Chapter 13 The Spinal Cord, Spinal Nerves, and Spinal Reflexes

Organization: President division heads department chair faculty VP (dean)

sensesmuscles processing centers more processing centers brain

fig. 13-1

Chapter 12neurons Chapter 13spinal cord reflexes Chapter 14brain and cranial nerves Chapter 15brain-spinal cord interaction Chapter 16autonomic NS

Spinal cord anatomy ~ 18 inches (not as long as vertebral column) ends about L 1 or L 2 (conus medullaris) deep groove on anterior (ventral) side roots attached to it (dorsal and ventral)

cervical thoracic lumbar fig. 13-2

fig. 13-3

fig. 13-5

white matter

fig descending tracts (motor) ascending tracts sensory

fig epineurium perineurium endoneurium

epaxial hypaxial e h Charlie axis

fig. 7-22c

fig. 11-3

fig 13-7

distribution of spinal nerves dorsal and ventral dermatomes

fig 13-8

Clinically significant: damage to spinal nerve or DRG may cause sensory loss to just a restricted part of the skin… …by mapping the deficit you may be able to pinpoint where the nerve has been damaged

Clinically significant: virus attacks DRG painful rash area of dermatome of that spinal nerve example:shingles

Distribution of spinal nerves T 1 to T 12 is “typical” spinal cord

fig. 13-9

fig C 3, 4and 5 keep the diaphragm alive cervical plexus

brachial plexus

lumbar plexussacral plexus S 2, 3and 4 keep your anus off the floor

The human body contains: 10,000,000sensory neurons 500,000motor neurons 20,000,000,000 interneurons information to CNS control effectors (muscles) interpret, plan, coordinate incoming and outgoing info

all these neurons are organized into neuronal pools - functional groups of interconnected neurons 100’s to 1000’s ??

neuronal polls each has limited input/output can be excitatory or inhibitory contain different circuit patterns

Divergence fig a one neuron (neuronal pool) multiple neurons (neuronal pools) (vision)

Convergence fig b many neurons single neuron (muscle control, breathing)

Serial Processing fig c stepwise spread of info (pain)

Parallel Processing fig d several neurons (pools) access same info divergence serial (ouch)

Reverberation fig e positive feedback loop

reflex rapid, automatic response to specific stimulus conditions in or around the body can change suddenly and unexpectedly… make adjustments to maintain homeostasis

remember chapter 1…

Homeostatic regulation: autoregulation extrinsic regulation adjustment within organ… nervous/endocrine system

Homeostatic regulation: Three part mechanism: receptor (stimulus) control center effector

wiring of a reflex is called… reflex arc

step 1 arrival of stimulus activation of receptor

reflex arc step 2 activation of sensory neuron

reflex arc step 3 information processing

reflex arc step 4 activation of motorneuron

reflex arc step 5 response of effector

fig

Classification of reflexes developmental origin nature of motor response complexity site of information processing

Classification of reflexes developmental origin innate acquired nursing driving

Classification of reflexes nature of motor response somatic visceral skeletal muscle later (#16)

Classification of reflexes complexity monosynaptic polysynaptic quicker slower

Classification of reflexes site of processing site spinal cord brain spinal reflexes cranial reflexes (later)

Monosynaptic reflexes very little delay rapid responses example:stretch reflex automatic regulation of length of skeletal muscle

fig

stimulus increases muscle length activates sensory neuron receptor stimulates effector muscle contracts to counter the stimulus (within msec) stretch reflex

receptor is called muscle spindle intrafusal fibers surrounded by extrafusal fibers stretch reflex

fig 13-16

intrafusal fibers myofibrils at ends sensory area in center gamma efferent gamma efferent sensory branch stretch reflex has a “normal” resting length

sensory branch stretching membrane distorts dendrites ^(AP) intrafusal fibers stretch reflex

if intrafusal fiber is compressed the frequency of AP’s is decreased sensory axon from intrafusal fiber is always active (AP) if intrafusal fiber is stretched the frequency of AP is increased (membrane in central area of) ^ stretch reflex

sensory axon synapses in spinal cord: motorneurons that innervate the extrafusal fibers of that muscle collaterals that send info to brain sensory axon from intrafusal fiber is always active (AP) stretch reflex

stretch muscle increase spindle sensory neuron AP increase activity of motorneuon to extrafusal fiber increase muscle tone (contraction) increase resistance to being stretched stretch reflex

Many stretch reflexes are postural postural muscles have firm muscle tone very sensitive stretch receptors fine adjustments are always being made

stretch reflex gamma efferent gamma efferent myofibrils at ends What is the role of the gamma efferents?

stretch reflex adjust the tension on the sensory membrane area What is the role of the gamma efferents?

stretch reflex What is the role of the gamma efferents? if muscle is contracted… …gamma efferents stimulate contraction of myofibrils of spindle…shorten spindle

stretch reflex What is the role of the gamma efferents? if muscle is lengthened… …gamma efferents stop stimulation of myofibrils of spindle…relax spindle

start sr spindle contract relax

stretch reflex regulates the length of muscle monosynaptic

polysynaptic reflexes more complicated responses can involve multiple muscle groups can be inhibitory or excitatory (ipsp’s)(epsp’s)

tendon reflex ?? receptors (different than spindles, etc) sense if collagen (tendon) is being stretched too much if so,… will stimulate inhibitory interneurons in spinal cord that will inhibit motorneuron activity

+ - tendon reflex too much stretch reduce tension - inhibitory interneuron sensory neuron motorneuron

withdrawl reflex move affected parts of the body away from a stimulus pain, touch, pressure eg., flexor reflex

fig activateinterneuron sensory neuron AP stimulate flexors

sensory neuron interneuron stimulates motorneuron contraction of flexor stretch of extensor ? stimulates

fig activateinterneuron sensory neuron AP stimulate flexors inhibit extensors “reciprocal inhibition”

What kind of processing did you see in the withdrawl reflex ?

fig divergence parallel

stretch tendon withdrawl reflexes all ipsilateral (same side)

crossed extensor reflex contralateral reflex arc (opposite side) this reflex compliments the flexor reflex

ipsilateral flexor reflex (crossed) contralateral extensor reflex reverberation (positive feedback)

characteristics of polysynaptic reflexes use pools of interneurons are intersegmental involve reciprocal inhibition have reverberating circuits several reflexes cooperate

Integration of reflexes all these reflexes occur without input from the brain (higher centers)… …but the higher centers do have an influence

Integration of reflexes facilitation epsp ipsp move a cell closer to threshold move cell further from threshold

Integration of reflexes higher centers stimulate excitatory or inhibitory interneurons adjust sensitivity of reflexes

Integration of reflexes reinforcement when excitatory synapses are chronically active inhibition when reflexes are inhibited example:

Integration of reflexes plantar reflex stroke sole of foot toes curl down (normal for adults) (negative Babinski)

Integration of reflexes in infants stroke sole of foot fanning of toes Babinski

Integration of reflexes Babinski disappears normally (is inhibited) with development of descending motor pathways If Babinski shows up in adult, it means that descending pathways are damaged