nervous system - weebly
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Nervous System
Structural Organization of Nervous System
Myelinated Neuron
Myelin
• White, fatty material which covers nerve fibers(axons)
• Protects and insulates fiber• Increases the rate of
transmission of nerve impulses
Myelination of axons outside the CNS
• Schwann cells – specialized supporting cells; wrap themselves around axon
• Coil of wrapped membrane = myelin sheath
• Sheath formed by many cells; has gaps(indentations) between cells –
• Nodes of Ranvier
Schwann cell-myelin sheath
Myelinated neuron
Impulse Conduction
• Fibers with myelin sheaths conduct impulses much faster than unmyelinatedfibers.
Nerve impulse jumps from node to node along fiber – this occurs because no current can flow across axon where there is a fatty myelin sheath.“Saltatory Conduction”
Saltatory conduction in myelinated axons
• http://www.blackwellpublishing.com/matthews/actionp.html
• http://outreach.mcb.harvard.edu/animations/
Multiple Sclerosis
• Degeneration of myelin sheath• Disrupt nerve transmission
Physiology of Nerve Impulses• Resting – Inactive – State
1. Neuron is polarized = fewer positive ions on inner face of cell membrane than on outer face in tissue fluid2. Major + ions inside cell = K+
Major + ions outside cell = Na+3. As long as inside more negative than outside – neuron is inactive
Generation of impulse
• Stimulus – may be heat, light , pressure, sound or chemical
• Result – permeability properties of plasma membrane change for very brief period1. “Gates” of Na+ channels open2. Na+ rushes into cell3. DEPOLARIZATION occurs = change in polarity across membrane
Nerve Impulse
• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html
Propagation of Impulse
• IF stimulus strong enough and Na+ influx great enough, local depolarization activates transmission of long distance impulse (action potential, nerve impulse)
• All or none response (propagated over entire axon or does not happen at all)
Repolarization
• Almost immediately, membrane becomes impermeable to Na+ and permeable to K+ ions.
• K+ flows out restoring the membrane to polarized or resting potential
• Finally, initial concentrations of Na+ and K+ restored by Na, K pump (pumps Na+ out and brings K+ in).
Conduction of Impulse from one neuron to another
• Generally, transmission of information from one neuron to another is chemical –via neurotransmitter molecules
• Neurotransmitters are chemicals released from an axon terminal, that diffuse across synaptic cleft and stimulate depolarization of the next neuron
• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__transmission_across_a_synapse.html
Conduction of Nerve Impulsefrom one neuron to another
• Electrochemical event -transmission down neuron is electrical -transmission from neuron to neuron is chemical
Reflexes
CNS Functions of major areas
Cerebrum– Consciousness, voluntary movement,
interpretation of sensation, speech, memory, logical and emotional response
Brain Anatomy
Diencephalon
• Thalamus, Hypothalamus, Epithalamus– 1. Thalamus – relay station for sensory
impulses– 2. Hypothalamus – autonomic nervous
system control center• Regulate temperature, water balance, metabolism• Center for drives and emotions• Regulates pituitary gland
Brain Stem
• Midbrain, pons and medulla oblongata• Ascending and descending nerve tracts• Control centers for heart rate, blood
pressure, breathing, swallowing, vomiting, reflex centers for vision and hearing, wake/sleep cycles
Cerebellum
• Precise timing for skeletal muscle activity and controls balance and equilibrium
Protection of CNS
• Meninges– Dura mater – outermost, leathery layer– Arachnoid mater – weblike, attaches
innermost and outermost– Pia mater – innermost layer , clings to curface
of brain
Meningitis – inflammation of meninges
Cerebrospinal Fluid (CFS)
• Formed from blood by choroid plexes in “roof” of ventricles
• Similar to blood plasma• Function: Cushioning, protection
Blood Brain Barrier
• Neurons separated from blood borne substances by blood brain barrier
• Capillaries in brain are relatively impermeable– Of water soluble substances, allow only water,
glucose and essential amino acids to pass through (many toxins, wastes and drugs cannot enter)
– Permeable to gases and fat soluble substances (therefore, alcohol, nicotine and anesthetics can affect brain)
Autonomic Nervous System
Parasympathetic NS –Origin: Cranial nerves and Sacral nerves
Sympathetic NS –Origin: Thoracic and Lumbar nerves
Autonomic Nervous System
• Together, the sympathetic and parasympathetic systems regulate autonomic body systems (including heart, smooth muscle in visceral organs, and glands) to maintain homeostasis.
• Generally,– Parasympathetic regulates systems “at rest”– Sympathetic regulates systems under stress
(e.g. excitement, fear, exercise)
Harmful Effects on Brain Development
• Nervous system is formed during the first month of embryonic development
• Any maternal infection early in pregnancy may impair nervous system development
e.g. rubella(measles)Other factors: lack of oxygen (may occur
due to smoking), radiation, drugs like alcohol, opiates, cocaine)