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Anatomy & Physiology I Lecture 12 Chapter 13: The Peripheral Nervous System and Reflex Activity

The Peripheral Nervous System

• Provides links from and to world outside body • All neural structures outside brain – Sensory receptors – Peripheral nerves and associated ganglia – Efferent motor endings

Figure 13.1 Place of the PNS in the structural organization of the nervous system.

Central nervous system (CNS) Peripheral nervous system (PNS) Sensory (afferent) division Motor (efferent) division Somatic nervous system Autonomic nervous system (ANS) Sympathetic division Parasympathetic division

Sensory Receptors

• Specialized to respond to changes in environment (stimuli) • Activation results in graded potentials that trigger nerve impulses • Sensation – the awareness of stimulus

Classification of Receptors

• Based on – Type of stimulus they detect – Location in body – Structural complexity

Stimulus Classification

• Mechanoreceptors—respond to touch, pressure, vibration, and stretch • Thermoreceptors—sensitive to changes in temperature • Photoreceptors—respond to light energy – retina

Stimulus Classification

• Chemoreceptors—respond to chemicals – smell, taste, changes in blood chemistry • Nociceptors—sensitive to pain-causing stimuli – extreme heat or cold, excessive pressure, inflammatory chemicals

Classified by Location

•

Exteroceptors

– Respond to stimuli arising outside body – Receptors in skin for touch, pressure, pain, and temperature – Most special sense organs

Classified by Location

• Interoceptors (visceroceptors) – Respond to stimuli arising in internal viscera and blood vessels – Sensitive to chemical changes, tissue stretch, and temperature changes

Classified by Location

•

Proprioceptors

– Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles – Inform brain of one's movements

Sensory Integration

• Survival depends upon sensation and perception • Sensation - the awareness of changes in the internal and external environment • Perception - the conscious interpretation of those stimuli

Somatosensory System

• Somatosensory system – part of sensory system serving body wall and limbs • Receives inputs from – Exteroceptors, proprioceptors, and interoceptors • Input relayed toward head, but processed along way

Neural Integration

• Levels of neural integration in sensory systems: 1. Receptor level—sensory receptors 2. Circuit level—processing in ascending pathways 3. Perceptual level—processing in cortical sensory areas

Figure 13.2 Three basic levels of neural integration in sensory systems.

3 Perceptual level sensory centers) (processing in cortical Motor cortex Somatosensory cortex Thalamus Reticular formation Pons 2 Circuit level (processing in ascending pathways) Medulla Spinal cord Cerebellum Free nerve endings (pain, cold, warmth) Muscle spindle

1 Receptor level (sensory reception and transmission to CNS) Joint kinesthetic receptor

Receptor Level

• Receptors have specificity for stimulus energy • Stimulus changed to graded potential • Graded potentials must reach threshold for AP

Processing at Receptor Level

In general sense receptors, graded potential called generator potential Stimulus  Generator potential in afferent neuron  Action potential

Circuit Level Processing

• Pathways of three neurons conduct sensory impulses upward to appropriate cortical regions • Three sensory neurons – First-order sensory neurons – Second-order sensory neurons – Third-order sensory neurons

Circuit Neurons

• First-order sensory neurons – Conduct impulses from receptor level to spinal reflexes or second-order neurons in CNS • Second-order sensory neurons – – Spinal reflex neuron Transmit impulses to third-order sensory neurons • Third-order sensory neurons – Conduct impulses from thalamus to the somatosensory cortex (perceptual level)

Perceptual Level

• Interpretation of sensory input depends on specific location of target neurons in sensory cortex • Aspects of sensory perception: – Perceptual detection – Magnitude estimation – Spatial discrimination

Sensory perception

• Perceptual detection – ability to detect a stimulus • Magnitude estimation – intensity coded in frequency of impulses • Spatial discrimination – identifying site or pattern of stimulus

Figure 13.2 Three basic levels of neural integration in sensory systems.

1 Receptor level (sensory reception and transmission to CNS) Joint kinesthetic receptor

Feature abstraction

• Identification of more complex aspects and several stimulus properties • • Quality discrimination – ability to identify submodalities of a sensation – sweet or sour tastes Pattern recognition – – recognition of familiar or significant patterns in stimuli melody in piece of music as opposed to the individual notes

Structure of a Nerve

• Cordlike organ of PNS • Bundle of myelinated and nonmyelinated peripheral axons enclosed by connective tissue

Structure of a Nerve

• Connective tissue coverings include: • • • Endoneurium – loose connective tissue that encloses axons and their myelin sheaths Perineurium – coarse connective tissue that bundles fibers into fascicles Epineurium – tough fibrous sheath around a nerve

Figure 13.4a Structure of a nerve.

Endoneurium Perineurium Nerve fibers Blood vessel Fascicle

Epineurium

Figure 13.4b Structure of a nerve.

Endoneurium Perineurium Epineurium Axon Myelin sheath Fascicle Blood vessels

Classification of Nerves

• Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers • Classified according to direction they transmit impulses – Mixed nerves – Sensory (afferent) nerves – Motor (efferent) nerves

Classification of Nerves

• Mixed nerves – both sensory and motor fibers; impulses both to and from CNS • Sensory (afferent) nerves – impulses only toward CNS • Motor (efferent) nerves – impulses only away from CNS

Mixed Nerves classified according to region they innervate

• Types of fibers in mixed nerves: – Somatic afferent – Somatic efferent – Visceral afferent – Visceral efferent • All peripheral nerves classified as cranial or spinal nerves

Ganglia

• Contain neuron cell bodies associated with nerves in PNS • Ganglia associated with afferent nerve fibers contain cell bodies of sensory neurons – Dorsal root ganglia (sensory, somatic) (Ch. 12) • Ganglia associated with efferent nerve fibers contain autonomic motor neurons – Autonomic ganglia (motor, visceral) (Ch. 14)

The Cranial Nerves

• Twelve pairs of nerves associated with brain – Two attach to forebrain; rest with brain stem • Each numbered (I through XII) and named from anterior to posterior

Helpful Reminders

On occasion, our trusty truck acts funny, very good vehicle anyhow” Oh once one takes the anatomy final, very good vacation are heavenly” Only Owls Observe Them Traveling And Finding Voldemort Guarding Very Secret Horcruxes

The 12 Cranial Nerves

• • • • • • Olfactory Optic Oculomoter Trochlear Trigeminal Abducens • • • • • • Facial Vestibulocochlear Glossopharyngeal Vagus Accessory Hypoglossal

Figure 13.6a Location and function of cranial nerves.

Filaments of olfactory nerve (I) Olfactory bulb Olfactory tract Optic nerve (II) Optic chiasma Optic tract Oculomotor nerve (III) Trochlear nerve (IV) Trigeminal nerve (V) Abducens nerve (VI) Cerebellum Medulla oblongata

Frontal lobe Temporal lobe Infundibulum Facial nerve (VII) Vestibulocochlear nerve (VIII) Glossopharyngeal nerve (IX) Vagus nerve (X) Accessory nerve (XI) Hypoglossal nerve (XII)

Figure 13.6b Location and function of cranial nerves.

Cranial nerves I – VI II III IV V I Olfactory Optic Oculomotor Trochlear Trigeminal VI Abducens Sensory function Yes (smell) Yes (vision) No No Yes (general sensation) No Motor function No No Yes Yes Yes Yes PS* fibers No No Yes No No No Cranial nerves VII – XII Sensory function VII VIII Facial Vestibulocochlear IX Glossopharyngeal X Vagus XI Accessory XII Hypoglossal Yes (taste) Yes (hearing and balance) Yes (taste) Yes (taste) No No Motor function Yes Some Yes Yes Yes Yes PS* fibers Yes No Yes Yes No No *PS = parasympathetic

• You have one nose, so the olfactory nerve is CN I, and it controls the sense of smell and innervates the nose.

• You have two eyes, so the optic nerve is CN II, which functions to produce vision.

• You have six abs, so the abducens nerve is CN VI, which abducts the eye.

• CN VII: A mnemonic for the names of the five branches of the Facial nerve is To Zanzibar By Motor Car. (Temporal, Zygomatic, Buccal, Mandibular and Cervical) • The number 8 resembles an ear, so CN VIII, the vestibulocochlear/acoustic nerve is a sensory nerve for hearing.

• The number 11 can resemble a pair of arms or shoulders, and CN XI innervates the sternocleidomastoid and trapezius muscles and controls shoulder and neck movements.

• The tongue licks the wound is a reminder that when the hypoglossal nerve (CNXII) is damaged, the tongue deviates to the same side of a lesion to the cranial nerve – if the tongue deviates to the right, the right side of CNXII is damaged).

I: The Olfactory Nerves

• • • • • Sensory nerves of smell Run from nasal mucosa to olfactory bulbs Fibers synapse in olfactory bulbs Pathway terminates in primary olfactory cortex Purely sensory (olfactory) function

II: The Optic Nerves

• • • • • Arise from retinas Pass through optic canals, converge and partially cross over at optic chiasma Optic tracts continue to thalamus, where they synapse Optic radiation fibers run to occipital (visual) cortex Purely sensory (visual) function

III: The Oculomotor Nerves

• Fibers extend from ventral midbrain through superior orbital fissures to four of six extrinsic eye muscles • Function in raising eyelid, directing eyeball, constricting iris (parasympathetic), and controlling lens shape

IV: The Trochlear Nerves

• Fibers from dorsal midbrain enter orbits via superior orbital fissures to innervate superior oblique muscle • Primarily motor nerve that directs eyeball – rotation of eye

V: The Trigeminal Nerves

• • Largest cranial nerves; fibers extend from pons to face Three divisions – Ophthalmic (V 1 ) – Maxillary (V 2 ) – Mandibular (V 3 ) • Convey sensory impulses from various areas of face (V 1 ) and (V 2 ) • Supply motor fibers (V 3 ) for mastication

VI: The Abducens Nerves

• Fibers from inferior pons enter orbits via superior orbital fissures • Primarily a motor, innervating lateral rectus muscle to move eye laterally

VII: The Facial Nerves

• Chief motor nerves of face with 5 major branches • Motor functions include facial expression, parasympathetic impulses to lacrimal and salivary glands • Sensory function (taste) from anterior two thirds of tongue

The Facial Nerves

Remembering the Facial Nerve

VIII: The Vestibulocochlear Nerves

• Afferent fibers from hearing receptors (cochlear division) and equilibrium receptors (vestibular division) • Enter brain stem at pons-medulla border • Mostly sensory function; small motor component for adjustment of sensitivity of receptors

IX: The Glossopharyngeal Nerves

• Fibers run to throat • Motor functions – innervate part of tongue and pharynx for swallowing, and provide parasympathetic fibers to parotid salivary glands • Sensory functions – conduct taste and general sensory impulses from pharynx and posterior tongue

X: The Vagus Nerves

• Only cranial nerves that extend beyond head and neck region • Most motor fibers are parasympathetic fibers that help regulate activities of heart, lungs, and abdominal viscera • Sensory fibers carry impulses from thoracic and abdominal viscera, heart rate, respiratory rate, and taste buds of posterior tongue and pharynx

XI: The Accessory Nerves

• Formed from ventral rootlets from C 1 –C 5 region of spinal cord (not brain) • Accessory nerves exit skull to innervate trapezius and sternocleidomastoid muscles

XII: The Hypoglossal Nerves

• Fibers from medulla exit innervate extrinsic and intrinsic muscles of tongue that contribute to swallowing and speech

The Spinal Nerves

• 31 pairs of mixed nerves named for point of issue from spinal cord – Supply all body parts but head and part of neck – 8 cervical (C 1 –C 8 ) – 12 thoracic (T 1 –T 12 ) – 5 Lumbar (L 1 –L 5 ) – 5 Sacral (S 1 –S 5 ) – 1 Coccygeal (C 0 )

Spinal Nerves

• Only 7 cervical vertebrae, yet 8 pairs cervical spinal nerves – 7 exit vertebral canal superior to vertebrae for which named – 1 exits canal inferior to C 7 • Other vertebrae exit inferior to vertebra for which named

Figure 13.7 Spinal nerves.

Cervical plexus Brachial plexus Cervical enlargement Intercostal nerves Lumbar enlargement Lumbar plexus Sacral plexus Cauda equina

Cervical nerves C 1 – C 8 Thoracic nerves T 1 – T 12 Lumbar nerves L 1 – L 5 Sacral nerves S 1 – S 5 Coccygeal nerve Co 1

Spinal Nerves: Roots

• Each spinal nerve connects to spinal cord via two roots • Ventral roots – Contain motor (efferent) fibers from ventral horn motor neurons – Fibers innervate skeletal muscles

Spinal Nerves: Roots

• Dorsal roots – Contain sensory (afferent) fibers from sensory neurons in dorsal root ganglia and conduct impulses from peripheral receptors • Dorsal and ventral roots unite to form spinal nerves, which emerge from vertebral column via intervertebral foramina

Figure 13.8a Formation of spinal nerves and rami distribution.

Gray matter White matter Ventral root Dorsal root Dorsal root ganglion Dorsal ramus of spinal nerve Ventral ramus of spinal nerve Spinal nerve Rami communicantes Sympathetic trunk ganglion Dorsal and ventral rootlets of spinal nerve Anterior view showing spinal cord, associated nerves, and vertebrae. The dorsal and ventral roots arise medially as rootlets and join laterally to form the spinal nerve.

Spinal Nerves: Rami

• Spinal nerves quite short (~1-2 cm) • Each branches into mixed rami – – Dorsal ramus Ventral ramus (larger) – Meningeal branch • reenters vertebral canal, innervates meninges and blood vessels – Rami communicantes • autonomic pathways that join ventral rami in thoracic region

Nerve Plexuses

• All ventral rami except T 2 –T 12 form interlacing nerve networks called nerve plexuses – cervical, brachial, lumbar, and sacral • Ventral rami of T 2 –T 12 as intercostal nerves supply muscles of ribs, anterolateral thorax, and abdominal wall

Ventral Plexus vs Dorsal Rami

• Dorsal Rami innervates muscles of the back • Ventral rami form interlacing nerve networks called plexuses

Spinal Nerves: Plexuses

• Within plexus fibers criss-cross and redistribute nerves – Each branch contains fibers from several spinal nerves – Fibers from ventral ramus go to body periphery via several routes • Each limb muscle innervated by more than one spinal nerve

Figure 13.8b Formation of spinal nerves and rami distribution.

Dorsal ramus Ventral ramus Spinal nerve Rami communicantes Sympathetic trunk ganglion Intercostal nerve Dorsal root ganglion Dorsal root Ventral root Branches of intercostal nerve Lateral cutaneous Anterior cutaneous Sternum Cross section of thorax showing the main roots and branches of a spinal nerve.

Cervical Plexus

• Formed by ventral rami of C1–C4 • Most branches form cutaneous nerves – Innervate skin of neck, ear, back of head, and shoulders – Other branches innervate neck muscles • Phrenic nerve – Major motor and sensory nerve of diaphragm (receives fibers from C3–C5)

Figure 13.9 The cervical plexus.

Ventral rami Hypoglossal nerve (XII) Lesser occipital nerve Greater auricular nerve Transverse cervical nerve Ansa cervicalis Accessory nerve (XI) Phrenic nerve Supraclavicular nerves

Segmental branches C 1 C 2 C 3 C 4 C 5

Brachial Plexus and Upper Limb

• Formed by ventral rami of C 5 –C 8 and T 1 • Gives rise to nerves that innervate upper limb

Brachial Plexus

• Major branches of this plexus: – – Roots—five ventral rami (C 5 –T 1 ), which form Trunks—upper, middle, and lower, which form – Divisions—anterior and posterior, which form – Cords—lateral, medial, and posterior

Figure 13.10a The brachial plexus.

Anterior divisions Posterior divisions Trunks Dorsal scapular Nerve to subclavius Suprascapular Posterior divisions Lateral Cords Posterior Roots Medial Axillary Musculo cutaneous Radial Median Ulnar Roots (rami C 5 –T 1 ), trunks, divisions, and cords

Roots (ventral rami): C 4 C 5 C 6 C 7 Upper Middle C 8 Lower Trunks T 1 Long thoracic Medial pectoral Lateral pectoral Upper subscapular Lower subscapular Thoracodorsal Medial cutaneous nerves of the arm and forearm

Figure 13.10b The brachial plexus.

Major terminal branches (peripheral nerves) Cords Divisions Trunks Roots (ventral rami) Musculocutaneous Median Ulnar Radial Axillary Lateral Medial Posterior Anterior Posterior Anterior Posterior Anterior Posterior Upper Middle Lower C 5 C 6 C 7 C T 1 8 Flowchart summarizing relationships within the brachial plexus

Figure 13.10c The brachial plexus.

Axillary nerve Humerus Radial nerve Musculocutaneous nerve Ulna Radius Median nerve Radial nerve (superficial branch) Dorsal branch of ulnar nerve Superficial branch of ulnar nerve Digital branch of ulnar nerve Muscular branch Digital branch Median nerve The major nerves of the upper limb

5 Important Nerves of Brachial Plexus

• • • • • Axillary Musculocutaneous Median Ulnar Radial

Anterolateral Thorax and Abdominal Wall

• Ventral rami in thorax (T 1 -T 12 ) – Form intercostal nerves that supply intercostal muscles, muscle and skin of anterolateral thorax, most abdominal wall – Give off cutaneous branches to skin along course • Dorsal rami innervate posterior body trunk

Lumbar Plexus

• • Arises from L1–L4 Innervates thigh, abdominal wall, • • Femoral nerve – innervates quadriceps and skin of anterior thigh and medial surface of leg Obturator nerve – innervate adductor muscles

Figure 13.11 The lumbar plexus.

Ventral rami Iliohypogastric Ilioinguinal Genitofemoral Ventral rami: L 1 Iliohypogastric Ilioinguinal L 2 L 3 Femoral Lateral femoral cutaneous Obturator Lateral femoral cutaneous L 4 Obturator Femoral Lumbosacral trunk L 5 Ventral rami and major branches of the lumbar plexus Anterior femoral cutaneous Saphenous

Distribution of the major nerves from the lumbar plexus to the lower limb

6 Important Nerves of Lumbar Plexus

• • • • • • Femoral Obturator Lateral femoral cutaneous Iliohypogastric Ilioinguinal Genitofemoral

Sacral Plexus

• • • Arises from L4–S4 Serves the buttock, lower limb, pelvic structures, and perineum Sciatic nerve – Longest and thickest nerve of body – Innervates hamstring muscles, adductor magnus, and most muscles in leg and foot – Composed of two nerves: tibial and common fibular

Figure 13.12a The sacral plexus.

Ventral rami Ventral rami: L 4 Superior gluteal Lumbosacral trunk Inferior gluteal L S 5 1 Common fibular Tibial Posterior femoral cutaneous Pudendal Sciatic S 2 S 3 S 4 S 5 Co 1 Ventral rami and major branches of the sacral plexus

Figure 13.12b The sacral plexus.

Superior gluteal Inferior gluteal Pudendal Sciatic Posterior femoral cutaneous Common fibular Tibial Sural (cut) Deep fibular Superficial fibular Plantar branches

Distribution of the major nerves from the sacral plexus to the lower limb

5 Important Nerves of Sacral Plexus

• • • • •

Sciatic nerve

–

Tibial

–

Common fibular

Superior gluteal Inferior gluteal

Posterior femoral cutaneous

Pudendal

Innervation of Skin: Dermatomes

• Area of skin innervated by cutaneous branches of single spinal nerve • All spinal nerves except C 1 participate in dermatomes • Extent of spinal cord injuries ascertained by affected dermatomes • Most dermatomes overlap, so destruction of a single spinal nerve will not cause complete numbness

Figure 13.13 Map of dermatomes.

T2 C5 C6 C6 C7 C8 L1 L2 C2 C3 C4 C5 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 S2 S3 L1 L2 L3 L3 L4 L4 T2 C5 C6 C8 C6 C7 L5 L5 S1 S1 C6 C7 C8 L2 L4 C2 C3 C4 C5 C6 C7 C8 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 C5 S1 S3 S2 S4 S5 T11 T12 L1 L3 L5 C7 C8 C6 L5 S1 S2 L1 L2 S2 S1 L5 L3 L4 L5 L4 Posterior view L4 L5 S1

Innervation of Joints

• To remember which nerves serve which synovial joint – Hilton's law: Any nerve serving a muscle that produces movement at joint also innervates joint and skin over joint

End of Part I

• Part II on Wednesday (6/19) • Peripheral Motor Endings and the Reflex Arc (Ch. 15) • Chapter 14: The Autonomic Nervous System (ANS)

Today’s Lab

• Lab Exercise 21 – Activity 1 – 3 only (Activity 3 Wednesdays' lecture) • Lab Exercise 22 – Introduction to the reflex arc (Wednesday) – Optional Activities 1 - 4 • Review your material. The final is next week!