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 gland  Emerges 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 axons Gray matter: cell bodies, dendrites, unmyelinated axons, axon terminals, neuroglia Locations:  Spinal cord: white matter (tracts) surround centrally located gray matter “H” or “butterfly”  Brain: gray matter in thin cortex surrounds white matter (tracts)

NEURON PHYSIOLOGY The Action Potential

Action potential = nerve impulse APs 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 concentration o Leakage channels o 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 outside Caused 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 phase Typically 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 Potentials Interactions Animations Membrane Potentials You must be connected to the internet to run this animation.

Conduction of Nerve Impulses Nerve 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 Ca 2+ channels  Ca 2+ flows into presynaptic cytosol Increased [Ca 2+ ]  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 – 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)