Neuroanatomy Lesson 2: Spinal Cord Anatomy PDF

NEUROANATOMY Lesson 2: Anatomy of the Spinal Cord – Spinal Tracts CNS Review CNS Review CNS Review CNS Review CNS Review Overview of the Spinal Cord The spinal cord is the most important structure between the body and the brain. The spinal cord extends from the foramen ma...

NEUROANATOMY Lesson 2: Anatomy of the Spinal Cord – Spinal Tracts CNS Review CNS Review CNS Review CNS Review CNS Review Overview of the Spinal Cord The spinal cord is the most important structure between the body and the brain. The spinal cord extends from the foramen magnum where it is continuous with the medulla to the level of the first or second lumbar vertebrae. The spinal cord is 40 to 50 cm long and 1 cm to 1,5 cm in diameter. In men, it extends up to 45 cm and in women up to 43 cm. Development of the Spinal Cord The cells of the neural tube migrate to form the mantle layer of the gray matter which differentiates into: – An Alar plate, mostly sensory neurons. – A Basal plate, mostly motor neurons. Overview of the Spinal Cord Two consecutive rows of nerve roots emerge on each of its sides. These nerve roots join distally to form 31 pairs of spinal nerves with their respective spinal root ganglia. Spinal nerves contain the motor, sensory, and autonomic fibers. These nerves exit through the intervertebral foramen. The spinal cord divides into 31 segments: – Cervical: 8 (C1 – C8) – Thoracic: 12 (T1 – T12) – Lumbar: 5 (L1 – L5) – Sacral: 5 (S1 – S5) – Coccygeal: 1 (Co1) Each spinal cord segment innervates a dermatome. Dermatomes – their corresponding nerve roots A dermatome is an area of skin supplied by peripheral nerve fibers originating from a single dorsal root ganglion. If a nerve is cut, one loses sensation from that dermatome. Because each segment of the cord innervates a different region of the body, dermatomes can be precisely mapped on the body surface, and loss of sensation in a dermatome can indicate the exact level of spinal cord damage in clinical assessment of injury. Dermatomes – origin Dermatomes – Clinical relevance Ø TETRAPLEGIA = QUADRIPLEGIA Some degree of paralysis in all four limbs. May not be able to breath without the assistance of a ventilator Ø PARAPLEGIA Paralysis or decreased mobility from the waist down, including the legs.  Overview of the Spinal Cord The spinal cord consists of both white matter and gray matter. The amount of white matter becomes sparse towards the end, and the gray matter converges to form the conus medullaris. The cord is anchored at the caudal end to the coccyx by the filum terminale, which is an extension of the pia mater. The spinal nerves L2 to S1 makes up the cauda equina present within the subarachnoid space called the lumbar cistern. The spinal cord has two significant enlargements at the cervical and lumbar regions for brachial and lumbosacral plexus. Cross-Section of the Spinal Cord Both white matter and gray matter comprise the spinal cord. The gray matter is a collection of cell bodies, and the white matter is a collection of axons. The amount of gray matter is scarce at the thoracic levels as opposed to the cervical and lumbosacral segments. Cross-Section of the Spinal Cord The spinal cord has fissure and sulci. – The anterior median fissure is located centrally. – The anterior white commissure is present at its base. – The posterior median sulcus is present posteriorly. – The posterolateral sulcus is present on either side of it. Cross-Section of the Spinal Cord The anterior nerve roots exit at the anterolateral sulcus. The posterior nerve roots exit at the posterolateral sulcus. The central canal runs between the two halves of the gray matter. It is continuous with the fourth ventricle and contains cerebrospinal fluid. Internal Morphology of the Spinal Cord: Gray matter Spinal cord grey matter can be functionally classified in three different ways: – Into four main columns – Into six different nuclei – Into ten Rexed laminae Internal Morphology of the Spinal Cord: Gray matter - Four main columns The dorsal horn (the posterior horn) contains neurons that receive somatosensory information from the body, which is then transmitted via the ascending pathways, to the brain. The ventral horn (the anterior horn) largely contains motor neurons that exit the spinal cord to innervate skeletal muscle. The intermediate column and lateral horn contains neurons that innervate visceral and pelvic organs.  Internal Morphology of the Spinal Cord: Gray matter - Six nuclei Marginal zone (MZ, posterior marginalis): At the tip of the dorsal horn. Relaying pain and temperature sensation to the brain. Substantia gelatinosa (SG): At the top of the dorsal horn. Relaying pain, temperature and light touch sensation to the brain. Nucleus proprius (NP): In the ‘neck’ of the dorsal horn. Relaying mechanical and temperature sensation to the brain. Dorsal nucleus of Clarke (DNC): T The most dorso-medial nuclei. Relaying unconscious proprioceptive information to the brain. Only found in spinal segments C8 to L3. Interomediolateral nucleus (IMN): In the intermediate column and lateral horn. Relaying sensory information from viscera to the brain, and autonomic signals from the brain to the visceral organs. Lateral motor neurons and medial motor neurons (MNs): In the ventral horn. Composed of motor neurons that innervate visceral and skeletal muscles.  Internal Morphology of the Spinal Cord: Gray matter - Rexed Laminae Lamina I (Dorsal Horn) Tip of the dorsal horn Cells respond to noxious or thermal stimuli Sends information to the brain by the contralateral spinothalamic tract Corresponds to the marginal zone Lamina II (Dorsal Horn) Involved in sensation of noxious and non- noxious stimuli, and modulating sensory input to contribute to the brain’s interpretation of incoming signals as painful, or not. Sends information to Lamina III and IV Corresponds to substantia gelatinosa  Internal Morphology of the Spinal Cord: Gray matter - Rexed Laminae Lamina III (Dorsal Horn) Involved in proprioception and sensation of light touch. Cells in this layer connects with cells in layers IV, V and VI. Partially corresponds to nucleus proprius Lamina IV (Dorsal Horn) Involved in non-noxious sensory information relay and processing. Cells connect with those in lamina II Partially corresponds to nucleus proprius Internal Morphology of the Spinal Cord: Gray matter - Rexed Laminae Lamina V (Dorsal Horn) Relays sensory, including nociceptive (potentially painful), information to the brain via the contralateral and spinothalamic tracts Receives descending information from the brain via the corticospinal and rubrospinal tracts. Lamina VI (Dorsal Horn) Contains many small interneurons involved in spinal reflexes Receives sensory information from muscle spindles (involved in proprioception). Sends information to the brain via ipsilateral spinocerebellar pathways Internal Morphology of the Spinal Cord: Gray matter - Rexed Laminae Lamina VII (Lateral horn) Large, heterogenous zone that varies through the length of the spinal cord. Receives information from Lamina II to VI, and from viscera Relays motor information to the viscera Gives rise to cells involved in the autonomic system Dorsal nucleus of Clarke is part of Lamina VII Lamina VIII (Ventral horn) Varies depending on spinal cord level, but is most prominent in cervical and lumbar enlargements Cells are involved in modulating motor output to skeletal muscle Internal Morphology of the Spinal Cord: Gray matter - Rexed Laminae Lamina IX (Ventral horn) Size and shape varies between spinal cord levels Distinct groups of motor neurons that innervate skeletal muscle. Lamina X (Central zone) Surrounds the central canal – the grey commissure Axons decussate (cross over) from one side of the spinal cord to the other  External Morphology of the Spinal Cord: White matter White matter surrounds the gray matter and is formed by tracts that transmit information up and down the spinal cord. It divides into three funiculi. – The Anterior (Ventral) Funiculus (VF) lies between the two anterolateral sulcus and anterior median fissure. – The Posterior (Dorsal) Funiculus (DF) is between the posterolateral and posterior median sulcus. – The Lateral Funiculus (LF) is between the anterior and posterior roots. The cord segments have a dorsal root or the sensory ganglia. It contains cell bodies for the sensory pathway. Ascending and Descending Fiber System Ascending Fiber System The ascending tracts refer to the neural pathways by which sensory information from the peripheral nerves is transmitted to the cerebral cortex. In some texts, ascending tracts are also known as somatosensory pathways or systems. Ascending Fiber System Unconcious tracts  Ascending Fiber System The Dorsal Column-Medial Lemniscus System (DCML) or The Posterior Column is made up of laterally located fasciculus cuneatus and medially located fasciculus gracilis. They receive afferent impulses from C1 to T6 and from below T6, respectively. Thus the cuneate fasciculus relays information from the upper limb extremities and upper thoracic spine, and the gracile fasciculus relays information from the lower limb extremities and mid-thoracic spine. There is a single dorsal column in the lumbar segments of the cord where only the gracile tract is present; there are no subdivisions. The dorsal column has two subdivisions only from the cervical segments of the cord. Function: They transmit vibration sense; position sense is also called proprioception and two-point discrimination. CONCIOUS TRACTS  Ascending Fiber System The Spinothalamic Tract is also referred to as the anterolateral system. The ventral spinothalamic tract transmits crude touch and pressure sensation, and the lateral spinothalamic tract transmits pain and temperature sensation. The impulses from the periphery reach the dorsal root ganglia, then enter the spinal cord in the Lissauer tract. Lissauer's dorsolateral tract carries pain and temperature sensation. This tract travels short distances of 1-2 segments. The impulse ascends or descends within the Lissauer tract to synapse commonly at the lamina II of the dorsal horn. Postsynaptically their axons cross to the contralateral side through the anterior white commissure and ascend to synapse at the thalamus, following which they terminate at the postcentral gyrus. CONCIOUS TRACTS Ascending Fiber System Ventral The Spinocerebellar Tracts are located Spinocerebellar tract at the lateral funiculus. Anteriorly the ventral spinocerebellar tract is lateral to the anterolateral system, and posteriorly the dorsal spinocerebellar tract is lateral to the corticospinal tract. They carry unconscious proprioceptive information from the muscle spindles and Golgi tendon organs to the cerebellum. Dorsal The other ascending tracts are Spinocerebellar spinoreticular tract, spinoolivary tract, tract spinomesencephalic tract, and spinohypothalamic tract. UNCONCIOUS TRACTS  Descending Fiber System The descending tracts are the pathways by which motor signals are sent from the brain to lower motor neurones. The lower motor neurones then directly innervate muscles to produce movement. Descending Fiber System Ø The descending tracts are the pathways by which motor signals are sent from the brain to lower motor neurones. Ø The lower motor neurones then directly innervate muscles to produce movement. Ø The motor tracts can be functionally divided into two major groups: Pyramidal tracts – These tracts originate in the cerebral cortex, carrying motor fibres to the spinal cord and brain stem. They are responsible for the voluntary control of the musculature of the body and face. Extrapyramidal tracts – These tracts originate in the brain stem, carrying motor fibres to the spinal cord. They are responsible for the involuntary and automatic control of all musculature, such as muscle tone, balance, posture and locomotion. Ø There are no synapses within the descending pathways. At the termination of the descending tracts, the neurones synapse with a lower motor neurone. Thus, all the neurones within the descending motor system are classed as upper motor neurones. Their cell bodies are found in the cerebral cortex or the brain stem, with their axons remaining within the CNS.  Descending Fiber System The pyramidal tracts derive their name from the medullary pyramids of the medulla oblongata, which they pass through. These pathways are responsible for the voluntary control of the musculature of the body and face. Functionally, these tracts can be subdivided into two: Corticospinal tracts – supplies the musculature of the body. Corticobulbar tracts – supplies the musculature of the head and neck. Descending Fiber System Descending Fiber System The corticospinal Tract (Pyramidal tract) originates from the precentral motor cortex. It includes two components, the lateral corticospinal tract, comprised of 90% crossed fibers, and the anterior corticospinal tract, which is 10% uncrossed fibers. They decussate at the lower medulla. They are named pyramidal tract as the tract travel through the pyramids of the medulla. They directly innervate the motor neurons of the spinal cord, unlike the extrapyramidal tract that modulates the action indirectly. The fibers that carry impulses to upper extremities (cervical) is present centrally, and the fibers that carry impulses to the lower extremities (lumbosacral region) is present laterally. Function: It provides impulses responsible for the limb and axial body movement, carrying motor information for voluntary movement.  Descending Corticospinal Tract (Pyramidal tract) The lateral corticospinal tract is present in the lateral funiculi. It is medially adjacent to the dorsal spinocerebellar tract. Some of the uncrossed fibers of the anterior corticospinal tract crosses via the anterior white commissure to the contralateral side interneurons and lower motor neurons. The anterior corticospinal tract is present close to the anterior median fissure, mostly in the upper segments of the spinal cord, which gradually diminishes as we go towards the lower segments of the cord. They innervate the postural muscles. The size of the corticospinal tract gradually shrinks towards the lower segments of the cord. Descending Corticobulbar Tract (Pyramidal tract) The corticobulbar tracts arise from the lateral aspect of the primary motor cortex. They receive the same inputs as the corticospinal tracts. The fibres converge and pass through the internal capsule to the brainstem. The neurones terminate on the motor nuclei of the cranial nerves. Here, they synapse with lower motor neurones, which carry the motor signals to the muscles of the fase and neck. Clinically, it is important to understand the organisation of the corticobulbar fibres. Many of these fibres innervate the motor neurones bilaterally. Descending Fiber System The Extrapyramidal Tract is crucial for involuntary movements. They mostly occur in the anterior portion of the cord. They function by indirectly controlling the anterior motor horn cells. Together they help maintain changes in posture, muscle tone, and reflexes.  Descending Fiber System – The Rubrospinal Tract The rubrospinal tract primarily arises from the red nucleus on the contralateral side of the brainstem, and they descend anterior to the corticospinal tract. This tract is commonly involved with modulating the flexor movements, predominantly the proximal muscles of the upper limbs. This tract is responsible for the characteristic decorticate posture (abnormal flexion reaction of upper limbs and extension of the lower limbs) in response to a painful stimulus following trauma. Thus the rubrospinal tract sends impulses to the upper limbs to cause the flexion, which overshadows the tracts that usually opposes and balances with extension. The extension of the lower limb is due to the additional disruption of the corticospinal tract leading to imbalance. Descending Fiber System – The Vestibulospinal Tract The vestibulospinal tract originates from the brainstem vestibular nuclei. These fibers go to the alpha motor neurons via the interneurons. Their primary function is to maintain tone and posture by sending impulses to the extensor and antigravity muscles. Descending Fiber System – The Reticulospinal Tract The reticulospinal tract helps control muscle spindles; this is essential for the bilateral coordination of both postural and locomotion muscles. Descending Fiber System – The Tectospinal Tract The tectospinal tract provides neural impulses to the ventral gray interneurons. This tract is involved with controlling reflex head movement in response to sudden external stimuli such as auditory, tactile, and visual Spinal cord: blood supply - Arterial Supply The anterior spinal artery and a pair of posterior spinal arteries supply the spinal cord. – The anterior spinal artery supplies the anterior two-thirds of the cord. – The posterior spinal arteries give vascular supply the posterior one-third of the cord. These arteries arise from the distal vertebral arteries though some variations may exist. The three vessels anastomose around the cord to form the pial plexus, also called the vasocorona, to provide a robust blood supply to the spinal cord. The artery of Adamkiewicz, which is the largest radiculomedullary artery, supplies the lumbar cord The central area supplied only by the anterior spinal artery is predominantly a motor area. MRI images: the axial slice shows a snake eye-lesion that suggest spinal cord infarction. Spinal cord: blood supply - Venous Drainage Intrinsic veins (sulcal and radial veins) drain the parenchyma of the spinal cord into the extrinsic venous system. – The extrinsic venous system is composed of a network of pial veins: The anterior median spinal vein runs adjacent to the anterior spinal artery coursing along the entire length of the cord. The dorsal median spinal vein runs in the posterior median sulcus, and two dorsolateral spinal veins run adjacent to the posterior spinal arteries. Radicular veins drain the anterior and dorsal median veins into the extradural vertebral venous plexus (Batson's plexus). This network is composed of the internal venous plexus (posterior to the vertebral body and anterior to the vertebral arch) and the external venous plexus (anterior to the vertebral body and posterior to the vertebral arch). – Basivertebral veins within the vertebral body connect internal and external systems. The veins that comprise Batson's plexus are valveless and thus allow for retrograde flow. This therefore serves a potential route for the spread of urinary infections or metastatic tumors from pelvic organs to the spine.