Ch.13 Spinal Cord/Nerves and Somatic Reflexes

cauda equina

"horse's tail" bundle fo nerve roots that occupy the vertebral column from L2 to S5 -innervates pelvic organs and lower limbs stretches out from conus medullaris bc vertebral column grows faster than spinal cord

spinal nerve number, layout, structure, attachment, innervation

31 pairs of mixed spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal) bc of vertebral column outgrowing spinal cord, cervical nerves C1-C7 are stretched higher than their corresponding level and nerves below C7 are stretched lower than their corresponding level each nerve attaches at 2 points, dorsal rootlets from dorsal root and ventral rootlets from ventral root -dorsal root ganglion -roots combine into mixed spinal nerve before breaking into *mixed* ventral and dorsal rami and meningeal branch -meningeal branch reenters cord to innervate meninges and cord -posterior rami innervates muscles/joints of posterior body -anterior rami innervates anterior body and gives rise to sympathetic chain

spinal cord functions

Conduction: bundles of nerve fibers conduct info up and down cord; sensory and motor Neural integration: pools of spinal nerves receive input from multiple sources, integrate the info, and execute appropriate output locomotion: groups of neurons coordinate the simple repetitive muscle contractions that allow movement Reflexes: nerves carry signals that start and execute somatic reflexes

general structure of a somatic reflex arc

Somatic receptors in skin, muscles, and tendons sense changes Afferent nerve fibers send this into from receptors to posterior horn of spinal cord or to brainstem Integrating center (of 1 or more interneurons) is point of synaptic contact b/w neurons in gray matter of spinal cord or in brainstem Efferent nerve fibers carry motor impulses back to the muscles effectors are muscles that carry out the response -more interneurons involved means information processing is more complex and delay is longer

Stretch reflex

When a muscle lengthens, the muscle spindle is stretched and its nerve activity increases causing the muscle fibers to contract and thus resist the stretching; maintains constant length within the muscle muscle fights back, contracts, increases tone, and feels stiffer than unstretched muscle, helping to maintain equilibrium and posture often feeds back to a set of synergists and antagonists, rather than a single muscle -flexion at a joint causes stretch flex in extensors; extension creates stretch reflex in flexors -valuable in stabilizing joints by balancing tension of extensors and flexors monosynaptic reflex arcs where fibers synapse directly with alpha motor neurons returning to muscle Ex. Tapping patellar ligament excites nerve endings of muscle spindle in quadriceps femoris (extensors), causing a stretch signal to travel to spinal cord through afferent fiber. Primary afferent neuron stimulates alpha motor neuron in spinal cord, and efferent signals in alpha motor neuron stimulate quadriceps (extensors) to contract, producing knee jerk. At the same time, a branch of the afferent fiber stimulates an inhibitory motor neuron to inhibit hamstring (flexors) from contracting and antagonizing quadriceps. -this reciprocal inhibition prevents muscles from working against each other Quadriceps femoris extensors being stimulated and hamstring flexors being inhibited to produce knee jerk

spinal cord segment

a region of the spinal cord that gives rise to a specific pair of spinal nerves segmentation is hallmark feature of chordates and vertebrates

spinal tracts

ascending tracts carry sensory fibers up the cord and descending tracts carry motor fibers down all nerve fibers in a given tract have similar origins, destinations, and functions -frequently the brainstem several ascending and descending tracts decussate -means left side of body may receive sensory info from right side and send motor signals to it -contralateral is origin and destination are on opposite sides of body (decussated) or ipsilateral if they are on the same side

descending tracts

carry motor signals down the brainstem and spinal cord involves 2 neurons: upper and lower motor neurons Upper neuron: begins with soma in the cerebral cortex or brainstem and has axon terminating on a lower motor neuron in brainstem or spinal cord Lower neuron: has axon that carries signal the rest of the way to the muscle or other target organ decussates in medulla, midbrain, spinal cord, or not at all lateral tract (darker) has 90% of descending tracts

ascending tracts

carry sensory information to the brain travel across 3 neurons from their origin in the receptors to their destination in the brain 1st order neuron: detects stimulus and transmits signal to spinal cord or brainstem 2nd order: continues from spinal cord or brainstem to the thalamus 3rd order: carries signal from thalamus to cerebral cortex -frequently in the somatosensory cortex -most destinations are gray matter integration centers of brain receptors for body movement, limb position, fine touch, discrimination, and pressure frequently decussate and do so in the medulla/spinal cord and then continue in medial lemniscus***

ganglia

cluster of neurosomas outside the CNS (called nuclei if in the CNS) know dorsal root ganglions that contain neurosomas of sensory neurons and how to identify anterior/posterior view of spinal cord based on this ganglia wrapped in epineurium that is continuous with that of the nerve mostly unipolar nerves, bc these are sensory nerves receiving info from skin and transmitting directly to spinal cord notice its a sensory pathway and not motor -relate to dorsal horn gray matter and ventral horn gray matter in spinal cord (lateral horn is ?)

spinal cord injuries

common from C1-C5 but not from L3-L5 C3-C5 is area of lot of stretching and strain which can harm spinal cord L3-L5 is area of vertebral column that is below conus medullaris of the spinal cord -spinal cord ends before this region usually -lumbar punctures to obtain CSF usually down in this region to avoid spinal cord injury

spina bifida

congenital defect in which one or more vertebrae fail to form a complete vertebral arch for enclosure of the spinal cord during embryonic development. Especially common in lumbosacral region. in most serious form, a sac protrudes from the spine and may contain meninges, CSF, and parts of spinal cord/nerve roots -in extreme cases, inferior spinal cord function is absent, causing a lack of bowel control and paralysis of the lower limbs and urinary bladder, which can lead to chronic urinary infections and renal failure usually requires spinal closure within 72 hours of birth. prognosis ranges from near normal life to a lifetime of treatment for multi system complications or to infant death in extreme cases risk can be reduced by taking folic acid supplements before pregnancy. folic acid added to many foods for this reason. -thus, UV radiation a concern

terminus of spinal cord

conus medullaris is the inferior terminus of he spinal cord Cauda Equina is the nerves that emerge from the end of the spinal cord (conus medullaris) and innervate the pelvic region and lower limbs vertebral column longer than spinal cord because the spinal cord stops growing at one year old and the vertebral column segments continue to each get longer -adaptive bc lessens chance of injury to spinal cord and allows safe lumbar punctures (below L3) -final spinal cord position is around T12 to L3 -as nerve exits are stretched lower and lower, they form a bundle stretching down from the cord that is the Cauda Equina notice in pic how spinal nerve initially exits at level with vertebral segment but as the vertebral column grows, the nerve exit is stretched lower and lower

spinal nerve structure

dorsal ramus (sensory and motor) fuses with ventral ramus (sensory and motor) to make mixed spinal nerve going towards spinal cord as it gets close to the cord, the spinal nerve branches into dorsal (sensory) and ventral (motor) roots, and then each branches into 6-8 rootlets connecting to its respective part of the spinal cord dorsal horn (gray matter) is primarily interneurons with some sensory neuron axons ventral horn (gray matter) is primarily motor neuron axons with some interneurons don't get dorsal/ventral rami and roots confused ganglion only on dorsal root; contains somas of sensory neurons

spinal cord structure

emerges from brainstem (medulla oblongata) at foramen magnum 2/3 length of vertebral column. explain why. conus medullaris and Cauda Equina 31 pairs of spinal nerves cervical, thoracic, lumbar, and sacral regions -cord extends only down to about T12-L3 -lumbar puncture below L3 cord thicker at cervical enlargement (contains nerves of upper limbs) and lumbar enlargement (contains nerves of pelvic region and lower limbs) and Cauda Equina emerges from here

denticulate ligaments

extensions of the pia mater that extend laterally through the arachnoid and dura and anchor the cord laterally while limiting side-to-side movement

filum terminale

fibrous extension of the pia mater beyond the conus medullaris -contained within lumbar cistern (subarachnoid space below the medullary cone) fuses with dura mater at S2 and forms coccygeal ligament that anchors the spinal cord to the coccyx

structures that stabilize the spinal cord

filum terminale, coccygeal ligament, and denticulate ligaments filum terminale: elaboration of the Pia mater; fibrous strand of Pia mater that extends beyond conus medullaris and is within lumbar cistern coccygeal ligament: anchors cord and meninges to Co1 -formed by filum terminale exiting lower end of cistern at S2 and fusing with dura mater to form coccygeal ligament denticulate ligaments: extensions of Pia mater that extend laterally through the arachnoid and dura and anchor the cord while limiting side to side movements -found at regular intervals along the spinal cord coccygeal ligament and denticulate ligaments provide both straight up and down and lateral support/stability, respectively

flexor (withdrawal) reflex and crossed extension reflex

flexor reflex is flexing of hamstring flexors of injured leg away from stimulus, and crossed extension reflex is extension of quadriceps extensors of opposite leg to maintain balance contraction of hamstring flexor muscles in one limb in response to injurious stimulus; also involves contraction of extensors in opposite limb -reciprocal inhibition means that 1st limb also has quadriceps extensors relaxed while 2nd limb has hamstring flexors relaxed ipsilateral neurons function in contracting leg that is injured while contralateral neurons function extending other leg to maintain balance involves a poly-synaptic reflex arc, with signals traveling over many synapses on their way back to the muscle -more routes=longer delay in this case, the stimulus is pain, not proprioception (which would be the case in the stretch reflex) adaptive significance is to obviously avoid injury hamstrings are extensors (relaxed in first leg and contracted in 2nd leg) quadriceps are flexors (contracted in 1st leg and relaxed in 2nd) ipsilateral reflex arc and contralateral reflex arc working together to produce coordinated and protective response to pain

decussation

for ascending tracts, occurs between first order and second order neurons, which may be in spinal cord or in medulla for descending tracts, occurs between upper and lower motor neurons, which is in brainstem or spinal cord some tracts may not decussate at all (proprioception)

coccygeal ligament

formed from fusion of terminal filum and dura mater; anchors the cord and meninges to vertebra Co1 filum terminale exits lumbar cistern at S2 and fuses with dura to make this ligament

gray and white matter in CNS

gray represents regions containing neruon cell bodies, dendrites and unmyelinated axons white gets its color from the myelinated axons in spinal cord, gray matter is in center (butterfly) and white matter is surrounding it -opposite layout in brain gray matter is *site of synaptic contact* b/w neurons and site of all neural integration with spinal cord white matter contains ascending and descending tracts

how to tell if view of spinal cord is anterior or posterior

look for dorsal root ganglion and see if it is the front or back spinal nerve ganglions only on dorsal root; never ventral pic shows anterior view because dorsal root is in the back

contralateral and ipsilateral nerves

nerves that decussate and have their origin and destination on opposite sides of the body are called contralateral -receptors for body movement, limb position, fine touch, discrimination, and pressure frequently decussate and do so in the spinal cord or medulla and then continue in medial lemniscus -gracilis and cuneate fasciculi decussate in medulla and rest of ascending tracts decussate in spinal cord, if at all -descending tracts may decussate in midbrain, medulla, spinal cord, or not at all nerves with origin and destination on same side of body are ipsilateral -receptors for pain, heat, and cold are usually ipsilateral

spinal nerve connective tissue

neurilemma of each fiber surrounded by basal lamina and endoneurium multiple fibers organized into fascicles surrounded by perineurium fascicles organized into entire nerve surrounded by epineurium -dense, irregular connective tissue protecting nerve from stretching and injury blood vessels penetrate epineurium and perineurium

nerve plexuses

neurons of nerve plexuses have somatosensory and motor functions, but formed only from ventral rami cervical, brachial (shoulder), lumbar, sacral, and coccygeal plexuses somatosensory functions: carry signals from bones, joints, muscles, and skin, but not viscera -includes touch, heat, cold, stretch, pressure, pain, etc -proprioception motor functions: primarily to stimulate contraction of skeletal muscles -also innervate bones of corresponding regions -carry autonomic (ANS) innervation fibers to some viscera and blood vessels, adjusting blood flow to local needs

dorsal and ventral rami

rami are branches off of spinal nerve going away from the cord bc spinal nerves are mixed, these rami are also mixed, carrying both sensory and motor nerve fibers dorsal and ventral just named for part of body they supply don't get confused with dorsal and ventral roots and rootlets, which are not mixed

basic properties and general components of somatic reflexes

require stimulation, quick, involuntary, and stereotyped include somatic receptors, afferent nerve fibers, integrating center, efferent nerve fibers, and effectors quick bc they only involve a few interneurons or non, and have minimum synaptic delay adaptive function is that it is rapid and doesn't require a lot of conscious control includes proprioceptors, which monitor body position, movement of body parts, and tension -muscle spindles: issue sensor input to CNS and receive motor output, functioning in somatic reflexes

weird brain picture

ridges are called gyri and valleys are called sulci somatosensory cortex (where many ascending tracts terminate) is located in postcentral gyrus in brain (parietal lobe) -especially sensory info for touch -gray matter? postcentral gyrus is named as such because it is behind the central sulcus, which divides the frontal lobe from parietal lobe -precentral gyrus is in frontal lobe and postcentral gyrus is in parietal lobe

lumbar puncture

safe procedure that allows one to obtain a sample of CSF b/w L3 and L5 so that the spinal cord will not be damaged can be used to assay for infections or pathogens in immune system, which show up in CSF

epidural space

space b/w dural sheath and vertebral bones. occupied by blood vessels, adipose tissue, and loose connective tissue. Also sinuses. No CSF (subarachnoid space) -outside of dura mater sheath. by bone. not subdural space. epidural anesthetics applied here during surgery or childbirth because they can directly reach nerves and vascularization of spinal cord pay attention to large concentration of blood vessels, adipose, and loose connective tissue in pic

subarachnoid space

space below the outer arachnoid membrane but above the pia mater consists of *collagenous and elastic fibers* creating a space that *allows for circulation of CSF* and blood vessels called the lumbar cistern below the conus medullaris and is occupied by Cauda Equina, CSF, and filum terminale (pia)

Meninges of the spinal cord

spinal cord and brain are enclosed in 3 fibrous membranes called meninges -dura mater, arachnoid mater, and pia mater dura: tough membrane composed of multiple layers of dense irregular connective tissue -epidural space: space b/w dural sheath and vertebral bones. occupied by blood vessels, adipose tissue, and loose connective tissue. Also sinuses. Anesthetics applied here -major function is to protect cord and allow epidural space for blood vessels, tissue, and anesthetic injections. -subdural space may be between dura and arachnoid arachnoid: consists of outer arachnoid membrane and inner loose array of collagenous and elastic fibers that make up subarachnoid space b/w arachnoid and Pia maters -space is filled with *CSF* and blood vessels -called the lumbar cistern below the conus medullaris and is occupied by Cauda Equina, CSF, and filum terminale (pia) pia: delicate, transparent membrane of 1-2 layers of squamous to cuboidal cells and delicate collagenous and elastic fibers; very tight and largely impermeable membrane that adheres tightly to every contour of spinal cord -closely follows contours of spinal cord (every dip) -called filum terminale below conus medullaris -fuses with dura at S2 and makes coccygeal ligament that anchors spinal cord to coccyx -also has denticulate ligaments, which are lateral extensions of pia that go through other meninges and anchor cord and prevent side-to-side movements meninges are not part of CNS but just surround, protect, and stabilize it

spinal cord development

spinal cord extends down to T12-L3 (conus medullaris) but vertebral column extends much further -part of spinal cord below conus medullaris is Cauda Equina -happens because vertebral column and dura mater lengthen more rapidly than neural tube and terminal end of spinal cord gradually shifts to higher level spinal nerves run obliquely from segment of cord origin due to disproportionate growth -spinal nerves grow so they can pass through same vertebral column foramina as the vertebral column lengthens more and more dura mater continues far past end of spinal cord and creates well at bottom

dorsal and ventral roots

spinal nerve branches out into dorsal and ventral roots, then into dorsal or ventral rootlets (6-8) that connect to spinal cord ganglion only on dorsal root; use as key to ID anterior or posterior view dorsal is sensory and ventral is motor

lumbar cistern

subarachnoid space inferior to medullary cone that contains cauda equina and CSF

cervical and lumbar enlargements

two areas where the cord is thicker cervical is where nerves of upper limbs emerge from lumbar is where nerves of pelvic region and lower limbs emerges from -Cauda Equina emerges from here notice large number of nerves exiting from enlargements

Muscle spindles

type of proprioceptor involved in many somatic reflexes -embedded in muscles -sense organs specialized to monitor position and movement of body parts stretch receptors embedded in muscle tissue, converting info about *how much muscle is being stretched and at what rate* -allows for protective reflexes/actions and contribute to ability to have smooth, coordinated movement -especially abundant in muscles requiring fine control (hands and feet) and at the ends of muscles or near tendons usually a bundle of about 7-8 small, modified muscle fibers enclosed in an elongated fibrous capsule of connective tissue muscle fibers within spindle are called intrafusal fibers; extrafusal fibers are normal fibers outside spindle each end of intrafusal fiber has a few sarcomeres. *Gamma motor neurons* innervate each end to contract. Also maintains tension and sensitivity of intrafusal fiber -alpha motor neurons supply extrafusal fiber *Primary Afferent fibers monition muscle length and how rapidly it changes, while Secondary Afferent fibers monitor length only -only ends of intrafusal fibers with sarcomeres can contract. Middle has no sarcomeres but has these 2 types of fibers informing brain -enter dorsal horn of spinal cord unconscious monitoring have gamma motor neurons innervating them and also 2 types of sensory neurons (primary afferent and secondary afferent fibers)