Download OVERVIEW OF PERIPHERAL NERVOUS SYSTEM Peripheral

OVERVIEW
OF
PERIPHERAL
NERVOUS SYSTEM
Peripheral Nervous System
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The Peripheral Nervous System
Nervous structures outside the brain and spinal cord
Nerves allow the CNS to receive information and take action
Functional components of the PNS
– Sensory inputs and motor outputs categorized as somatic or
visceral
– Sensory inputs also classified as general or special
Sensory Input and Motor Output
Sensory (afferent) signals picked up by sensor receptors, carried by
nerve fibers of PNS to the CNS
Motor (efferent) signals are carried away from the CNS, innervate
muscles and glands
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
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PNS Afferent Division
Afferent (sensory) division – transmits impulses from receptors to the
CNS.
– Somatic afferent fibers – carry impulses from skin, skeletal
muscles, and joints
– Visceral afferent fibers – transmit impulses from visceral organs
PNS Efferent Division
Motor (efferent) division – transmits impulses from the CNS to effector
organs. Two subdivisions:
– Somatic nervous system – provides conscious control of skeletal
muscles
– Autonomic nervous system – regulates smooth muscle, cardiac
muscle, and glands
Types of Sensory and Motor Information
Sensory
General somatic senses – include touch, pain, vibration, pressure,
temperature
Proprioceptive senses – detect stretch in tendons and muscle provide
information on body position, orientation and movement of body in
space
Special Senses - hearing, balance, vision, olfaction (smell), gustation
(taste)
Motor
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General somatic motor
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• Signals contraction of skeletal muscles
• Under our voluntary control
– Visceral motor
• Makes up autonomic nervous system (ANS)
• Regulates the contraction of smooth and cardiac muscle,
controls function of visceral organs
• ANS has two divisions
– Parasympathetic
– Sympathetic
Divisions of the ANS
Sympathetic - ―fight or flight‖
– Catabolic (expend energy)
– Mass activation prepares for intense activity.
• Heart rate (HR) increases.
• Bronchioles dilate.
• Blood [glucose] increases.
Parasympathetic - ―feed & breed‖, ―rest & digest‖
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Maintain homeostasis
Normally not activated as a whole, stimulation
of separate parasympathetic nerves.
– Relaxing effects:
• Decreases HR.
• Dilates visceral blood vessels.
• Increases digestive activity.
Dual innervation of many organs — having a brake and an accelerator
provides more control
Sympathetic Division Organization
Preganglionic neurons in segments T1 to L2
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– Ganglia near the vertebral column
– Sympathetic ganglia
• Paired sympathetic chain ganglia
• Unpaired collateral ganglia
– Preganglionic fibers to adrenal medullae
• Epinephrine (adrenalin) into blood stream
The Autonomic Nervous System
Effects of Sympathetic Activation
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Generalized response in crises
Increased alertness/energy
Increased cardiovascular activity
Increased respiratory activity
Increased muscle tone
Parasympathetic Division Organization
Preganglionic neurons in brain stem and sacral spinal segment
Ganglionic neurons (peripheral ganglia) in or near target organ
Sacral fibers form pelvic nerves
The Autonomic Nervous System
Effects of Parasympathetic Activation
– Relaxation
– Food processing
– Energy absorption
– Brief effects at specific sites
Basic Structural Components of the PNS
Sensory receptors – pick up stimuli from inside or outside the body
Motor endings – axon terminals of motor neurons innervate effectors
(muscle fibers and glands)
Nerves and ganglia
– Nerves – bundles of peripheral axons
– Ganglia – clusters of peripheral neuronal cell bodies
Nerves
Nerves – cablelike organs in the PNS
Consists of numerous axons wrapped in connective tissue
– Endoneurium – layer of delicate connective tissue
surrounding the axon
– Perineurium – connective tissue wrapping surrounding
a nerve fascicle
– Nerve fascicles – groups of axons bound into bundles
– Epineurium – whole nerve is surrounded by tough
fibrous sheath
Axon is surrounded by Schwann cells
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Cranial Nerves
Attach to the brain and pass through foramina of the skull
Numbered from I–XII
Cranial nerves I and II attach to the forebrain
– All others attach to the brain stem
Primarily serve head and neck structures
– The vagus nerve (X) extends into the abdomen
The 12 Pairs of Cranial Nerves
Spinal Nerves
31 pairs – contain thousands of nerve fibers
Connect to the spinal cord
Named for point of issue from the spinal cord
– 8 pairs of cervical nerves (C1–C8)
– 12 pairs of thoracic nerves (T1–T12)
– 5 pairs of lumbar nerves (L1–L5)
– 5 pairs of sacral nerves (S1–S5)
– 1 pair of coccygeal nerves (Co1)
Spinal Nerves
Connect to the spinal cord by the dorsal root and ventral root
– Dorsal root – contains sensory fibers
– Dorsal root ganglion – of afferent cell bodies
– Ventral root – contains motor fibers arising from anterior gray
column
Branch into dorsal ramus and ventral ramus both contain sensory
and motor fibers
Rami communicantes connect to the base of the ventral ramus and
lead to the sympathetic chain ganglia
Spinal Nerves
Innervation of the Back
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Dorsal rami
– Innervate back muscles
– Follow a neat, segmented pattern
– Innervate a horizontal strip of muscle and skin
– In line with emergence point from the vertebral column
Innervation of the Thoracic region
– Ventral rami arranged in simple, segmented pattern
– Intercostal nerves – supply intercostal muscles, skin, and
abdominal wall
• Each gives off lateral and anterior cutaneous branches
Introduction to Nerve Plexuses
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Nerve plexus – a network of nerves
Ventral rami (except T2 – T12)
– Branch and join with one another
– Form nerve plexuses
• Cervical
• Brachial
• Lumbar
• Sacral
– Primarily serve the limbs
– Fibers from ventral rami crisscross
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