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Chapter 12
Nervous System Cells
Introduction
• The function of the nervous
system, along with the endocrine
system, is to communicate
– Controls and integrates many
different functions performed by
different organs, tissues, and cells
– Communication, Control, Integration,
Homeostasis,Survival
• The nervous system is made up
of the
– Brain
– Spinal cord
– Nerves
Organization of the Nervous System
• Organized to…
– Detect changes in internal and external environments
– Evaluate the information
– Initiate an appropriate response
• Central nervous system (CNS)
– Structural and functional center of entire nervous system
– Consists of the brain and the spinal cord
– Integrates sensory information, evaluates it, and initiates
an outgoing response
• Peripheral nervous system (PNS)
– Nerves that lie in “outer regions” of nervous system
– Cranial nerves—originate from brain
– Spinal nerves—originate from spinal cord
Organization of the Nervous System
• Divisions of the nervous system
– Afferent division - incoming sensory pathways
– Efferent division - outgoing motor pathways
• Somatic Nervous System (SNS) – skeletal muscles
– Somatic motor division (efferent)
– Somatic sensory division (afferent)
– Somatic integration centers
• Autonomic Nervous System (ANS) – smooth/cardiac muscles, glands
– Autonomous of voluntary control
– Sympathetic division – “fight or flight” (efferent)
• Middle portions of the spinal cord
– Parasympathetic division – “rest and repair” (efferent)
• Brain and lower portions of spinal cord
– Visceral sensory division (afferent)
– Autonomic integration centers
Cells of the Nervous System
• Glia
– Support the functions of neurons
– 900 billion in the human nervous system
– Ability to do cell division throughout adulthood
– Susceptible to abnormalities…cancer
– Five types
• Astrocytes
• Microglia
• Ependymal cells
• Oligodendrocytes
• Schwann cells
Cells of the Nervous System
• Astrocytes
– Star-shaped
– Largest
– Most numerous type
– Cell extensions connect to both
neurons and capillaries
– Transfer nutrients from the blood to
the neurons
– Constitute the blood-brain barrier
(BBB)
Cells of the Nervous System
• Microglia
– Small, usually stationary
– In inflamed brain tissue, they
enlarge, move about, and carry on
phagocytosis
• Ependymal cells
– Resemble epithelial cells and form
thin sheets that line fluid-filled
cavities in the CNS
– Some produce fluid; others aid in
circulation of fluid
Cells of the Nervous System
• Oligodendrocytes
– Smaller than astrocytes with fewer
processes
– Hold nerve fibers together and
produce the myelin sheath
Cells of the Nervous System
• Schwann cells
– Found only in PNS
– Support nerve fibers and form myelin sheaths
(Figure 12-4)
– Gaps in the myelin sheath are called nodes of Ranvier
– Neurilemma is formed by cytoplasm of Schwann cell (neurilemmocyte)
wrapped around the myelin sheath; essential for nerve regrowth
– Satellite cells are Schwann cells that cover and support cell bodies in the
PNS
Cells of the Nervous System
• Neurons
–
–
–
–
Excitable cells that initiate and conduct impulses
Components of neurons
100 billion in the brain (10% of nervous system cells)
Structure
• Cell body (perikaryon)
– Nucleus, cytoplasm, mitochondria, Golgi apparatus, ribosomes,
endoplasmic reticulum (Nissl bodies)
• Dendrites
– Receive the stimuli and initiate nerve signals
– Conduct nerve signals to the cell body
• Axon
–
–
–
–
–
A single process extending from the axon hillock
Axon collaterals (side branches)
Sometimes covered by a fatty layer called a myelin sheath
Conducts nerve impulses away from the cell body of the neuron
Distal tips of axons are telodendria, each of which terminates in a
synaptic knob
– Thicker diameter = faster impulse
Cells of the Nervous System
• Classification of neurons
– Structural classification—classified according to
number of processes extending from cell body
• Multipolar
– One axon and several dendrites
• Bipolar
– Only one axon and one dendrite
– Least numerous kind of neuron
• Unipolar (pseudounipolar)
– One process comes off neuron cell body but divides almost
immediately into two fibers
» Central fiber (toward CNS)
» Peripheral fiber (away from CNS)
Cells of the Nervous System
• Classification of neurons
– Functional classification
• Afferent (sensory) neurons
– Conduct impulses to spinal
cord or brain
• Efferent (motor) neurons
– Conduct impulses away
from spinal cord or brain
toward muscles or glandular
tissue
• Interneurons
– Entirely within CNS
Cells of the Nervous System
• Reflex arc
– A signal conduction route to and from the CNS
• Electrical signal beginning in receptors and ending in
effectors
– Three-neuron arc
• Most common
• Consists of afferent neurons, interneurons, and efferent
neurons
• Afferent neurons—conduct impulses to the CNS from
the receptors
• Efferent neurons—conduct impulses from the CNS to
effectors (muscle or glandular tissue)
– Two-neuron arc
• Simplest form
• Consists of afferent and efferent neurons
Cells of the Nervous System
• Reflex arc (cont.)
– Synapse
• Where nerve signals are transmitted from one neuron to
another
• Located at the junction of the synaptic knob of one neuron
and the dendrites or cell body of another neuron
• Electrical and chemical synapses (more on this later)
– Ipsilateral reflex arc
• Receptors and effectors located on the same side of body
– Contralateral reflex arc
• Receptors and effectors located on opposite sides of the
body
– Many signals fail to be conducted across a synapse
– Not all signals that terminate in effectors start in
receptors
Nerves and Tracts
• Nerves
– Bundles of peripheral nerve fibers held together by several
layers of connective tissue
• Endoneurium—delicate layer of fibrous connective tissue
surrounding each nerve fiber
• Perineurium—connective tissue holding together
fascicles (bundles of fibers)
• Epineurium—fibrous coat surrounding numerous
fascicles and blood vessels to form a complete nerve
• Tracts
– Within the CNS, bundles of nerve fibers are called tracts
rather than nerves
Nerves and Tracts
• White matter
– PNS—myelinated nerves
– CNS—myelinated tracts
• Gray matter
– Made up of cell bodies and unmyelinated fibers
– CNS—referred to as nuclei
– PNS—referred to as ganglia
• Mixed nerves
– Contain sensory and motor neurons
– Sensory nerves—predominantly sensory neurons
– Motor nerves—predominantly motor neurons