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)