Additional information

Box for spinal tap: The spinal cord ends between L1-L2 while 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 in L4-L5 interspace in children. The interspace is at the level of the iliac crests.

Box for cauda equina syndrome: The cauda equina is a bunch of spinal nerves consisting of lumbar and sacral spinal nerves and the coccygeal nerve. It is shaped like a horse tail, hence the name. The cauda equina nerves collectively innervate the pelvic organs and lower limbs including internal and external anal sphincters, perineum, bladder parasympathetics to detrusor muscle in the pelvic splanchnic nerves and motor supply to the lower limb. Any lesion that compresses the cauda equina like disc herniation, spinal stenosis, spinal trauma, local ependymomas and schwannomas, metastatic tumors etc, can cause cauda equina syndrome. It presents as 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 dorsal nucleus, it 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 fibres carrying input from muscle spindles and Golgi tendons, involved in proprioception. It gives rise to the dorsal spinocerebellar tract. The 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), using the following categories with A being the worst prognosis and E having the 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

Upper MN cell bodies are located in the premotor and primary motor region of the cerebral cortex also known as the “motor strip.” Since upper MNs make glutamatergic connections with lower MNs located in the CNS, they are exclusively confined to the latter. Typical clinical symptoms of 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 as well as in the ventral horn of the spinal cord. Lower MNs are cholinergic and receive inputs from upper MNs, sensory neurons (SNs) as well as from interneurons (INs). Paralysis is a typical clinical symptom of lower MN lesions since once damaged there is no alternative route to convey the information to the muscle targets in the periphery. Lower MNs are classified into three groups according to the type of target they innervate - branchial, visceral, and somatic MNs.

Branchial MNs are located in the brainstem and form, together with SNs, the cranial nuclei. They innervate branchial arch derived muscles of the face and neck through 5 cranial nuclei: the trigeminal (V), facial (VII), glossopharyngeal (IX), vagus (X) and accessory (XI) nerves. Despite their similar function, muscles of the neck and the face differ from other skeletal muscles in their embryological origin since they do not derive from the somites, but instead from the branchial arches.

Motor neurons of the sympathetic system: The sympathetic nervous system is involved in the traditional “fight or flight” responses, recruiting energy storage, increasing awareness, and leading to a global activation of the body metabolism. Central MNs of the sympathetic system are located in the spinal cord from the thoracic segment 1 (T1) to the lumbar segment 2 (L2). These MNs have an intermedio-lateral position and constitute the preganglionic column (PGC). They connect to 3 different targets: two chains of ganglia adjacent to the spinal cord named paravertebral and prevertebral as well as directly to the chromaffin cells of the adrenal medulla responsible for the release of the catecholamines in the circulation, in response to stress stimuli. On the other hand, paravertebral and prevertebral ganglia connect to a wide variety of targets including the heart, lungs, kidneys, intestines and the colon.

Motor neurons of the parasympathetic system: The parasympathetic system controls glandular secretion and activates the gastrointestinal tract as well as sexual behavior, which are summarized as “rest and digest” functions. Central MNs of the parasympathetic system are located in the brainstem and contribute to the formation of the 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 in the proximity of the peripheral targets such as the heart, bladder, lungs, kidneys, and pancreas.

Somatic MNs are located in the Rexed lamina IX in the brainstem and the spinal cord and innervate skeletal muscles responsible for movements. Somatic MNs can be divided into 3 groups - alpha, beta, and gamma according to the muscle fiber type they innervate.

A motor unit defines 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) both UMN and LMN are affected; 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: They are inhibitory interneurons. They secrete 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 muscle spasms and rigidity.

Quick tip: The pyramidal tract arises from layer-V pyramidal cells in the cerebral cortex. Pyramidal tracts include corticospinal and corticobulbar tracts. The corticobulbar tract synapses on the cranial nerves controlling the muscles of the face, head, and neck.

Box for Batson’s venous plexus: it is a plexus of veins with no valves that connect deep veins of the bladder, prostate and rectum to the internal vertebral venous plexus. Cancers and infections can spread from these organs to the vertebral column directly and from there to the brain and meninges. In the thoracic region, the plexus can cause breast and lung cancers to spread to the spine.

Nociceptors: They are specialized peripheral sensory receptors that detect potentially damaging stimuli to the skin such as extremes of temperature, pressure and chemicals.

Prefrontal cortex: It has lateral and medial areas. The lateral area is involved in executive functions like working memory, planning, judgement, 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: It is a non-fluent aphasia characterized by partially losing the ability to produce both spoken and written language. It is also called expressive aphasia. Speech will still contain the important content, but they may omit articles, prepositions, and other words that only have grammatical significance. Thus, they are said to have “telegraphic speech.” Patients know what they want to say but they cannot say it. Differentiate it from dysarthria where inability to speak is due to problems with the muscles of the tongue and mouth/face area.

Conduction aphasia: It is seen in damage to the arcuate fasciculus (it connects Broca’s and Wernicke’s areas) and left parietal region. A type of fluent aphasia with impaired repetition. There are problems with word finding while speech expression and comprehension are fairly intact.

Projection fibres: connect structures over long distances like brain to spinal cord. Association fibres connect structures within the same hemisphere. Commissural fibres connect homologous structures on either side of the cerebral hemispheres e.g. corpus callosum.

Gliomas are a group of brain tumors arising from the glial cells. Oligodendromas and astrocytomas are more common. Glioblastoma multiforme is a very aggressive tumor with a bad 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 internal carotid artery or its branches posterior communicating and ophthalmic arteries while 22% occur in the middle cerebral artery. Rupture of intracranial aneurysm can lead to intracranial or acute subarachnoid hemorrhage.

Emissary veins: they connect the extracranial veins to the intracranial venous sinuses. They drain the scalp and skull bones. They are valveless. There is a risk of extracranial infections being transmitted intracranially to the dural venous sinuses via the emissary veins.

Bridging veins: These veins carry venous blood from the cerebral cortex to the superior sagittal sinus. They travel across the subdural space and are attached at right angles making them prone to trauma. Rupture of these veins in head trauma can cause a subdural hematoma.

Pulsating exophthalmos: It is a unique feature seen in cases of rupture of cavernous aneurysm (which is an aneurysm of the cavernous part of the internal carotid artery) with formation of a carotid-cavernous fistula.

Nucleus accumbens: It is often included as a basal nucleus, 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 has been strongly associated with motivation and action, having 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 like depression, bipolar disorder, anxiety etc. 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 may lead to the pituitary gland not being visualized on brain imaging, hence the name. It is usually asymptomatic or may present with chronic headaches and visual field defects. Causes are idiopathic,benign intracranial hypertension (pseudotumor cerebri), obesity, head trauma, tumors etc.