The Nervous System

Chapter 13

Introduction

• Communication system of an animal’s body.• Study is called Neurology• Composed of two main divisions:

• Central Nervous System (CNS)• Composed of brain and spinal cord

• Peripheral Nervous System (PNS)• Consist of nerves that come off of the CNS

and innervate rest of body• Function of nervous system activities:

• Sensory functions• Integrating functions• Motor functions

Neurons

• Basic functional unit of the nervous system.

• Have high oxygen requirement• Can not reproduce but in some cases can

regenerate.• Neuroglia or glial cells- supporting cells to

neurons• Structurally and functionally support

and protect the neurons• Are more numerous than neurons• Do not transmit impulses

Neuron composition• Soma (perikaryon): central cell body• Dendrites: receive stimuli or impulses from other nuerons and

transmit info to soma.• Sensory components feel things like heat and cold• Very short and branched

• Axons: Conduct nerve impulses away from the cell body to another neuron. Have single, long body with branches at end.• Secondary cells are called effector cells because they do

something when stimulated.• Covered in myelin.

• White matter, specialized glial cells called oligodendroctytes in brain and spinal cord and Schwann cells in nerves outside brain and spinal cord.

• Between the adjacent glial cells there are small gaps in meylin sheath called Nodes of Ranvier.

• Gaps help to increase speed of impulse along the axons.

• http://www.youtube.com/watch?v=i-NgGKSNiNw

Organization of Nervous System

1. 1. Anatomical• Central nervous system (CNS)

• Brain and spinal cord• Peripheral nervous system (PNS)

• Extends outward from the central axis toward the periphery of the body

• Cranial nerves originate directly from the brain

• Spinal nerves emerge from the spinal cord

Organization of Nervous System

22. Direction of Impulses• Afferent nerves - conduct impulses TOWARD CNS

• Also called sensory nerves - conduct sensations from sensory receptors in skin and other locations to CNS

• Efferent nerves - conduct impulses AWAY from CNS• Also called motor nerves - cause skeletal muscle

contraction/movement• Some nerve fibers are sensory (optic), motor (oculomotor), or

both (glossopharyngeal)

Organization of Nervous System

3. 3. Function: Autonomic vs. Somatic• Somatic nervous system –

• actions under conscious, or voluntary, control• Autonomic nervous system –

• controls and coordinates automatic functions• Example: slowing of the heart rate in response to an

increased blood pressure• Sympathetic Division (fight or flight)• Parasympathetic Division (feed or breed)

Neuron Function: Depolarization and Repolarization

• Resting state - when neuron is not stimulated• But, not truly resting – still working to maintain resting

state• Sodium-Potassium Pump – proteins in the neuron’s

cell membrane pump sodium ions out and potassium ions into the cell• Sodium can’t diffuse through membrane on its own

which creates a higher concentration of sodium ions outside and higher concentration of potassium inside

• Positive charges outside and negative charges inside create a polarized membrane

Neuron Function

• Resting membrane potential - difference in electrical charge across neuronal membrane

• Due to differences in distribution of positive and negative charges from sodium, potassium, proteins, and other charged ions on either side of neuronal membrane

• Resting membrane potential is a negative number – indicating the negative charge inside the cell

• Sodium-potassium pump maintains this negative charge

Depolarization

Steps:• Neuron receives stimulus/impulse• Na+ channel opens in neuron cell membrane• Na+ flow into cell by passive diffusion

• Down concentration gradient• Electrical gradient (opposites attract)

Depolarization Continued

• During depolarization, inside of neuron goes from negative to a net positive charge due to inflow of Na+ ions

• ACTION POTENTIAL = significant change in electrical charge from a negative to positive

Repolarization

Steps:

• Na+ channels close

• K+ channels open

• K+ diffuses out of cell

• Down electrical and concentration gradients, just like sodium

• Resting potential (charge) restored

• Cell is REPOLARIZED – sodium and potassium once again on opposite sides

• But, Na+ is inside, K+ outside (switched places)

AFTER Repolarization

• Na+/K+ pump moves ions back to their original sides

• Resting state restored

Threshold Stimulus

• Stimulus must be sufficient to make the neuron respond and cause complete depolarization – weak stimuli do not cause depolarization• “All-or-nothing principle” - neuron depolarizes to its

maximum strength or not at all• Strong stimuli cause depolarization and sodium channels

open which causes adjacent channels to open – wave of depolarization

• Conduction of action potential— spreading wave of opening Na+ channels in sufficient numbers to allow Na+ influx and depolarization

• Nerve impulse= wave of depolarization=conduction of AP

Refractory Period

• Time when neuron is not sensitive to a stimulus

—Cell is still in depolarization/early repolarization• Absolute refractory period – • during Na+ influx and early K+ outflow

• No amount of stimulus can elicit another AP• Relative refractory period – • during end of repolarization period

• Strong stimulus may elicit response

Saltatory Conduction

• Saltatory means “to leap”• Rapid means of conducting an AP (more rapid than in

unmyelinated axons)• Depolarization in myelinated axons can only take place

at nodes of Ranvier

http://www.blackwellpublishing.com/matthews/actionp.html

Local Anesthetics

• Work by blocking the propagation of the action potential.

• Blocks Na channels, so no net positive charge flows into cell, so no threshold is achieved.

• Example: Lidocaine

Synaptic Transmission

• Synapse - junction between two neurons or a neuron and target cell

• Synaptic cleft - gap between adjacent neurons/effector cell

• Presynaptic neuron - neuron bringing the depolarization wave to the synapse • Releases neurotransmitter

• Postsynaptic neuron - contains receptors for the neurotransmitter

Synaptic Transmission

• Axon - structure on presynaptic neuron• Terminal bouton (axon terminal) - slightly enlarged bulb at

end of axon• Vesicles contain neurotransmitter (a chemical)

• When depolarization wave reaches axon terminal, calcium channels open and cause vesicles to fuse with cellular membrane and release neurotransmitter into the synapse

Synaptic Transmission

• Neurotransmitters diffuse across synaptic cleft toward postsynaptic membrane

• Receptors on postsynaptic membrane bind neurotransmitter• Receptors are VERY specific for each neurotransmitter

(similar to a lock and key)

Types of Neurotransmitters

• Excitatory neurotransmitters

• Usually cause an influx of Na+; postsynaptic membrane moves toward threshold (more positive)

• Inhibitory neurotransmitters

• Move the charge of postsynaptic cell farther away from threshold (more negative)

• May open K+ channels/Cl- channels

Examples of Neurotransmitters

• Acetylcholine • Can be excitatory or inhibitory

depending on its location in the body• Catecholamines

• Norepinephrine/epinephrine - associated with "fight or flight" reactions of sympathetic nervous system

• Dopamine - involved with autonomic functions and muscle control

Recycling the Neurotransmitter

• Acetylcholinesterase – • found on postsynaptic membrane; breaks down acetylcholine • Remember this??

• Monoamine oxidase (MAO) –• breaks down norepinephrine

• Catechol-O-methyl transferase (COMT) – • breaks down norepinephrine that is not reabsorbed

Central Nervous System

• Brain• Cerebrum• Cerebellum• Diencephalon • Brain stem

• Spinal Cord

Cerebrum• Gray matter –

• cerebral cortex; outer layer of the brain

• White matter – • fibers beneath cortex and

corpus callosum (fibers that connect the two halves of cerebral cortex)

• Area of brain responsible for higher-order behaviors (learning, intelligence,

awareness, etc.)

Cerebrum Terminology• Gyri (gyrus):

• folds (rises) in cerebral hemispheres• Sulci (sulcus):

• shallow grooves (remember interventricular sulci?)• Divides the cerebral hemispheres into lobes

• Fissures: • deep grooves separating the gyri• Longitudinal fissure: prominent groove

that divides cerebrum into right and left

cerebral hemispheres

Cerebellum• Located just caudal to cerebrum• Area of brain responsible for coordinated movement,

balance, posture, and complex reflexes

Diencephalon• Passageway between brain stem and cerebrum

• Structures associated with diencephalon:• Thalamus, hypothalamus, and pituitary gland

Brain Stem• Connection between brain and spinal

cord contains:• Medulla oblongata• Pons• Midbrain

• Area of brain responsible for basic (subconscious, autonomic) functions of the body• Heart Rate• Breathing, coughing, sneezing, • Blood Pressure

• Many of the cranial nerves originate from this area of brain

Meninges• Connective tissue layers that surround brain and

spinal cord • Contain blood vessels, fluid, and fat

• Supply nutrients and oxygen to the superficial tissues of the brain and spinal cord

• Provide some cushioning and distribution of nutrients for CNS.

Meninges• Three layers:

1.Dura mater – tough, fibrous

2.Arachnoid - delicate, spiderweb-like

3.Pia mater - very thin; lies directly on surface of brain and spinal cord

Cerebrospinal Fluid—in subarachnoid space

(Contains CSF)

Cerebrospinal Fluid• Fluid between Arachnoid and Pia mater and in canals and

ventricles inside brain and central canal of spinal cord • Provides cushioning function• May play role in regulation of autonomic functions such as

respiration and vomiting

Blood-Brain Barrier• Separates the capillaries in the brain from the nervous tissue • Capillary walls in the brain have no fenestrations; covered by

cell membranes of glial cells • Prevents many drugs, proteins, ions, and other molecules

from readily passing from the blood into the brain

Cranial Nerves• 12 nerve pairs in PNS that originate directly from brain • Numbered in Roman numerals from I through XII (1 through 12) • Each nerve may contain axons of motor neurons, axons of

sensory neurons, or combinations of both

Spinal Cord• Medulla: central part of spinal cord

• Composed of gray matter • A lot of nerve processing occurs here (not just

in brain)• Central canal – center of medulla

• Cortex: outer part of spinal cord• White matter

• Myelinated and unmyelinated nerve fibers• Surrounds the gray matter

Spinal Cord Roots• Dorsal and Ventral Nerve Roots emerge as Spinal Nerves from

between each pair of adjacent vertebrae• Dorsal nerve roots contain sensory fibers • Ventral nerve roots contain motor fibers• Spinal nerve is a mix of sensory and motor neurons

Autonomic Nervous System• Controls automatic functions at subconscious level

• Sympathetic nervous system - nerves emerge from thoracic and lumbar vertebral regions (thoracolumbar system)

• Parasympathetic nervous system - nerves emerge from brain and sacrum (cranial-sacral)

Neurotransmitters and Receptors

• Sympathetic nervous system

• Primary neurotransmitter—norepinephrine • Adrenergic neurons - neurons that release norepinephrine

• Epinephrine/norepinephrine also released from adrenal medulla

• To elicit an effect, effector organ must contain receptor for epinephrine/norepinephrine

Neurotransmitters and Receptors• Smooth/Cardiac muscles may constrict or dilate in

response to epi/norepi

1. α1- adrenergic receptors – cause vasoconstriction of skin, GI tract, and kidney [don’t need to digest, make urine, or bleed profusely}

2. β1-adrenergic receptors - increase heart rate and force of contraction

3. β2-adrenergic receptors - cause bronchodilation (relaxation)

Neurotransmitters and Receptors• Parasympathetic nervous system

• 1º neurotransmitter—acetylcholine • Cholinergic neurons - release acetylcholine• 2 types:

• Nicotinic receptors• Muscarinic receptors

Reflexes

• Somatic reflexes –• involve contraction of skeletal muscles

• Autonomic reflexes – • regulate smooth muscle, cardiac muscle, and

endocrine glands• Contralateral reflex –

• starts on one side of body and travels to opposite side

• Ipsilateral reflex – • stimulus and response are on same side of

body

Reflex Arc5 Components:• Sensory receptor—activated by stimulus

• Sensory neuron—transports Action Potential to gray matter of spinal cord or brain stem (cranial n.) and synapses with other neurons

• Interneuron—sensory info integrated with info from other sensory neurons

• Motor Neuron—response is sent out via motor neuron

• Target organ—effector cell

Stretch Reflex (tap knee)• Monosynaptic (1) reflex arc; ipsilateral reflex

• Involves 1 sensory neuron and 1 motor neuron • Signals also sent to:

• Antagonistic m (inhibitory)• Cerebellum/Cerebrum

Withdrawal Reflex (toe pinch)• Also called flexor reflex; ipsilateral reflex• Several interneuron synapses • Several segments of spinal cord• Results in:

• Contraction of muscles• Before cerebrum is aware

• Inhibition of antagonist m

Crossed Extensor Reflex• Contralateral reflex • Withdrawal reflex initiated

• afferent sensory neuron synapses with interneurons

• Causes contraction of opposite extensor muscles

CNS Moderation of Reflexes

• Upper CNS (brain) normally produces an inhibitory effect on the reflex arcs (muffled effect)

• With injury, intact reflex arcs caudal to spinal cord trauma become hyperreflexive

• Trauma to a portion of the reflex arc results in either hyporeflexive or absent reflexes