CH 13 spinal cord and more the cord spinal nerves somatic reflexes

spinal cord four primary functions conduction = carries information up and down the cord integration = spinal neurons receive info from many sources locomotion = simple repetitive movements are coordinated by groups of neurons (central pattern generators) in the spinal cord and pass this information cranially reflexes = spinal reflexes for posture, muscle tone, protection

31 pairs of spinal nerves cervical 8, thoracic 12, lumbar 5, sacral 5, coccygeal 1 cervical and lumbar have thicker areas

brain and spinal cord are covered by three fibrous layers – the meninges dura mater - lies just under the skull or surrounds cord, thick rubbery collagenous membrane arachnoid – simple squamous epithelial membrane lies just under the dura & has a loosely organized space between the arachnoid membrane and pia mater filled with CSF pia mater - delicate transparent membrane which follows closely the cord and brain surfaces, in lumbar region becomes terminal filum & at sacral area fuses with dura to become coccygeal ligament which anchors cord and meninges to coccyx 1

spinal cord terms white matter – bundles of axons called tracts, lots of myelin gray matter – little myelin, lots of somas, dendrites, synapses tracts – bundles carry specific info to and from specific areas horns – anterior, posterior, lateral, give gray matter a H shape central canal – filled with cerebrospinal fluid (CSF) posterior (dorsal) root – sensory nerves anterior (ventral) root – motor fibers lateral horn – sympathetic NS sends axons to anterior root afferent – towards the CNS, sensory efferent - towards the periphery, motor decussation – cross over from one side to opposite body side contralateral – origin & destination are on opposite body sides ipsilateral – origin and destination are on same side of body

denticulate ligament, epidural space - BV, fat, CT Ganglion – lots of cell bodies

words to know commissure – bundle that crosses over tecto – roof, cover reticulo – brainstem area involved with many involuntary functions ganglia – groups of nerve cell bodies usually outside of CNS fissure – anterior groove, not as deep & wider than post. sulcus sulcus – posterior groove, not as wide & deeper than ant. fissure

spinal tracts ascending – afferent, sensory, carry info to CNS descending – efferent, motor, carry info to effectors specific tracts carry information from a similar origin a similar destination and a similar function

ascending tracts (sensory) typically 3 neurons 1st order detects stimulus & sends signal to spinal cord or brainstem 2nd order carries information to thalamus 3rd order sends info to cerebral cortex

descending tracts - corticospinal - tectospinal - lateral & medial reticulospinal - lateral & medial vestibulospinal

spinal nerves - this is how the brain via the spinal cord communicates with the body - a nerve can have a few to thousands of individual nerve fibers - most nerves are a mix of afferent (sensory) and efferent (motor) fibers - ganglions contain many neurosomas (cell bodies) outside of CNS - peripheral nervous system (PNS) have myelinated fibers

spinal nerves - there are 31 pairs of spinal nerves - cervical = 8, thoracic = 12, lumbar = 5, sacral = 5, coccyx = 1

classification of nerves afferent = carry sensory info from receptors to CNS efferent = carry motor info from CNS to effectors somatic = innervate skin, bones, joints, skeletalmuscles visceral = innervates blood vessels, glands, and viscera general = innervates blood vessels, glands, viscera, skin, & muscles special = innervate more localized organs in the head, eyes, ears, smell, taste, and muscles for chewing, smiling, expression,

anatomy of a nerve nerve – bunch of individual nerve fibers held together by CT endoneurium – CT covering of an individual nerve fiber unmyelinated nerve – one coat of myelin (neurilemma) myelinated - many wrappings of myelin, last one = neurilemma fascicle – a bundle of nerve fibers surrounded by CT perineurium - CT wrapping about a bundle of nerve fibers spinal nerve - 31 pairs leaving the spinal cord at different levels anterior root – carry motor fibers to effectors posterior root – carry sensory fibers to cord and CNS ganglion – in posterior root filled with somas of sensory nerves rootlets – 6 to 8 nerves which together form the ant & post roots

branches of spinal nerves spinal nerve – 31 afferent and efferent fibers of the spinal cord meningeal nerve – reenters vertebral canal and innervates the meninges, vertebrae, and spinal ligaments with sensory and motor fibers ramus (rami) – branch, spinal nerves split into ant & post rami communicating rami – fibers leave ant ramus to join sympathetic chain of ganglia sympathetic ganglia – thoracolumbar division from T1 to L2 but communicating fibers travel up and down so that all levels of the cord have sympathetic fibers

nerve plexus – multiple anterior rami spinal nerves anastomosing, communicating with each other, and generating multiple peripheral functions - they provide somatosensory function from bones, joints, muscles, skin, for touch, heat, cold, stretch, pressure, pain, proprioception - their motor functions is primarily for skeletal muscles, bones, and ANS fibers to the viscera and for control of BV

dermatomes - each spinal nerve except C1 receives sensory input from a specific area of the skin - dermatome maps show which area of the skin a spinal nerve receives sensory information - the maps are not perfect because there is always some overlap sometimes as much as 50% with the dermatome above and below

reflexes - reflexes are quick, involuntarily, stereotypical reactions of glands and muscles to a stimulation - reflexes need stimulation therefore not spontaneous - are quick, there are few or no interneurons - involuntary so difficult to supress - stereotypical so the same every time unless pathological

somatic reflex - reflexes of skeletal muscles visceral reflexes – reflexes of heart and intestines the simplest reflex involves a receptor, a sensory neuron, and an effector (monosynaptic reflex arc) usually there are one or more interneurons and information is used for the reflex as well as integration with the spinal cord and brain

muscle spindle reflex (myotatic reflex) - nerve ending or muscle spindle is excited - primary afferent nerve sends info to posterior root & cord - efferent fiber in cord activated - which stimulates muscle to act - branch of afferent fiber stimulates inhibitory motor neuron - which inhibits muscle antagonistic to initial reflex, known as reciprocal inhibition

muscle spindle (somatic reflex) - stretch receptors embedded in normal (extrafusal) muscles - spindle info used to monitor body part position & movement - a bundle of 7 or 8 modified muscle fibers (intrafusal fibers) - spindles have a few sarcomeres and have a fibrous capsule - are concentrated at ends of muscles near tendons - gamma motor neuron innervates each end of the spindle - 30% of all spinal motor neurons are gamma motor neurons - primary afferents monitor muscle length & speed of change - secondary afferents monitor length of spindle only

- through spindle fibers the brain is consciously and unconsciously made aware of the length and tension on skeletal muscles as well as the speed with which these changes take place - this information is used to control posture, motor control, and corrective reflexes

flexor & crossed extension reflex - when a reflex causes you to pull away from a source of injury your body suddenly becomes unstable - so as one flexor reflex is used to pull a body part away from injury a second extension reflex acts on the opposite side of the body to stabilize and support the body

tendon reflex - proprioceptors at junction of tendons and muscles - when muscle contracts tendon organs let CNS know - if too strong/rapid CNS inhibits alpha motor neurons to muscle does not contract as strongly - if one part of a muscle contracts much more than another part it inhibits those fibers so it is in line with the contractions of the rest of the muscle