ch13

peripheral nervous system: provides links from and to world outside body

• sensory receptors
• peripheral nerves and associated ganglia
• efferent motor endings

• specialized to respond to changes in environment (stimuli)
• activation results in graded potentials that trigger nerves impulses
• sensation (awareness of stimulus) and perception (interpretation of meaning of stimulus occur in brain

awareness of stimuli

interpretation of meaning of stimulus

• based on type of stimulus they detect
• location in body
• structural complexity

• mechanoreceptors
• thermoreceptors
• photoreceptors
• chemoreceptors
• nociceptors

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, change sin blood chemistry)

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

exteroceptors, interoreceptors, and proprioceptors

• respond to stimuli arising outside body
• receptors in skin for touch, pressure, pain, and temp
• most special sense organs

• respond to stimuli arising in internal viscera and blood vessels
• sensitive to chemical changes, tissue, stretch, and temp changes
• sometimes cause discomfort but usually unaware of their workings

• respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles
• inform brain of one's movements

general senses and special senses

• tactile sensations (touch, pressure, stretch, vibration) temp, pain, and muscle sense
• modified dendritic endings of sensory neurons

vision, hearing, equilibrium, smell, and taste

nonencapsulated (free) or capsulated

• free
• abundant in epithelia and connective tissues
• most nonmyelinated, small-diametered group C fibers; distal endings have knoblike swellings
• respond mostly to temp. and pain; some to pressure-induced tissue movement; itch

• general senses;
• cold receptors (10-40 C) in superficial dermis
• heat receptors (32-48 C) in deeper dermis
• outside those temperature ranges the nociceptors activated and it is perceived as pain

• unencapsulated
• player in detection (vanillioid receptors); ion channel opened by heat, low pH, chemicals (things like red pepper=capsaicin)
• respond to pinching, chemcials from damaged tissue, capsaicin

light touch receptors: tactile (merkel) discs, hair follicle receptors

• all mechanoreceptors in connective tissue capsule
• tactile corpuscles: discriminative touch
• lamellar corpuscles: deep pressure and vibration
• bulbous corpuscles: deep continuous pressure
• muscle spindles: muscle stretch
• tendon organs: stretch in tendons
• joint kinesthetic receptors: joint position and motion

• sensation: awareness of changes in the internal and external environment
• perception: the conscious interpretation of those stimuli

• somatosensory system: part of the sensory system serving body wall and limbs
• receives inputs from exteroceptors, proprioceptors, and interoceptors
• input relayed toward head, but processed along way

• receptors level: sensory receptors
• circuit level: processing in ascending pathways
• perceptual level: processing in cortical sensory areas

• 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 or receptor potential)
• graded potentials must reach threshold for action potential

graded potential called generator potential

stimulus -> generator potential in afferent neuron -> action potential

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 of stop

• phasic (fast adapting) receptors: signal beginning or end of stimulus; e.g. receptors for pressure touch, smell
• tonic receptors: adapt slowly or not at all; e.g. nociceptors and most proprioceptors

• pathways of three neurons conduct sensory impulses forward to appropriate cortical regions;
• first order sensory neurons, second order sensory neurons, and third order sensory neurons

conduct impulses from receptors level to spinal reflexes or second order neurons in CNS

transmit impulses to third order sensory neurons

conduct impulses from thalamus to the somatosensory cortex (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

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 properties
• 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 moldy in piece of music)

• warns of actual or impending tissue damage (protective action)
• stimuli include extreme pressure and temp, histamine (inflammation), K+, ATP, acids, and bradykinin
• impulses travel on fibers that releases neurotransmitters glutamate and substance P
• some pain impulses are blocked by inhibitory endogenous opioids (endorphins)

• all perceive paint at same stimulus intensity
• pain tolerance varies
• sensitive to pain means low pain tolerance, not low pain threshold
• genes help determine pain tolerance, response to pain medications

long-lasting/intense pain -> hyperalgesia, chronic pain, phantom limb pain

• stimulation of visceral organ receptors
• felt as vague aching, gnawing, burning
• activated by tissue stretching, ischemia, chemicals, muscle spasms

• pain from one body region perceived from drifferent region
• visceral and somatic pain fibers travel in same nerves; brain assumes stimulus from common somatic region
• e.g. left arm pain during heat attack

• cordlike organ of PNS
• bundle of myelinated and unmyelinated peripheral axons enclosed by connective tissue
• connective tissue coverings (endo, peri, epineuriums)

• 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

• mixed nerves: both sensory and motor fibers; impulses both to and from CNS
• sensory (Afferent): impulses only toward CNS
• motor (efferent) nerves: impulses only away from CNS

• somatic afferent
• somatic efferent
• visceral afferent
• visceral efferent

cranial or spinal nerves

dorsal root ganglia

autonomic ganglia

• axon fragments spreads distally form injury
• macrophages clean dead axon; myelin sheath intact
• axon filaments grow through regeneration tube
• axon regenerates; new myelin sheath forms

oligodendrocytes

astrocytes

contain motor (efferent) fibers from ventral horn motor neurons.  fibers innervate skeletal muscles

contain sensory (afferent) fibers from sensory neurons in dorsal root gnaglia and conduct impulses form peripheral receptors

• spinal nerves
• which emerge form vertebral column via intervertebral foramina

area of skin innervated by cutaneous branches of single spinal nerve

any nerve serving a muscle that produces movement at joint also innervates joint and skin over joint

• takes palce at neuromuscular junction
• neurotransmitter acetylcholine released when nerve impulse reaches axon terminal
• ACh binds to receptors, resulting in: movement of Na+ and K+ across membrane, depolarization of muscle cell, an end plate potential, which triggers an action potential (muscle contraction)

• action potential arrives at axon terminal of motor neuron
• voltage gated Ca2+ channels open. Ca2+ enters axon terminal moveing down its electrochemical gradient
• Ca2+ entry causes ACh to be released by exocytosis (a neurotransmitter)
• ACh diffuses across the sympatic cleft and binds to its receptors on the sarcolemma
• ACh binding opens ion channels in the receptors that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber.  More Na+ ions enter than K+ ions exit, which produces a local change in the membrane potential called the end plate potential
• ACh effects are terminated by its breakdown in the synaptic cleft by acetylcholinesterase and diffusion away from the junction

visceral motor

innervation of visceral muscle and glands

innervation of visceral muscle and glands

• segmental level
• projection level
• precommand level

cerebellum and basal nuclei are the ultimate planners and coordinators of complex motor activities

• lowest level of motor hierarchy
• reflexes and automatic movements
• central pattern generators: segmental circuits that activate networks of ventral horn neurons to stimulate specific groups of muscles
• controls locomotion, and specific, often repeated motor activity

• consists of upper motor neurons that initiate direct pyramidal system to produce voluntary skeletal muscle movements and the brain stem motor areas that oversee indirect extrapyramidal system to control reflex and CPG controlled motor actions
• projection motor pathways send info to lower motor neurons and keep higher command levels informed of what is happening

• neurons in cerebellum and basal nuclei
• regulate motor activity
• precisely start or stop movements
• coordinate movements with posture
• block unwanted movements
• monitor muscle tone
• perform unconscious planning and discharge in advance of willed movements
• cerebellum: acts on motor pathways through projection areas of brain stem; acts on motor cortex via thalamus to fine tune motor activity
• basal nuclei: inhibit various motor centers under resting conditions

true

inborn (intrinsic) and learned (acquired)

• rapid, involuntary, predictable motor response to stimulus
• e.g. posture, control visceral activities
• can be modified by learning and conscious effort

• result form practice of repetition
• e.g. driving skills

• receptor: site of stimulus action
• sensory neuron: transmits afferent impulses to CNS
• integration center: either monosynaptic of polysynaptic region within CNS
• motor neuron: conducts efferent impulses from integration center to effector organ
• effector: muscle fiber or gland cell that responds to efferent impulses by contracting or secreting

• somatic reflexes: activate skeletal muscles
• autonomic (visceral) reflexes: activate visceral effectors (smooth or cardiac muscle or glands)

• integration center in spinal cord
• effectors are skeletal muscles

• to smoothly coordinate skeletal muscle nervous system must receive proprioceptor input regarding:
• length of muscle: form muscle spindles
• amount of tension in muscle: from tendon organs

composed of 3-10 modified skeletal muscle fibers (intrafusal muscle fibers) wrapped in connective tissue capsule

• noncontractile in central regions (lack myofilaments)
• two types of afferent endings: anulospiral endings and flower spray endings

• primary sensory endings
• endings wrap around spindle; stimulated by rate and degree of stretch

• secondary sensory endings
• small axons at spindle ends; respond to stretch

• external stretch of muscle and muscle spindle
• internal stretch of muscle spindle

it causes increased rate of impulses to spinal cord

• maintains muscle tone in large postural muscles and adjusts it reflexively
• causes muscle contraction in response to increased muscle length (stretching)

• stretch activates muscle spindle
• sensory neurons synapse directly with motor neurons in spinal cord
• motor neurons cause stretched muscle to contract
• all stretch reflexes are monosynaptic and ipsilateral

• fibers synapse with interneurons that inhibit motor neurons of antagonistic muscles
• e.g. patellar reflex, stretched muscle (quads) contract and antagonists (hamstrings) reflex

• sensory and motor connections between muscle and spinal cord intact
• strength of response indicates degree of spinal cord exciteability

peripheral nerve damage or central horn injury

there are lesions of corticospinal tract

• 1. when stretch activates muscle spindles, the associated sensory neurons transmit afferent impulses at higher frequency to the spinal cord
• 2. the sensory neurons synapse directly with alpha motor neurons, which excite extrafusal fibers of the stretched muscle. sensory fibers also synapse with interneurons that inhibit motor neurons controlling antagonistic muscles
• 3a. efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists of reverses the stretch
• 3b. efferent impulses of alpha motor neurons to antagonist muscles are reduced (recipricol inhibition)

• polysnaptic reflexes
• helps prevent damage due to excessive stretch
• important for smooth onset and termination of muscle contraction
• produces muscle relaxation (lengthening) in response to tension

• 1. quadriceps strongly contracts. tendon organs are activated
• 2. afferent fibers synapse with interneurons int he spinal cord
• 3a. efferent impulses to muscle with stretched tendon are damped. muscle relaxes, reducing tension
• 3b. efferent impulses to antagonist muscle cause it to contract

• initiated by painful simulus
• causes automatic withdrawal of threatened body part
• ipsilateral or polysnaptic
• protective; important
• brian can override (e.g. stick for blood)

• occurs with flexor reflexes in weight-bearing limbs to maintain balance
• consists of ipsilateral withdrawal reflex and contralateral extensor reflex
• e.g. step barefoot on broken glass

• elicited by gentle cutaneous stimulation
• depend on upper motor pathways and cord level reflex arcs
• plantar reflex and abdominal reflex

• superficial 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
• normal in infants til 1 year due to incomplete myelination

• superficial reflex
• 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