2. Neurons
• Neurons = nerve cells
– Cells specialized to transmit messages
– Major regions of neurons
• Cell body – nucleus and metabolic center of the cell
• Processes – fibers that extend from the cell body
4. Neuron Anatomy
• Extensions outside
the cell body
– Dendrites –
conduct impulses
toward the cell
body
– Axons – conduct
impulses away
from the cell
body
5. Axons and Nerve Impulses
• Axons end in axonal terminals
• Axonal terminals contain vesicles with
neurotransmitters
• Axonal terminals are separated from the
next neuron by a gap
– Synaptic cleft – gap between adjacent neurons
6. Synapses
• Site at which neurons communicate
• Signals pass across synapse in one direction
• Presynaptic neuron
– Conducts signal toward a synapse
• Postsynaptic neuron
– Transmits electrical activity away from a synapse
10. Myelin Sheaths
• Segmented structures composed of the
lipoprotein myelin
• Surround the axon except at its endings
• Form an insulating layer
– Prevent leakage of electrical current
• Increase the speed of impulse conduction
11. Myelin Sheaths
• Formed by Schwann
cells
• Schwann cells wrap
in concentric layers
around the axon
– Cover the axon in a
tightly packed coil of
membranes
12. Myelin Sheaths
• Thick axons are myelinated
– Fast conduction velocity
• Thin axons are unmyelinated
– Slow conduction velocity
13. Nerve Fiber Coverings
• Schwann cells –
produce myelin
sheaths in jelly-roll
like fashion
• Nodes of Ranvier –
gaps in myelin sheath
along the axon
14. Nervous System: Functions
• Three overlapping functions
– Sensory receptors monitor changes inside and
outside the body
• Change – a stimulus
• Gathered information – sensory input
– CNS Processes and interprets sensory input
• Makes decisions – integration
– Dictates a response by activating effector organs
• Response – motor output
20. Structural Classification of the Nervous
System
• Central nervous system (CNS)
– Brain
– Spinal cord
• Peripheral nervous system (PNS)
– Nerve outside the brain and spinal cord
21. Sensory Input and Motor Output
• Sensory signals picked up by sensory receptors
– Carried by afferent nerve fibers of PNS to the CNS
• Motor signals are carried away from the CNS
– Carried by efferent nerve fibers of PNS to effectors
– Innervate muscles and glands
22. Sensory Input and Motor Output
• Divided according to region they serve
– Somatic body region
– Visceral body region
• Results in four main subdivisions
– Somatic sensory
– Visceral sensory
– Somatic motor
– Visceral motor
23. Somatic Sensory
• Somatic sensory
– General somatic senses – receptors are widely
spread
• Touch, pain, vibration, pressure, and temperature
• Body sense – position and movement of body in
space
24. Somatic Motor
• Somatic motor
– General somatic motor – signals contraction of
skeletal muscles
• Under voluntary control
• Often called “voluntary nervous system”
25. Visceral Sensory
• Visceral sensory
– General visceral senses – stretch, pain,
temperature, nausea, and hunger
• Widely felt in digestive and urinary tracts, reproductive
organs
26. Visceral Motor
• Visceral motor
– Regulates the contraction of smooth and cardiac
muscle and gland secretion
– Makes up autonomic nervous system
– Controls function of visceral organs
– Often called “involuntary nervous system”
28. Nerve Fibers That Transmit Different Types of Signals, and Their
Physiologic Classification
• Nerve fibers come in all sizes between 0.5 and
20 micrometers in diameter-the larger the
diameter, the greater the conducting velocity.
• The range of conducting velocities is between
0.5 and 120 m/sec