The Peripheral Nervous System (PNS)

Peripheral Nervous System (PNS)

§    PNS – all neural structures outside the brain and spinal cord

§    Includes sensory receptors, peripheral nerves, associated ganglia, and motor endings

§    Provides links to and from the external environment

PNS in the Nervous System

Sensory Receptors

§    Structures specialized to respond to stimuli

§    Activation of sensory receptors results in depolarizations that trigger impulses to the CNS

§    The realization of these stimuli, sensation and perception, occur in the brain

Receptor Classification by Stimulus Type

§    Mechanoreceptors – respond to touch, pressure, vibration, stretch, and itch

§    Thermoreceptors – sensitive to changes in temperature

§    Photoreceptors – respond to light energy (e.g., retina)

§    Chemoreceptors – respond to chemicals (e.g., smell, taste, changes in blood chemistry)

§    Nociceptors – sensitive to pain-causing stimuli

Receptor Class by Location: Exteroceptors

§    Respond to stimuli arising outside the body

§    Found near the body surface

§    Sensitive to touch, pressure, pain, and temperature

§    Include the special sense organs

Receptor Class by Location: Interoceptors

§    Respond to stimuli arising within the body

§    Found in internal viscera and blood vessels

§    Sensitive to chemical changes, stretch, and temperature changes

Receptor Class by Location: Proprioceptors

§    Respond to degree of stretch of the organs they occupy

§    Found in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles

§    Constantly “advise” the brain of one’s movements

Receptor Classification by Structural Complexity

§    Receptors are structurally classified as either simple or complex

§    Most receptors are simple and include encapsulated and unencapsulated varieties

§    Complex receptors are special sense organs

Simple Receptors: Unencapsulated

§    Free dendritic nerve endings

§     Respond chiefly to temperature and pain

§    Merkel (tactile) discs

§    Hair follicle receptors

Simple Receptors: Encapsulated

§    Meissner’s corpuscles (tactile corpuscles)

§    Pacinian corpuscles (lamellated corpuscles)

§    Muscle spindles, Golgi tendon organs, and Ruffini’s corpuscles

§    Joint kinesthetic receptors

From Sensation to Perception

§    Survival depends upon sensation and perception

§    Sensation is the awareness of changes in the internal and external environment

§    Perception is the conscious interpretation of those stimuli

Organization of the Somatosensory System

§    Input comes from exteroceptors, proprioceptors, and interoceptors

§    The three main levels of neural integration in the somatosensory system are:

§     Receptor level – the sensor receptors

§     Circuit level – ascending pathways

§     Perceptual level – neuronal circuits in the cerebral cortex


Processing at the Receptor Lever

§    The receptor must have specificity for the stimulus energy

§    The receptor’s receptive field must be stimulated

§    Stimulus energy must be converted into a graded potential

§    A generator potential in the associated sensory neuron must reach threshold

Adaptation of Sensory Receptors

§    Adaptation occurs when sensory receptors are subjected to an unchanging stimulus

§     Receptor membranes become less responsive

§     Receptor potentials decline in frequency or stop

§    Receptors responding to pressure, touch, and smell adapt quickly

§    Receptors responding slowly include Merkel’s discs, Ruffini’s corpuscles, and interoceptors that respond to chemical levels in the blood

§    Pain receptors and proprioceptors do not exhibit adaptation


Processing at the Circuit Level

§    Chains of three neurons conduct sensory impulses upward to the brain

§    First-order neurons – soma reside in dorsal root or cranial ganglia, and conduct impulses from the skin to the spinal cord or brain stem

§    Second-order neurons – soma reside in the dorsal horn of the spinal cord or medullary nuclei and transmit impulses to the thalamus or cerebellum

§    Third-order neurons – located in the thalamus and conduct impulses to the somatosensory cortex of the cerebrum

Processing at the Perceptual Level

§    The thalamus projects fibers to:

§     The somatosensory cortex

§     Sensory association areas

§    First one modality is sent, then those considering more than one

§    The result is an internal, conscious image of the stimulus

Main Aspects of Sensory Perception

§    Perceptual detection – detecting that a stimulus has occurred and requires summation

§    Magnitude estimation – how much of a stimulus is acting

§    Spatial discrimination – identifying the site or pattern of the stimulus

§    Feature abstraction – used to identify a substance that has specific texture or shape

§    Quality discrimination – the ability to identify submodalities of a sensation (e.g., sweet  or sour tastes)

§    Pattern recognition – ability to recognize patterns in stimuli (e.g., melody, familiar face)


Structure of a Nerve

§    Nerve – cordlike organ of the PNS consisting of peripheral axons enclosed by connective tissue

§    Connective tissue coverings include:

§     Endoneurium – loose connective tissue that surrounds axons

§     Perineurium – coarse connective tissue  that bundles fibers into fascicles

§     Epineurium – tough fibrous sheath around a nerve

Classification of Nerves

§    Sensory and motor divisions

§    Sensory (afferent) – carry impulse to the CNS

§    Motor (efferent) – carry impulses from CNS

§    Mixed – sensory and motor fibers carry impulses to and from CNS; most common type of nerve

Peripheral Nerves

§    Mixed nerves – carry somatic and autonomic (visceral) impulses

§    The four types of mixed nerves are:

§     Somatic afferent and somatic efferent

§     Visceral afferent and visceral efferent

§    Peripheral nerves originate from the brain or spinal column


Regeneration of Nerve Fibers

§    Damage to nerve tissue is serious because mature neurons are amitotic

§    If the soma of a damaged nerve remains intact, damage can be repaired

§    Regeneration involves coordinated activity among:

§     Macrophages – remove debris

§     Schwann cells – form regeneration tube and secrete growth factors

§     Axons – regenerate damaged part


Cranial Nerves

§    Twelve pairs of cranial nerves arise from the brain

§    They have sensory, motor, or both sensory and motor functions

§    Each nerve is identified by a number (I through XII) and a name

§    Four cranial nerves carry parasympathetic fibers that serve muscles and glands

The Peripheral Nervous System (PNS)

Spinal Nerves

§    Thirty-one pairs of mixed nerves arise from the spinal cord and supply all parts of the body except the head

§    They are named according to their point of issue

§     8 cervical (C1-C8)

§     12 thoracic (T1-T12)

§     5 Lumbar (L1-L5)

§     5 Sacral (S1-S5)

§     1 Coccygeal (C0)

Spinal Nerves: Roots

§    Ventral roots arise from the anterior horn and contain motor (efferent) fibers

§    Dorsal roots arise from sensory neurons in the dorsal root ganglion and contain sensory (afferent) fibers

Spinal Nerves: Rami

Nerve Plexuses

§    All ventral rami except T2-T12 form interlacing nerve networks called plexuses

§    Plexuses are found in the cervical, brachial, lumbar, and sacral regions

§    Each resulting branch of a plexus contains fibers from several spinal nerves

§    Fibers travel to the periphery via several different routes

§    Each muscle receives a nerve supply from more than one spinal nerve

§    Damage to one spinal segment cannot completely paralyze a muscle

Cervical Plexus

§    The cervical plexus is formed by ventral rami of

§    Most branches are cutaneous nerves of the neck, ear, back of head, and shoulders

§    The most important nerve of this plexus is the phrenic nerve

§    The phrenic nerve is the major motor and sensory nerve of the diaphragm

Brachial Plexus

§    Formed by C5-C8 and T1 (C4 and T2 may also contribute to this plexus)

§    It gives rise to the nerves that innervate the upper limb

Lumbar Plexus

§    Arises from L1-L4 and innervates the thigh, abdominal wall, and psoas muscle

§    The major nerves are the femoral and the obturator

Sacral Plexus

§    Arises from L4-S4 and serves the buttock, lower limb, pelvic structures, and the perineum

§    The major nerve is the sciatic, the longest and thickest nerve of the body

§    The sciatic is actually composed of two nerves: the tibial and the common fibular (peroneal) nerves


§    A dermatome is the area of skin innervated by the cutaneous branches of a single spinal nerve

§    All spinal nerves except C1 participate in dermatomes

Innervation of Joints

§    Hilton’s law: any nerve serving a muscle that produces movement at a joint also innervates the joint itself and the skin over the joint


Motor Endings

§    PNS elements that activate effectors by releasing neurotransmitters at:

§     Neuromuscular junctions

§     Varicosities at smooth muscle and glands

Innervation of Skeletal Muscle

§    Takes place at a neuromusclular junction

§    Acetylcholine is the neurotransmitter that diffuses across the synaptic cleft

§    ACh binds to receptors resulting in:

§     Movement of Na+ and K+ across the membrane

§     Depolarization of the interior of the muscle cell

§     An end-plate potential that triggers an action potential

Innervation of Visceral Muscle and Glands

§    Autonomic motor endings and visceral effectors are simpler than somatic junctions

§    Branches form synapses en passant via varicosities

§    Acetylcholine and norepinephrine are used as neurotransmitters

§    Visceral responses are slower than somatic responses




§    A reflex is a rapid, predictable motor response to a stimulus

§    Reflexes may:

§     Be inborn (intrinsic) or learned (acquired)

§     Involve only peripheral nerves and the spinal cord

§     Involve higher brain centers as well

Reflex Arc

§    There are five components of a reflex arc

§     Receptor – site of stimulus

§     Sensory neuron – transmits the afferent impulse to the CNS

§     Integration center – either monosynaptic or polysynaptic region within the CNS

§     Motor neuron – conducts efferent impulses from the integration center to an effector

§     Effector – muscle fiber or gland that responds to the efferent impulse









Stretch Reflex

§    Stretching the muscle activates the muscle spindle

§    Excited g motor neurons of the spindle cause the stretched muscle to contract

§    Afferent impulses from the spindle result in inhibition of the antagonist

§    Example: patellar reflex

§     Tapping the patellar tendon stretches the quadriceps and starts the reflex action

§     The quadriceps contract and the antagonistic hamstrings relax

Golgi Tendon Reflex

§    The opposite of the stretch reflex

§    Contracting the muscle activates the Golgi tendon organs

§    Afferent Golgi tendon neurons are stimulated, neurons inhibit the contracting muscle, and the antagonistic muscle is activated

§    As a result, the contracting muscle relaxes and the antagonist contracts

Flexor and Crossed Extensor Reflexes

§    The flexor reflex is initiated by a painful stimulus (actual or perceived) that causes automatic withdrawal of the threatened body part

§    The crossed extensor reflex has two parts

§     The stimulated side is withdrawn

§     The contralateral side is extended