Chapter 13 - Spine, Nerves - A&P II - MADYS functions of brain and spinal cord § Receive sensory input from receptors § reflex centers § Send motor output to effectors reflex Rapid... Chapter 13 - Spine, Nerves - A&P II - MADYS functions of brain and spinal cord § Receive sensory input from receptors § reflex centers § Send motor output to effectors reflex Rapid, automatic response triggered by stimuli spinal reflexes § Function without input from the brain § Controlled in the spinal cord how many segments and nerves 31 segments, 31 pairs of nerves outer white matter myelinated, faster signals due to insulation, superficial, unmyelinated too, carries information inner grey matter with central canal process and release information through axon signalling found in the white matter, control movement, memory, and emotions, deep and forms an H/butterfly shape, neuroglia, neuron cell bodies, unmyelinated, integrates sensory and motor functions central canal - has CSF spinal cord length 42-45 cm (16-18 in) spinal cord width 14 mm (0.5 in.) spinal cord ends at **** L1-L2 (conus medullaris) how many C segments 8 cervical how many T segments 12 thoracic how many L segments 5 lumbar how many S segments 5 sacral how many coccygeal segments 1 coccygeal conus medullaris cone-shaped end of cord at L1-L2 *** filium terminale 'terminal thread', extension of pia (fibrous tissue strand from tip of conus to S2), anchors spinal cord to coccyx (longitudinal support) cauda equina extended anterior and posterior roots of spine segments L2-S5 and cilium terminal, horse tail naming spinal nerves designated by region and number spinal nerves C1 is above 1st cervical, C8 is below 7th cervical, all others named for vertebra above (T1 is below 1st thoracic vertebra) posterior median sulcus shallow, longitudinal groove on posterior surface anterior median fissure deep groove on anterior surface spinal nerve axons of sensory and motor neurons anterior root motor neurons posterior root sensory neurons posterior root ganglion (spinal ganglion) cell bodies of sensory neurons in posterior root spinal meninges surround spinal cord, continuous with cranial meninges, pia, arachnoid, and dura mater, stabilizes, absorbs shock, carries blood supply dura mater outermost covering, tough and fibrous, dense collagen fibres on longitudinal axis of cord arachnoid mater middle layer, arachnoid membrane (layer of simple squamous epithelium), subarachnoid space (between arachnoid membrane and pia) subarachnoid space has arachnoid trabeculae (collagen and elastic fibres anchoring arachnoid to pia), has blood vessels for spinal cord, has CSF to absorb shock, diffuse gases, and supply nutrients epidural space between dura mater and vertebrae, has areolar and adipose tissue, blood vessels denticulate ligaments extend from pia mater through arachnoid mater to dura mater, prevents lateral movement dural connections at foramen magnum, prevent superior/inferior movements coccygeal ligaments at sacrum, prevent superior/inferior movements lumbar puncture (spinal tap) withdrawal of CSF, needle into subarachnoid space (in lumbar region below conus medullaris to avoid spinal cord) posterior grey horn somatic and visceral sensory nuclei (organs) lateral grey horn only in T and L segments, has visceral motor nuclei anterior grey horn somatic motor nuclei nuclei functional groups of neuronal cell bodies in grey matter of spinal cord sensory nuclei receive and relay sensory information from peripheral receptors motor nuclei issue motor commands to peripheral effectors grey commissures have axons that cross over spinal cord (connects one side to the other like the brain), located posterior and anterior to central canal posterior white column between posterior gray horns and posterior median sulcus lateral white column between the anterior and posterior columns anterior white column between the anterior gray horns ascending tracts in posterior columns, carry sensory information descending tracts in anterior columns, carry motor commands epineurium outermost tissue layer, dense network of collagen fibres perineurium middle tissue layer, separates nerve into fascicles (bundles of axons) endoneurium innermost layer, surrounds individual axons connective layer blood supply***** arteries/veins go through the epineurium, branch in peri, and the capillaries enter endo spinal nerve rami each spinal nerve divides to form two rami posterior ramus muscles, joints, skin of back anterior ramus lateral and anterior trunk; limbs rami communicantes in nerves T1-L2*******, carry motor output of sympathetic division of ANS (responsible for fight or flight) dermatome bilateral area of skin supplied by a pair of spinal nerves, C1 usually lacks a sensory branch, face is CN V (trigeminal), boundaries can overlap, area affected can determine where issue is from sensory information collected from periphery, delivered to sensory nuclei in SC sensory sympathetic nerve sensory info from visceral organs sensory anterior ramus from where ventrolateral body surface, body wall, and limbs sensory posterior ramus from where skin/skeletal muscles of back sensory posterior root of spinal nerves carry sensory info to SC motor commands originate in motor nuclei motor anterior root axons of somatic and visceral motor neurons motor spinal nerve forms at junction of anterior and posterior roots motor posterior ramus somatic/visceral motor fibers to skin/skeletal muscles of back motor anterior ramus ventrolateral body surface, body wall, limbs motor white ramus communicans short branch with preganglionic visceral motor fibres to sympathetic ganglia motor grey ramus communicans postganglionic fibers to glands/smooth muscle motor sympathetic nerve pre/postganglionic fibers to thoracic cavity shingles - Varicella-zoster virus - herpes virus; causes chickenpox and shingles - Attacks neurons in posterior roots and sensory ganglia - Painful rash/blisters along dermatome of affected nerve - History of chickenpox increases chances nerve plexus complex interwoven nerve network (braid), formed during development, anterior rami of adjacent spinal nerves blend fibers to form these 4 major nerve plexuses****** cervical - neck and diaphragm brachial - pectoral girdle, upper limb lumbar - pelvic girdle, lower limb sacral - pelvic girdle, lower limb cervical plexus - From anterior rami of spinal nerves C1-C5 - Branches innervate skin/muscles of neck - Extends into thoracic cavity phrenic nerve - Provides entire nerve supply to the diaphragm (breathing) brachial plexus - Innervates pectoral girdle, upper back, upper limbs - Anterior rami of C5–T1, form trunks; trunks split into divisions; divisions combine to form cords - Most nerves of brachial plexus come off cords; a few originate at the trunks major brachial plexus nerves**** § Musculocutaneous nerve § Median nerve § Ulnar nerve § Axillary nerve § Radial nerve § Cutaneous nerve distribution to wrist/hand is clinically important § Nerve damage can be precisely localized by testing sensory function of the hand lumbar and sacral plexuses § From lumbar and sacral segments of spinal cord § Innervate lower trunk, pelvic girdle, lower limbs lumbar plexus - Formed from spinal nerves T12-L4 - Lumbosacral trunk—L4 branches contribute to sacral plexus sacral plexus - Formed from spinal nerves L4–S4 - Contains the sciatic nerve********—largest/longest nerve in body nerves supplying foot and ankle**** • Saphenous nerve - lateral (outer) • Sural nerve - medial • Common fibular nerve - top of foot • Tibial nerve - bottom of sole neuronal pools neurons in several regions of brain, or in 1 specific CNS location, # estimated from a few 100s-1000s, pattern of interaction reflects pools function, most complex processing in brain, simplest circuits are in PNS and SC (reflex) neural circuits § 'Wiring diagram' of neuron interaction § Common patterns include divergence, convergence, parallel processing, serial processing, reverberation ********** divergence spreads info from one neuron to several, or one pool to several, allows broad distribution of specific input - radiates in area - one signal can turn into 3 convergence several neurons synapse on a single postsynaptic neuron, motor neurons experience both conscious and unconscious control, ex: adjusting breathing (makes smaller, 3 signals to 1) parallel processing many neurons or pools process same info simultaneously, requires divergence prior to this process serial processing info is relayed in a step-fashion from one pool to another, ex. relay race reverberation § Collateral branches of axons along circuit; extend back toward the source of impulse and keep stimulating presynaptic neurons § Positive feedback loop - circle (ex. Tinnitus) § Continues until synaptic fatigue or inhibitory stimuli end cycle reflexes rapid and automatic response, preserves homeostasis through rapid adjustments, little variability in response 5 components of a reflex arc • Stimulation of a receptor • Activation of a sensory neuron • Information processing in the CNS • Activation of a motor neuron • Response of a peripheral effector activation of a receptor by stimulus Receptor can be specialized cell or dendrites of a sensory neuron; detect physical or chemical changes activation of a sensory neuron - Stimulation of dendrites produces graded polarization - Generates action potentials in sensory neurons; signal enters SC through posterior root info processing in the CNS • Sensory neuron releases neurotransmitter, causes excitatory postsynaptic potentials (EPSPs) at interneuron • Signals integrated with others arriving at same time. activation of a motor neuron Interneuron stimulates motor neurons that carry action potentials to periphery simultaneously, collaterals from interneuron may send pain sensations to other areas in CNS response of a peripheral effector motor neurons release neurotransmitters at axon terminals; stimulates effector to respond 4 classes of reflex***** 1. Developmental - innate or acquired 2. Nature of response - somatic or visceral 3. Complexity of circuit - mono or polysynaptic 4. Processing site - spinal or cranial development of reflexes - innate basic neural reflexes formed before birth, predictable developmental sequence from simplest to most complex development of reflexes - acquired (conditioned) rapid automatic learned motor patterns, repetition enhances them nature of responses - somatic involuntary control of skeletal muscles, important to emergencies b/c immediate, ex. withdrawal reflex nature of responses - visceral (autonomic) control involuntary effectors: smooth muscle, cardiac muscle, glands, adipose tissue complexity of circuit - monosynaptic • Single synapse—simplest reflex arcs • Sensory neuron synapses directly with motor neuron (no interneuron) • Fast response complexity of circuit - polysynaptic • At least one interneuron between sensory neuron and motor neuron • Slower response; delay increases with the number of synapses involved (longer path = longer delay) processing site - spinal • Processing occurs in spinal cord • Intersegmental reflexes—involve multiple segments of spinal cord processing site - cranial • Processing occurs in the brain stretch reflex best known monosynaptic reflex, automatically regulates skeletal muscle length, stimulus = increased muscle length, send. neuron triggers immediate motor response (muscle contraction), ex. patellar reflex Patellar reflex steps • Stimulation of a receptor • Tapping patellar ligament stretches fibers in quadriceps femoris muscle • Activation of a sensory neuron • Distortion of stretch receptors stimulates sensory neurons • Sensory neurons enter spinal cord; synapse directly with motor neurons of the motor units in stretched muscle • Information processing in CNS • Occurs at cell body of the motor neuron—no interneuron • Sufficient stimulation activates the motor neuron • Activation of a motor neuron • Once activated, motor neuron sends action potential to the effector—quadriceps femoris • Response of peripheral effector • Skeletal muscle fibers of stretched muscle are stimulated to contract • Quadriceps femoris extends knee in a brief kick muscle spindles sensory receptors for stretch reflex, made of intrafusal fibers (small bundled specialized fibers), supplied by both sens. and motor neurons, surrounded by muscle fibers that maintain rest muscle tone and can contract whole muscle gamma motor neuron innervates each muscle spindle •Alters tension in intrafusal fibers; allows CNS to increase/decrease muscle tone stretched/compressed muscle spindles Stretched length stimulates more frequent action potentials from sensory neuron • Stimulates motor neurons • Increases muscle tone Compressed length inhibits sensory neuron • Reduces stimulation of motor neuron • Decreases muscle tone postural reflexes § Category of stretch reflexes - Help maintain a normal, upright position • Many muscle groups work in opposition to one another to maintain balance • Leaning forward stimulates stretch receptors in calf muscles • They respond by contracting to pull you back upright • Postural muscle adjustments are usually unconscious withdrawal reflexes § Move away from stimulus • Strongest triggered by painful stimuli • Sometimes initiated by stimulus from touch or pressure receptors § Versatile because sensory neurons activate many pools of interneurons • Distribution and strength of response depend on intensity and location of stimulus flexor reflex § Type of withdrawal reflex § Affects muscles of a limb § Example: Pain stimulus of grabbing a hot pan • Pain receptors stimulated • Sensory neurons activate interneurons in spinal cord Interneurons stimulate motor neurons in anterior gray horns resulting in: – Contraction of flexor muscles—withdraws hand from stimulus – Reciprocal inhibition***********—keeps extensors relaxed (blocks opposition) Crossed extensor reflexes § Stretch reflexes and withdrawal reflexes use ipsilateral reflex arcs (ipsi, same) • Sensory stimulus and motor responses are on the same side of the body § Crossed extensor reflexes involve contralateral reflex arcs (contra, opposite) • An additional motor response occurs on the side opposite the stimulus § Coordinated with flexor reflex § Flexion of the affected side accompanied by extension of the opposite side crossed extensor reflex example • Step on something sharp • Before the flexor reflex lifts the injured foot, the crossed extensor reflex straightens the opposite limb to receive body weight • Then the flexor reflex can lift foot • Stimulus elicits flexor reflex to pull affected foot away • Interneurons stimulate flexors/inhibit extensors • Crossed extensor reflex occurs simultaneously • Collaterals of excitatory and inhibitory interneurons cross spinal cord to motor neurons in the unaffected leg • Excitatory interneurons stimulate extensors • Inhibitory interneurons relax the flexor muscles • Opposite leg straightens (crossed extension) to support the shift in weight § These are maintained by reverberating circuits Polysynaptic reflexes § Responsible for automatic actions involved in complex movements (examples—walking and running) Five******** properties of polysynaptic neurons • Involve pools of interneurons—may be excitatory or inhibitory • Involve multiple spinal segments—may activate muscle groups in many areas • Involve reciprocal inhibition—coordinates movement and reduces resistance • Have reverberating circuits that prolong motor response • Several reflexes may cooperate to produce coordinated, controlled response Biceps reflex, triceps reflex, ankle-jerk reflex • Each controlled by specific segments of spinal cord • Reflex responses provide information about the status of the corresponding spinal segments • Not very important, more just nice to know**** Babinski reflex § Stroking foot on lateral side of sole triggers extension of hallux (big toe) and spreading of other toes (up to age 1) § Normal in infants—occurs due to lack of inhibition by descending motor fibers § Response disappears as descending pathways develop plantar reflex § Same stimulus causes toe curling (plantar flexion) once inhibitory descending pathways develop § Normal in adults abdominal reflex § Light stroking of skin produces reflexive twitch in abdominal muscles; moves navel toward the stimulus § Depends on the facilitation of descending tracts § Absence may indicate damage to descending tracts Read more