A&P chap 13

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A&P chap 13
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Classification of Receptors
• Based on –

-Type of stimulus they detect
- Location in body
– Structural complexity



—respond to touch, pressure, vibration, and stretch



sensitive to changes in temperature



respond to light energy (e.g., retina)



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



sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals


Classification by Location
• Exteroceptors

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


Classification by Location
• Interoceptors (visceroceptors)

– Respond to stimuli arising in internal viscera and blood vessels
– Sensitive to chemical changes, tissue stretch, and temperature changes – Sometimes cause discomfort but usually unaware of their workings


Classification 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


Classification by Receptor Structure
• Simple receptors for general senses

– Tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense
– Modified dendritic endings of sensory neurons•


Receptors for special senses

– Vision, hearing, equilibrium, smell, and taste


Simple Receptors of the General Senses

• Either nonencapsulated (free) or encapsulated


Nonencapsulated (free) nerve endings

– Abundant in epithelia and connective tissues
– Most nonmyelinated
– Respond mostly to temperature and pain; some to pressure-induced tissue movement; itch


Unencapsulated Dendritic Endings
• Nociceptors

– Player in detection
– vanilloid receptor • Ion channel opened by heat, low pH, chemicals, e.g., capsaicin (red peppers)
– Respond to: • Pinching, chemicals from damaged tissue, capsaicin


Light touch receptors

– Tactile (Merkel) discs
– Hair follicle receptors


Tactile (Meissner's) corpuscles

—discriminative touch


Lamellar (Pacinian) corpuscles

—deep pressure and vibration


Bulbous corpuscles (Ruffini endings)

—deep continuous pressure


Joint kinesthetic receptors

—joint position and motion


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


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


Processing at the Receptor Level
• To produce a sensation

– Receptors have specificity for stimulus energy
– Stimulus must be applied in receptive field
– Transduction occurs • Stimulus changed to graded potential
– Generator potential or receptor potential
– Graded potentials must reach threshold ===> AP


In general sense receptors, graded potential called generator potential

Generator potential in afferent neuron 
Action potential


Processing at the Receptor Level
• In special sense organs:

Stimulus----> Graded potential in receptor cell called receptor potential ----> Affects amount of neurotransmitter released ----->Neurotransmitters generate graded potentials in sensory neuron


Adaptation is change in sensitivity in presence of constant stimulus

– Receptor membranes become less responsive
– Receptor potentials decline in frequency or stop


Phasic (fast-adapting) receptors

signal beginning or end of stimulus
– Examples - receptors for pressure, touch, and smell


Tonic receptors

adapt slowly or not at all
– Examples - nociceptors and most proprioceptors


Processing at the Circuit Level
• First-order sensory neurons

– Conduct impulses from receptor level to spinal reflexes or second-order neurons in CNS


Second-order sensory neurons

– Transmit impulses to third-order sensory neuron


Third-order sensory neurons

– Conduct impulses from thalamus to the somatosensory cortex (perceptual level)


Aspects of sensory perception:
– Perceptual detection
--Magnitude estimation
– Spatial discrimination

—ability to detect a stimulus (requires summation of impulses)

—intensity coded in frequency of impulses

—identifying site or pattern of stimulus (studied by two-point discrimination test)


Feature abstraction

identification of more complex aspects and several stimulus propertie


Quality discrimination

ability to identify submodalities of a sensation (e.g., sweet or sour tastes)


Pattern recognition

recognition of familiar or significant patterns in stimuli (e.g., melody in piece of music)


Perception of Pain

• Warns of actual or impending tissue damage ->protective action
• Stimuli include extreme pressure and temperature
• Impulses travel on fibers that release neurotransmitters glutamate and substance P
• Some pain impulses are blocked by inhibitory endogenous opioids (e.g., endorphins)


Visceral and Referred Pain • Stimulation of visceral organ receptors

– Felt as vague aching, gnawing, burning
– Activated by tissue stretching, ischemia (low blood flow), chemicals, muscle spasms


Referred pain

– Pain from one body region perceived from different region
– Visceral and somatic pain fibers travel in same nerves; brain assumes stimulus from common (somatic) region • E.g., left arm pain during heart attack


Structure of a Nerve

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


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


Classification of Nerves

• Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers


Nerves Classified according to direction transmit impulses

– 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


Pure sensory (afferent) or motor (efferent) nerves

are rare; most mixed


Types of fibers in mixed nerves

– Somatic afferent
– Somatic efferent
– Visceral afferent
– Visceral efferent


Peripheral nerves classified as

cranial or spinal nerves



• 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) (Chapter 12)


– Ganglia associated with efferent nerve fibers contain

autonomic motor neurons
• Autonomic ganglia (motor, visceral) (Chapter 14)


Cranial Nerves

• Twelve pairs of nerves associated with brain
– Two attach to forebrain; rest with brain stem
• Most mixed nerves; two pairs purely sensory
• Each numbered (I through XII) and named from rostral to caudal


The Olfactory Nerves

• Sensory nerves of smell
• Purely sensory (olfactory) function


The Optic Nerves

• Arise from retinas; really a brain tract
• Converge and partially cross over at optic chiasma
• Purely sensory (visual) function


The Oculomotor Nerves

• Motor: Innervate four of six extrinsic eye muscles
• Function in raising eyelid, directing eyeball, constricting iris (parasympathetic), and controlling lens shape


The Trochlear Nerves

• Fibers from midbrain to innervate superior oblique muscle (eye muscle)
• Primarily motor nerve that directs eyeball


The Trigeminal Nerves

• Largest cranial nerves; pons to face
• Three divisions – Ophthalmic (V1) – Maxillary (V2) – Mandibular (V3)
• Convey sensory impulses from various areas of face (V1) and (V2)
• Supply motor fibers (V3) for mastication


The Abducens Nerves

• Fibers from pons
• Primarily a motor, innervating lateral rectus muscle (eye muscle)


The Facial Nerves

• Fibers from pons
• 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 Vestibulocochlear Nerves

• Afferent fibers from hearing receptors (cochlear division) and equilibrium receptors (vestibular division)
• Mostly sensory function; small motor component for adjustment of sensitivity of receptors


The Glossopharyngeal Nerves

• Fibers from medulla leave skull
• Motor functions - innervate part of tongue and pharynx for swallowing, and provide parasympathetic fibers to parotid salivary glands
• Sensory functions - from pharynx and posterior tongue (general and taste), and from carotid chemoreceptors and baroreceptors


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, baroreceptors, chemoreceptors, and taste buds of posterior tongue and pharynx


The Accessory Nerves

• Formed from ventral rootlets from C1–C5 region of spinal cord (not brain) • Motor: innervate trapezius and sternocleidomastoid muscles


The Hypoglossal Nerves

• Motor: Innervate extrinsic and intrinsic muscles of tongue that contribute to swallowing and speech


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 (C1–C8)
– 12 thoracic (T1–T12)
– 5 Lumbar (L1–L5)
– 5 Sacral (S1–S5)
– 1 Coccygeal (C0)


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
Dorsal roots – Contain sensory (afferent) fibers
• Dorsal and ventral roots unite to form spinal nerves


Spinal Nerves: Rami

Spinal nerves quite short (~1-2 cm)
• Each branches into mixed rami
– Dorsal ramus
– Ventral ramus - larger –
Rami communicantes (autonomic pathways)
~~All ventral rami except T2–T12 form interlacing nerve networks called nerve plexuses (cervical, brachial, lumbar, and sacral)


Spinal Nerves: Plexuses

• Within plexus fibers criss-cross – 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 – Damage to one does not lead to paralysis


Cervical Plexus and the Neck

• 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) – Irritation = hiccups


Brachial Plexus and Upper Limb

• Formed by ventral rami of C5–C8 and T1 (and often C4 and/or T2)
• Gives rise to nerves that innervate upper limb
• Major branches of this plexus: – Roots—five ventral rami (C5–T1), which form – Trunks—upper, middle, and lower, which form – Divisions—anterior and posterior, which form – Cords—lateral, medial, and posterior


Lumbar Plexus

• Arises from L1–L4 • Innervates thigh, abdominal wall, and psoas muscle
• Femoral nerve—innervates quadriceps and skin of anterior thigh and medial surface of leg
• Obturator nerve—passes through obturator foramen to innervate adductor muscles


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


Anterolateral Thorax and Abdominal Wall

• Ventral rami in thorax in simple segmental pattern – 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


Innervation of Skin: Dermatomes

• Dermatome - area of skin innervated by cutaneous branches of single spinal nerve • All spinal nerves except C1 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


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


Peripheral Motor Endings

PNS elements that activate effectors by releasing neurotransmitters


Levels of Motor Control

• Cerebellum and basal nuclei are ultimate planners and coordinators of complex motor activities
• Complex motor behavior depends on complex patterns of control
– Segmental level – Projection level – Precommand level



• Inborn (intrinsic) reflex - rapid, involuntary, predictable motor response to stimulus
– Example – maintain posture, control visceral activities – Can be modified by learning and conscious effort
• Learned (acquired) reflexes result from practice or repetition, – Example – driving skills


Reflex Arc

• Components of a reflex arc (neural path)
1. Receptor—site of stimulus action
2. Sensory neuron—transmits afferent impulses to CNS
3. Integration center—either monosynaptic or polysynaptic region within CNS
4. Motor neuron—conducts efferent impulses from integration center to effector organ
5. Effector—muscle fiber or gland cell that responds to efferent impulses by contracting or secreting


• Functional classification

– Somatic reflexes • Activate skeletal muscle
– Autonomic (visceral) reflexes • Activate visceral effectors (smooth or cardiac muscle or glands)


Spinal Reflexes

• Spinal somatic reflexes – Integration center in spinal cord – Effectors are skeletal muscle
• Testing of somatic reflexes important clinically to assess condition of nervous system – If exaggerated, distorted, or absent  degeneration/pathology of specific nervous system regions


Stretch and Tendon Reflexes

• To smoothly coordinate skeletal muscle nervous system must receive proprioceptor input regarding
– Length of muscle
• From muscle spindles
– Amount of tension in muscle
• From tendon organs


The Stretch Reflex

Maintains muscle tone in large postural muscles, and adjusts it reflexively – Causes muscle contraction in response to increased muscle length (stretch)


The Tendon Reflex

• Polysynaptic reflexes
• Helps prevent damage due to excessive stretch
• Important for smooth onset and termination of muscle contraction


The Flexor and Crossed-Extensor Reflexes

• Flexor (withdrawal) reflex
– Initiated by painful stimulus
– Causes automatic withdrawal of threatened body part
– Ipsilateral (same side of body) and polysynaptic (involving many synapses)
– Protective; important
– Brain can override • E.g., finger stick for blood test


Flexor and Crossed-Extensor Reflexes

• Crossed extensor reflex
– Occurs with flexor reflexes in weight-bearing limbs to maintain balance
– Consists of ipsilateral withdrawal reflex and contralateral (opposite side) extensor reflex • e.g., step barefoot on broken glass


Superficial Reflexes

• Elicited by gentle cutaneous stimulation
• Depend on upper motor pathways and cord-level reflex arcs
• Best known:
– Plantar reflex – Abdominal reflex


Superficial Reflexes: Plantar Reflex

• Test integrity of cord from L4 – S2
• Stimulus - stroke lateral aspect of sole of foot
• Response - downward flexion of toes
• Damage to motor cortex or corticospinal tracts--->abnormal response = Babinski's sign
– Hallux dorsiflexes; other digits fan laterally
– Normal in infant to ~1 year due to incomplete myelination


Superficial Reflexes: Abdominal Reflexes

• Test integrity of cord from T8 – T12
• Cause contraction of abdominal muscles and movement of umbilicus in response to stroking of skin
• Vary in intensity from one person to another
• Absent when corticospinal tract lesions present