Anatomy and Physiology of the Spinal Cord PDF

Anatomy and Physiology of the Spinal Cord - The spinal cord → begins at occipital bone and extends from the **foramen magnum** to the **conus medullaris** - L1-2 in adults, L3 in newborns - Below this the lumbar and sacral roots form the **cauda equina** - Length of the spinal cord \...

Anatomy and Physiology of the Spinal Cord - The spinal cord → begins at occipital bone and extends from the **foramen magnum** to the **conus medullaris** - L1-2 in adults, L3 in newborns - Below this the lumbar and sacral roots form the **cauda equina** - Length of the spinal cord \< length of the vertebral column - **Length:**- 18 inches in men; 17 inches in women - **Width**:- ½ inch thick in the cervical & lumbar - ¼ inch thick in the thoracic area Structures Associated with the Spinal Cord - **[Cauda Equina]:** bundle-like structure of nerve roots and rootlets extending caudally from the end of the spinal cord - Formed by lumbar and sacral **dorsal and ventral roots** surrounding **the filum terminale** - Located in subarachnoid space below **conus medullaris** - Free floats in CSF in lumbar cistern - **The number of nerve fibers gradually declines as pairs of nerves leave the spinal column** - **10 fiber pairs at the base of the cauda equina** Cauda Equina Syndrome - Caused from compression of nerve roots in this area - Considered to be a medical surgical emergency! - Treatment = surgical decompression ASAP - Symptoms can occur suddenly or over a few weeks or a few years - Symptoms = - Radiculopathy (pain, numbness, tingling, weakness) caused by compressed nerves - Altered sensation or severe or progressive weakness or numbness in the lower extremities, legs, and/or feet. - Urinary or bowel incontinence or retention - Severe low back pain Transient Neurologic Syndrome - Transient neurologic syndrome: Mild lower back pain following SAB & epidural - Appear within a few hours up to 24 hours after spinal anaesthesia and may last up to 2-5 days - Symptoms = Mild to severe pain and/or dysesthesia in the legs and buttocks - Dyesthesia- =abnormal touch sensation - No sensory or motor deficits - Normal MRI **Comparison of Cauda Equina Syndrome & Transient Neurologic Syndrome** - CES is medical emergency - Both occur more with Lidocaine 1:7(13%) but can be caused by all local anesthetics - **[Transient Neurologic Syndrome (TNS)-]** 1993- Transient radicular irritation- mild to severe **pain**, or dysesthesia or both in the legs and buttocks after SAB - TNS is self-limited disease - Treatment of symptoms for TNS is NSAIDs and trigger point injections - May be resistant to treatment and last several months Spinal Cord Coverings - The spinal cord is sheathed in the same **[three]** meninges as the brain: the **pia, arachnoid and dura mater**. - The **dura mater** is the tough outer sheath - The **arachnoid mater** lies beneath it, - The **pia mater** closely adheres to the surface of the cord. - The spinal cord is attached to the **dura** by a series of lateral denticulate ligaments that arise from the pial folds Clinical Considerations: Defects of the Spine - Spina bifida = developmental abnormalities that result in defects of the spine - Meningocele = sac like herniation of meninges - Meningomyelocele = sac like herniation of meninges and spinal cord Spinal Cord Blood Supply - 1 Anterior Spinal Artery - 2 Posterior Spinal Arteries - 6-8 Radicular arteries - Artery of Adamkiewicz = main radicular artery Blood Supply to the Spinal Cord - **[Anterior spinal artery- supplies anterior 2/3 of cord]** - 1 artery - Inconsistent flow in the thoracolumbar region - Highly dependent on radicular flow - Runs the length of the spinal cord longitudinally along the anterior median fissure - Aorta →Subclavian artery→-vertebral artery→ anterior spinal artery - Aorta →Segmental artery→ anterior radicular artery →anterior spinal artery - **[Posterior spinal arteries- supplies posterior 1/3 of cord]** - 2 arteries - Runs the length of the spinal cord longitudinally bilaterally - Aorta →Subclavian artery→-vertebral artery→ posterior spinal artery - Aorta →Segmental artery→ posterior radicular artery→ posterior spinal artery Blood Supply to the Spinal Cord - **[Radicular arteries-]** provide additional blood flow to the anterior and posterior spinal arteries - 6-8 radicular arteries - Thoracolumbar regions - The main radicular artery is the **[Artery of Adamkiewicz ]** - Largest & most important radicular artery - Provides most of the blood supply to the anterior, lower 2/3- thoracolumbar region - More commonly arises from the left - Variation in origination of this artery - More commonly arises from left T11-12 - 75% population- T8-12 - 10% of population will have origin at - L1-2 - **Watershed areas- single blood supply and are vulnerable to ischemia** - **What are the spinal watershed areas? Thoracolumbar areas because they are highly dependent on radicular arteries** Regulation of Blood Flow in the Spinal Cord - **Normal autoregulatio**n- MAP between 50-150 mm Hg or 60 to 160 torr - **Map \< 50 mm Hg leads to ischemia** - **Map \> 150 mm Hg leads to tissue edema, hyperemia** - What happens to the autoregulatory range of patients with chronic hypertension? Autoregulation shifted to the right and will need a higher MAP to maintain CPP - **Alterations of PaCO2** and **PaO2** disrupt spinal cord autoregulation **Clinical Application: Anterior Spinal Artery Syndrome** - **[Anterior spinal artery syndrome/Beck's syndrome/Central Cord syndrome/anterior cord syndrome]** = anterior-central cord ischemia - Extremely rate cause of acute ischemic cord infarction - **Causes =** occlusion or hypoperfusion of the anterior spinal artery due to flexion, vascular injury, atherosclerotic disease, aortic cross-clamp - **What do we see? (S/S Beck's syndrome)** - Flaccid paralysis of the lower extremities -- paraplegia or quadriplegia - Motor paralysis bilateral below the level of the lesion - **Problems** autonomic dysfunction- ↓ BP, neurogenic bowel & bladder, sexual dysfunction - Loss of temperature & pain sensation - Bladder/bowel control issues - **Preserved function of the posterior column =** Intact proprioception and vibration sense, 2 -point discrimination & light touch Damage to the Artery of Adamkiewicz - Hemorrhage can cause ischemia in thoracolumbar segments - Occlusion can cause flaccid paralysis - Surgical risks = Spinal, kidney, aortic surgeries Functions of the Spinal Cord - **The spinal cord has three major functions:** - Serves as a **conduit for motor** information traveling down the spinal cord - Serves as a **conduit for sensory** information in the reverse direction - Acts as a **center for coordinating certain reflexes** - The **anterior portion** of the cord gives rise to the **motor nerves** - **Sensory nerves** originate **posteriorly** Vertebral Column - **[33 vertebrae]** - 7 cervical - 12 thoracic - 5 lumbar - 5 fused sacral - 4 fused coccygeal - Curvatures: - **Primary:** retained from fetal curvatures - Sacral - Thoracic - **Secondary:** develop after birth - Cervical - Lumbar Components of a Typical Vertebra - Vertebral Body - Supports weight (main bearing) - Connected by disks - Vertebral (neural) arch- posterior - Paired pedicles laterally - Paired laminae posteriorly form spinous processes - Processes -- 7 - Articular process 4 - Spinous process 1 - Transverse process 2 - Facets - Foramina - Vertebral - Intervertebral - Transverse Intervertebral Disks: Components - Each disc consists of: - Nucleus pulposus - Anulus Fibrosus - Vertebral end plates - Components = protein, collagen, and water Intervertebral Disc & Herniation - Comprise ¼ length of the spinal column - Movement between the vertebrae - Acts as a shock absorber - Changes to intervertebral discs occur due to trauma or degenerative changes - **Herniation =** protrusion of nucleus pulposus→ posterior lateral anulus fibrosis - **HNP= herniated nucleus pulposus** Abnormal Curvatures of the Spine - Kyphosis: abnormal curvature of thoracic spine - Lordosis: swayback, abnormal curvature of lumbar spine - Scoliosis: abnormal curvature of spine from side to side Regional Characteristics of Vertebrae - [Atlas: C1] - Supports the skull/ head - No body or spine - Articulates with Occipital Condyles, **Atlanto-occipital joint**, and Axis - Atlanto-occipital joint is the rotation point for flexion/extension of the head & neck - **Atlantoaxial joint** provides head/neck rotation - [Axis: C2] - Smallest transverse process - **Vertebra Prominens C7** - Long spinous process - Attachments for: - Supraspinous ligaments - Ligamentum nuchae - **Thoracic vertebra** - Costal facets (superior and inferior) - Transverse Process - L5: - Largest body - 2 processes - Mammillary processes - Accessory processes - Sacrum: - 5 fused sacral vertebrae - 4 pairs of foramina - Articulates with 4 bones: - Last lumbar vertebra above - Coccyx below - Ilium portion of the hip bone on either side (2) - Coccyx: - 4 coccygeal vertebrae - Attachment for ligaments Muscles that Affect the Spine - Back muscles stabilize the spine - [EX: ] - Sternocleidomastoid -- head movement - Psoas Major- thigh flexion - Flexion, rotation, or extension - [Extensors]- → back of the spine →enable us to stand up and lift objects - [Flexors]- → located at front →abdominal muscles→ enable us to flex or bend forward & lift and control the arch in the lower back - Muscles → supported by fascia **Examples of Specific Muscles Associated with Movement of the Spinal Cord** - **Thoracic Region:** - Longissimus Thoracis- extension & lateral flexion of the vertebral column; rib rotation - Iliocostalis Thoracis- same - **Lumber Region**: - Psoas Major- flexes the thigh at the hip joint and the vertebral column - Quadratus Lumborum- lateral flexion of the vertebral column - **Cervical region (Anterior):** - Sternocleidomastoid- head movement- extension & rotation; flexion of the vertebral column - **Posterior Cervical**: - Longissimus Cervicus- extends cervical vertebrae - Longissimus Capitus- head rotation/pulls backward What are Reflex Arcs? - **Reflex arc = Neural pathways that control an action reflex** - Sensory neurons receive stimulation and activate effectors (i.e. muscle cells) - **Reflex actions** = involuntary responses to a specific stimulus - **2 types of reflex arcs:** - Autonomic which are related to inner organs - Somatic which are related to skeletal muscles - 2 types of neuron reactions - Monosynaptic- sensory + motor neurons - Polysynaptic- multiple interneurons - **Consist of:** - Sensor organ - Sensory neuron - 1 or more synapse - Motor neuron - Effector -muscle - Myotatic Reflex = Deep Tendon Reflexes = stretch reflex - Example of myotatic reflex: Knee Jerk Reflex- stretch reflex - When a load is placed on a muscle, → stretched → reflex contraction of the muscle→ muscle strength - Automatic regulation of muscle length- monitored by muscle spindles - Monosynaptic reflex- one synapse - Physiologic function of myotatic reflex = resist gravity - A lesion in any part of the myotatic reflex circuit will result in areflexia - Common reflex arcs - Brachioradialis- C5-C6 - Biceps- C6-C7 - Triceps- C6-C7 - Knee- L2-L4 - Ankle- S1 Inverse Myotatic Reflex - Inverse Myotatic Reflex = Reflex inhibition of muscle contraction that is stimulated by the active contraction of the muscle itself - Protects a muscle from overload during extreme contraction - Provides a tension feedback system →regulates muscle tension in sustained contraction. - Mediated →Golgi tendon organs - AKA Golgi reflex- opposes contraction Reciprocal Inhibition - Antagonistic muscle pairs - Agonist- movement by shortening - Antagonist- opposes the action - In reciprocal inhibition → simple movement occurs → antagonistic muscle pairs - Mediated by spinal interneurons - Ex: Biceps/Triceps; Hamstring/Quadriceps - **The inverse myotatic reflex is a type of autogenic inhibition within a single muscle, while reciprocal inhibition involves the interaction between antagonist muscle groups** Muscles, Tendons, & Ligaments - Ligaments connect bone to bone and help stabilize joints - Tendons attach muscle to bone - Both tendons and ligaments have limited blood supply - Difference between tendon and ligament composition? Spinal Cord Ligaments - Vertebral column is stabilized by ligaments - provide flexibility & limit excessive movement which limit damage - **2 Primary Spinal Ligament Systems** - Intra-segmental holds individual vertebrae together - Intersegmental holds many vertebrae together - Strongest ligament= ligamentum flavum Clinical Considerations: Tissues involved in Epidural Placement - Lumbar puncture: - Tap of lumbar SA space → L3 and L4 or L4 and L5 - Allows for: - Measurement of CSF pressure - Bacteriologic and chemical examination - Introduction of anesthetics - Caudal Anesthesia: - Blocks spinal nerves in the epidural space - Injection of anesthetic agents in the sacral hiatus - SAB- local anesthesia injected into the subarachnoid space - CSF acts as carrier for anesthetic drug so you will need a smaller volume - Smaller volume - Epidural- local anesthesia injected into the epidural space- lumbar, sacral, thoracic - Larger volume needed, no carrier fluid Somatic Motor Systems - The CNS has 2 neural outputs - The somatic motor system - The autonomic nervous system - **Somatic motor system (voluntary)** - Part of Peripheral Nervous System - Controls locomotion, fine movements, body posture and equilibrium- - nearly all voluntary muscle movements - Acts on motor neurons - Carries motor and sensory information to/from the CNS - Consists of nerves →skin, sensory organs, and all skeletal muscles - Responsible for processing sensory information -- hearing, touch, & sight Neural Pathway for Skeletal Muscle Contraction - Upper motor neurons located in the central nervous system - Lower motor neurons of the somatic nervous system. - Lower motor neurons can be a part of cranial or spinal nerves - Innervate muscle fibers &directly cause contraction Somatic Nervous System Motor Unit- Alpha Motor Neurons - **alpha motor neurons aka lower motor neurons** - Innervate skeletal muscle - Cause muscle contractions and therefore movement - **Motor Neuron Pool:** Group of motor neurons that innervate fibers within the same muscle (Lower motor neurons) - Originate in spinal cord or brain stem - End on skeletal muscle - Release Ach - Stimulates muscle contraction - Motor neuron recruitment = more tension generated Spinal control of Alpha Motor Neurons - 3 Inputs = upper motor neurons, spinal interneurons, and sensory neurons - Upper Motor Neurons - Located in Cortex & brainstem - Synapse with a lower motor neuron - Control the lower motor neurons → control movement - Spinal Interneurons - Extensive spinal circuitry - Central pattern generators - Sensory neurons - Provide Feedback about muscle length and tension - Motor neurons release ACH at the NMJ Descending Motor Tracts - **Lateral Pathways-** - Consists of upper and lower motor neurons - Involved in distal limb control - Pyramidal tracts (voluntary) - Corticospinal tracts - Corticobulbar tracts - Motor pathway for upper motor neuron signals from cerebral cortex - **Ventromedial/extrapyramidal Pathways-** midbrain-send upper motor neuron axons down the spinal cord - 4 major tracts: - Rubrospinal tract - Vestibulospinal tract - Tectospinal tract - Reticulospinal tract Pyramidal (corticospinal) Tract - **Corticospinal (Pyramidal) Tract**: - Cortical upper motor neurons- Brodmann's areas 1-4- 90% axons cross to the contralateral side - Lesions above the medullary pyramids (site of decussation) will result in contralateral muscle weakness - Remaining 10 % axons descend on the ipsilateral side - Lesions below the medullary pyramids will produce ipsilateral muscle weakness Extrapyramidal (ventromedial) Tracts - Control the large, postural muscles of the axial skeleton - Originate in the brainstem - Help maintain head position and posture - 4 tracts: - [Rubrospinal-] responsible for regulation of flexion and extension tone, fine motor control; minor in humans - [Vestibulospinal tra]ct- provides a link between the sensors for balance and the extensor muscles - [Tectospinal tract-] directs the head and eyes to move toward a selected object in the visual field - [Reticulospinal tract-] facilitates fine control of posture by acting on the extensor muscles of the lower limb Lesions on Motor Neurons - **Upper motor neuron lesion: loss of inhibitory control from upper motor neurons resulting in hyperreflexia and hypertonia** - Spastic paresis- weakness or partial paralysis - Hyperreflexia - Hypertonia - \+ Babinski sign after age 2 - **Lower motor neuron signs and symptoms :** - Flaccid paralysis - Hypotonia - Absent Babinski sign- - Fasciculations - Atrophy Clinical Application of motor neuron disease - Poliomyelitis = Pure lower motor neuron disease - Can progress to weakening ventilatory muscles - Amyotrophic Lateral Sclerosis = Progressive upper and Lower Motor Neuron Disease Somatosensory System - 5 cutaneous sensory modalities - Vibration - Touch - Pressure - Pain - Temperature - Proprioception = Sense of movement, action, location - Dermatomes: 30- part of somatosensory system = Area of skin supplied with afferent nerve fibers by a single dorsal spinal nerve root - Myotomes = group of muscles innervated by a single ventral nerve root Spinal Segments - 33 vertebrae →in the human spinal cord - 31 pairs of spinal nerves - 30 dermatomes- start at C2- Why? C1 -- no sensory nerve root - Dermatomes have a segmented distribution- unique to each person - Torso and core dermatomes have horizontal distribution - Limb dermatomes have vertical distribution Spinal Nerves & Dermatomes - 31 pairs- 8 cervical, 12 thoracic, 5 lumbar, 1 coccygeal - Dermatomes exist for each spinal nerve starting at C2 - Dermatomes help localize sensory levels Myotome Nerve Roots & Muscle Actions - C2: neck flexion and extension - C3: neck lateral flexion - **C4: **shoulder shrugs - **C5:** shoulder abduction and external rotation; elbow flexion - **C6: **wrist extension - **C7: **elbow extension and wrist flexion - **C8: **thumb extension and finger flexion - **T1: **finger abduction - **L2: **hip flexion - **L3: **knee extension - **L4: **ankle dorsiflexion - **L5: **big toe extension - **S1: **ankle plantarflexion - **S4: **bladder and rectum motor supply - **C3,4,5 keeps the diaphragm alive** - **S2,3,4 keeps the peen off the floor** Dermatomic Symptoms of Spinal Nerve Infections - Viruses may infect spinal nerves→ painful dermatomic area - Herpes zoster→ migrates along spinal nerve→ affects area of skin served by the nerve Dorsal Column Medial Lemniscus Pathway - Conveys discriminative touch, vibration, and proprioceptive sensory input - Decussation at the medulla - Any unilateral damage below the decussation at medulla leads to ipsilateral loss of sensation - Damage above decussation at medulla leads to contralateral loss of sensation - Main pathway involves a **three- neuron chain** - **First order neuron-** somatosensory receptor neuron - **Second order neuron-** dorsal column nuclei of the caudal medulla - **Third order neuron-** located in the thalamus Specialized touch receptors - **[Merkel's disks]**: slow adapting, Sensing steady pressure - **[Meissner's corpuscles]**: more rapidly adapting, Rapid changes in skin contact - **[Ruffini's Endings]**: slowly adapting, Local stretching of the skin - **[Pacinian Corpuscles]**: very rapidly adapting receptors, vibration - **[Hair follicles]**: Transduce displacement of hair Secondary Somatosensory Areas - **[Posterior parietal cortex-]** integrates touch with other sensations - **[Agnosia]**- inability to recognize objects despite the presence of normal sensations - **[Astereognosia]**- inability to recognize objects through touch Diseases Affecting the DCMLS - **Vitamin B12 neuropathy leads to** ipsilateral sensory deficits below the lesion -pathway decussates at the medulla - **[Tabes dorsalis]** -late manifestation of syphilis - impaired sensations from demyelination of dorsal column- - ataxic wide gait with loss of touch and proprioception, - paresthesias with altered sensations, - bladder dysfunction such as urinary retention - positive Romberg sign Anterolateral System- Spinothalamic - Sensory pathway from skin to thalamus - Conveys pain and temperature sensory input - Decussates-1-2 spinal cord segments above and below the entry of the peripheral afferent neuron - Damage = contralateral loss of sensation 1-2 spinal cord segments below lesion - 2 types of spinothalamic tracts = Neospinothalamic tract and Paleospinothalamic tract Anterolateral or Spinothalamic Tracts - **Neospinothalamic tract** carries fast type A delta nerve fibers - Nerves terminate in the dorsal horn of the spinal cord- - The neurotransmitter involved is **glutamate** - **Paleospinothalamic tract** carries slow type C nerve fibers. - Nerves terminate in the **substantia gelatinosa** - Morphine and its derivatives work by inhibiting mu receptors located in this tract - **Syringomyelia =** Cavity formation in the cervical spine - Bilateral loss of pain and temp - Compression of motor neurons→ flaccid paralysis of upper extremities Transmission of Pain and Temperature Sensation - Pain & temperature pathway- 3 neuron chain - Pain sensation is protective and alerts body of tissue damage - **Superficial pain** is body surface pain - **Initial pain**-sharp/highly localized- fast- "prickly" - **Small type A delta fibers (myelinated)** - Thermal or mechanical stimuli - **Delayed pain**- diffuse/burning, dull, aching- poorly localized, slow - Thermal, mechanical, or chemical stimuli - **Type C nerve fibers (unmyelinated)** - **Deep pain-** muscles & joints - **Nociceptors**- pain receptors Other Types of Pain - **[Visceral pain]**- pain arising from the internal organs- - dull, burning, poorly localized sensation - **[Referred pain-]** pain arising from the viscera- - perceived to be on the body surface - **[Phantom sensations --]**perceptions occurring after limb or organ removal - sensations of pain, touch, temperature, pressure, itchiness, movement - **[Phantom limb pain-]** feeling of pain in an absent limb or portion of a limb after amputation, congenital limb deficiency or spinal cord injury - **[Phantom limb sensation-]** sensory phenomenon other than pain - Felt in an absent limb or a portion of the limb - \~ 80% of amputees experience phantom sensations Temperature Sensation - **[Cold fibers- ]** - most numerous type of thermoreceptor - respond to a broad range of temperatures - **[Warm fibers-]** respond to a narrower range of temperatures - Both types are rapidly adapting - Both help protecting against hot and cold and recognizing wet and slipperiness - Play a minor role in control of core temperature