THE PERIPHERAL NERVOUS SYSTEM AND

REFLEX ACTIVITY

THE PERIPHERAL NERVOUS SYSTEM

• PNS: includes all neural structures outside the brain and spinal cord, that is, the sensory receptors, peripheral nerves, and their associated ganglia, and efferent motor endings

PNS in the Structural Organization of the Nervous system

SENSORY RECEPTORS AND

SENSATION

SENSORY RECEPTORS

• Sensory receptors are specialized to respond to changes in their environment called stimuli– Sensation: awareness of the stimulus (PNS)– Perception: interpretation of the meaning of

the stimulus (CNS: brain)

SENSORY RECEPTORS

• Sensory receptors may be classified 3 ways:– Receptors my be classified according to the

activating stimulus– Receptors may be classified based on their

location or the location of the activating stimulus

– Receptors may be classified based on their overall structural complexity

SENSORY RECEPTORS

• Classification based on Stimulus Type: overstimulation of anyone of the following receptors can result in pain:– Mechanoreceptors: generates nerve impulses when stimulated

by forces (touch, pressure, vibration, stretch, itch)– Thermoreceptors: sensitive to temperature changes– Photoreceptors: respond to light energy (e.g. retina of the eye)– Chemoreceptors: respond to chemicals in solution (molecules

smelled or tasted, or changes in blood chemistry)– Nociceptors: respond to potentially damaging stimuli that result

in pain

SENSORY RECEPTORS

• Classification based on Location:– Exteroceptors:

• Sensitive to stimuli arising outside the body• Receptors near or at the body surface

– Interoceptors:• Respond to stimuli arising from within the body• Internal viscera and blood vessels

– Propriceptors:• Respond to internal stimuli from musculoskeletal organs

– Skeletal muscle, joints, tendons, ligaments, connective tissue coverings of bone and muscle

• Monitors degree of stretch of the organs

SENSORY RECEPTORS

• Classification based on Complexity:– Simple:

• Modified dendritic endings of sensory neurons– Found in the skin, mucous membranes, muscles, and

connective tissues

• Monitor most types of general sensory information

– Complex: • Actually sense organs

– Localized collection of cells working together to accomplish a specific receptive process (vision, hearing, smell, and taste)

SENSORY RECEPTORS

• Anatomically:– Classified as free dendritic endings or

encapsulated dendritic endings

SENSORY RECEPTORS

• Free Dendritic Endings:– Naked– Present nearly everywhere in the body, but abundant in

epithelia and connective tissue– Most unmyelinated– Small diameters– Free, or naked, nerve endings are present everywhere in the

body and respond primarily to pain and temperature– Merkel discs: deep layers of skin epidermis

• Function as light touch receptors

– Root Hair Plexuses: light touch receptors that detect bending of hairs

– Itch receptors: seemed to be stimulated by chemicals (histamine and bradykinin) present at inflamed sites

SENSORY RECEPTORS

• Encapsulated Dendritic Endings: consist of one or more fiber terminals of sensory neurons enclosed in a connective tissue capsule

• Virtually all are mechanoreceptors– Meissner’s corpuscles are receptors for discriminatory and light touch in hairless areas of

the body• Tactile corpuscles• Skin

– Krause’s end bulbs: variation of Meissner’s corpuscles• Abundant in mucous membranes• Mucocutaneous corpuscles

– Pacinian, or lamellated, corpuscles, are stimulated when deep pressure is first applied• Deep in epidermis• Respond only when pressure is first applied

– Ruffini’s corpuscles respond to deep and continuous pressure• Dermis

– Muscle spindles detect when a muscle is being stretched and initiate a reflex that resists the stretch

• Neuromuscular spindles– Golgi tendon organs are stimulated when the associated muscle stretches the tendon

• Proprioceptors – Joint kinesthetic receptors monitor the stretch in the articular capsules of synovial joints

OVERVIEW: FROM SENSATION TO PERCEPTION

• The somatosensory system: the part of the sensory system serving the body wall and limbs, involve the receptor level, the circuit level, and the perceptual level– Processing at the receptor level involves a stimulus

that must excite a receptor in order for sensation to occur

– Processing at the circuit level is involved with delivery of impulses to the appropriate region of the cerebral cortex for stimulus localization and perception

– Processing at the perceptual level involves interpretation of sensory input in the cerebral cortex

SOMATOSENSORY SYSTEM

TRANSMISSION LINES: NERVES

AND THEIR STRUCTURE

AND REPAIR

NERVES AND

ASSOCIATED GANGLIA• A nerve is a cordlike organ consisting of parallel

bundles of peripheral axons (some myelinated and some not) enclosed by connective tissue wrappings:– Most mixed: both sensory and motor– Some are only sensory– Some are only motor

• Ganglia are collections of neuron cell bodies associated with nerves in the PNS:– Ganglia associated with afferent nerve fibers contain cell

bodies of sensory neurons– Ganglia associated with efferent nerve fibers mostly contain

cell bodies of autonomic motor neurons

NERVE STRUCTURE

NERVE REGENERATION• Mature neurons do not divide• If damage to a neuron occurs to the axon and the cell body

remains intact, cut or compressed axons can regenerate:– Post-trauma axon regrowth is never exactly the same as what existed

before the injury– Much of the functional recovery after nerve injury involves retraining the

nervous system to respond appropriately so that stimulus and response are coordinated

– Unlike peripheral nerve fibers, most of those within the CNS never regenerate under normal circumstances (damage to the brain or spinal cord has been viewed as irreversible)

• The key to CNS regeneration will most likely be found in the hippocampus, a brain region important in learning and memory because this area produces significant numbers of new neurons throughout life

NERVE REGENERATION

NERVE REGENERATION

• a.Nerve Regeneration:– Peripheral axon has

been severed or crushed

– Separated ends seal themselves off

– Substances being transported along the axon begin to accumulate in the sealed ends

NERVE REGENERATION

• b.Nerve Regeneration:– Within a few hours, the

axon and its myelin sheath distal to the site of injury begin to disintegrate because they cannot receive nutrients (Wallerian degeneration)

• Spreads distally from the injury site, completely fragmenting the axon

– Macrophages phagocytize disintegrating myelin and axonal debris

NERVE REGENERATION

• c.Nerve Regeneration:– Once the debris has

been disposed of, surviving Schwann cells proliferate

– Axonal growth takes place

NERVE REGENERATION

• d.Nerve Regeneration:– The Schwann cells

protect, support, and remyelinate the regenerating axons

NERVE REGENERATION

• The greater the distance between the severed nerve endings, the less the chance of recovery because adjacent tissues block growth by protruding into the larger gaps

• Neurosurgeons align cut nerve endings surgically to enhance the chance of successful regeneration (silicon tubes filled with biodegradable collagen)

• The post-trauma axon regrowth is never exactly the same as what existed before the injury– pinpoint accuracy in nerve fibers (realignment) is impossible

• Much of the functional recovery after nerve injury involves retraining the nervous system to respond appropriately so that stimulus and responses are coordinated

CRANIAL NERVES

CRANIAL NERVES

• Twelve pairs of cranial nerves are associated with the brain and pass through various foramina of the skull

• The first two pairs attach to the forebrain; the rest originate from the brain stem

• Other than the vagus nerves, which extend well into the abdomen, the cranial nerves serve only head and neck structures

• Names reveal the most important structures they serve or their primary functions

• Traditionally they are numbered using Roman Numerals

CRANIAL NERVES• Twelve pairs of cranial nerves are

associated with the brain and pass through various foramina of the skull

• The first two pairs attach to the forebrain; the rest originate from the brain stem

• Other than the vagus nerves, which extend well into the abdomen, the cranial nerves serve only head and neck structures

• Names reveal the most important structures they serve or their primary functions

• Traditionally they are numbered using Roman numerals

CRANIAL NERVES• I: Olfactory nerves are responsible for smell• II: Optic nerves are responsible for vision• III: Oculomotor nerves play a role in eye movement• IV: Trochlear nerves play a role in eye movement• V: Trigeminal nerves are general sensory nerves of the face• VI: Abducens nerves play a role in eye movement • VII: Facial nerves function as the chief motor nerves of the face• VIII: Vestibulocochlear nerves are responsible for hearing and equilibrium• IX: Glossopharyngeal nerves innervate part of the tongue and pharynx• X: Vagus nerves innervate the heart, lungs, and the abdominal organs• XI: Accessory nerves move structures associated with the head and neck• XII: Hypoglossal nerves are mixed nerves that arise from the medulla and

serve the tongue

CRANIAL NERVES• I: Olfactory nerves are responsible for smell• II: Optic nerves are responsible for vision• III: Oculomotor nerves play a role in eye

movement• IV: Trochlear nerves play a role in eye

movement• V: Trigeminal nerves are general sensory

nerves of the face• VI: Abducens nerves play a role in eye

movement • VII: Facial nerves function as the chief

motor nerves of the face• VIII: Vestibulocochlear nerves are

responsible for hearing and equilibrium• IX: Glossopharyngeal nerves innervate part

of the tongue and pharynx• X: Vagus nerves innervate the heart, lungs,

and the abdominal organs• XI: Accessory nerves move structures

associated with the head and neck• XII: Hypoglossal nerves are mixed nerves

that arise from the medulla and serve the tongue

CRANIAL NERVES

• On occasion, our trusty truck acts funny– very good vehicle anyhow

CRANIAL NERVES

• On occasion, our trusty truck acts funny– very good vehicle anyhow

• I: Olfactory nerves are responsible for smell• II: Optic nerves are responsible for vision• III: Oculomotor nerves play a role in eye

movement• IV: Trochlear nerves play a role in eye

movement• V: Trigeminal nerves are general sensory

nerves of the face• VI: Abducens nerves play a role in eye

movement • VII: Facial nerves function as the chief

motor nerves of the face• VIII: Vestibulocochlear nerves are

responsible for hearing and equilibrium• IX: Glossopharyngeal nerves innervate part

of the tongue and pharynx• X: Vagus nerves innervate the heart, lungs,

and the abdominal organs• XI: Accessory nerves move structures

associated with the head and neck• XII: Hypoglossal nerves are mixed nerves

that arise from the medulla and serve the tongue

CRANIAL NERVES

CRANIAL NERVES

• I II III IV V VI VII

• Some say marry money, but my brother

• VIII IX X XI XII

• says (its) bad business (to) marry money

CRANIAL NERVES

• I II III IV V VI VII• Some say marry money, but my brother• VIII IX X XI XII• says (its) bad business (to) marry money

SPINAL NERVES• Thirty-one pairs of mixed spinal nerves arise from the spinal cord and

serve the entire body except the head and some areas of the neck• All are mixed nerves• Named according to their point of issue from the spinal cord

– 8 pairs of cervical spinal 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 tiny coccygeal nerves ( C0)

• Notice that there are 8 pairs of cervical nerves but only 7 cervical vertebrae

– First 7 pairs of cervical nerves exit the vertebral canal superior to the vertebrae for which they are named (C8 emerges inferior to the 7th cervical vertebra– between C7 and T1)

• All the remaining spinal nerves leave the vertebral column inferior to the same-numbered vertebra

SPINAL NERVES

SPINAL NERVES• Innervation of Specific Body Regions:

– Each spinal nerve connects to the spinal cord by a dorsal root and a ventral root

– Rami lie distal to and are lateral branches of the spinal nerves that carry both motor and sensory fibers

– The back is innervated by the dorsal rami with each rami innervating the muscle in line with the point of origin from the spindle column

– The cervical plexus is formed by the ventral rami of the first four cervical nerves

– The brachial plexus is situated partly in the neck and partly in the axilla and gives rise to virtually all the nerves that innervate the upper limb

– Only in the thorax are the ventral rami arranged in a simple segmental pattern corresponding to that of the dorsi rami

– The area of skin innervated by the cutaneous branches of a single nerve is called a dermatone

– Hinton’s law states that any nerve serving a muscle that produces movement at a joint also innervates the joint and the skin over the joint

FORMATION OF SPINAL NERVES

SPINAL NERVES

• Innervation of Specific Body Regions:– Each spinal nerve

connects to the spinal cord by a dorsal root and a ventral root

– Rami lie distal to and are lateral branches of the spinal nerves that carry both motor and sensory fibers

SPINAL NERVES• The cervical plexus is formed by the

ventral rami of the first four cervical nerves• The brachial plexus is situated partly in the

neck and partly in the axilla and gives rise to virtually all the nerves that innervate the upper limb

• Only in the thorax are the ventral rami arranged in a simple segmental pattern corresponding to that of the dorsi rami

• The sacral and lumbar plexuses overlap and because many fibers of the lumber plexus contribute to the sacral plexus via the lumbosacral trunk, the two plexuses are often referred to as the lumbosacral plexus

• The area of skin innervated by the cutaneous branches of a single nerve is called a dermatone

• Hinton’s law states that any nerve serving a muscle that produces movement at a joint also innervates the joint and the skin over the joint

FORMATION OF SPINAL NERVES

• Formation of one pair of spinal nerves by the union of the ventral and dorsal roots of the spinal cord

FORMATION OF SPINAL NERVES

• Spinal nerve is very short

• Roots lie medial to and form the spinal nerves– Each root is strictly

sensory or motor in function

• Ramus (branch of a nerve, artery, vein, or bone) lie distal to and are lateral branches of the spinal nerves– Like spinal nerves, carry

both sensory and motor fibers

RAMI DISTRIBUTION OF THE SPINAL NERVE

RAMI DISTRIBUTION OF THE SPINAL NERVE

• Cross-sectional view of the left side of the body at the level of the thorax, showing the distribution of the dorsal and ventral rami of the spinal nerve

• Notice the rami communicantes branches of the spinal nerve

• Spinal nerve is very short– Emerging from its foramen it

divides into the dorsal, ventral, and meningeal branches (reenters the vertebral canal to innervate the meninges and blood vessels within)

PLEXUS

• A network of converging and diverging nerve fibers, blood vessels, or lymphatics

CERVICAL PLEXUS

CERVICAL PLEXUS

• Single most important nerve from this plexus is the phrenic nerve– Input from C3 and C4

– Supplies both motor and sensory fibers to the diaphragm which is a major muscle in the breathing

– Irritation of this nerve causes spasms resulting in hiccups

– If severed, the diaphragm is paralyzed and respiration arrest occurs

• Kept alive by mechanical respirators

BRACHIAL PLEXUS

BRACHIAL PLEXUS• Gives rise to virtually all the nerves

that innervate the upper limb• Rami (roots) gives rise to trunks

then divisions then cords (Really Tired Drink Coffee)

• Cords are named for their relationship to the axillary artery (lateral / posterior / medial)

– The three cords gives rise to the main nerves of the upper limb:

• Axillary (deltoid/teres major/skin/shoulder joint)

• Musculocutaneous (biceps brachii/brachialis/skin)

• Median (skin/most flexors/lateral palm)• Ulnar (medial hand/intrinsic hand/finger

and wrist flexion)• Radial: largest branch (extensors)

– Nerve rest against the medial epicondyle of the humerus (funny bone)

» Striking results in tingling of the little finger

– Compression and ischemia (decrease blood supply) can result in temporary paralysis

BRACHIAL PLEXUS

LUMBAR PLEXUS

LUMBAR PLEXUS

• Overlaps with sacral plexuses

• Innervates abdominal and psoas (loins: lower part of the back and sides between the ribs and pelvis) muscles

• Innervates anterior (quadriceps) / medial (adductors) / posterior thigh

SACRAL PLEXUS

SACRAL PLEXUS• Serves the buttock, lower limbs,

pelvic, and perineum (structures between the anus and external genitalia)

• Sciatic nerve: largest branch– Supplies nearly the entire lower

limb except the anteromedial thigh

– Actually two nerves (tibial and fibular) wrapped in a common sheath

– Motor branches to the hamstring muscles (all thigh extensors and knee flexors) and to the adductor magnus

• Pudendal nerve: perineum and external genitalia (clitoris and penis)

– Voluntary control of urination and erection

DERMATOMES

• Dermatomes: area of skin innervated by the cutaneous branches of a single spinal nerve

• All spinal nerves except C1 participate in dermatomes

• Regions not cleanly separated as map indicates• Considerable overlapping:

– Thus, destruction of a single spinal nerve will not result in complete numbness in a specific area

DERMATOMES

DERMATOMES

• Dermatomes: area of skin innervated by the cutaneous branches of a single spinal nerve

• All spinal nerves except C1 participate in dermatomes

• Regions not cleanly separated as map indicates

• Considerable overlapping:– Thus, destruction of a

single spinal nerve will not result in complete numbness in a specific area

DERMATOMES

• Lumbar nerves supply most of the anterior surfaces of the thigh and legs

• Sacral nerves serve most of the posterior surfaces of the lower limbs

• In the limbs, the overlap is less complete and some skin regions are innervated by just one spinal nerve

MOTOR ENDINGSAND

MOTOR ACTIVITY

PERIPHERAL MOTOR ENDINGS

• Peripheral motor endings are the PNS element that activates effectors by releasing neurotransmitters

• The terminals of the somatic motor fibers that innervate voluntary muscles form elaborate neuromuscular junctions with their effector cells and they release the neurotransmitter acetylcholine

• The junctions between autonomic motor endings and the visceral effectors involve varicosities and release either acetylcholine or epinephrine as their neurotransmitter

PNS in the Structural Organization of the Nervous system

PERIPHERAL MOTOR ENDINGS

• The terminals of the somatic motor fibers that innervate voluntary muscles form elaborate neuromuscular junctions with their effector cells and they release the neurotransmitter acetylcholine

• The junctions between autonomic motor endings and the visceral effectors involve varicosities and release either acetylcholine or epinephrine as their neurotransmitter

MOTOR CONTROL

• Motor Control– Three levels:

• Segmental• Projection• Programs/Instruction

LEVELS OF MOTOR CONTROL

• The segmental level is the lowest level on the motor control hierarchy and consists of the spinal cord circuits (stimulating a specific group of muscle fibers)– Called central pattern generators (CRGs)

• The projection level: the spinal cord has direct control– Control reflex and fixed-action motor actions and produce

discrete voluntary movements of skeletal muscles

• The precommand level is made up of the cerebellum and the basal nuclei and is the highest level of the motor system hierarchy

LEVELS OF MOTOR CONTROL

LEVEL OF MOTOR CONTROL

• Segmental Level:– Lowest level of

hierarchy– Consists of segmental

circuits of the spinal cord

– Spinal cord neurons arranged in reverberating circuits

– Inherited, not learned

LEVEL OF MOTOR CONTROL

• Projection Level:– Consists of cortical

motor areas that produce the direct pyramidal system

– Axons of these neurons project to the spinal cord

– Houses command neurons that modify and control the segmental apparatus

LEVEL OF MOTOR CONTROL

• Programs and Instructions Level:– Cerebellum and basal

nuclei– Regulate motor movement

(start/stop, coordination, block unwanted, and monitor)

– Sensory and motor integrated

– Cerebellum: lacks direct connection to the spinal cord

• Acts through the projection areas of the thalamus and brain stem

REFLEX ACTIVITY

THE REFLEX ARC

• Reflexes are unlearned, rapid, predictable motor responses to a stimulus, and occur over highly specific neural pathways called reflex arc

THE REFLEX ARC

COMPONENTS of a REFLEX ARC

• 1. Receptor: site of stimulus action

• 2. Sensory Neuron: transmits the afferent impulse to the CNS

• 3. Integration Center: – Always within the CNS– Synapse between the sensory

and motor neurons• 4. Motor Neuron: conducts

efferent impulses from the integration center to an effector organ

• 5. Effector: muscle fiber or gland cell that responds to the efferent impulse

SPINAL REFLEXES

• Spinal reflexes are somatic (body skeletal muscle) reflexes mediated by the spinal cord (many not involving the brain)– In the stretch reflex the muscle spindle is stretched and excited

by either an external stretch or an internal stretch– The Golgi tendon reflex produces muscle relaxation and

lengthening in response to contraction– The flexor, or withdrawal, reflex is initiated by a painful

stimulus and causes automatic withdrawal of the threatened body part from the stimulus

– They crossed extensor reflex is a complex spinal reflex consisting of an ipsilateral withdrawal reflex (affecting the same side of the body) and a contralateral extensor reflex (affecting the opposite side of the body)

– Superficial reflexes are elicited by gentle cutaneous stimulation

Anatomy of the Spindle and Golgi Tendon Organ

Operation of the Muscle Spindle

STRETCH REFLEX

STRETCH REFLEX• 1. Stretching the muscle

activates a muscle spindle (encapsulated receptor found in skeletal muscle that is sensitive to stretch)

• 2. Impulses transmitted by afferent fibers from muscle spindle to motor neurons in the spinal cord causes the stretched muscle to contract

• 3.Impulses transmitted by afferent fibers from muscle spindle to interneurons in the spinal cord result in reciprocal inhibition of the antagonist muscle

STRETCH REFLEX

• Patellar Reflex:– Tapping the patellar tendon

excites muscle spindles in the quadriceps muscle

– Afferent impulses travel to the spinal cord, where synapses occur with motor neurons and interneurons

– Motor neurons send impulses to the quadriceps resulting in contraction and inhibition of its antagonist (hamstring), which causes extension of the knee and forward movement of the foot

PATELLAR REFLEX

• Knee-jerk

• Results in information:– Proves that the sensory and motor

connections between that muscle and the spinal cord are intact

– The vigor of the motor response indicates the degree of excitability of the spinal cord

STRETCH REFLEX

• Absent:– Diabetes– Neurosyphilis– Coma

• Hyperactive:– Lesions of corticospinal tract which reduces

the inhibitory effect of the brain• Polio (inflammation of the gray matter of brain and

spinal cord)• Stroke (sudden loss of neurological function)

DEEP GOLGI TENDON REFLEX

DEEP GOLGI TENDON REFLEX

• Golgi tendon organ:– proprioceptors

located in tendons, close to the point of skeletal muscle insertion

– Important to smooth onset and termination of muscle contraction

DEEP GOLGI TENDON REFLEX

• Help ensure smooth onset and termination of muscle contraction involving rapid switching between flexion and extension such as in running:

• 1. Golgi tendon receptors in the contracted muscles tendon are stimulated

• 2. Afferent impulse to the spinal cord

• 3. Information sent to the cerebellum where muscle tension is adjusted

• 4. Simultaneously, motor neurons in spinal cord circuits supplying the contracting muscles are inhibited and antagonist muscles are activated (Reciprocal Activation)

FLEXOR REFLEX

• Flexor reflex initiated by a painful stimulus (actual or perceived)– Automatic withdrawal of the threatened body

part from the stimulus– Protective reflexes important to our survival– They override the spinal pathways and

prevent any other reflexes from using them at the same time

CROSSED EXTENSOR REFLEX

• Complex spinal reflex consisting of an ipsilateral (same side) withdrawal reflex and a contralateral (opposite side) extensor reflex

CROSSED EXTENSOR REFLEX

CROSSED EXTENSOR REFLEX

• Example illustrated when a stranger suddenly grabbed your arm

• Incoming afferent fibers synapse with interneurons that control the flexor withdrawal response on the same side of the body and with other interneurons that control the extensor muscles on the opposite side

PAIN TRANSMISSIOM

DEVELOPMENTAL ASPECTS OF

THE PERIPHERAL NERVOUS SYTEM

• The Spinal Nerves branch from the developing spinal cord and adjacent neural crest and exit between the forming vertebrate– Each nerve becomes associated with the adjacent

muscle mass

• Cranial Nerves innervate muscles of the head in a similar way

• Sensory Receptors atrophy to some degree with age, and there is a decrease in muscle tone in the face and neck, reflexes occur a bit more slowly