Chapter 14 Lecture Outline See separate PowerPoint slides for all figures and tables preinserted into PowerPoint without notes. Copyright McGraw-Hill Education. Permission required for reproduction or display. 1
14.1 Spinal Cord Gross Anatomy Spinal cord Extends inferiorly from brain s medulla through vertebral canal Ends at L 1 vertebra with conus medullaris and below extend inferiorly as cauda equina Two widened regions with greater number of neurons o Cervical enlargement: contains neurons innervating upper limbs o Lumbar enlargement: contains neurons innervating lower limbs 2
14.1 Spinal Cord Gross Anatomy Spinal cord subdivided into five parts from top to bottom Cervical part (superiormost part) o 8 pairs of cervical spinal nerves Thoracic part o 12 pairs of thoracic spinal nerves Lumbar part o 5 pairs of lumbar spinal nerves Sacral part o 5 pairs of sacral spinal nerves Coccygeal part (inferior tip of spinal cord) o 1 pair of coccygeal spinal nerves 3
Gross Anatomy of the Spinal Cord and Spinal Nerves Figure 14.1a, c 4
14.1 Spinal Cord Gross Anatomy Spinal cord parts do not align with vertebrae names Vertebrae growth continues after spinal cord growth complete Rootlets from parts L2 and below extend inferiorly as cauda equina o Filum terminale: thin strand of pia attaching conus medullaris to coccyx Spinal nerves are named for attached spinal cord part Superiormost spinal nerve is C1 nerve Inferiormost spinal nerve is Co1 nerve 5
14.2 Protection and Support of the Spinal Cord Spinal cord meninges Pia mater: delicate layer adhering to spinal cord o Made of elastic and collagen fibers o Denticulate ligaments: lateral extensions of pia; help suspend spinal cord o Filum terminale: pia anchoring inferior end of spinal cord to coccyx Arachnoid mater: web-like layer, external to pia o Arachnoid trabeculae: fibrous extensions of the membrane o Subarachnoid space: area deep to arachnoid through which CSF flows Dura mater: tough, outermost layer o One layer of dense irregular connective tissue that stabilizes spinal cord o Subdural space is between dura and arachnoid o Epidural space is between dura and vertebra Houses adipose, areolar connective tissue, blood vessels 6
Spinal Meninges and Structure of the Spinal Cord Figure 14.3a 7
Spinal Meninges and Structure of the Spinal Cord Figure 14.3b 8
Clinical View: Lumbar Puncture Procedure for obtaining CSF for medical diagnosis Needle passes through Skin, back muscles, ligamentum flavum Epidural space, dura mater Arachnoid mater into subarachnoid space Adult spinal cord ends at L 1 Lumbar puncture below this, just above or below L 4 Spinous process of L 4 at highest points of iliac crests 9
Gray matter 14.3a Distribution of Gray Matter Made of neuron s cell bodies, dendrites, and unmyelinated axons; also glial cells Masses of grey matter project from center of spinal cord Anterior horns house cell bodies of somatic motor neurons Lateral horns house cell bodies of autonomic motor neurons o Only present in parts T1 L2 Posterior horns house axons of sensory neurons and cell bodies of interneurons 10
14.3a Distribution of Gray Matter Gray commissure Horizontal band of gray matter surrounding central canal Contains unmyelinated axons connecting left and right gray matter Nuclei: groups of cell bodies Sensory nuclei in posterior horn contain interneurons o Somatic sensory nuclei receive signals from skin, muscle, joints o Visceral sensory nuclei receive signals from blood vessels, viscera Motor nuclei in anterior and lateral horns contain motor neurons o Somatic motor nuclei (anterior) innervate skeletal muscle o Autonomic motor nuclei (lateral) innervate smooth muscle, heart, glands 11
Neuron Pathways and Nuclei Locations Figure 14.5 12
14.3b Distribution of White Matter White matter: myelinated axons to and from the brain Regions of white matter Posterior funiculus o Sits between posterior gray horns and posterior median sulcus o Contains sensory tracts (axon bundles called fasciculi) Lateral funiculus o Sits on lateral sides of spinal cord o Contains ascending (sensory) and descending (motor) tracts Anterior funiculus o Sits between anterior gray horns and anterior median fissure o Left and right anterior funiculi are interconnected by white commissure o Contains ascending (sensory) and descending (motor) tracts 13
Clinical View: Treating Spinal Cord Injuries May leave individuals paralyzed and unable to perceive sensations Prompt use of steroids after injury May preserve muscle function Early antibiotics Have reduced number of deaths due to pulmonary and urinary infections Neural stem cells May be used in future to regenerate CNS axons 14
14.4a Overview of Conduction Pathways Spinal pathways are sensory or motor Sensory pathways ascend toward brain Motor pathways descend from brain Common pathway characteristics Cell locations: axons are in spinal cord tracts; cell bodies are in ganglia, spinal cord gray horns, and brain gray matter Each pathway is made of a chain of two or more neurons Pathways are paired: there is a left and a right tract Most pathways decussate: axons cross midline so brain processes information for contralateral side o Uncrossed pathways work on the ipsilateral side of body 15
14.4b Sensory Pathways Sensory (ascending) pathways Signals for proprioception, touch, temperature, pressure, pain Somatosensory pathways carry signals from skin, muscles, joints Viscerosensory pathways carry signals from viscera Use a series of neurons to relay signal to brain o Primary (1 st order) neuron has peripheral ending, cell body in posterior root ganglion, and axon leading to secondary neuron o Secondary (2 nd order) neuron is an interneuron; receives primary input and extends to tertiary neuron or to cerebellum o Tertiary (3 rd order) neuron is an interneuron; receives secondary neuron input and extends to somatosensory cortex of parietal lobe of cerebrum 16
14.4b Sensory Pathways Posterior funiculus medial lemniscal pathway Signals about proprioception, touch, pressure, and vibration with a three neuron chain Primary neuron relays signal from skin to brainstem o Peripheral receptor has axon in spinal nerve, posterior root, spinal cord o Within the cord, axon is in the posterior funiculus o In the medulla the axon contacts a secondary neuron Secondary neuron relays signal from medulla to thalamus o Cell body in either nucleus cuneatus or nucleus gracilis of medulla o Axon decussates and joins medial lemniscus o In thalamus, the axon contacts a tertiary neuron Tertiary neuron relays signal to primary somatosensory cortex (postcentral gyrus) 17
Posterior Funiculus Medial Lemniscal Pathway Figure 14.7 18
14.4b Sensory Pathways Anterolateral spinothalamic pathway Signals related to crude touch, pressure, pain, and temperature with a three-neuron chain Primary neuron relays signal from skin to spinal cord o Axon is in spinal nerve and posterior root o Axon contacts secondary neuron in spinal cord s posterior horn Secondary neuron relays signal from spinal cord to thalamus o Axon decussates and ascends in contralateral white matter (either the anterior spinothalamic tract or the lateral spinothalamic tract) o Axon contacts tertiary neuron in thalamus Tertiary neuron relays signal from thalamus to cerebral cortex o Axon contacts target neuron in appropriate part of primary somatosensory cortex 19
Anterolateral Pathway Figure 14.8 20
14.4b Sensory Pathways Spinocerebellar pathway Signals about proprioception with a two-neuron chain Primary neuron relays signal from skin to spinal cord o Axon is in spinal nerve and posterior root o Axon contacts secondary neuron in spinal cord s posterior horn Secondary neuron relays signal from spinal cord to cerebellum o Some secondary neuron axons cross, while others remain ipsilateral o Axon ascends in either the anterior spinocerebellar tract or posterior spinocerebellar tract o Axon contacts cell within the cerebellum 21
Spinocerebellar Pathway Figure 14.9 22
Sensory Pathways in the Spinal Cord Figure 14.6 23
14.4c Motor Pathways Motor (descending) pathways Control effectors such as skeletal muscles Start in brain and include at least two neurons o Upper motor neuron in motor cortex, cerebral nucleus or brainstem nucleus; contacts lower motor neuron o Lower motor neuron in cranial nerve nucleus or spinal cord anterior horn; excites muscle Figure 14.10 24
14.4c Motor Pathways Direct (pyramidal) pathway Begins with upper motor neurons in primary motor cortex Axons end in brainstem (corticobulbar tracts) or spinal cord (corticospinal tracts) Corticobulbar tracts o Axons extending to brainstem where they synapse with lower motor neurons in cranial nerve nuclei Corticospinal tracts o Synapse on lower motor neurons in anterior horn o Lateral corticospinal tracts Lower motor neurons innervate limb muscles for skilled movements o Anterior corticospinal tracts Lower motor neurons innervate axial skeletal muscle 25
Corticospinal Tracts Figure 14.11 26
Indirect pathway 14.4c Motor Pathways Upper motor neurons originate in brainstem nuclei Rubrospinal tracts originating in midbrain (red nucleus) o Regulates precise movement and tone in flexor limb muscles Reticulospinal tracts from reticular formation o Help control reflexes related to posture and balance Tectospinal tracts from superior and inferior colliculi o Regulate reflexive orienting responses to visual and auditory stimuli Vestibulospinal tracts from vestibular nuclei of brainstem o Help maintain balance during sitting, standing, walking 27
Differences Between Sensory and Motor Pathways Figure 14.12 28
14.5 Overview of Spinal Nerves Spinal nerve characteristics 31 pairs of spinal nerves (C1 to Co1) Each nerve formed from merger of anterior (ventral) root and posterior (dorsal) root o Anterior root is many axons of motor neurons whose somas are in anterior and lateral horns o Posterior root is many axons of sensory neurons whose somas are in posterior root ganglion 29
14.5a Overview of Spinal Nerves Each nerve is named for part of spinal cord it comes from and a number Cervical nerves exit intervertebral foramina superior to the vertebra of the same number (e.g., C2 nerve exits between C 2 and C 1 vertebrae) Below C8, nerves exit inferior to the vertebra of the same number (e.g., T 2 nerve exits between T 2 and T 3 vertebrae) Lumbar, sacral, and coccygeal spinal nerves have long roots that extend inferiorly before exiting vertebrae o These roots form the cauda equina 30
14.5a Overview of Spinal Nerves Distribution of spinal nerves After intervertebral foramen, spinal nerve splits Posterior ramus small branch o Innervates muscles and skin of back Anterior ramus large branch o Splits into multiple other branches o At different levels, this ramus innervates anterior and lateral trunk, upper limb, lower limb o Participates in plexuses Rami communicantes small branches of autonomic fibers o Extend between spinal nerve and sympathetic trunk ganglion Ganglia interconnected in sympathetic trunk parallel to vertebral column 31
Spinal Nerve Branches Figure 14.13 32
14.5a Overview of Spinal Nerves Dermatomes Segment of skin supplied by single spinal nerve o Some overlap in innervated regions E.g., T10 dermatome = horizontal ring of skin around umbilicus Can help localize damage to one or more spinal nerves o E.g., loss of sensation on medial arm and forearm indicates C8 damage Involved in referred visceral pain o E.g., appendicitis pain often referred to T10 dermatome 33
Dermatome Maps Figure 14.14 34
Clinical View: Shingles Reactivation of chickenpox infection Virus remaining latent in posterior root ganglia Reactivated, travels through sensory axons to dermatome Rash and blisters along the dermatome Burning and tingling pain Antiviral medication to reduce severity Vaccine to prevent or reduce disease severity 35
Nerve plexus 14.5b Nerve Plexuses Network of interweaving anterior rami of spinal nerves Four main plexuses occur bilaterally: cervical, brachial, lumbar, and sacral plexuses o Most thoracic spinal nerves and nerves S5 Co1 do not form plexuses Individual rami branch repeatedly o Damage to one nerve or spinal segment does not deprive a muscle or skin region of all innervation 36
14.5d Cervical Plexuses Cervical plexuses anterior rami of C1 C4 C5 contributes a few axons Branches innervate: anterior neck muscles, skin of neck, portions of head and shoulders From rami of C3 C5 it gives rise to phrenic nerve innervating diaphragm 37
Cervical Plexus Figure 14.16 38
14.5e Brachial Plexuses Brachial plexuses from anterior rami of C5 T1 Network of fibers extending laterally from neck into axilla Composed of anterior rami, trunks, divisions, cords Cords give rise to 5 major terminal branches 1. Axillary nerve: to deltoid, teres major muscles; sensory input from superolateral arm 2. Median nerve: to most anterior forearm muscles, thenar muscles, lateral lumbricals; sensory input from palmar side and dorsal tips of most fingers (not pinkie) 3. Musculocutaneous nerve: to anterior arm muscles (e.g., biceps brachii); sensory input from lateral forearm 4. Radial nerve: to posterior arm and forearm muscles; sensory input from posterior arm and forearm and dorsolateral hand 5. Ulnar nerve: to anterior forearm muscles, most intrinsic hand muscles; sensory input from palmar and dorsal aspect of two medial fingers 39
Brachial Plexus Figure 14.17a 40
Brachial Plexus Figure 14.17c 41
Clinical View: Brachial Plexus Injuries Axillary nerve injury (crutch palsy) Can be compressed in axilla or damaged if neck of humerus broken Difficulty abducting the arm and anesthesia along superolateral skin Radial nerve injury (waiter s tip) By humeral shaft fractures or injuries to lateral elbow Causes paralysis of extensor muscles of forearm, wrist, fingers Causes anesthesia along posterior arm, forearm, part of hand Median nerve injury May be compressed in carpal tunnel syndrome Ulnar nerve injury May be injured by fractures or dislocations of elbow Causes paralysis of most intrinsic hand muscles; sensory loss on medial hand 42
14.5f Lumbar Plexuses Lumbar plexuses from anterior rami of L1 L4 Femoral nerve o Main nerve in posterior division of plexus o Innervates anterior thigh muscles and sartorius o Sensory input from anterior and inferomedial thigh and medial leg Obturator nerve o Main nerve in anterior division of plexus o Innervates medial thigh muscles (adductors) o Sensory input from superomedial skin of thigh 43
Lumbar Plexus Figure 14.18a 44
Lumbar Plexus Figure 14.18c 45
14.5g Sacral Plexuses Sacral plexuses anterior rami of L4 S4 Sciatic nerve o Largest and longest nerve in body o Formed from portions of anterior and posterior sacral plexus o Composed of tibial division and common fibular division The two divisions split into two separate nerves just above popliteal fossa o Tibial nerve (from anterior division of sciatic) Innervates hamstrings and hamstring part of adductor magnus muscle o Common fibular nerve (from posterior division of sciatic) Innervates short head of biceps femoris muscle 46
Sacral Plexus Figure 14.19a 47
Sacral Plexus Figure 14.19c 48
Clinical View: Sacral Plexus Injuries Sciatica: injury to sciatic nerve Characterized by extreme pain down posterior thigh and leg May be caused by herniated intervertebral disc Common fibular nerve Prone to injury due to fracture of neck or compression from cast May cause paralysis of anterior and lateral leg muscles Person unable to dorsiflex and evert the foot (foot drop) 49
14.6a Characteristics of Reflexes Reflexes: involuntary responses A stimulus is required to initiate a reflex Response is rapid; involves a chain of only a few neurons The response is preprogrammed; always the same The response is involuntary; no intent or awareness of the reflex before it happens A reflex is a survival mechanism We respond to a potentially detrimental stimulus immediately and awareness comes later 50
14.6b Components of a Reflex Arc Reflex arc: neural pathway responsible for generating the response Figure 14.20 51
14.6c Classifying Spinal Reflexes Reflexes vary and can be classified in different ways Spinal or cranial: Is the spinal cord or brain the reflex integration center? Somatic or visceral: Is the effector a skeletal muscle or is it cardiac muscle, smooth muscle, a gland? Monosynaptic or polysynaptic: Do sensory neurons synapse directly with motor neurons or are there interneurons in the reflex arc? Ipsilateral or contralateral: Are receptor and effector on the same side of the body or on opposite sides? Innate or acquired: Are you born with the reflex or do you develop it after birth? 52
Monosynaptic and Polysynaptic Reflexes Figure 14.21 53
Stretch reflex 14.6d Spinal Reflexes Reflexive contraction of a muscle after it is stretched Stretch (or tendon tap) is detected by a muscle spindle receptor o o o o Spindle contains intrafusal muscle fibers innervated by gamma motor neurons and wrapped by sensory neurons Fibers not within spindle are extrafusal muscle fibers innervated by large alpha motor neurons When stretched, spindle s sensory axon fires impulses that are conducted to the spinal cord In spinal cord, the sensory axon excites alpha motor neurons of the same muscle, causing contraction (monosynaptic) Simultaneously, the sensory axon excites interneurons that inhibit motor neurons of antagonist muscle (polysynaptic reciprocal inhibition) 54
Biceps Stretch Reflex Figure 14.22 55
14.6d Spinal Reflexes Golgi tendon reflex Prevents muscles from contracting excessively Golgi tendon organs detect excessive tension o o They are proprioceptors with sensory ending at muscle tendon junction Their sensory axons excite interneurons in the spinal cord Some excited interneurons inhibit motor neurons of same muscle o Muscle relaxes (polysynaptic reflex), preventing it from damage Some excited interneurons excite motor neurons of antagonist muscle (reciprocal activation) o Antagonist muscle contracts (polysynaptic reflex) 56
Golgi Tendon Reflex Figure 14.23 57
Withdrawal reflex 14.6d Spinal Reflexes Pulls a body part away from a painful stimulus Stimulus excites nociceptor sensory neuron that transmits signal to spinal cord and excites interneurons Interneurons excite motor neurons of flexors so flexor muscles (e.g., hamstrings) contract and limb is withdrawn Simultaneously, other interneurons reciprocally inhibit motor neurons of extensors so that extensor muscles (e.g., quadriceps) relax and withdrawal happens quickly 58
Crossed-extensor reflex 14.6d Spinal Reflexes Occurs in conjunction with withdrawal reflex Some interneurons excited by the nociceptor sensory neuron cross midline and excite extensor motor neurons on other side o E.g. as left leg is withdrawn, right leg s quadriceps is excited Allows the opposite side limb to support body weight while the hurt limb withdraws 59
Withdrawal and Crossed- Extensor Reflexes Figure 14.24 60
Pupilary Light Reflex 14.6d Cranial Reflexes Shine a light in the eyes and initiate constriction of the pupil Used to test for concussion injuries on the field Babinski Reflex Stroke the lateral surface of the foot and the toes flare out Innate reflex normal in children under 2 years of age Positive reflex may indicate CNS damage or meningitis 61
14.6e Reflex Testing in a Clinical Setting Reflexes are useful for diagnoses Can test function of specific muscles, nerves, spinal segments Hypoactive reflex: diminished or absent o May indicate damage to spinal cord, or muscle disease, or damage to neuromuscular junction Hyperactive reflex: abnormally strong response o May indicate damage to brain or spinal cord, especially if accompanied by clonus (rhythmic oscillating movements with reflex testing) 62