The Peripheral Nervous
System
Peripheral Nervous System
31 pairs of spinal
nerves
12 pairs of cranial
nerves
All of the smaller
nerves that branch
from larger nerves
Afferent and Efferent Pathways
Afferent Pathways
Efferent Pathways
Spinal Nerves
31 Pairs
Numbered according to the level of the vertebral
column at which they emerge from the spinal
cavity
Lumbar, Sacral, and Coccygeal nerves descend
from their point of origin at the lower end of the
spinal cord
Lower end of the cord is called the cauda
equina, which means “horses tail” in Latin
Structure of Spinal Nerves
Mixed Nerves (contain
motor and sensory fibers)
Attach by means of two
short roots
Dorsal root
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Has ganglion (cell bodies)
Carries sensory neruons
Ventral root
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Carries motor neurons
Dorsal and Ventral Rami
Soon after each spinal nerve emerges from the
spinal cavity, it forms several large branches,
each of which is called a ramus.
Dorsal Ramus

Supplies somatic motor and sensory fibers to serveral
smaller nerves
Ventral Ramus

More complex
Rami of Spinal Nerves
Nerve Plexuses
Ventral rami of most spinal nerves
subdivide to form complex networks called
plexuses.
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Cervical plexus
Brachial plexus
Lumbar plexus
Sacral plexus
Damage to one
spinal nerve does
not mean complete
loss of function in
any one region.
Dermatomes and Myotomes
Each skin surface
area supplied by
sensory fibers of a
given spinal nerve is
called a dermatome.
A myotome is skeletal
muscle or group of
muscles that receives
motor axons from a
given spinal nerve.
Dermatome Distribution of Spinal
Nerves
Myotomes and Body Movement
Cranial Nerves
12 pairs
Connect to the undersurface of the brain
Pass through small foramina (holes) in the cranial cavity
and skull
Identified by names and numbers
3 Types
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Mixed
Sensory
Motor
Divisions of the
Peripheral Nervous
System
Somatic Motor Nervous System
All the voluntary motor pathways outside
the CNS (pathways to skeletal muscles)
Single motor neuron whose axon stretches
from the cell body in the CNS all the way
to the effector
Stimulates effectors by means of the
neurotransmitter acetylcholine
Somatic Reflexes
The action that results from a nerve impulse passing
over a reflex arc is called a reflex.
Predictable response to a stimulus
May or may not be conscious
Somatic reflexes are contraction of skeletal muscle
Autonomic reflexes consist of contractions of smooth or
cardiac muscle, or secretion by glands
Used in the diagnosis of disease
Knee Jerk Reflex
Autonomic Nervous System
Regulates involuntary effectors
Major function of the ANS is to regulate
heartbeat, smooth muscle contraction, and
glandular secretion in ways to maintain
homeostasis.
Sympathetic and Parasympathetic
Divisions
Sympathetic and Parasympathetic have
separate pathways
Effectors may have dual innervation, that
is they have input from both types of
pathways
Parasympathetic – “rest-and-repair”
Sympathetic – “fight-or-flight”
Autonomic Conduction Paths