodium
hyperkalemia • hyper- means high (contrast with hypo-, meaning low). • kalium, which is neo-Latin for potassium. • -emia, means "in the blood". • Death by lethal injection, kidney failure • If neurons can not maintain a K gradient, they will not generate an action potential.
Graded local potentials • Mechanical or Chemical Event Affects Neuronal Membrane • Neuron Becomes Perturbed (Perturbation) • Channels Open Causing Negative Ions to Flow Out or Positive Ions to Flow in
Changes in resting potential • Resting Potential Becomes Less than -70 mvolts = Depolarization • Resting Potential Becomes More than -70 mvolts = Hyperpolarization • If voltage exceeds threshold (~-55mV) the neuron fires.
Movement of Graded Potentials • Potential changes can occur in soma, along dendrite or initial portions of axon • Spreads along membrane, effect becomes smaller. • If depolatrization is at least 10mv at axon hillock, action potential is triggered • Smaller changes in potential will not influence neuron.
Action potential • During an action potential • Membrane is Depolarized, then Sodium (Positive Charge) Flows into Cell Causing Interior Potential to Become Positive. • Impulse Occurs – travels down axon to terminals • Absolute Refractory Period • After Impulse Fires, Over Reaction Drives Interior Charge to -80 or -90 mV • Any Additional Charge Would be Hard to Activate Until Cell Returned to Normal Resting State of -70mV
Impulse conduction • Neighboring Areas of the Cell Undergo Positive Charge Changes • The Impulse is Carried Through Continuous Short Distance Action Potentials • Myelin Speeds up the Impulse Through Saltatory Conduction • Unmyelinated: .5 to 2 meters/sec • Myelinated: 5 to 120 meters/sec
Impulses Between Cells • Synapse • When a neuron fires, it pours neurotransmitters into the synaptic clefts of its terminals. • These neurotransmitters influence the post-synaptic membrane, either polarizing (inhibiting) or depolarizing (exciting) the target neuron.
Conduction Velocities • Dependent on Size of Axon and Whether it is Myelinated or Not • Myelinated Fibers Conduct at 6m/sec Times Size of Fiber • ( 3um x 6m/sec=18m/sec) • Unmyelinated Fiber Diameter of 1 um Conducts Impulse at <1m/sec
Neuronal Response to Injury • Two Types • Axonal (Retrograde) Reaction: Occurs When Sectioning of Axon Interrupts Information that returns to Cell Body and Interferes with Support Reprogramming • Wallerian Degeneration: Occurs When Axon Degenerates in Region Detached from cell Body
Axonal Reaction • Chromatolysis: degenerative process of a neuron as a result of injury, fatigue, or exhaustion. • Begins between axon hillock and cell nucleus • Nissl bodies disintegrate • Displacement of nucleus from center of soma • If RNA Production and Protein Synthesis Increase, Cell May Survive and Return to Normal Size
Wallerian Degeneration • Axon Dependent on Cytoplasm from Cell Body • Without Nourishment, Distal Portion of Axon Becomes Swollen and Begins Degenerating in 12-20 Hours • After 7 Days, Macrophagic Process (Cleanup) Begins and Takes 3-6 Months
Neuroglial Responses • Glial cells multiply in Number: Hyperplasia • Increase in Size: Hypertrophy • Neurophils (Scavenger White Blood Cells) Arrive at Injury • Astrocytes Form a Glial Scar • Microglia Cells Ingest Debris • Cells May Return to Function
Axonal Regeneration • PNS: • Ends of Axon are Cleaned • Sheath of Schwan Cell Guides Axon to Reconnect • Grows 4 mm/day • May Have Mismatch of Axons • CNS: • Minimal restoration after injury • Growth occurs, but not significant enough to be functional
Neuro-transmitters • Two Types • Small molecules: transient effects • Acetylcholine, Norepinephrine, Dopamine, Serotonin, Glutamate, Y-aminobutyric acid (GABA) • Large Molecules - Longer Effects • Peptides : Table 5.4
Neurotransmitter: Acetylcholine • Major Player in the PNS • Released in Synapses Where it is Released to Facilitate Stimulation of Synapse • Needed for Continuous Nerve Impulses • Most Studied Neurotransmitter • After Use, Picked Up By Acetylcholinesterase • Regulates Forebrain and Inhibits Basal Ganglia • Example: Scopolamine used for motion sickness. Blocks acetylcholine receptors
Related Diseases • Myasthenia Gravis • Affects Acetylcholine receptors • Behavioral Example: Fatigue in Speaking • Alzheimer's Disease • Implication of Deficient Projections in Cortex, Hippocampus, and Orbito-frontal Cortex
Dopamine • Cells are Located in Upper Midbrain and Project Ipsilaterally • Mesostriatal - Midbrain and Striatum • Substantia Nigra to Basal Ganglia • Results in Parkinson’s Disease • Mesocortical - Midbrain and Cortex • Can Result in Problems of Cognition and Motivation • Can be Affected by Drug Abuse to Gain Pleasurable Feelings
Dopamine • Parkinson's disease: loss of dopamine in the neostriatum • Treatment: increase dopamine • Schizophrenia: Too much dopamine • Treatment: Block some (D2) dopamine receptors. • Problem: Overdose or prolonged dose leads to Parkinson's disease-like tremors (tardive dyskinesia) Not enough DA Parkinsons ‘Normal’ Too much DA Schizophrenia
Norepinephrine • Pons and Medulla • Reticular Formation and Locus Ceruleus • Project to Diencephalon, Limbic Structures and Cerebral Cortex, Brainstem, Cerebellar Cortex and Spinal Cord • Maintain Attention and Vigilance • May be Related to Handedness Due to Asymmetry in Thalamus
Serotonin • Found Primarily in Brain. Blood Platelets and GI Tract • Terminals at Most Levels of Brainstem and in Cerebrum • Involved in General Activity of CNS and in Sleep Patterns • Increased Concentration of Serotonin in Synaptic Cleft, Decreases Depression and Pain (Prozac)
Y-Aminobutyric Acid (GABA) • Major Player in the CNS • Pyramidal (Motor Cortex) Cells Rich in GABA • Present in Hippocampus, Cortex of Cerebrum and Cerebellum • Suppress Firing of Projection Neurons • Implicated in Huntington’s Disease • Reduced GABA Causes High Amount of Dopamine and Acetylcholine and Uncontrolled Movements
Peptides • Important in Pain Management • Examples • Enkephalin • Endorphins • Substance P
Drug Treatments • Blocking Enzymatic Breakdown of Neurotransmitter • Allows for Increased Neurotransmitter to Continue Function • e.g. Myasthenia Gravis • Regulating Activity of Postsynaptic Membrane • Blocking Effects of Released Neurotransmitter Causing Problem