The Nervous System
Chapter 9
Nervous System
• The master controlling and communicating
system of the body
• Functions:
– Sensory input – monitoring stimuli
– Integration – interpretation of sensory input
– Motor output – response to stimuli
Organization of the Nervous System
• Central nervous system (CNS)
– Brain & spinal cord
– Integration and command center
• Peripheral nervous system (PNS)
– Paired spinal & cranial nerves
– Carries messages to and from the spinal cord and
brain
Peripheral nervous system (PNS)
• Two functional divisions:
– Sensory (afferent) division
• Sensory afferent fibers – carry impulses from skin,
skeletal muscles, and joints to the brain
• Visceral afferent fibers – transmit impulses from
visceral organs to the brain; Ex) feeling full
– Motor (efferent) division
• Transmits impulses from the CNS to effector organs
Motor Division: Two Main Parts
• Somatic nervous system
– Conscious control of skeletal muscles
• Autonomic nervous system (ANS)
– Regulates smooth muscle, cardiac muscle, and glands
(unconscious)
– Divisions:
• Sympathetic – fight or flight system; increase in heart rate,
blood pressure, blood sugar
• Parasympathetic – housekeeping systems
(resting/digestive); keeps functions going under normal
conditions
Histology of the Nerve Tissue
• The two principle cell types of the nervous system
are:
– Neurons  excitable cells that transmit electrical
signals
– Supporting cells  cells that surround and wrap
neurons (neuroglial or glial cells)
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Provide a supportive scaffolding for neurons
Segregate and insulate neurons
Guide young neurons to the proper connections
Promote health and growth
Neurons (Nerve Cells)
• Lose their ability to divide
• Very long lived
• High metabolic rate – need astrocytes to get
nutrients from capillaries
• NEED OXYGEN
• No centrioles
• Dendrites and Cell bodies receive graded
potentials
Neuron Structure
• Neurons vary considerably in size & shape, but
they have certain features in common:
– Cell body
– Nerve fibers
– Axon
– Dendrites
Nerve Cell Body
• Contains the nucleus and nucleolus
• The major biosynthetic center
• Is the focal point for the outgrowth of
neuronal processes
• Has well-developed Nissl bodies (rough ER)
• Contains an axon hillock – cone-shaped area
from which axons arise
Dendrites of Motor Neurons
• Short, tapering, and diffusely branched
processes
• They are the receptive, or input, regions of the
neuron
• Electrical signals are conveyed as graded
potentials (not action potentials)
Axons: Structure
• Slender processes of uniform diameter arising
from the axonal hillock of the cell body
• Long axons are called nerve fibers
• Usually there is only one unbranched axon per
neuron
• Axonal terminal – branched terminus of an
axon
Axons: Function
• Generate and transmit action potentials
• Secrete neurotransmitters from the axonal
terminals
• Movement along axons occurs in two ways
– Anterograde – toward the axonal terminal
– Retrograde – away from the axonal terminal
Myelin Sheath
• Whitish, fatty (protein-lipoid), segmented
sheath around most long axons
• It functions to:
– Protect the axon
– Electrically insulate fibers from one another
– Increase the speed of nerve impulse transmission
Myelin Sheath & Neurilemma:
Formation
• Formed by Schwann cells in the PNS
• A Schwann cell:
– Envelopes an axon in a trough
– Encloses the axon with its plasma membrane
– Has concentric layers of membrane that make up
the myelin sheath
• Neurilemma – remaining nucleus and
cytoplasm of a Schwann cell
Nodes of Ranvier
• Gaps in the myelin sheath between adjacent
Schwann cells
• They are the sites where axon collaterals can
emerge
Types of Neurons and Neuroglial Cells
• Neuron Classification (structural):
– Multipolar – three or more processes
– Bipolar – two processes (axon & dendrite)
– Unipolar – single, short process
Types of Neurons & Neuroglial Cells
• Neuron Classification (function):
– Sensory (afferent) – transmit impulses toward the
CNS (stimulus)
– Motor (efferent) – carry impulses away from the
CNS (reaction)
– Interneurons (association neurons) – shuttle
signals through CNS pathways
Types of Neurons & Neuroglial Cells
• Classification of Neuroglial Cells
– Microglial cells
– Oligodendrocytes
– Astrocytes:
– Ependymal cells
Microglial Cells
• Small, oval cells w/ spiny processes
• Scattered throughout the CNS
• Support neurons & phagocytize bacterial cells
& cellular debris
Oligodendrocytes
• Occur in rows along nerve fibers
• Form myelin within the brain & spinal cord
• Insulation
Astrocytes
• Most abundant, versatile, & highly branched
glial cells
• Cling to neurons at the synaptic endings &
cover capillaries
• Functions:
– Support & brace neurons
– Anchor neurons to their nutrient supplies
– Guide migration of younger neurons
Ependymal Cells
• Range in shape from squamous to columnar
• Line the central cavities of the brain and spinal
column
• Make a barrier for spinal fluid
Spinal Cord (structure)
• Consists of 31 segments – each gives rise to a
pair of spinal nerves
• Cervical enlargement – in the neck region,
supplies nerves to the upper limbs
• Lumbar enlargement – in the lower back,
supplies nerves to the lower limbs
Spinal Cord (function)
• Has 2 main functions:
– Conducting nerve impulses
– Serving as a center for spinal reflexes
• Nerve tracts provide a two-way communication system
between the brain and body parts outside the nervous
system
Spinal Cord (function)
• Ascending tracts – carry sensory information
to the brain
• Descending tracts – conduct motor impulses
from the brain to muscles and glands
• Nerve fibers within ascending and descending
tracts are axons