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Chapter 7
Lecture Notes
Sensory Input > Integration > Motor Output
Structural Classification:
CNS = Brain and Spinal Cord. The integrating and command center.
PNS = Nerves that extend from the Brain and Spinal Cord. Communication lines
that link all parts of the body.
Functional Classification:
Sensory (afferent) Division:
Nerve fibers that convey impulses to the CNS from the sensory receptors.
Motor (efferent) Division:
Carries impulses from the CNS to effector organs, the muscles and glands.
Subdivisions:
1. Somatic Nervous System- allows us voluntary control of
skeletal muscles.
2. Autonomic Nervous System- Regulates events that are
automatic/involuntary
Nervous Tissue: Structure and Function
Supporting Cells: CNS
Neuroglia (nerve glue): Includes many types of cells that generally
support, insulate, and protect the delicate neurons.
Glia: Different types of neuroglia which have special functions
Astrocytes: star-shaped and accounts for nearly half of the neural
tissue. They have numerous projections with swollen ends that
cling to neurons and anchor them to their nutrient supply.
They form a living barrier between capillaries and neurons and
play a role in making exchanges between them. They pick up
excess ions and recapture released neurotransmitters.
Microglia: spider like phagocytes that dispose of debris (dead
brain cells, bacteria etc.).
Ependymal Cells: Line the cavities of the brain and spinal cord.
The beating of their cilia helps circulate the cerebrospinal fluid
around the CNS.
Oligodendrocytes: Wrap their flat extensions tightly around the
nerve fibers, producing fatty insulating coverings called myelin
sheaths.
*Neuroglia never lose their ability to divide, whereas neurons do.
Consequently, most brain tumors are gliomas or tumors formed by glial
cells.
Supporting Cells: PNS
Schwann Cells: Form myelin sheaths around nerve fibers that are found in the
PNS.
Satellite Cells: Act as protective, cushioning cells.
Neurons: (nerve cells)
Anatomy:
Nerve Cells: highly specialized to transmit messages (nerve impulses).
Cell Body: Contains the nucleus and is the metabolic center of the
cell.
Processes/fibers: Each cell contains one or more of these that
extend from the cell body. Vary in length from microscopic to 3-4
feet.
Dendrites: Neuron processes that conduct electrical currents
toward the cell body. A neuron may have hundreds of these.
Axons: Neuron processes that generate nerve impulses typically
conduct them away from the cell body. A neuron will only have
one axon.
Each axon branches profusely at their terminal end forming
thousands of axonal terminals which stimulate the release
of chemicals called neurotransmitters. (i.e. ACh and
muscle contraction).
Synaptic cleft: The tiny gap between axonal terminals and
neuron. This functional junction is called a synapse.
Myelin: Fatty material which covers most long nerve
fibers. Myelin protects and insulates the fibers and
increases the transmission rate of nerve impulses.
*Axons outside the CNS are myelinated by Schwann
Cells, specialized supporting cells that wrap themselves
tightly around the axon. When the wrapping process is
done a tight coil of wrapped membranes, the myelin
sheath, encloses the axon.
Neurilemma: Outermost part of the plasma membrane of
the Schwann Cell, that is external to the myelin sheath.
Nodes of Ranvier: Gaps/indentations between the
individual Schwann Cells.
Functional Classifications:
Sensory: (afferent) convey impulses to the CNS
Cutaneous Receptors: skin
Proprioreceptors: Muscles and Tendons (stretch receptors)
Motor: (efferent) carries impulses from CNS to effector organs, muscles and
glands.
Somatic: Voluntary system. Voluntary control of skeletal muscles.
Autonomic: Involuntary System. i.e. smooth and cardiac muscles/glands
Interneurons: (association neurons) these connect the motor and sensory
neurons in neural pathways. Their cell bodies are always located in the
CNS.
Motor neurons carry impulses from CNS to the viscera/muscles and
glands. The cell bodies of motor neurons are always located in the CNS.
Nerve Impulse:
Irritability: ability to respond to a stimulus and convert it to a nerve impulse.
Conductivity: ability to transmit the impulse to other neurons, muscles or glands.
More on irritability:
Plasma membrane of resting (inactive) neuron is polarized. (Fewer + ions
on the inner face of the membrane than the outer face)
+ions inside are predominately K+
+ions outside are predominately Na+
*most neurons in the body are excited by neurotransmitters released by other
neurons.
Nerve impulses along unmyelinated fibers:
When neurons are excited the permeability properties of the plasma membrane
change for a brief period. Normally Na+ cannot diffuse through the membrane but when
the neuron is stimulated the “sodium gates” open. Because Na+ is in higher concentration
outside the cell, they quickly diffuse inside. This changes the polarity and the inside is
now more + than outside. This is termed Depolarization, which activates the neuron to
transmit an action potential, also called a nerve impulse (in neurons). Almost
immediately after Na+ rush in, the permeability of the membrane changes back, but now
K+ are allowed to diffuse out which restores the electrical condition and the cell is
Repolarized. After repolarization the sodium-potassium pump restores the cell back to
initial Na+ and K+ concentrations inside and outside cell. (Using ATP)
*Impulses along myelinated fibers conduct much quicker because the nerve
impulse literally jumps or leaps from node to node along the length of the fiber. (saltatory
conduction).
Factors that impair conduction:
 Alcohol, sedatives and anesthetics block impulses by reduction membrane
permeability to Na+.
 Cold and continuous pressure hinders conduction because they interrupt
blood circulation (delivery of oxygen and nutrients) to neurons.
*Impulse conducts from neuron to neuron by releasing neurotransmitter (at axonal
terminals) which diffuses across synapse and binds to receptors on membrane of
next neuron.
Transmission of an impulse is an electrochemical event.
Reflex Arc: Reflexes are rapid, predictable, and involuntary responses to stimuli.
Autonomic Reflexes: Regulate the activity of smooth muscles, the heart,
and glands. (i.e.: secretion o f saliva and changes in the size of the eye
pupils) These reflexes regulate body functions such as digestion,
elimination, blood pressure, and sweating.
Somatic reflexes: include al reflexes that stimulate the skeletal muscles.
When you quickly pull your hand away from a hot object a somatic reflex
is working.
5 minimum elements of a reflex arc:
Sensory receptor: reacts to stimulus
Effector organ: muscle or gland eventually stimulated
Afferent and Efferent neurons: To connect the two.
The CNS integration center