Neuroanatomy PDF - Spinal Cord

Isabella Ronchi Ñëürøåñåtømÿ The spinal cord is contained in the vertebral canal, starting from the foramen magnum until the inferior border of the first lumbar vertebra (L1/L2). Because of its position, the spinal cord and its roots can be damaged when the vertebra...

Isabella Ronchi Ñëürøåñåtømÿ The spinal cord is contained in the vertebral canal, starting from the foramen magnum until the inferior border of the first lumbar vertebra (L1/L2). Because of its position, the spinal cord and its roots can be damaged when the vertebral column undergoes a strain. The spinal cord is linked to the periphery by spinal nerves. Each segment of the vertebral is characterised by four spinal roots and two spinal nerves. There are: o 8 cervical segments o 12 thoracic segments o 5 lumbar segments o 5 sacral segments o 1 coccygeal segment (according to the Anglo-Saxon tradition) or 3 coccygeal segments (according to the Latin tradition) The spinal cord has a total of 31 (or 33) segments. Thoracic, lumbar and sacral nerves roots exit caudally to the correspondingly numbered vertebral bone. The cervical roots exit above the corresponding vertebra with the exception of C8. C8 spinal nerve exits below C7 vertebra, because there are only 7 cervical vertebrae. Since the vertebral column grows longer than the spinal cord, the caudal nerves has to exit more obliquely. Roots exit at the level of the intervertebral foramina, formed by the superior and inferior notches of the articular processes joined by the zygapophysial joint. Two roots on each side fuse together and form the spinal nerve. At the level of the intervertebral foramina there are the dorsal root ganglia. The spinal nerve immediately divides into a dorsal ramus and a ventral ramus, because each myotome is divided into a dorsal division and a ventral division. cervical enlargement and a lumbar enlargement. The reason for this is that those regions have to accommodate a larger number of neurons to innervate the upper and lower limbs. The surface of the spinal cord presents longitudinal grooves that extend throughout the whole length of the spine, like the anterior median fissure, the anterolateral sulcus (caused by the exit of the ventral roots), the dorsolateral sulcus (caused by the entrance of the dorsal roots) and the median posterior sulcus. The anterior median fissure contains the anterior spinal artery. The dorsolateral sulci contain the posterior spinal arteries. The floor of the anterior median fissure is formed by a transverse band of white matter, the anterior white commissure. The region of white matter between the median posterior sulcus and the dorsolateral sulcus is called posterior funiculus; the portion between the  Isabella Ronchi dorsolateral sulcus and the anterolateral sulcus is called lateral funiculus and the portion between the anterolateral sulcus and the anterior median fissure is called anterior funiculus. The pia mater is the innermost and thinnest of the meninges. Then there is the arachnoid mater, made of a thin layer of fibroblasts from where strands of tissue extend to reach the pia mater. This space forms the subarachnoid space, filled with cerebrospinal fluid. The outermost of the meninges is the dura mater. Up to the intervertebral foramina the roots are enveloped and protected by the meninges too. When the nerve exits the intervertebral foramen, it is only covered by connective tissue. The pia mater gives rise to extensions that go through the subarachnoid space to reach the dura mater; they are the denticulate ligaments. Between the dura mater and the bone there is a space, called epidural space, occupied by adipose tissue and by the epidural venous plexus of Batson. This plexus harvests the red marrow of the vertebrae and drains into segmental veins (cervical, intercostal, lumbar). This is clinically important because these segmental veins drain tissues of the breast, lungs and prostate gland (the most common sites for tumours) and have no valves. Tumours can metastasize to the vertebrae via the segmental veins and the plexus of Batson. In the vertebral body these cancer cells proliferate and may compress a nerve root. Meningitis presents with fever, headache, vomiting, muscle pain, cold hands and feet. One of the physically demonstrable symptoms of meningitis is sign knees to flex when the neck is flexed. Another one is : severe stiffness of the hamstrings causes an inability to straighten the leg when the hip is flexed to 90 degrees. Meningococcal bacteria can reproduce in the bloodstream and cause a diffuse septicaemia: blood vessels are damaged and can cause bleeding under the skin. This causes blood pressure to fall and the body reduces the blood sent to the limbs, which can cause ischemia and tissue injuring. Blood vessel damage looks like a skin rash, but when a clear tumbler is pressed against the rash, it does not change colour or appearance. Nerve root compression may lead to back pain and referred pain (pain in the place from where that segment collects information). Compression of roots causes problems in the myomeric and dermatomeric territories of innervation. Muscles are innervated by more than a single nerve, because myotomes give rise to multiple weakness and decreased reflexes, because there are other nerves innervating it. Compression of a single compression can be caused by: o Herniation of intervertebral disc (prolapse of the nucleus polposus). 95% of all disk prolapses occur immediately above or below the last lumbar vertebra. The typical herniation is posterolateral, with  Isabella Ronchi compression of the nerve roots passing to the next intervertebral foramen. Symptoms include back ache cause by rupture of the annulus fibrosus, and pain the buttock, thigh or leg cause by pressure on posterior root fibres contributing to the sciatic nerve. The pain is increased by stretching the affected root, for example by having the straightened leg raised by the examiner. Motor weakness may be detected during dorsiflexion of the great toes and during eversion of the foot. o Spondylosis: degenerative disease of the spine with inflammation o Osteophytes: bony outgrowths o Degeneration of articular capsule with synovial inflammation The termination of the spinal cord is called conus medullaris. After this point, there are only spinal nerves, called cauda equina. The subarachnoid space from L2 to S2 is called lumbar cistern. Lumbar punctures and spinal anaesthesia are performed with the needle in the subarachnoid space between L3 and L4 or between L4 and L5, to avoid injuring the spinal cord. Normal cerebrospinal fluid in the lumbar cistern is 8-15 mm of Hg and the quantity should be 140-270 ml. Epidural anaesthesia, instead, are performed in the epidural space. The patient is asked to flex the back anteriorly, so that the interlaminar window is larger. The filum terminale is fibrous tissue covered by the pia mater, proceeding downward from the conus medullaris. The filum terminale internum is covered by the dura mater and arachnoid mater, and surrounded by the nerves forming the cauda equina; the lower part, the filum terminale externum, fuses with the investing dura mater and is attached to the first segment of the coccyx in a structure sometimes called coccygeal ligament. Cauda equina syndrome refers to a pattern of neuromuscular and urogenital symptoms resulting from the simultaneous compression of multiple lumbosacral nerve roots below the conus medullaris. These symptoms include low back pain, sciatica, saddle sensory disturbances, bladder and bowel dysfunction and variable lower extremity motor and sensory loss. The internal structure The internal structure of the spinal cord has a common layout with some differences along its length. The white matter decreases caudally. The lumbar and sacral regions contain the cell bodies of the motor neurons and interneurons that innervate the lower limbs and trunk. By contrast, most of the ascending axons terminate at higher levels of the cord. The ventral horns are larger at the cervical and lumbar enlargments, because they contain the motor neurons that innervate the muscles of the limbs. The thoracic segments have a lateral horn, in addition to the ventral and dorsal horns that are typical of all segments. It contains pre-ganglionic sympathetic neurons.  Isabella Ronchi The grey matter of the spinal cord can be divided in laminae according to the morphologic characteristics and function of the cells. The Rexed laminae comprise a system of ten layers of grey matter, idientified to label portions of the grey columns of the spinal cord. The laminae are numbered from dorsal to ventral with roman numerals. Thoracic , because it contains a lot of interneurons involved in motor control, which are more present in the cervical and lumbar regions. Lamina X is located around the central canal. In the enlargements, the ventral horns have a less lamellar organisation, because there are groups of motor neurons involved in the innervation of the limbs. These clusters are arranged sparsely in the ventral horn. The laminae contain columns of cells that extend for different legnths. Some columns are present all throughout the length of the spinal cord, while others are only present in certain segments. For example, the posteromarginal nucleus (lamina I) is everywhere along the length of the spinal cord, while the nucleus dorsalis (medial region of lamina VII) is present only in some portions. Neurons of the spinal cord can be classified as: o Radicular neurons: the neurons that give rise to roots (somatic motoneurons and visceromotor neurons). Primary sensory neurons that form the dorsal roots, however, are outside the spinal cord, in the dorsal root ganglia. Radicular neurons are the only ones having contact with the periphery. The axons of visceromotor preganglionic leave the spinal cord through the ventral roots and then synapse with neurons of the paravertebral or prevertebral chains, which in turn signal to smooth muscle. They can be found in the thoracic (lateral horn) and sacral regions of the spinal cord. The somatic motoneurons (motoneurons proper) leave the spinal cord through the ventral roots and synapse directly with the skeletal muscles. o Funicular neurons: they send their axons in the white matter of the spinal cord, which is organised in funiculi. Some are intersegmental associative, meaning that they project the signal to other segments of the spinal cord (e.g., neurons of the cervical region projecting to neurons of the lumbar region). Commissural associative neurons cross the anterior commissure to communicate with the other side. Projecting neurons project rostral to the spinal cord, to the brainstem. o Golgi II type neurons: their axon remains in the grey matter for local associative circuits. Funicular neurons and Golgi II type neurons are interneurons, and they make up about 99% of all neurons. A motor unit is made of a single motor neuron and the population of muscle fibres it innervates. Each muscle is controlled by several motor units. If a muscle is innervated by many motor neurons, it will perform finer movements. For example, large muscles of the abdomen do not need as much control as the muscles of the hand, which means that they will be innervated by relatively fewer motor units. Groups of motoneurons are somatotopically organised, meaning that the neurons innervating a certain portion of the body are concentrated in one spot of the ventral horn and do not mix with other neurons innervating different areas. The flexors are more dorsally located than the extensors; motoneurons innervating axial muscles are more  Isabella Ronchi medially located than neurons that innervate distal muscles. The column of motoneurons innervating a muscle extends for more than one segment and each segment of the spinal cord contains more than one column of motoneurons. There is also overlapping of columns. The neuromuscular spindle is a complex receptor made of modified skeletal muscle fibres. In normal muscle fibres (extrafusal muscle fibres), the contractile component is located all throughout the length of the fibre. Normal muscle fibres are innervated by alpha motoneurons. In intrafusal muscle fibres the contractile component is only located at the extremities, while at the centre there are no contractile myofilaments. These fibres are innervated by gamma motoneurons. Beta motoneurons innervate both types of fibres. Amyotrophic Lateral Sclerosis (ALS) affects lower motor neurons (spinal cord and brainstem) as well as upper motoneurons (cortical neurons at the origin of the corticospinal/pyramidal tract). Most patients with ALS present with random, asymmetric symptoms, consisting of cramps, weakness and muscle atrophy of the hands or feet. Weakness progresses to the forearms, shoulders, and lower limbs. Fasciculations, spasticity, hyperactive deep tendon reflexes, extensor plantar reflexes, clumsiness, stiffness of movement, fatigue and difficulty controlling facial expression and tongue movements soon follow. Other symptoms include hoarseness, dysphagia and slurred speech. Late in the disorder, a pseudobulbar affect occurs, with inappropriate involuntary and uncontrollable excesses of laughter or crying. Sensory systems, consciousness, cognition, voluntary eye movements, sexual function and urinary and anal sphincters are usually spared. Death is usually caused by failure of the respiratory muscles; 50% of patients die within 3 years of onset. Interneurons are involved in reflex arches. Interneurons involved in the stretch reflex are called 1a inhibitory. They receive inputs from 1a sensory fibres, which are the largest sensory fibres. In the patellar tendon reflex, you tap the tendon of the quadriceps muscle, slightly stretching the muscle. As a response, the muscle spindles send the information to the spinal cord. Consequently, the quadriceps contracts and the leg extends. The 1a fibres enter the spinal cord and branch: one branch mono-synaptically excites the motoneurons that go to the quadriceps, while the other branch excites a set of inhibitory interneurons, which target the hamstrings. This happens because in order for the quadriceps to contract, its antagonist, the hamstring, has to relax. One stimulus causes both an excitatory reflex and an inhibitory response. Some interneurons of the spinal cord are involved in the flexion withdrawal reflex. When stepping on something pointy, the extended leg suddenly retracts and flexes. In the meantime, to avoid falling, the other leg that has started flexion has to extend. The stimulus from the sole of the right foot reaches the dorsal horn and comes into contact with a set of excitatory interneurons, which in turn synapse on another set of interneurons. On the right side the stimulus uses inhibitory interneurons to inhibit motoneurons that are  Isabella Ronchi involved in the extension of the leg, while excitatory interneurons excite motoneurons that are involved in the flexion of the leg. On the left side, the opposite happens. are involved in recurrent inhibition of motoneurons. While a stimulus exits via the ventral root, the same axon gives off a collateral that excites an inhibitory interneuron. The axon of the interneuron synapses on the same motoneuron t from which it received the excitatory input. Some interneurons are called central pattern generators and they are involved in generating rhythmic activity for locomotion. Finally, projecting neurons send their axons to the brainstem, where they synapse on specific sets of interneurons. The white matter of the spinal cord is organised in 3 funiculi on each side: the dorsal, lateral and ventral funiculi. In the funiculi there are descending (axons of neurons in the brainstem or cortex) and ascending (projecting neurons) pathways. The dorsal funiculus is characterised by the presence only of ascending gracile fasciculus and cuneate fasciculus. On the other hand, in the lateral funiculus there are both descending and ascending pathways. This means that if there is a lesion of the dorsal funiculus there are only sensory problems, while injuries to the lateral funiculus cause both motor control and sensory issues. The funiculi also contain axons of associative neurons, which are located close to the grey matter. The spinal cord is supplied by the anterior spinal artery, which runs in the anterior spinal sulcus and by two posterior spinal arteries than run in the posterolateral sulci. The anterior spinal artery supplies the anterior 2/3 of the spinal cord. The anterior spinal artery originates from the descending branches of the vertebral arteries. The posterior spinal arteries originate from the vertebral arteries or posterior cerebellar arteries. approximately 10 to 12 segmental arteries join the spinal arteries along their course via radicular arteries. There is also a variable presence of segmental medullary arteries that feed directly the anterior and posterior spinal arteries. The segmental arteries can originate from several places: the posterior intercostal arteries, the deep cervical arteries, the ascending cervical arteries, from the lumbar arteries, from the lateral sacral arteries The artery of Adamkiewicz is the dominant segmental artery, which usually originates from the posterior intercostal arteries more typically from the left side between T9 and L2. This artery anastomoses with the anterior and posterior spinal arteries and contributes to the supply of the lower 2/3 of the spinal cord. The segments from T3 to T9 are more vulnerable to ischemia, because the radicular vessels give a minimal contribution to the anterior and posterior spinal arteries.