Box for spinal tap: The spinal cord ends between L1-L2, while the arachnoid mater ends at S2. This creates a lumbar cistern below the level of L2. In lumbar puncture, CSF is tapped from the lumbar cistern. Lumbar puncture is done at the L3-L4 interspace in adults and at the L4-L5 interspace in children. This interspace is identified at the level of the iliac crests.
Box for cauda equina syndrome: The cauda equina is a bundle of spinal nerves consisting of the lumbar and sacral spinal nerves and the coccygeal nerve. It resembles a horse’s tail, which is why it’s called the cauda equina. Collectively, these nerves innervate pelvic organs and the lower limbs, including the internal and external anal sphincters, perineum, bladder parasympathetics to the detrusor muscle (via the pelvic splanchnic nerves), and motor supply to the lower limb. Any lesion that compresses the cauda equina - such as disc herniation, spinal stenosis, spinal trauma, local ependymomas and schwannomas, metastatic tumors, etc. - can cause cauda equina syndrome. It presents with low back pain, sciatica, saddle anesthesia in the perineal area, lower limb paresis with hyporeflexia, and bowel/bladder incontinence. It is a surgical emergency and should be treated with immediate surgical decompression.
Box for Clarke’s nucleus: Also called the dorsal nucleus, Clarke’s nucleus is a group of interneurons located in the intermediate zone of the thoracic and upper lumbar spinal segments, extending from C8-L3. It functions as a relay for nerve fibers carrying input from muscle spindles and Golgi tendons (proprioception). It gives rise to the dorsal spinocerebellar tract. Clarke’s nucleus degenerates in Federich’s ataxia.
ASIA impairment scale
The extent of spinal cord injury (SCI) is defined by the American Spinal Injury Association (ASIA) Impairment Scale (modified from the Frankel classification). The categories range from A (worst prognosis) to E (best prognosis).
A = Complete: No sensory or motor function is preserved in sacral segments S4-S5
B = Incomplete: Sensory, but not motor, function is preserved below the neurologic level and extends through sacral segments S4-S5
C = Incomplete: Motor function is preserved below the neurologic level, and most key muscles below the neurologic level have a muscle grade of less than 3
D = Incomplete: Motor function is preserved below the neurologic level, and most key muscles below the neurologic level have a muscle grade that is greater than or equal to 3
E = Normal: Sensory and motor functions are normal
Comparison of upper and lower motor neurons
| Features | Upper motor neurons | Lower motor neurons |
| Location | Cortex | Brainstem; Spinal cord |
| Neurotransmitter | Glutamate | Acetylcholine |
| Lesion effect | Spasticity | Flaccid paralysis |
Upper MN cell bodies are located in the premotor and primary motor regions of the cerebral cortex (the “motor strip”). Upper MNs make glutamatergic connections with lower MNs located in the CNS, so upper MN pathways are confined to the CNS. Typical clinical features of an upper MN lesion include uncontrolled movement, decreased sensitivity to superficial reflex stimulation, and spasticity.
Lower MN cell bodies are located in specific nuclei in the brainstem and in the ventral horn of the spinal cord. Lower MNs are cholinergic and receive inputs from upper MNs, sensory neurons (SNs), and interneurons (INs). Paralysis is a typical clinical feature of lower MN lesions because, once damaged, there is no alternative route to convey information to the muscle targets in the periphery. Lower MNs are classified into three groups based on the type of target they innervate: branchial, visceral, and somatic MNs.
Branchial MNs are located in the brainstem and, together with SNs, form the cranial nuclei. They innervate branchial arch-derived muscles of the face and neck through 5 cranial nerves: trigeminal (V), facial (VII), glossopharyngeal (IX), vagus (X), and accessory (XI). Although these muscles function like other skeletal muscles, they differ in embryologic origin: muscles of the face and neck do not derive from somites; they derive from the branchial arches.
Motor neurons of the sympathetic system: The sympathetic nervous system mediates the classic “fight or flight” responses, including mobilizing energy stores, increasing awareness, and producing global activation of body metabolism. Central MNs of the sympathetic system are located in the spinal cord from thoracic segment 1 (T1) to lumbar segment 2 (L2). These MNs lie in an intermediolateral position and form the preganglionic column (PGC). They connect to 3 different targets: two chains of ganglia adjacent to the spinal cord (paravertebral and prevertebral) and directly to the chromaffin cells of the adrenal medulla, which release catecholamines into the circulation in response to stress stimuli. Paravertebral and prevertebral ganglia then connect to a wide variety of targets, including the heart, lungs, kidneys, intestines, and colon.
Motor neurons of the parasympathetic system: The parasympathetic system controls glandular secretion and activates the gastrointestinal tract and sexual behavior, often summarized as “rest and digest” functions. Central MNs of the parasympathetic system are located in the brainstem and contribute to the formation of cranial nerves (III, VII, IX, and X). Parasympathetic MNs are also found in sacral segments 2 to 4 (S2-S4) of the spinal cord. They innervate ganglia located near the peripheral targets, such as the heart, bladder, lungs, kidneys, and pancreas.
Somatic MNs are located in Rexed lamina IX in the brainstem and spinal cord and innervate skeletal muscles responsible for movement. Somatic MNs can be divided into 3 groups - alpha, beta, and gamma - based on the muscle fiber type they innervate.
A motor unit is a single MN together with all the muscle fibers it innervates.
Box for motor neuron disease involving spinal motor neurons: Amyotrophic lateral sclerosis (Lou Gehrig disease) affects both UMNs and LMNs. Progressive bulbar palsy affects LMNs in the brainstem. Poliomyelitis and spinal muscular atrophy (SMA) affect spinal motor neurons. SMA type 1 is called Werdnig Hoffmann disease, while type 2 is called Dubowitz disease. (P.S. look out for these names on USMLEs).
Box for renshaw cells: Renshaw cells are inhibitory interneurons. They secrete the neurotransmitter glycine to inhibit alpha motor neurons in the spinal cord. Tetanus toxin inhibits the release of glycine from Renshaw cells, leading to loss of inhibition of motor neurons and resulting in muscle spasms and rigidity.
Quick tip: The pyramidal tract arises from layer V pyramidal cells in the cerebral cortex. Pyramidal tracts include the corticospinal and corticobulbar tracts. The corticobulbar tract synapses on the cranial nerves that control the muscles of the face, head, and neck.
Box for Batson’s venous plexus: Batson’s venous plexus is a network of veins with no valves that connects deep veins of the bladder, prostate, and rectum to the internal vertebral venous plexus. Cancers and infections can spread from these organs directly to the vertebral column and from there to the brain and meninges. In the thoracic region, this plexus can allow breast and lung cancers to spread to the spine.
Nociceptors: Nociceptors are specialized peripheral sensory receptors that detect potentially damaging stimuli to the skin, such as extremes of temperature, pressure, and chemicals.
Prefrontal cortex: The prefrontal cortex has lateral and medial areas. The lateral area is involved in executive functions such as working memory, planning, judgment, sequencing of activity, abstract reasoning, and dividing attention. The medial area is involved in impulse control, personality, mood, and reactivity to surroundings. The anterior cingulate gyrus is associated with mood, especially depression and mania. Lesions in the dominant cortex tend to produce depression, while lesions in the nondominant hemisphere cause mania. Frontal lobe injury can lead to personality changes.
Broca’s aphasia: Broca’s aphasia is a non-fluent aphasia characterized by partial loss of the ability to produce spoken and written language. It is also called expressive aphasia. Speech still contains important content, but patients may omit articles, prepositions, and other words that mainly serve grammatical functions. This is described as “telegraphic speech.” Patients know what they want to say but cannot express it. Differentiate this from dysarthria, in which difficulty speaking is due to problems with the muscles of the tongue and mouth/face area.
Conduction aphasia: Conduction aphasia is seen with damage to the arcuate fasciculus (which connects Broca’s and Wernicke’s areas) and the left parietal region. It is a type of fluent aphasia with impaired repetition. Word-finding is difficult, while speech expression and comprehension are fairly intact.
Projection fibres: Projection fibers connect structures over long distances (for example, brain to spinal cord). Association fibers connect structures within the same hemisphere. Commissural fibers connect homologous structures on either side of the cerebral hemispheres (e.g., corpus callosum).
Gliomas are a group of brain tumors arising from glial cells. Oligodendromas and astrocytomas are more common. Glioblastoma multiforme is a very aggressive tumor with a poor prognosis.
Box for intracranial aneurysms: 85% of saccular aneurysms of the cerebral circulation occur in the circle of Willis. 35% occur in the anterior communicating artery, 30% in the internal carotid artery or its branches (posterior communicating and ophthalmic arteries), while 22% occur in the middle cerebral artery. Rupture of an intracranial aneurysm can lead to intracranial hemorrhage or acute subarachnoid hemorrhage.
Emissary veins: Emissary veins connect extracranial veins to intracranial venous sinuses. They drain the scalp and skull bones and are valveless. Because they are valveless, extracranial infections can be transmitted intracranially to the dural venous sinuses via emissary veins.
Bridging veins: Bridging veins carry venous blood from the cerebral cortex to the superior sagittal sinus. They travel across the subdural space and attach at right angles, which makes them prone to trauma. Rupture of these veins in head trauma can cause a subdural hematoma.
Pulsating exophthalmos: Pulsating exophthalmos is a distinctive feature seen when a cavernous aneurysm (an aneurysm of the cavernous part of the internal carotid artery) ruptures and forms a carotid-cavernous fistula.
Nucleus accumbens: The nucleus accumbens is often included as a basal nucleus and is often called the ventral striatum. It is the most inferior part of the striatum. Dopamine is the major neurotransmitter in this region. The nucleus accumbens is strongly associated with motivation and action and plays a key role in food intake, sexual behavior, reward-motivated behavior, stress-related behavior, and substance dependence. It is involved in the pathogenesis of several psychiatric disorders such as depression, bipolar disorder, and anxiety. Deep brain stimulation of this nucleus can treat clinically resistant depression.
Box for empty sella syndrome: In empty sella syndrome, the sella turcica is either partially filled with cerebrospinal fluid with a very small associated pituitary gland lying in the floor of the sella (partially empty sella) or completely filled with cerebrospinal fluid with no visualized pituitary gland (completely empty sella). This can lead to the pituitary gland not being visualized on brain imaging, which explains the name. It is usually asymptomatic but may present with chronic headaches and visual field defects. Causes include idiopathic disease, benign intracranial hypertension (pseudotumor cerebri), obesity, head trauma, tumors, etc.