Chapter 14 Lecture Outline

See separate PowerPoint slides for all figures and tables pre- inserted into PowerPoint without notes.

Copyright © McGraw-Hill Education. Permission required for reproduction or display. 1 14.1 Gross Anatomy

• Spinal cord – Extends inferiorly from brain’s medulla through vertebral canal

– Ends at L1 vertebra with and below extend inferiorly as – Two widened regions with greater number of neurons o : contains neurons innervating upper limbs o : 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 : 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 – 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 – : 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 – : 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 L1

– Lumbar puncture below this, just above or below L4

– Spinous process of L4 at highest points of iliac crests

9 14.3a Distribution of Gray Matter

• Gray matter – Made of neuron’s cell bodies, dendrites, and unmyelinated ; also glial cells • Masses of 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

10 14.3a Distribution of Gray Matter

• Gray commissure – Horizontal band of gray matter surrounding – 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 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: myelinated axons to and from the brain • Regions of white matter – Posterior o Sits between posterior gray horns and posterior median sulcus o Contains sensory tracts ( bundles called fasciculi) – 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 (1st order) neuron has peripheral ending, cell body in posterior root ganglion, and axon leading to secondary neuron o Secondary (2nd order) neuron is an ; receives primary input and extends to tertiary neuron or to cerebellum o Tertiary (3rd 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 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 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; Figure 14.10 excites muscle

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 14.4c Motor Pathways • Indirect pathway – 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 o Help control 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 C2 and C1 vertebrae) – Below C8, nerves exit inferior to the vertebra of the same

number (e.g., T2 nerve exits between T2 and T3 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 14.5b Nerve Plexuses • Nerve plexus – 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 – 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: 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 14.6d Spinal Reflexes • – Reflexive contraction of a muscle after it is stretched – Stretch (or tendon tap) is detected by a receptor 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 o When stretched, spindle’s sensory axon fires impulses that are conducted to the spinal cord o In spinal cord, the sensory axon excites alpha motor neurons of the same muscle, causing contraction (monosynaptic) o 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 – Prevents muscles from contracting excessively – Golgi tendon organs detect excessive tension o They are proprioceptors with sensory ending at muscle tendon junction o 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 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 14.6d Spinal Reflexes

• Crossed-extensor reflex – 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 14.6d Cranial Reflexes

• Pupilary Light Reflex – 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