A&P TEST 4

Dermatome

area of skin supplied w sensory info by a pair of spinal nerves

Myotome

muscles that a particular spinal nerve innervates

Nerve Plexus

arise from the interweaving of the anterior (ventral) rami of the spinal nerves on each side of the body; acts to combine several nerves that innervate the same part of the body into one larger nerve 

• good thing abt this is that damage to single spinal nerve does not result in complete loss in innervation

Spinal nerve organization

cord → rootlets → roots → nerves → rami → nerve plexus

motor axons originate from…

• spinal nerve

• ventral rootlets containing this exit from spinal cord

• ventral root forms from ventral rootlets

• cell bodies are found in ventral and lateral horns of spinal cord 

sensory axons enter…

• spinal cord via dorsal aspect

• dorsal roots arise from dorsal rootlets

• dorsal root contain short central axons

• dorsal root ganglion houses cell bodies 

Rami

distal to the spinal nerve and are formed by branches of the spinal nerves

Dorsal ramus

innervate the deep muscles of the back and the skin of the back

Ventral ramus

• splits into multiple branches

• innervate the ventral and lateral portions of the trunk, upper and lower limbs 

• forms nerve plexuses 

Rami communicans 

• specific to ventral/anterior rami

• associated with autonomic nervous system

• extends between the spinal nerve and the sympathetic ganglions

Intercostal Nerves 

• thoracic nerves that do not form plexuses

• T2: sensory info from skin of axilla (armpit) and medial surface of the arm 

• T3- T6: sensory info from the anterior and lateral chest wall

• T7-T12: innervates abdominal muscles and overlying skin 

Cervical Plexus

• located deep at back of neck (formed primarily by anterior rami of C1- C4) 

• Nerves innervate superficial nerve structures 

  • muscles attached to the hyoid bone

  • the surface of the neck 

  • lower portions of the head 

Cervical Plexus Branches

• Cutaneous branches

  • Occipital nerve (C2- C3; occipital scalp)

  • Auricular nerve (C2- C3; skin around the auricle- ear and external auditory meatus) 

  • Cervical nerve (C2-C3; skin of anterior neck)

  • Supraclavicular nerve (C3-C4; skin of shoulder region)

• Motor branches 

  • ansa cervicalis (C1- C3; muscles of the throat)

  • phrenic nerve: MOST IMPORTANT; C4 (some C3 and C5); innervates the diaphragm to regulate breathing 

Brachial Plexus

• located superior to clavicle 

• more complex than the cervical plexus 

Brachial Plexus Five Levels of Organization

• Rami: continuation of anterior rami of spinal nerves C5- T1

• Trunks: unification of rami; superior- C5 & C6, middle- C7, inferior- C8 & T1

• Divisions: fibers of trunk diverge to travel to anterior or posterior portions of the upper arm; two divisions- anterior and posterior; found deep to the clavicle 

• Cords: convergence of anterior and posterior divisions; named for their position relative to the axillary artery

  • posterior: formed by posterior divisions of all 3 trunks (contains portions of C5- T1)

  • medial: formed by anterior division of the inferior trunk (contains portions of C8- T1)

  • lateral: formed by anterior division of superior and middle trunk (contains portions of C5- C7)

• Terminal branches: arise from cords (five major branches) 

Axillary Nerve (five terminal branches of Brachial Plexus) 

• arises from posterior cord

• innervates deltoid

• receives sensory from superolateral portion of arm 

Musculocutaneous nerve (five terminal branches of brachial plexus)

• arises from lateral cord

• innervates anterior arm muscles 

• receives sensory from lateral forearm

Radial Nerve (five terminal branches of brachial plexus) 

• arises rom posterior cord

• innervates posterior arm and forearm muscles

• receives sensory from posterior arm and forearm and lateral 3 digits

Median Nerve (five terminal branches of the brachial plexus)

• arises from medial and lateral cords

• innervates most of anterior forearm lateral hand

• receives sensory from lateral palmar and dorsal tips of lateral 3 fingers

Ulnar Nerve (Five Terminal Branches of the Brachial Plexus)

• arises from medial cord

• innervates anteromedial forearm and intrinsic hand muscles

• receives sensory from medial 1 ½ digits (anterior and posteriorly) 

Lumbar Plexus

• located in anterior pelvis 

• formed by anterior rami of spinal nerves L1- L4

  • overlaps with sacral plexus

• less complex than brachial but has anterior and posterior divisions 

• contains nerves innervating the anterior aspect of lower limbs as well as the abdomen, pelvis and buttock 

Femoral Nerve (Lumbar Plexus)

• main nerve of the posterior division

• innervates muscles of the anterior thigh 

  • allows flexion and extension of the knee and hip

• receives sensory from the anterior and inferomedial thigh and medial aspect of the leg 

Obturator Nerve (Lumbar plexus)

• main nerve of the anterior division

• innervates the medial thigh

• receives sensory from the superomedial thigh 

Sacral Plexus

• located in posterior pelvis (inferior to lumbar pelvis)

  • overlaps with lumbar plexus

• formed by the anterior rami of spinal nerves L4- S4

• organized into anterior and posterior divisions

  • anterior division tend to innervate muscles that flex 

  • posterior division innervate mucles that extend 

• contains nerves innervating the posterior aspect of the lower limbs as well as the abdomen, pelvis and buttock

Sciatic nerve (Sacral Plexus)

• longest and largest nerve in the body

• formed from portions of both anterior and posterior divisions of the sacral plexus

• has 2 divisions wrapped in a common sheath, tibial division and common fibular division that give rise to the tibial and common fibular nerve 

Tibial Nerve (Sacral Plexus)

• formed by anterior division of sciatic nerve

• innervates posterior thigh and leg and plantar foot muscles

  • hip extension, thigh adduction, knee and foot flexion, and toe flexion

  • plantar nerves cause flexion and adduction of the toes

• receives sensory input from the plantar region of the foot (via sural nerve) 

Common fibular nerve (Sacral Plexus)

• formed by posterior division of sciatic nerve

• innervates a knee muscle, anterior and lateral leg muscles

• receives sensory input from the anteroinferior region of the foot (via its branches) and dorsal interspace bw the 1st and 2nd toe 

Stimulus

sensory input that initiates reflex

Rapid response

requires few neurons involved

Pre-programmed response

Response is the same every time

Involuntary response

requires no conscious intent, cant be suppressed 

The reflex arc

• basic functional unit of the nervous system

• smallest, simplest circuit capable of receiving a stimulus and producing a response

• action potential never travels to the brain for processing

• five parts

Ipsilateral

receptor and effector are on the same side of the spinal cord 

contralateral 

receptor and effector are on opposite sides of the spinal cord 

Monosynaptic reflex

• most simple reflex

• has only one synapse in between receptor and effector

• direct communication bw sensory and motor neuron

• can’t be modified by other input from the CNS

Polysynaptic reflex

• more complex

• has more than 1 synapse bw receptor and effector

• interneurons facilitate communication bw sensory and motor neuron

• may be affected by other input from CNS

Autonomic reflex

reflex that affects organs

Somatic reflex

reflex that affects muscles

Stretch reflex

• monosynaptic

• monitors and regulates muscle length

• monitored by stretch receptors called muscle spindles

  • internal: sensory neurons wrapped around intrafusal muscle fibers; all surrounded by CT capsule

  • external: extrafusal muscle fibers 

• When muscle spindle is stretched

  • sensory neurons detect stretch of intrafusal muscle fibers → signal sent to spinal cord (CNS) → synapse w α motor neurons → extrafusal fibers contract to resist stretch

Golgi Tendon reflex

• polysynaptic

• prevents excessive contraction in response to increased tension 

• sensory receptor is the golgi tendon organ 

• When a muscle contracts, the tendon stretches

  • golgi tendon organ detects stretch → sends sensory info to CNS → synapse w interneurons in spinal cord → interneurons inhibit alpha motor neurons to prevent muscle contraction

• sensory neuron also stimulates the alpha motor neuron in the antagonistic muscle (called reciprocal activation)

Withdrawal (flexor) reflex

• polysynaptic

• reflex that is initiated by painful stimuli

• causes a flexor muscle to contract (and antagonist muscle to relax) thus causing the limb to be removed from the offending stimulus

Crossed-extensor reflex

• polysynaptic

• occurs in conjunction w withdrawal reflex (when withdrawal reflex is initiated in one limb, the opposing limb is stimulated to extend)

• sensory neurons branch to synpase with internuerons involved in this activity and withdrawal activity (interneuron involved in this activity crosses to other side of spinal cord to synapse w alpha neurons innervating opposing limb)

Sensory Nervous System

• afferent nervous system

• convey info (impulses) to the CNS

• somatic sensory: sensory input consciously perceived from receptors (special senses, skin, etc); like touch, pain, temperature 

• visceral sensory: sensory input that is NOT consciously perceived from the viscera and blood vessels; ex: blood pressure, pH levels, osmorality 

Motor Nervous System 

• Efferent Nervous system

• Convey info (impulses) from the CNS to the effector organs

• Somatic motor: motor output that is consciously or voluntarily controlled (effector is skeletal muscle)

• Autonomic motor: motor output that is involuntarily controlled; maintains homeostasis, effectors: glands, smooth, and cardiac muscle; divided into sympathetic and parasympathetic systems

Somatic nervous system

• target tissues: skeletal muscle

• response: excitation of skeletal muscle contraction

• regulation: voluntary/ conscious

• origins: motor cortex, medulla, dorsal horns of the cord

• myelination: yes 

• neurotransmitters: ACh

• receptor subtype: nicotinic 

Autonomic nervous system

• target tissues: viscera

• response: inhibition and excitation of smooth and cardiac muscle and glandular secretion

• regulation: involuntary/unconscious

• origins: brain stem nuclei and varying regions of the cord

• myelination: preganglionic fibers are myelinated, postganglionic fibers are not

• neurotransmitters: acetylcholine, norepinephrine, epinephrine

• receptor subtype: nicotinic, muscarinic, alpha and beta adrenergic 

Autonomic nerve pathway

• two- neuron chains

• Cell body of 1st neuron located within CNS

  • synapses with cell body of 2nd neuron located in a ganglion

• axon of 2nd neuron (post-ganglionic fiber) innervates the effector organ 

Sympathetic Nervous System

• one subdivision of autonomic nervous system

• “fight-or-flight”

• primarily mobilized during period of exertion (activity), excitement (stress), or emergency 

• increases in heart and respiration rate, blood flow to skeletal and cardiac muscle, and sweating 

Parasympathetic Nervous System 

• one subdivision of autonomic nervous system

• “rest-and-digest” or “feed and breed” 

• primarily promotes maintenance functions and energy conservation

  • SLUD= salivation, lacrimation, urination, and defection

  • decreases heart rate and blood pressure

  • increases motility (mvmt) and secretion in the digestive system, the excretion of waste products, and sexual arousal

• body spends most of the time under parasympathetic dominance

Parasympathetic anatomical differences 

• Location of Preganglionic cell bodies: Cranial and sacral origins

• Length of preganglionic fibers: Long

• Location of ganglia: Located near the target organ or within wall of organ

• Length of postganglionic fibers: Short

• Number of preganglionic axon branches: Few (1 preganglionic fiber to <4 postganglionic fiber)

• Degree of response: local 

Sympathetic anatomical differences

• Location of preganglionic cell bodies: Thoracic and lumbar origins

• Length of preganglionic fibers: short 

• Location of ganglia: located near the spinal cord 

• Length of postganglionic fibers: Long

• Number of preganglionic axon branches: extensive (1 preganglionic fiber to >20 postganglionic fiber)

• Degree of response: mass activation (many systems simultaneously) OR local 

Parasympathetic anatomy

• oculomotor nerve (CN III), facial nerve (CN VII), glossopharyngeal nerve (CN IX)- parotid salivary gland, vagus nerve (CN X)- viscera, splachnic nerves (S2-S4)- lower abdominal viscera and pelvic viscera 

Sympathetic anatomy

• more complex

• cell bodies housed in the lateral horn of the spinal cord (bw T1 and L2) and fibers exit through anterior roots

• sympathetic fibers leave spinal nerve to enter either left or right sympathetic trunk (immediately lateral to the spinal cord)

  • the trunks contain the sympathetic ganglia

  • sympathetic ganglia are interconnected by bundles of axons to form the trunk 

  • roughly one sympathetic ganglia per spinal nerve

• one of three things can happen

  • a fiber can synapse with neurons in same trunk ganglion

  • it can ascend or descend to synapse with another trunk ganglion

  • it can pass through wo synapsing (collateral ganglia)

• Cervical ganglia

  • superior: postganglionic fibers to head and neck

  • middle and inferior: postganglionic fibers to thoracic viscera

• Rami communicantes: connect spinal nerves to each sympathetic trunks; white and gray

• Sympathetic splanchnic nerves

  • preganglionic fibers do not synapse in a sympathetic trunk ganglion

  • travel to most of the abdominal and pelvic viscera

  • terminate in collateral (prevertebral) ganglia

White Ramus communicans 

Carry pre-ganglionic axons from T1-L2 to the sympathetic trunk 

Gray Ramus communicans

carry post-ganglionic axons from the sympathetic trunk to the spinal nerve; lack myelin, hence the color and the name 

Greater thoracic splanchnic nerve

• preganglion fibers originate from T5- T9

• synapse in celiac ganglion 

Lesser thoracic splanchnic nerve

• preganglion fibers originate from T10- T11

• synapse in superior mesenteric ganglion 

Least thoracic splanchnic nerve

• preganglion fiers origiante from T12

• synapse in superior mesenteric ganglion

Lumbar splanchnic nerve

• preganglion fibers originate from L1-L2

• synapse in inferior mesenteric ganglion 

Sacral splanchnic nerves

post ganglion fibers originate from sympathetic sacral ganglion

Prevertebral (collateral) ganglia

• located anterior to the vertebral column on the aneterolateral wall of the aorta

• located only in the abdominopelvic cavity

• celiac ganglion: postganglionic fibers innervate stomach, spleen, liver, gall bladder and proximal portion of duodenum and part of pancreas

• superior mesenteric ganglion: postganglionic fibers innervate distal half of dueodenum, rest of small intesitne, proximal portion of large intestine, part of pancreas, kidneys and proximal part of ureters

• inferior mesenteric ganglion: postganglionic fibers innervate distal part of large intestine, rectum, urinary bladder, distal ureters, and most of the repro organs 

Sympathetic pathway

path by which a sympathetic fiber exits the sympathetic ganglion

Spinal nerve pathway

• preganglionic fiber: synapses in the sympathetic trunk ganglion with postganglionic fiber

• postganglionic fiber: travels out of the ganglion via the gray ramus at the same “level”; fiber joins the spinal nerve and extends to its target organ

• targets: skin of the torso, neck and limbs including sweat glands, blood vessels of the skin, and arrector pili muscles 

Postganglionic sympathetic nerve

• preganglionic fiber: synapses in the sympathetic trunk ganglion with postganglionic fiber

• postganglionic fiber: does not use gray ramus; fiber exits the ganglion and directly extends to its target organ

• targets: esophagus, heart, lungs, and thoracic blood vessels; innervated structures of the head including sweat glands and blood vessels; also some eye muscles

Splanchnic nerve pathway 

• preganglionic fiber: pass through the sympathetic trunk ganglion without synapsing; extend to the collateral (prevertebral) ganglia to synapse with postganglionic fiber

• Postganglionic fiber: travels out of the collateral ganglion and extends to its target organ

• Targets: abdominal and pelvic organs

Adrenal medulla pathway

• preganglionic fiber

  • directly innervates the adrenal medulla

  • extends through the sympathetic trunk and collateral ganglion

  • terminate on neurosecretory cells

• postganglionic fibers: none

• targets adrenal medulla

  • modified sympathetic ganglion

  • does not give rise to postganglionic fibers

  • secretes hormone into the bloodstream: 20% NorE and 80% Epi

  • reinforces the activity of the sympathetic nervous system 

Adrenal medulla

• adrenal gland is located superior to the kidney (two parts: cortex= outer portion; medulla= inner portion)

• modified sympathetic ganglion that does not give rise to postganglionic fibers 

Neurotransmitters of the ANS

• two main neurotransmitters of ANS are NorE and ACh

• neurons named for which NT they synthesize and release

  • cholinergic fibers release ACh

    • all sympathetic and parasympathetic preganglionic fibers

    • all parasympathetic postganglionic fibers

    • sympathetic postganglionic fibers that innervate sweat glands in skin and blood vessels in muscle

  • adrenergic fibers release NorE (almost all sympathetic postganglionic fibers)

Varicosity

• terminal end of a sympathetic fiber

• analogous to a synaptic knob

• network of swellings along the terminal branch that overlays the innervated tissue

• NT is released along the terminal length of the axon as opposed to the terminal end

• yields a greater response

Cholinergic Receptors: Nicotinic

• named bc they are sensitive to nicotine

• location: post-ganglionic cell bodies, adrenal medulla, skeletal muscles

• activated by ACh

• Binding of ACh to nicotinic receptor is always excitatory → produces EPSPs

Cholinergic Receptors: Muscarinic

• named bc they are sensitive to muscarine, a mushroom toxin

• location: effector cell membranes in parasymp system, selected cells in symp system

• activated by ACh

  • from parasymp postganglionic fibers

  • from some symp fibers innervating sweat glands and blood vessels (of skeletal muscle)

• binding of ACh to muscarinic receptor can be either excitatory or inhibitory depending on the subclass of receptor 

Adrenergic Receptors

• two major types: Alpha receptors- a1 & a2; Beta receptors- B1, B2, B3

• bound by either Epi or NorE depending on subtype

• can be excitatory or inhibitory depending on subtype

• all are coupled with G-protein 

Interactions within the ANS

• Tone

  • basal lvl of activity of both systems

  • both sympathetic and paraysymp systems always active but dominance is dependent on homeostatic need 

• Antagonistic effects

  • symp and parasymp frequently innervate the same organ

  • generally exert opposite effects on a particular organ

  • called dual reciprocal innervation 

Autonomic Reflexes

• also called visceral reflexes

• similar to the somatic reflex arc, but has two neurons in motor component

• cardiovascular reflex: reduction of bp

• gastrointestinal reflex: prepare body for digestion and elimination

• micturition reflex: leads to urinary elimination 

Control of the ANS

• top lvl is the hypothalamus

• signals from hypothalamus are relayed through the brainstem and spinal cord

• cortex provides input to the hypothalamus so that the proper response is performed 

Transducers

• change energy from one form to the next: original energy form is what is detected by the receptor; converted into electrical/ chemical energy

• Two features

  • receptors establish and maintain resting membrane potential across their plasma membrane

  • receptors contain modality-gated channels (activated by diff stimuli?) in their plasma membranes

Receptive Field

• area through which a stimulus is detected 

• size inversely correlates with the density of receptors (more receptors, more frequent and smaller the fields)

• larger the field, the less we can localize the exact spot of stimulation 

Tonic Receptors

• respond continuously to stimuli at a constant rate

• respond to both the presence of and intensity of the stimulus

• receptor sensitivity stay constant over time or slowly adaptive 

Phasic Receptors

• receptors that deal with temp, pressure, and light touch

• detect new stimuli or changes in the state of the detected stimulus (detect onset and offset of a stimulus)

• undergo rapid adaptation, a reduction in sensitivity to a continually applied stimulus 

General sense receptor

• typically simple in structure

• somatic sensory receptor 

  • housed within the skin monitoring tactile sensations

  • housed within muscles and joints monitoring stretch and mvmt

• visceral sensory receptor

  • housed in walls of viscera

  • respond to temp, chemicals, stretch and pain

Special senses

• specialized, complex sense organs

• gustation, olfaction, audition, vision, equilibrium 

Exteroceptors 

• detect stimuli of external origin

• located near body surfaces

  • tactile receptors in the skin and mucous membranes 

Interoceptors

• detect stimuli of internal origin

• visceral sensory receptors 

• somatic sensory receptors (part of muscoskeletal structures and position of bones and muscles) 

Propioceptors

• located in muscles, tendons and joints

• detect body mvmt, skeletal muscle contraction and stretch

• enable our awareness of body position 

Thermoreceptors

• detect changes in temperature

  • 6x more cold receptors than warm receptors

  • cannot detect temp below 10 degrees C

• Receptors are transient receptor potential cation (TRP) channels

  • respond at different temps → changes in channels

  • allows Ca 2+ to depolarize the cell

  • some channels also respond to chemicals

Chemoreceptors

detect chemicals or specific molecules dissolved in fluid

Mechanoreceptors

• respond to touch, pressure, vibration, and stretch

• specialized receptors

  • baroreceptors detect changes in stretch or distention within body structures

  • osmoreceptors detect changes in solute concentration of bodily fluids

  • proprioceptors detect position of body in space

Nociceptors 

• subtype of free nerve endings

• concentrated in areas more prone to injury

• adapt very slowly or not at all

• two primary types

  • visceral detect internal damage within viscera

  • somatic detect chem, temp, or mech changes at body surface, joints, or skeletal muscles

• respond to cellular damage, noxious chemicals, cellular signals

Sensory cells 

possess receptors to detect stimulus and stimulates peripheral neuron 

Peripheral neuron

responds directly to stimulus

Tactile receptors

• most numerous type of receptor

• mechanoreceptors

• mostly located in dermis and subcutaneous layer 

• can be simple (unencapsulated) or complex (encapsulated) 

Tactile sensation

• touch: provides info abt location, texture, size, shape, and mvmt

• pressure: results from deformation of deeper tissues

• vibrations: rapid and repetitive sensory signals 

Unencapsulated tactile receptors

• dendritic ends of sensory neurons lack a protective coat

• 3 types: free nerve endings, root hair plexuses, tactile discs (merkel discs)

• abundant in epithelia and CT

• most are unmyelinated 

Free nerve endings and root hair plexus (figure this out)

Merkel (tactile) cells or discs

• flattened nerve endings associated with specialized sensory cells- abundant in the tips of fingers and lips 

• located in the stratum basale

• tonic receptors 

• detect fine touch 

  • distinguish texture and shapes of objects 

Encapsulated tactile receptors

• wrapped by connective tissue or surrounded by glial cells

• 4 types: tactile (meissner’s) corpuscles, bulbous (ruffini’s) corpuscles, end bulbs (krause’s corpuscles), lamellated (pacinian) corpuscles 

• almost all are mechanoreceptors 

• (increased size of the receptor field)

Tactile (Meissner’s) corpuscles

• intertwined dendrites

  • enclosed by neurolemmocytes arranged as horizontal lamellae 

  • surrounded by dense irregular CT

• location

  • papillary dermal layer

  • lips, palms, eyelids, nipples, and genitalia

• phasic receptors

• detects

  • light touch

  • vibrations (10- 50 MHz)

  • used to distinguish texture and shape of objects 

Bulbous (Ruffini’s) corpuscles

• spindle-shaped dendritic endings ensheathed in CT

  • nerve endings are intertwined with collagen fibers encased within a capsule

• location

  • dermal layer

  • highest density around the fingernails to monitor the slippage of objects 

• detects skin distortion and continuous deep pressure

  • important for our ability to grasp and hold things with our fingers

• tonic receptors 

  • do not exhibit adaptation 

End bulbs (Krause’s corpuscles)

• dendritic endings ensheathed in CT

• locations: reticular dermal layer; mucous membranes of oral, nasal, vaginal, and anal cavity

• detects: light pressure and temp 

• different from the others bc it focuses on membrane rather than skin 

Lamellated (Pacinian) corpuscles

• capsule: outer CT sheath (concentric layers of collagen fibers) + inner core of neurolemmocyte

• locations: 

  • hairless skin: subcutaneous layer of palms and soles, breasts and external genitalia

  • deep reticular layer of dermis

• rapidly adapting (phasic receptors) 

• detects: deep pressure and high frequency vibration