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Chapter 5b Nerve Cells

Chris RordenUniversity of South CarolinaNorman J. Arnold School of Public HealthDepartment of Communication Sciences and DisordersUniversity of South Carolina

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MCQ

Visual problem after superficial damage to this region of left hemisphere…

A. Blind

B. Blind left of fixation

C. Blind right of fixation

D. These regions not responsible for vision.

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MCQ

Movement problem after superficial damage to this region of left hemisphere…

A. Paralyzed on both sides

B. Weak on left

C. Weak on right

D. These regions not responsible for movement.

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MCQ

Somatosensory problem after superficial damage to this region of left hemisphere…

A. Unable to feel on either side

B. Numb on left

C. Num on right

D. These regions not responsible for touch.

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MCQ

Language problem after superficial damage to this region of left hemisphere…

A. Poor speech comprehension

B. Poor language comprehension

C. Poor speech production

D. Poor writen language production

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Hierarchy of Organism Structures

Organism– Organ Systems

Organs– Tissues

Cells Organelles Organic Molecules

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Cell components

ChannelsStructural ProteinsSodium-Potasium Pump (Na-K)Extracellular fluid Intracellular fluidMembranes – lipids attached to proteins.

– Lipids (fats) do not dissolve in water– Separates extra and intra-cellular fluids.

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Cell membranes

Lipoproteins line up in double layer with protein (head) to outside and lipid tail to inside of membrane

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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.

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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.

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Intra vs Extracellular fluid

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MCQ

What is hyperkalemia

A. Not enough potassium

B. Not enough sodium

C. Too much patassium

D. Too much sodium

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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.

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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

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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.

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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.

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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

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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

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An action potential

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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.

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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

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Neuronal Response to Injury

Two Types1. Axonal (Retrograde) Reaction: Occurs When

Sectioning of Axon Interrupts Information that returns to Cell Body and Interferes with Support Reprogramming

2. Wallerian Degeneration: Occurs When Axon Degenerates in Region Detached from cell Body

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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

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Wallerian Degeneration

Axon Dependent on Cytoplasm from Cell BodyWithout 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

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Neuroglial Responses

Glial cells multiply in Number: Hyperplasia Increase in Size: HypertrophyNeurophils (Scavenger White Blood Cells)

Arrive at InjuryAstrocytes Form a Glial ScarMicroglia Cells Ingest DebrisCells May Return to Function

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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

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Neuro-transmitters

Two TypesSmall molecules: transient effects

– Acetylcholine, Norepinephrine, Dopamine, Serotonin, Glutamate, Y-aminobutyric acid (GABA)

Large Molecules - Longer Effects– Peptides : Table 5.4

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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

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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

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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

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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 DAParkinsons

Too much DASchizophrenia

‘Normal’

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Norepinephrine

Pons and MedullaReticular Formation and Locus CeruleusProject to Diencephalon, Limbic Structures and

Cerebral Cortex, Brainstem, Cerebellar Cortex and Spinal Cord

Maintain Attention and VigilanceMay be Related to Handedness Due to

Asymmetry in Thalamus

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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)

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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

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Peptides

Important in Pain ManagementExamples

– Enkephalin– Endorphins– Substance P

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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