A&P II Exam 2, The Peripheral Nervous System

peripheral nervous systems (PNS)

• links the CNS to the body and external environment

• PNS detects sensory stimuli and delivers info to CNS as sensory input

• CNS processes input and transmits impulse through PNS to muscle cells and glands as motor output

what are the divisions of the PNS

sensory division, somatic sensory division, visceral sensory division, motor division, somatic motor division, visceral motor division (autonomic motor nervous system, ANS)

sensory division of the PNS

consists of sensory (afferent) neurons; sends sensory stimuli to CNS

somatic sensory division of the PNS

• detects both internal and external stimuli 

• general sense detect stimuli from skin 

• special sensory detect stimuli from special sense organs (eyes, skin, skeletal muscles, etc.) 

visceral sensory division of the PNS 

relays internal info (blood pressure) from organs (abdominopelvic and thoracic cavities); ex urinary bladder

motor division of the PNS

motor (efferent) neurons; carry out motor functions of nervous system 

somatic motor division of the PNS

voluntary motor functions; triggers skeletal muscle contractions

visceral motor division (autonomic motor nervous system, ANS) of the PNS

maintains many aspects of homeostasis (involuntary motor functions of the body); divided into sympathetic and parasympathetic

what events occur when sensory neurons detect stimuli

detected stimuli transmitted along sensory neurons (spinal or cranial) to cerebral cortex → in cortex, sensory info is interpreted, integrated, and appropriate motor response is selected and initiated (such as fight or flight, or rest and digest)

the motor division of the PNS is divided in the parasympathetic and sympathetic divisions. What does the sympathetic nervous system do?

• also known as the fight or flight division 

• does homeostasis activities surrounding physical work and visceral response of emotion 

the motor division of the PNS is divided in the parasympathetic and sympathetic divisions. What does the parasympathetic nervous system do?

• also known as the rest and digest division

• does digestion and maintains the body’s homeostasis at rest

why is the sympathetic nervous system referred to as the fight or flight division of the ANS in the PNS 

the sympathetic nervous system carries out the “fight or flight” response in the body by prepping the body for emergency situations. plays a vital role in maintaining homeostasis when the body is engaged in physical work. mediates the body’s response to motion.

how is the body effected when going into “fight or flight” via the sympathetic nervous system

• heart rate increase, increases diameter of airways and increases diameter of pupils

• blood flow diverted from the GI tract and kidneys to organs involved in activity, such as skeletal muscles

• liver increases conversion of glycogen to glucose for ATP production

why is the parasympathetic division of the ANS in the PNS referred to as “rest and digest”

the parasympathetic nervous system carries out the rest and digest function in the body; plays a role in body maintenance functions, such as digestion and urine formation

• also known as the craniosacral division based on association with cranial nerves and pelvic nerves from sacral plexus  

how is the body effected when going into “rest and digest” via the parasympathetic nervous system 

• heart rate decrease, decreased diameter of airways, and decreased diameter of pupils 

• promotes salivation, lacrimation, urination, digestion, and defecation

• liver converts absorbed glucose to glycogen stores 

peripheral nerves

main organ of the PNS; axons of many neurons bound together by connective tissue; nerves of PNS contact (innervate) majority of the structures in the body (which they control)

mixed nerves 

contain both sensory and motor neurons 

sensory nerves

contain only sensory neurons; also some involved in muscle stretch and tension

spinal nerves

originate from the spinal cord and innervate structures below the head and the neck; all 31 pairs of spinal nerves are mixed nerves

what are the functions of the parts of the spinal nerves

• posterior root - carry sensory neurons from posterior horn

• posterior root ganglion (dorsal root ganglion) - swollen area in posterior root; houses cell bodies of sensory neurons

• anterior root - motor neurons from anterior horn

what makes up the epineurium, fascicles, and endoneurium of the peripheral nerves and what is their function

• epineurium - outermost layer of connective tissue; hold motor and sensory axons together

• fascicles - small groups of bundled axons surrounded by connective tissue perineurium

• endoneurium - connective tissue surrounding each individual axon within fascicless

cranial nerves

• attach to brain; mostly innervate structures in the head and neck

• not formed by the fusion of sensory and motor roots (like spinal nerves)

• allow for purely sensory, mixed, and mostly motor nerve

spinal nerve

short; divides into two mixed nerves; both carry somatic motor and sensory info

• posterior ramus - travels of the posterior side of the body

• anterior ramus - travels to anterior side of the body and/or to an upper or lower limb

how many pairs of spinal nerves are there 

31 

of the 31 spinal nerves, how many are cervical, thoracic, lumbar and sacral, and coccygeal

• cervical = 8

• thoracic = 12

• lumbar and sacral = 5

• coccygeal = 1

nerve plexus

anterior rami of cervical, lumbar, and sacral spinal nerves merged together; complicated networks of nerves

right and left cervical plexuses

• anterior rami of C1-C5, with a small contribution from hypoglossal nerve (CN XII)

• each nerve has cutaneous branches that innervate skin of neck and sections of head, chest, and shoulders

• motor branches innervate specific muscle in neck

phrenic nerve

• major motor branch of C4 with contribution from C3 and C5 (3-5 to stay alive); innervates diaphragm

• part of the cervical plexus

hiccups

spasms of the diaphragm that cause of forceful inhalation of air; one way to end hiccups is to apply firm pressure to the muscles of the neck that overlie the phrenic nerve for about 5-10 seconds, the pressure will interrupt aberrant impulses causing the diaphragm to contract inappropriately, pressure is not adequate to stop the nerve from firing completely or interfering with breathing

right and left brachial plexuses 

• lateral to 5th cervical through 1st thoracic vertebrae; provides motor and sensory innervation to upper limbs; includes nerve roots from c1-T1

• brachial plexus begins with formation of large nerve trunks 

• C5 and C6 typically unite to form superior trunk 

• C7 usually forms middle trunk 

• C8 and T1 unite to form inferior trunk

medial cord

the anterior division of the inferior trunk of the brachial plexus that descends down the medial arm

lateral cord

the anterior division of superior and middle trunks of the brachial plexus combined; descends down the lateral arm

posterior cord

posterior division of each trunk of brachial plexus combined; lies in posterior arm

what are the 5 major nerves of the brachial plexus 

1. axillary nerve 

2. radial nerve 

3. musculocutaneous nerve 

4. median nerve 

5. ulnar nerve 

axillary nerve

structures near axilla, including deltoid and teres minor muscle and skin over deltoid region

radial nerve

innervates triceps barchii muscle and most of extensor muscles of forearm; also skin over posterior thumb, 2nd digit, 3rd digit, and lateral half of 4th digit

musculocutaneous digit

innervates bicets brachii and skin covering lateral arm

median nerve 

innervates wrist and digital flexors, some intrinsic muscles of hand and skin over anterior thumb, 2nd, 3rd digits, and lateral half of 4th digit 

ulnar nerve

innervates flexor muscles in forearm (not innervated by median nerve), most of intrinsic hand muscles, and skin of 5th digit and medial side of 4th digit

thoracic spinal nerves ——- form plexuses

do not (except T1)

the posterior ramus of thoracic spinal nerves innervates…

deep back muscles

the anterior ramus of the thoracic spinal nerves travels between two ribs as an ——-

intercoastal nerve 

left and right lumbar plexus

derived from anterior rami of L1-L; innervate pelvic structures and lower extremity after splitting into two divisions

what are the lumbar plexus divisions

obturator nerve and femoral nerve

obturator nerve

• anterior division’s largest member

• enters thigh from pelvis via obturator foramen

• branches of nerves innervate adductor muscles in thigh, hip joint, and skin over medial aspect of thigh

femoral nerve 

• posterior divisions largest member; largest branch of lumbar plexus 

• travels from psoas, through pelvis and under inguinal ligament to enter thigh to innervate; anterior thigh muscles and skin over anterior and medial thigh and leg, as well as knee joint 

right and left sacral plexuses

formed from anterior rami of spinal nerves L4-S4; nerve branches innervate structures of pelvis, gluteal region, and much of lower extremity; each plexus is divided into anterior and posterior divisions

what are the sacral plexuses

sciatic nerve, common fibular nerve (common peroneal(, and tibial nerve

sciatic nerve

• longest and largest nerve in the body

• travels through the greater sciatic notch in the pelvis into the thigh

• innervates hip joint in posterior thigh before it divides into tibial and common fibular nerves

common fibular nerve (common peroneal) 

• made up of axons from posterior division of sacral plexus 

• descends along lateral leg to supply part of knee joint and skin of anterior and distal leg 

• divides into superficial and deep branches; superficial branch serves lateral leg and dorsum of foot; deep branch supplies ankle dorsiflexors and two muscles of dorsum  

tibial nerve

• larger branch of sciatic nerve branches innervates most of hamstring muscles as nerve descends distally

• innervates parts of knee and ankle joint

• smaller nerve branches serves posterior and lateral skin of leg as well as skin and muscles of foot

what is the role o the RNS in sensation

stimuli are first detected by sensory neurons’ stimulus is transmitted by sensory neurons to CNS, where stimulus is integrated and interpreted by CNS neurons

sensory transduction 

stimulus is converted into electrical signal 

what are the steps of sensory transduction

1. ion channels in axolemma are closed

2. stimulus is detected by sensory receptor → sodium ion channels open → sodium ions flow into axoplasm → temp. depolarization (less -) (receptor potential)

3. if enough sodium ions enter, membrane potential may reach threshold → voltage-gated sodium ion channels open → actions potential is propagated along axon towards CNS

what are the types of sensory receptors in sensory transduction

rapidly adjusting receptors and slowly adapting receptors

rapidly adjusting receptors

receptors that respond rapidly with high intensity to stimuli; stop sending signals after certain time period (called adaptation); receptors detect initiation of stimuli but ignoer ongoing stimuli

slowly adapting receptors 

respond to stimuli with constant action potentials that dont diminish over time 

how are sensory receptors classified when concerning surrounding cells

encapsulated nerve endings (surrounded by specialized supportive cells) and free nerve endings (lack supportive cells)

how are sensory receptors classified when concerning location and origin of stimulus 

• exteroceptors - at or near external surface 

• interceptors - in vessels or tissues of organs (blood vessels, tissues, organs, think visceral) 

• proprioceptors - in muscles, joint, and inner ear

how are sensory receptors classified when concerning the type of stimulus detected

• mechanoreceptors - mechanical energy

• thermoreceptors - temperature energy

• nociceptors - damage to tissue (pain)

• photoreceptors - light energy

• chemoreceptors - chemical energy

• osmoreceptors - osmotic pressure of body fluids

what are the classes of mechanoreceptors (on the exterior surface of the skin)

merkel cell fibers, tactile corpuscles, ruffini endings, lamellated corpuscles, hair follicle receptors, proprioceptors

merkel cell fibers

slow adapting; detect discriminative touch stimuli; tell the difference between the objects touching you 

tactile corpuscles (meissner corpuscles) 

rapidly adapting; transmit discriminative touch stimuli; tell the difference between objects 

ruffini endings (bulbous corpuscles)

slowly adapting receptors that respond to stretch and movement 

lamellated corpuscles (pacinian corpuscle)

rapidly adapting receptors detect vibration and deep pressure stimuli (pain)

hair follicle receptors

free nerve endings surrounding the base of hair follicles in thin skin; not on palms and soles; respond to stimuli that cause hair to bend

proprioceptors

in musculoskeletal system; detect movement and position of joints and body parts

types of thermoreceptors 

• cold

  • in superficial dermis

  • temp range: 10-40 C

• warm 

  • in dermis 

  • temp range: 32-48 C 

how do thermoreceptors work

activated with temp detection; free nerve endings that adapt rapidly but generate impulses at lower frequency for prolonged stimulus; temps above or below thermoreceptors range stimulate nociceptors instead, resulting in painful sensations 

what are first order somatic sensory neurons and what are the three components

pseudounipolar neurons; composed on cell body, peripheral process, and central process

cell body

cell bodies of spinal nerves are in posterior (dorsal root ganglion), just lateral to spinal cord; cell boies of cranial nerves are in cranial nerve ganglia in head and neck

peripheral process

long axon that transmits action potentials from source of stimulus (receptor) to neuron’s central process

central process 

exits cell body and travels through posterior root; enters spinal cord at posterior horn (or brainstem for cranial nerves) where they deliver their action potentials 

how does an action potential travel down somatic sensory nerve

1. peripheral process transmits action poetnial from sensory receptors to neuron’s central process

2. central process transmits actions potential from peripheral process to posterior horn, eventually synapsing on second-neuron in spinal cord or brain stem

3. action potential propagated down peripheral process does not generally reach cell body ; instead, usually transmitted to central process in area when peripheral and central processes come into contact near cell body

receptive fields

areas served by particular neurons; neurons with more branches innervates larger receptive fields

body regions whose primary function is sensing environment (fingertips) contain — neurons with — receptive fields 

many, smaller 

body regions that are not as involved in sensing environment (skin of forearm) have —- neurons with — receptive fields 

fewer, larger 

two point discrimination threshold

mthod for measuring the relative size of receptive fields

referred pain

phenomenon whereby pain that originates in an organ is perceived as cutaneous pain

• occurs because many spinal nerves carry both somatic and visceral neurons; so visceral sensations travel along the same pathways as somatic sensations

• rgenerally located along dermatome for particular neurons

upper motor neurons 

neurons of primary motor cortex makes decision to move and initiate that movement; not in contact with muscle fibers itself 

lower motor neurons

recieve messages from upper motor neurons; in contact with skeletal muscle fibers; release acetylcholine onto muscle fibers to initiate contraction

lower motor neurons

multipolar neurons whose cell bodies are in either anterior horn of the spinal cord or brainstem; axons are in the PNS

motor neuron pools 

groups of lower motor neurons that innervate same muscles; clustered in anterior horn of spinal cord 

large motor neurons

majority of neurons within pools; stimulate skeletal muscle fibers to contract by excitation-contraction mechanism

smaller motor neurons

found within neuron pools; innervate intrafusal fibers; part of specialized stretch receptors

diagram of brain and motor response 

reflexes

programmed, autonomic responses to stimuli; occur in reflex arc (three-step sequence of events); usually protective negative feedback loops

how do reflexes work

1. reflexes begin with sensory stimulus and finish with rapid motor response

2. neural integration between sensory stimulus and motor response occurs in CNS, at spinal cord or brainstem

3. mechanoreceptors in muscles and tendons monitor muscle length and force of contraction; communicate information to spinal cord, cerebellum, and cerebral cortex

muscle spindles 

tapered structures found scattered among regular contractile muscle fibers (extrafusal muscle fibers) 

• intrafusal fibers have contractile filaments of actin and myosin at their poles; innervated by motor neurons 

• contractile filaments are absent in central area of intrafusal fibers 

what are the two structural and function classes of sensory neurons that innervate intrafusal fibers

• primary afferents - respond to stretch when first initiated

• secondary afferents - respond to both static length of muscle and position of limb

golgi tendon organs

mechanoreceptors located within tendons near muscle-tendon junction

• monitor tension generated by muscle contraction

• contain single somatic sensory axon that fires more rapidly as greater tension is generated with each contraction; info is sent to the CNS

how are reflexes classified

1. number of synapses between neurons involved in arc

2. type of organ in which the reflex takes place, either visceral or somatic

simplest reflex (monosynaptic reflexes) 

involve only a single synapse within spinal cord between sensory and motor neuron 

polysynaptic reflex

complicated reflex arc involving multiple synapses

patellar (knee-jerk). reflex and jaw-jerk reflex

examples of simple stretch reflexes

steps of a simple reflex stretch 

golgi tendon reflexes

polysynaptic reflexes; protects muscles and tendons from damaging forces

• causes muscle relaxation; opposite of simple stretch reflex action

• when tension in muscle and tendon increases dramatically, golgi tendon organs signal spinal cord and cerebellum

• motor neuron innervating muscle are inhibited while antagonist muscles are simultaneously activated

flexion or withdrawal reflex

involves rapidly conducting nociceptive afferents and multiple synapses in spinal cord; act to withdraw limb from painful stimuli

crosses-extension reflex 

occurs simultaneously on opposite sides of the body for balance and postural support while other limb is withdrawn from painful stimulus 

cranial nerve reflexes

polysnaptic reflex arcs that involve cranial nerves; gag reflex and corneal blink reflex