The Nervous System

ORGANIZATION

Functions of the Nervous System

SENSORY FUNCTION• Gathers information from internal and

external environs and transmits the information to the processing areas of the CNS

INTEGRATIVE FUNCTION• Processes the information (CNS)• Perception, information storage

Functions of the Nervous System

MOTOR FUNCTION• Sends command information to muscles,

glands, and organs (effectors) so they can respond correctly

Divisions

Central Nervous System• Brain and spinal cord• Integrating and command centers• Reflex centers

Divisions

Peripheral Nervous System• Spinal nerves and Cranial nerves• Input (sensory) & output (motor)

Structures of the Nervous System

• Brain• Spinal Cord: connects periphery to brain;

enclosed in spinal cavity• Nerves: bundles of many neurons/axons

(like a cable) outside CNS–Cranial nerves emerge from the brain (12

pairs)– Spinal nerves emerge from spinal cord (31

pairs)

Structures of the Nervous System

• Ganglia: groups of neuron cell bodies located outside brain and spinal cord

• Enteric plexuses: networks in digestive tract

• Sensory receptors: specialized nerve ending that monitor changes in the environment

The Nervous System

NEURON ANATOMY

Neuroglia

• Support, nourish and protect neurons• Critical for homeostasis of interstitial fluid

around neurons• More numerous than neurons• Can multiply and divide• Guide development

Neuroglia

• Astrocytes: help form blood brain barrier• Oligodendrocytes: produce myelin in CNS• Microglia: protect CNS cells from disease• Ependymal cells: form CSF in ventricles• Schwann: produce myelin around PNS

neurons; help to regenerate PNS axons• Satellite cells: support neurons in PNS

ganglia

Neurons

• Can respond to stimuli and convert stimuli to electrical signals (nerve impulses) that travel along neurons

• Cell body (soma): nucleus, cytoplasm with typical organelles

• Dendrites: highly branched structures that carry impulses to the cell body

Neurons

• Axon: conducts away from cell body toward another neuron, muscle or glandEmerges at cone-shaped axon hillock

• Axon terminals: contain synaptic vesicles that can release neurotransmitters

Typical Neuron

Structural Classification

Multipolar• Have several or many dendrites and one axon• Most common type in brain and spinal cord• Typical for interneurons

Bipolar• Have one dendrite and one axon• Example: in retina of eye and inner ear

Structural Classification

Unipolar • Have fused dendrite and axon• Sensory neurons of spinal nerves

Myelination

Axons covered with a myelin sheath• Many layers of lipid and protein: insulates

neurons• Increases speed of nerve conduction• Appears white (in white matter)

Nodes of Ranvier: gaps in the myelin• Nodes are important for rapid signal conduction

Myelination

Some diseases destroy myelin: • Multiple sclerosis• Tay-Sachs disease

Collections of Nervous Tissue

Clusters of nerve cell bodies• Ganglion: outside CNS• Nucleus: inside CNS

Bundles of neuron axons• Nerve: outside CNS• Tract: inside CNS

Gray and White matter

White matter: myelinated axonsGray matter: cell bodies, dendrites, unmyelinated

axons, axon terminals, neurogliaLocations: Spinal cord: white matter (tracts) surround

centrally located gray matter “H” or “butterfly” Brain: gray matter in thin cortex surrounds

white matter (tracts)

NEURON PHYSIOLOGYThe Action Potential

The Action Potential

Action potential = nerve impulseAPs require:o A membrane potential: a charge difference

across cell membrane (polarization)o Ion channels: allow ions to move by diffusion

from high to low concentrationo Leakage channelso Gated channels; Open and close on command,

respond to changes in membrane so can generate and conduct action potentials

Resting Membrane Potential

Typically it is –70 mV• Inside of membrane more negative than outsideCaused by presence of ions: • Inside (more negative) because cytosol has: – Many negative ions (too large to leak out): amino acids (in

cellular proteins) and phosphates (as in ATP)– K+ that easily leaks out through many K+ channels

• Outside (more positive) because interstitial fluid has: – Few negative ions – Na+ that does not leak into cell: few Na+ channels – Membrane “pumps” that quickly pump out Na+ that does

leak (diffuse) into cell

Resting Membrane Potential

ActionPotential

• Series of events that activate cell membrane in neuron or muscle fiber

• An initial event (stimulus) is required– Triggers resting membrane to become more

permeable to Na+

– Causes enough Na+ to enter cell so that cell membrane reaches threshold (~ –55 mV)

– If so, the voltage gated Na+ channels open, membrane potential reverses and an AP is generated

ActionPotential

• Depolarizing phase– Na+ channels open as more Na+ enters cell,

membrane potential rises and becomes positive (–70 0 + 30 mv)

• Repolarizing phase – K+ channels open as more K+ leave cell, membrane

potential is returned to resting value (+ 30 0 –70 mv)

– May overshoot: hyperpolarizing phaseTypically depolarization and repolarization take place in

about 1 millisecond (1/1000 sec)

Action Potential

ActionPotential

• Recovery– Levels of ions back to normal by action of Na+/K+

pump– Refractory period (brief): even with adequate

stimulus, cell cannot be activated• All-or-none principle– If a stimulus is strong enough to cause

depolarization to threshold level, the impulse will travel the entire length of the neuron at a constant and maximum strength

Copyright 2010, John Wiley & Sons, Inc.

Membrane PotentialsInteractions Animations

• Membrane Potentials

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Conduction of Nerve ImpulsesNerve impulse conduction (propagation)• Each section triggers the next locally as even more

Na+ channels are opened (like row of dominos)Types of conduction • Continuous conduction– In unmyelinated fibers; slower form of conduction

• Saltatory conduction– In myelinated fibers; faster as impulses “leap”

between nodes of Ranvier• Factors that increase rate of conduction– Myelin, large diameter and warm nerve fibers

Synaptic Transmission• Components of synapse:–Sending neuron: presynaptic neuron

(releases neurotransmitter)–Space between neurons: synaptic cleft–Receiving neuron: postsynaptic neuron

Synaptic Transmission

• Action potential arrives at presynaptic neuron’s end bulb

• Opens voltage gated Ca2+ channels Ca2+

flows into presynaptic cytosol• Increased [Ca2+ ] exocytosis of synaptic

vesicles• Neurotransmitter (NT) released into cleft• NT diffuses across cleft and binds to

receptors in postsynaptic cell membrane

Synaptic Transmission• NT serves as chemical trigger (stimulus) of ion

channels• Postsynaptic cell membrane may be

depolarized or hyperpolarized – Depends on type of NT and type of postsynaptic cell– 1000+ neurons converge on synapse; the sum of all

of their NTs determines effect• If threshold reached, then postsynaptic cell

action potential results

Synaptic Transmission• Finally, NT must be removed from the cleft.

Three possible mechanisms– Diffusion out of cleft– Destruction by enzymes (such as ACh-ase) in cleft– Transport back (recycling) into presynaptic cell

Synaptic Transmission

Neurotransmitters• Acetylcholine (ACh): common in PNS– Stimulatory (on skeletal muscles) – Inhibitory (on cardiac muscle)

• Amino acids– Glutamate, aspartate, gamma aminobutyric acid

(GABA), glycine• Modified amino acids – Norepinephrine (NE), dopamine (DA), serotonin

• Neuropeptides such as endorphins• Nitric oxide (NO)