Neuroanatomy Flashcards

Spinal Cord Anatomy

The spinal cord is typically 40-45 cm (16-18 inches) long.
It contains up to 100 million neurons.
The maximum diameter of the spinal cord is 1.3 cm.
At vertebral levels C5 & C6, the cord occupies 75% of the vertebral canal.
The superior boundary of the spinal cord is the Foramen Magnum or the superior-most ventral rootlet of the 1st cervical spinal nerve.
As an adult, the inferior boundary of the spinal cord is the disc between vertebral levels L1 & L2 (Conus Medullaris).
There are 31 spinal nerve pairs.
Bell-Magendie Law: Dorsal roots contain sensory (afferent) fibers, and ventral rootlets contain motor (efferent) fibers.
Mixed fibers are found in the rami.
Spinal nerve relationship to vertebrae:
- C1-C7 nerve is above the corresponding vertebrae.
- C8 nerve is below the C7 vertebrae.
- T1 and below nerves are below the corresponding vertebrae.
- The spinal nerve goes through the intervertebral foramen.
- C1 nerve: Between the occiput and C1.
- C2 nerve: Between C1 and C2.
- C3 nerve: Between C2 and C3.
- C4 nerve: Between C3 and C4.
- C5 nerve: Between C4 and C5.
- C6 nerve: Between C5 and C6.
- C7 nerve: Between C6 and C7.
- C8 nerve: Between C7 and T1.
- T1 nerve: Between T1 and T2.
- L1 nerve: Between L1 and L2.
- S4 nerve: Between S4 and S5.

Spinal Cord Development & Structure

During the first 3 months of embryonic development, the cord and vertebral column are approximately the same length.
At birth, the Co1 cord level is typically at the L1-L3 vertebra.
By adulthood, the Co1 cord level is found at the L1-L2 vertebral level in about 90% of people.
The tapering end of the cord is called the Conus Medullaris.
The extended distance of the cord through the lumbar cistern, resembling a horse’s tail, is called the Cauda Equina.
The central canal is continuous with the Medulla Oblongata and the 4th Ventricle.

Spinal Cord Horns & Matter

The dorsal horn synapses sensory information.
Motor neurons are found in the ventral horn.
The lateral horn has an autonomic function and is not located at every cord level.
The lateral horn is absent at vertebral levels C1-C8 & L4-S1.
The gross structure of gray matter is also known as Substantia Grisea.
The lateral horn carries autonomic sympathetic information at levels T1-L2.
The lateral horn carries autonomic parasympathetic information at levels S2-S4.
The gross structure of white matter is also known as Substantia Alba.
A longitudinal bundle of white matter fibers is called a Funiculus.
Composition of gray matter:
- Dense concentration of neuron cell bodies
- Thick dendritic mats
- Support glial cells
- Dense capillary beds

Spinal Cord Laminae

Lamina I: Forms a thin cap of gray over the posterior horn; home of the marginal nucleus.
Lamina II: Contains an important pain reception center called Substantia Gelatinosa; the dorsolateral tract synapses here.
Lamina III, IV: Contains the Nucleus Proprius; senses touch & pressure; the anterior spinothalamic tract synapses here.
Lamina V: Contains the reticular formation of the cord - cervical area only, but the lamina itself is found at all cord levels.
Lamina VI: Missing at some cord levels & is the most anterior aspect of the dorsal horn.
Lamina VII: Located in the intermediate gray area; descending tract fibers synapse here; home of the Nucleus Dorsalis (Clarke's Nucleus) & Intermediolateral Nucleus; surrounds the central canal.
Lamina VIII: Located on the medial aspect of the anterior horn; many descending tracts synapse here
Lamina IX: Most posterior aspect of the anterior horn; Motor innervation of skeletal musculature somatic motor horn; Contains Class A alpha motor neurons & looks like a series of disconnected neurons.
Lamina X: Surrounds the central canal & contains the anterior & posterior gray commissure.

Spinal Cord Nuclei & Matter Composition

Nucleus Dorsalis (Clarke’s Nucleus): Extends like a column from cord levels C8 - L3; famous ascending fasciculus called the posterior spinocerebellar tract.
Intermediolateral Nucleus: Column-like nucleus forms the bulk of the lateral horn cell bodies from T1 - T12; often called the sacral parasympathetic nucleus.
Between T1 - L2, lamina VII carries preganglionic sympathetic fibers.
Between S2,3,4, Lamina VII carries preganglionic parasympathetic neurons “Sacral Parasympathetic Nucleus”.
Lamina VIII is located on the medial aspect of the anterior horn.
Lamina IX: Innervates most of the body’s skeletal muscle & gives the ventral horn its name “Somatic Motor Horn”; carries class S alpha motor neurons & looks like a series of disconnected neurons.
Lamina X surrounds the central canal & contains the anterior & posterior gray commissure.
White matter has a dense concentration of neuron fibers (axons).
In white matter, neuroglial support & covering cells are mostly formed by Interfascicular Oligodendrocytes.
White matter has blood vessels but less dense than gray matter.
Bundles of functionally related axons within a funiculus are called Fasciculi (tracts).
Tracts associate with each other through surface proteins during development (NCAMs - Nerve Cell Adhesion Molecules).
Three discovery methods of tracts include:
- Animal Experiments - DRG destruction
- Modern Aids - HRP, tritiated amino acids, PET scanners, Human Pathologies

Spinal Cord Tracts

Tracts Gracilis & Cuneatus are both located in the posterior column.
Gracilis: Slender, present at all cord levels, & synapses in the nucleus gracilis of the MO.
Cuneatus: Wedge-shaped, T5/6 up, & synapses in the nucleus cuneatus of the MO.
Gracilis & Cuneatus carry 2-point touch, vibratory sensation, and kinesthetic sensation information.
The Romberg Test is considered the most misinterpreted neurological test; it will test the posterior columns but is thought to test the cerebellum.
Arcuate fibers extend from the dorsal column to the contralateral medial lemniscus.
The lateral & anterior spinothalamic tracts are formed by axons that come from cell bodies of the gray horns.
Lateral Spinothalamic: Located in the lateral funiculus, present at all cord levels, senses pain/temperature & crosses quickly.
Anterior Spinothalamic: Located in the anterior funiculus, present at all cord levels, senses light touch/pressure & crosses gradually.
The lateral & anterior spinothalamic tracts synapse in the VPL (ventral posterior lateral) of the thalamus.
Pain information comes into the cord & synapses in the posterior horn; then travels up the lateral funiculus.
Lesions in the spinothalamic tracts can be clinically important because they can lead to analgesia (loss of pain) & thermoanaesthesia (loss of temperature).
The combination name of the lateral & anterior spinothalamic tracts is the anterolateral system.
Anterior Spinocerebellar: fibers cross in the cord & cross back again as they enter the cerebellum
The anterior spinocerebellar originates in the lumbosacral cord’s gray matter.
The anterior spinocerebellar tract terminates in the cerebellum via the superior cerebellar peduncle.
The anterior spinocerebellar tract sends information about gross movement of the lower body & general activity (Anticipation of movement).
Posterior Spinocerebellar: Does not cross, originated from cell bodies in nucleus Dorsalis & not found below L3.
The posterior spinocerebellar tract terminates in the cerebellum via the inferior cerebellar peduncle.
The posterior spinocerebellar tract sends/reads proprioceptive input mainly dealing with fine movement.
The spinocortico tract terminates in the cerebral cortex.
The spinoolivary tract terminates in the olives.
The spinovestibular tract terminates in the vestibular nuclei.
The spinopontine tract terminates in the pons.
The spinotectal tract terminates in the tectum.
Anterior Corticospinal tract: Located in the anterior funiculus, cross in the spinal cord & terminate in the mid thoracics.
The anterior corticospinal tract only has about 5-15% of the total corticospinal fibers.
The function of the anterior corticospinal tract is unclear - seems to influence axial musculature of the neck & shoulders.
Lateral Corticospinal Tract: Located in the lateral funiculus, fibers cross in the pyramids of MO & run the entire length of the cord.
The lateral corticospinal tract carries the mass of corticospinal fibers, about 85-90%.
The function of the lateral corticospinal tract is initiating & accomplishing precise voluntary skilled movement (especially in the distal extremities).
Corticospinal fibers are not always using a-alpha motor neurons.
Lower Motor Neurons (LMN): Originate in the CNS & extend fibers into the PNS to innervate somatic musculature.
Upper Motor Neurons (UMN): Are from higher brain centers (cortex or brain stem) & influence lower motor neurons; found entirely in the CNS.
UMN Lesion can produce a reduction or absence of voluntary movement (ex. Increased muscle tone, hyperreflexia, clonus, Babinski sign).
LMN Lesion can produce a reduction of absence of voluntary movement (ex. Hyporeflexia/areflexia, decreased muscle tone, muscle fibrillations).
Pyramidal Neuron: UMN involved with the initiation of skilled voluntary movement, in the cord they are considered corticospinal.
Extrapyramidal: UMN originates in the brainstem; influence posture, muscle tone, reflexes & allows voluntary movements to be smooth & effective.
Tectospinal Tract: Originates in the superior colliculus of midbrain’s tectum, fibers cross as they descend & it is responsible for postural reflex enhancement, sight, auditory.
The muscles involved with the Tectospinal tract & moving the head via the XI CN are the Trapezius & sternocleidomastoid.
Rubrospinal Tract: Originates in the nucleus rubber of midbrain’s tegmentum, fibers cross in the midbrain as they descend, influencing hand & foot flexor musculature, strongly influenced by cerebellum & cerebral cortex.
Vestibulospinal Tract: Originates in the lateral part of the vestibular nucleus located in the medulla oblongata, fibers do NOT cross, runs entire cord length, functions in muscle tone, postural adjustment, Ipsilateral extensor (inhibits flexor).
Vestibulospinal can be important because it maintains proper orientation in falling & enhances spinal reflex capabilities.
Medial Reticulospinal Tract: Originates in the pons tegmentum, mostly uncrossed, terminates at all cord levels in ventral horn, influences heart, blood pressure & respiratory.
Lateral Reticulospinal Tract: Originates in the medulla oblongata, mostly uncrossed, terminated at all cord levels in ventral horn, influences heart, blood pressure, respiratory rates.
The medial & lateral reticulospinal tracts serve as an alternative pathway if corticospinal tract fibers are destroyed.
Medial/Lateral reticulospinal tracts are said to have Autonomic function.
Spinospinal Fibers: Cell bodies that originate in the cord & axons terminate in the cord.
Fasciculus Proprius: System that contains Spinospinal fibers that extend short distances up or down, may or may not cross, located next to gray horns, very important for coordination for spinal reflexes (may be first fibers that are myelinated in the fetus & responsible for earliest spontaneous movement at 10w).
Dorsolateral Tract of Lissauer: Located between RL I & the posterior lateral sulcus of the cord, small diameter collateral fibers of posterior root axons (mainly in RL II).

Clinical Aspects & Rhombencephalon

CNS lesions can be caused by Stroke, Trauma, Tumors, Infections.
Total Transection of Spinal Cord: Involves C5/C6-T12/L1, severe injury, neural inactivity, referred to as “spinal shock”.
Brown-Sequard Syndrome: Involves a total loss of either the right or left side of the spinal cord (hemisection).
Tabes Dorsalis: Involves the bacterial tertiary syphilis resulting in wasting away of the dorsal funiculus, typically the fibers of fasciculus gracilis.
Multiple Sclerosis: Results in the destruction of the CNS (not PNS) myelin, more common in females (2:1) between ages 20-40, symptoms can remit & relapse.
Amyotrophic Lateral Sclerosis (ALS): Results in the destruction of UMN & LMN in the lateral corticospinal tract, males are twice as affected (45yo), characterized by localized weakness/clumsiness & condition will progress until diaphragm is affected resulting in death (lack of breathing).
Injury through myelin alteration can be caused by Toxins.
Pernicious Anemia: Involves a deficiency of vitamin B12 due to intrinsic factors, resulting in CNS/PNS change.
Syringomyelia: A rare condition in which the cord’s central canal or adjacent areas begin to erode/hollow out, characterized by a loss of pain.
Poliomyelitis: Involves a virus altering the cytoplasm of the neuron’s cell body until cell death occurs, scar tissue replacement follows.
The 3 parts of the rhombencephalon (hindbrain) are:
- Medulla Oblongata
- Pons
- Cerebellum
The parts of the rhombencephalon (hindbrain) included in the myelencephalon is the Medulla Oblongata.
The parts of the rhombencephalon (hindbrain) included in the metencephalon are the Pons & Cerebellum.
The brain stem consists of the Medulla Oblongata & Pons.

Medulla Oblongata

The 3 functions of the M.O. are:
- Passive fiber conduction
- Relay nuclei
- Cranial Nerve Nuclei (V, VII, VIII, IX, X, XI, XII)
The M.O. is the control center for functions such as Respiratory, Cardiac function, and Swallowing.
The M.O. is located at the most inferior part of the rhombencephalon & brain stem.
The exact inferior border of the M.O. is at the superior most C1 anterior rootlet.
The M.O. is 2.5-3 cm long.
CN IX, X, XI apparently originate near the posterolateral sulcus.
CN VI apparently originates in the inferior pontine sulcus.
CN VII, VIII apparently originate in the pontocerebellar angle.
CN XII apparently originates in the ventral lateral sulcus.
The Area Postrema detects toxins in the blood & triggers vomiting.
Inferior Olivary Nucleus relays information into the cerebellum’s central nuclei & cortex.
Reticular Formation is “net like”, continuous, bilateral & responsible for general auroral (wakefulness & all states of attention).
Pyramids are bilateral & located on the ventromedial aspect along the entire length of the M.O.
Medial Lemniscus of the Brain terminates in the thalamus (VPL), they will enlarge half-way up the M.O. & receive a million internal arcuate fibers.
Accessory Oculomotor Nucleus (Edinger-Westphal) has parasympathetic control of ciliary & pupillary constrictor muscles of the eye.
Oculomotor Nuclei have somatic motor muscle control for 4/6 extraocular eye muscles & the upper eyelid.
Trochlear Nucleus has somatic motor muscle control for the superior oblique extraocular eye muscles.
Trigeminal Mesencephalic Nucleus involves proprioception from muscles of mystification & periodontal ligaments of the teeth.
Trigeminal Motor Nucleus involves somatic motor to muscles of mystification (chewing), tensor villi palatini, tensor tympani, anterior belly of disaster & mylohyoid.
Abducens Nucleus has somatic motor muscle control for the lateral rectus extraocular eye muscle.
Facial Nucleus has most superficial motor muscles of the scalp & face, includes posterior belly of digastric & platysma.
Superior Salivary Nucleus has parasympathetic control of lacrimal glands, submandibular & sublingual salivary glands.
Inferior Salivary Nucleus has parasympathetic control of the parotid salivary gland.
Vestibular & Cochlear Nuclei are involved with equilibrium & hearing, complex & subdivided into parts.
Hypoglossal Nucleus has somatic motor muscle control for 16/18 named tongue muscles.
Posterior (Dorsal) Nucleus of Vagus is sensory & motor for organs like voice, heart, lungs, intestines.
Nucleus Ambiguus involves nerves IX, X, XI & all share this nucleus in delivery of visceral efferent (motor) overs to the pharynx musculature.
Nucleus Solitarius (solitary nucleus) has sensory reception via VII, IX, X & primarily deals with taste from the tongue, palate & pharynx.
Main & Spinal Nucleus of the Trigeminal Nerve is the main sensory reception nucleus for the face, deals with pain, thermal, proprioception.
Spinal Nucleus of the Accessory Nerve has somatic motor for movement of the important neck/head muscles.
Lateral Medullary Stroke Syndrome aka Wallenberg’s Stroke Syndrome is a clinical aspect that results in a stroke that can come through the vertebrobasilar arteries or their branches, symptoms include loss of pain, temp sensation on side of the face or opposite side of the body.

Pons

The functions of the Pons are:
- Fiber conduction
- Nuclei of Origin for: V, VI, VII, VIII
- Relay Center for fibers going to the cerebellum
- Control center for cardiovascular & respiration
The Rhomboid Fossa is located at the dorsal part of the pons & forms part of the floor of the 4th ventricle.
The lateral aspect of the pons has a huge Middle cerebellar peduncle.
The lateral aspect of the pons is the apparent origin for the Largest CN, Trigeminal (V).
The two subdivisions of the internal part of the pons are the Treatmentum (dorsal) & Basilar (ventral).
The basilar (ventral) division contains:
- Corticospinal Tract Fibers
- Medial Lemniscus
- Pontine Nuclei
Pontine Nuclei create a ventral bulge of the pons (because they’re so numerous).
Pontine nuclei relay input from the cerebral cortex back to the cerebellum through the Middle Cerebellar Peduncle.
The tegmentum (dorsal) division of the pons contains:
- Lateral Lemniscus
- CN V, VI, VII, VIII
- Tracts (spinal trigeminal, spinothalamic etc)
- Pontine Reticular Formation
The Lateral Lemniscus is the most ventral part of the tegmentum & is an auditory pathway.

Cerebellum

The functions of the cerebellum are:
- Momentary status of muscle contraction
- Joint tension
- Visual & auditory input on equilibrium
Integration from the cerebellum allows it to aid & influence muscle tone, posture & voluntary movement.
Cerebellum input is entirely subconscious & each cerebellar hemisphere influences the ipsilateral side of the body.
One of the cerebellums greatest attributes is compensation.
The distributions of peduncle fibers are:
- Inferior - Afferent dominate (efferent is present)
- Middle - Afferent ONLY (largest)
- Superior - Efferent dominate (Afferent present)
Medulloblastomas are 20% of all childhood brain tumors & arise in the superior medullary velum, very invasive, mostly in males (2:1) ages 4-8.
The cerebellum has an outer gray matter cortex.
The cerebellum has a mass of internal white matter & small islands of gray matter called Deep/Central Cerebellar Nuclei.
The internal cerebellar White matter aka “Corpus Medullare” is naturally continuous with the 6 cerebellar peduncles.
Internal cerebellar white matter is made of afferent, efferent, commissural, and association fibers.
The extension of the white matter toward the cortex appear as a branching tree called Arbor Vitae.
Isolated in the white matter are 4 pairs of nuclei called the deep central cerebellar nuclei.
The external features of the cerebellar cortical gray matter are fissures (deep), sulci (shallow), and folia (raised).
The gray cortex is fairly uniform as to depth & cytoarchitecture.
The most common neuron cell types in the cerebellar cortical gray matter are Purkinje, Golgi II, Stelae, Basket, Granular.
The cells of the cerebellar cortical gray matter are arranged into three laminae: outer (molecular), middle (purkinje), and inner (granular).
Two types of outside axons bring input to cerebellar cortical laminae: Mossy Fibers and Climbing Fibers.
Purkinje fibers take messages away from the cerebellar cortex.
Characteristics of Purkinje cell layer:
- Middle layer
- 30 million cell bodies
- Large (50 microns) & Flask shaped
- Dendritic branches can be smooth or spined
- Myelinated purkinje axons are the only efferent (outgoing) fibers from the cerebellar cortex, they terminate in the deep central cerebellar nuclei & release inhibitory neurotransmitter GABA.
Characteristics of Granular layer:
- Innermost layer
- Granular neurons
- 3-7 million stimulated by mossy fibers
- Spined branches
- The granular neurons release the neurotransmitter Glutamate which is the brain’s most abundant neurochemical.
Characteristics of mossy fibers:
- Very numerous (more than climbing)
- Originate from spinocerebllar & corticopontocerebellar
Characteristics of climbing fibers:
- Less numerous (than mossy)
- Extremely excitatory
- Reach Purkinje cells smooth dendritic branches directly
- Originate from inferior Oliver’s nucleus (aspartate is involved)
The names of the 4 central nuclei are:
- Dentate - largest, most lateral
- Emboliform } collectively called interposed nuclei
- Globose }
- Fastigial - most primitive, medial
Purkinje axons from the vestibulocerebellum go to the vestibular nucleus.
Purkinje axons from the spinocerebellum go to the Interposed & Fastigial Nucleus.
Purkinje axons from the cerebrocerebellum go to the Dentate Nucleus.
The Inferior Olive sends excitatory input to all 4 nuclei.
Fastigial nucleus fibers terminate in the Vestibular nucleus and Reticular formation CN III, IV, VI.
Interposed nucleus axons extend to the Red Nucleus & Reticular Formations via the superior cerebellar peduncles.
Dentate axons reach the Thalamus with collateral branches being sent to the Red Nucleus.
Equilibrium Disturbances (flocculonodular syndrome): A clinical aspect that is common, from lesions of the archicerebellar lobe - patient must constantly fight against falling backwards.
Ataxia: A clinical aspect involving lesions of the neocerebellum, abnormal performance, stagger to side, inability to rapidly supinate/probate hands - a lack of coordination.
Intention Tremor: A clinical aspect involving exaggerated tremors, neocerebellar lobe, happens when trying to make precise voluntary movement with digits.
Dysmetria: A clinical aspect when measured movements are difficult, over or undershoot target.
Nystagmus: A clinical aspect involving repetitive jerking movements of the eyeballs.