Chapter 12

Intro to the Nervous System

The Nervous System• The most complex system• Coordinates activities of all body systems• Two divisions:

The Central Nervous System (CNS) = brain & spinal cordThe Peripheral Nervous System (PNS) = all neural tissue not included in CNS

Histology of Nervous Tissue

2 cell types:• Neurons

transmit nerve impulses

• Neuroglia:support neurons

Neurons• Very long lived• Amitotic

mature neurons lose the ability to divide

• High metabolic rateneed a large supply of O2 and C6H12O6

Neuron Structure• Cell Body = Soma

contains the usual cellular organelles

• Dendritesshort, highly branched extensions from the somatransfer information to the cell body

• Axontransmits information away from the somaends in many branches (axon terminals)

• Action potentials (nerve impulses) start at the axon hillock & travel along the axon to the axon terminal

• Arrival of action potential causes the release of neurotransmitters across a synapse to the dentrites of the next neuron

• Neurotransmitters can excite or inhibit the next neuron

• Myelin sheath = lipid-rich covering on axons, myelinated fibers conduct impulses 10-150x faster than unmyelinated fiber

Nodes of Ranvier = gaps between myelin; impulses jump from node to node

Functions of the Nervous System• Sensory Input

Sensory neurons deliver info (stimulus) from sensory receptors to the CNS

• IntegrationInterneurons in the CNS process & interpret sensory stimulus

• Motor Output Motor neurons cause response by carrying info from

the CNS to various effector organs

Sensory Neuron

Interneuron

Motor Neuron

Muscle Contracts

Synapse Synapse

Synapse

NERVES

Nerves = bundles of neurons• Can contain all motor neurons, all sensory

neurons, or be mixed