Phasic blood flow

Phasic blood flow

Comparison of phasic blood flow velocity characteristics of arterial and venous coronary artery bypass conduits.


Coronary artery bypass conduits derived from internal mammary arteries show relative resistance to atherosclerosis and significantly improved long-term patency compared with saphenous vein grafts. Atherothrombotic occlusion of venous conduits has previously been correlated with lower flow rates measured intraoperatively. To quantitate coronary bypass conduit flow velocity, we examined the phasic blood flow velocity patterns by intravascular Doppler spectral analysis in patients during cardiac catheterization to test the hypothesis that resting systolic and diastolic phasic blood flow velocity patterns differ significantly between arterial and venous bypass conduits.


Spectral phasic blood flow velocity was measured using an intravascular Doppler-tipped angioplasty guidewire in the proximal, mid, and distal segments of 18 internal mammary artery conduits and 11 saphenous vein grafts in 27 patients at a mean of 4 years (range, 1 to 11) postoperatively. In situ internal mammary artery conduits demonstrated a gradual longitudinal transition in the phasic flow pattern from predominantly systolic velocity proximally (diastolic/systolic peak velocity ratio, 0.6 +/- 0.2) to predominantly diastolic velocity distally (diastolic/systolic peak velocity ratio, 1.4 +/- 0.3; P < .001).

Phasic blood flow

Saphenous vein graft flow velocity pattern, however, showed a consistently diastolic predominance, both proximally and distally (diastolic/systolic peak ratios, 1.4 +/- 0.6 and 1.5 +/- 0.7, respectively; P = NS). Mean flow velocities, total velocity integral, and calculated maximal shear rates were significantly higher in all segments of internal mammary arteries compared with values in saphenous vein grafts.


Patterns of resting phasic blood flow, as well as mean velocity and total velocity integral, differ significantly between internal mammary artery and saphenous vein bypass conduits. These differences may have implications regarding blood-vessel wall interactions, the development of degenerative graft disease, and long-term conduit patency.

Cerebral Circulation

Cerebral Circulation
What is cerebral circulation?

Cerebral circulation is the blood flow in your brain. It’s important for healthy brain function. Circulating blood supplies your brain with the oxygen and nutrients it needs to function properly.

Blood delivers oxygen and glucose to your brain. Although your brain is a small part of your body’s total weight, it requires a lot of energy to function. According to the Davis Lab at the University of Arizona, your brain needs about 15 percent of your heart’s cardiac output to get the oxygen and glucose it needs. In other words, it needs a lot of blood circulating through it to stay healthy.

When this circulation is impaired, your brain can become damaged. Many conditions and disabilities related to neurological function can occur as a result.

How does blood flow through your brain?

The four main arteries that supply blood to your brain are the left and right internal carotid arteries and the left and right vertebral arteries. These arteries connect and form a circle at the base of your brain. This is called the circle of Willis. Smaller blood vessels also branch off from these arteries to nourish different sections of your brain.

Your brain also has venous sinuses. These types of veins carry blood containing carbon dioxide and other waste products away from your cranium. Some of them connect with the veins of your scalp and face.

Nutrient and waste exchange occurs across the blood-brain barrier. This barrier helps protect your brain.

What happens when your cerebral circulation is impaired?

When your cerebral circulation is impaired, less oxygen and glucose reach your brain. This can cause brain damage and neurological problems. Some conditions related to impaired cerebral circulation include:

  • stroke
  • cerebral hemorrhage
  • cerebral hypoxia
  • cerebral edema


When a blood clot blocks the flow of blood in your cranial artery, a stroke can occur. As a result, the brain tissue in that area can die. When that tissue dies, it can impair the functions that part of your brain normally controls. For example, it can affect your speech, movement, and memory.

The degree of impairment you experience after a stroke depends on how much damage has occurred, as well as how quickly you get treatment. Some people fully recover from a stroke. But many people have lasting disabilities or even die from strokes. According to the American Stroke Association, stroke is the fifth leading cause of death among Americans.

Cerebral Circulation

Cerebral hypoxia

Cerebral hypoxia occurs when part of your brain doesn’t get enough oxygen. This happens when you don’t have enough oxygen in your blood even if there’s enough blood flow. Causes of cerebral hypoxia include:

  • drowning
  • choking
  • suffocation
  • high altitudes
  • pulmonary diseases
  • anemia

If you experience it, it’s likely you’ll appear confused or lethargic. If you address the underlying cause quickly enough, your brain tissue probably won’t become damaged. But if you don’t address it quickly enough, coma and death can occur.

Cerebral hemorrhage

A cerebral hemorrhage is internal bleeding in your cranial cavity. It can occur when your arterial walls are weakened and burst. This forces blood into your cranial cavity. In turn, this can put pressure on your cranial cavity and cause you to lose consciousness. Other possible causes of cerebral hemorrhage include abnormally formed blood vessels, bleeding disorders, and head injuries.

A cerebral hemorrhage can potentially cause brain damage and death. It’s a medical emergency.

Cerebral edema

Edema is a type of swelling that occurs due to the collection of watery fluids. Cerebral edema is swelling that occurs due to an increase of water in your cranial cavity. Disturbances in the blood flow in your brain can also cause it.

Cerebral edema can put pressure on your brain. This can eventually crush or damage your brain if it’s not relieved in time.

What are the risk factors for poor cerebral circulation?

Anyone at any age can have problems with cerebral circulation. You’re at an increased risk of having these problems if you:

  • have high blood pressure
  • have high cholesterol
  • have heart disease
  • have atherosclerosis
  • have a family history of heart disease
  • have diabetes
  • are overweight
  • smoke
  • drink alcohol
The takeaway

You need good cerebral circulation to supply your brain with oxygen- and nutrient-rich blood. Cerebral circulation also helps remove carbon dioxide and other waste products from your brain. If your cerebral circulation becomes impaired, it can lead to serious health issues, including:

  • a stroke
  • cerebral hypoxia
  • cerebral hemorrhage
  • cerebral edema
  • brain damage
  • disability

It can even lead to death in some cases.

Some causes of impaired cerebral circulation may be hard to prevent. But you can lower your risk of stroke and some other conditions by practicing healthy habits and following these tips:

  • Maintain a healthy weight.
  • Eat a well-balanced diet.
  • Exercise regularly.
  • Avoid smoking.
  • Limit alcohol.

Objectives of ECG

Objectives of ECG

Electrocardiogram (ECG) deals with the study of electrical activity of the heart. The instrument used to record the activity is called electrocardiograph. It was developed by a Dutch physiologist Einthoven in the year 1903. The recording was known as electrokardiogram (EKG). Both ECG and EKG are valid terms that can be used for the recording. The study of ECG, tells us the heart rate, rhythm, conduction in the heart and presence of any abnormalities in them known as arrhythmias. It is also useful to know the presence of infarction in the myocardium and the effect of drugs, electrolytes on the heart.
The ECG waves represent the sum total of tiny action potentials developed from the cardiac muscle. The electrical activity is spread to the surface of the body through the body fluid, which acts as a volume conductor. These electrical potentials from the surface of the body can be recorded by placing surface electrodes or leads, on certain conventional positions in the body. They are amplified and connected to a string galvanometer, which records them on a moving strip of paper or displayed on the screen in cathode ray oscilloscope.

There are 12 leads used in the recording of ECG. Einthoven recorded the electrical activity of the heart by using bipolar limb leads. He considered right arm, left arm and left leg as the regions for surface recording and showed that, when these points are joined, an equilateral triangle could be obtained. In the center of this triangle, the heart is situated. The equilateral triangle obtained by this method is called Einthoven’s triangle. The bipolar limb leads record the potential difference between two limbs. Accordingly, there are three types of leads present.
They are:

  • Lead I (between right arm and left arm)
  • Lead II (between right arm and left leg)
  • Lead III (between left arm and left leg).

Objectives of ECG
In the bipolar limb leads, if we know the potentials in any two leads, the potential in the third lead can be determined. According to Einthoven’s law, the sum of the potentials in lead II.
Lead I + Lead III = Lead II

In Unipolar augmented limb leads method, there is an indifferent electrode (V), which is obtained by connecting the three limb leads and passing through 5000 ohms resistance to get 0 potential (Wilson’s terminal). Recording between one limb and the other two limbs increases the size of the potential by 50%. The two limbs are connected through electrical resistance to the negative terminal and the other limb is connected to the positive terminal. There are three types of leads such as aVR, aVL and aVF present in this category.
There is an indifferent electrode (V) and exploring electrode is placed on the anterior chest wall in six positions. They are given numerical numbers from 1 to 6. The leads are V1, V2, V3, V4, V5 and V6. In ECG recording, positive deflection is recorded, when the wave of excitation moves towards the positive or exploring electrode. If the depolarization wave moves away from the exploring electrode, a negative deflection is recorded. In aVR lead, the exploring electrode is facing the cavity of the ventricles and the wave of excitation moves away from the recording electrode and hence in this lead, all the deflections of ECG are negative.

ECG waves

ECG waves

What is an ECG?

ECG is short for electrocardiogram.

It is used to record the electrical activity of the heart from different angles to identify and locate pathology.

Electrodes are placed on different parts of a patient’s limbs and chest to record the electrical activity.

Parts of the ECG explained


P-waves represent atrial depolarisation.

In sinus rhythm, there should be a P-wave preceding each QRS complex.


PR interval

The PR-interval is from the start of the P-wave to the start of the Q wave.

It represents the time taken for electrical activity to move between the atria and ventricles.

QRS complex

The QRS-complex represents depolarisation of the ventricles.

It is seen as three closely related waves on the ECG  (Q,R and S wave).

ST segment

The ST-segment starts at the end of the S-wave and finishes at the start of the T-wave.

The ST segment is an isoelectric line that represents the time between depolarization and repolarization of the ventricles (i.e. contraction).


The T-wave represents ventricular repolarisation.

It is seen as a small wave after the QRS complex.



The RR-interval starts at the peak of one R wave and ends at the peak of the next R wave.

It represents the time between two QRS complexes.


The QT-interval starts at the beginning of the QRS complex and finishes at the end of the T-wave.

It represents the time taken for the ventricles to depolarise and then repolarise.


The 12 lead ECG: how it all works

The first thing to clear up is the definition of the word “lead” in an ECG context.

Lead refers to an imaginary line between two ECG electrodes.

The electrical activity of this lead is measured and recorded as part of the ECG.

A 12-lead ECG records 12 of these “leads” producing 12 separate graphs on the ECG paper.

However you only actually attach 10 physical electrodes to the patient.


The electrodes are wires that you attach to the patient to record the ECG.

These electrodes allow leads to be calculated.

For example Lead I is calculated using the electrodes on the right and left arm.

Below are the electrodes used in a 12 lead ECG.

Chest electrodes positions 

V1 – 4th intercostal space right sternal edge

V2 – 4th intercostal spaceleft sternal edge

V3 – midway between V2 and V4

V4 – 5th intercostal space midclavicular line

V5 – left anterior axillary line same horizontal level as V4

V6 – left mid-axillary line same horizontal level as V4 & V5

Limb electrodes

LA – left arm

RA right arm

LL – left leg

RL – right leg – neutral – not used in measurements


Lead refers to an imaginary line between two ECG electrodes.

There are 12 leads measured in a 12-lead ECG.

Chest leads

V1 – Septal view of heart

V2 – Septal view of heart

V3 – Anterior view of heart

V4 – Anterior view of heart

V5 – Lateral view of heart

V6 – Lateral view of heart

Chest electrode positions

Other leads

Lead I Lateral view (RA-LA)

Lead II – Inferior view (RA-LL)

Lead III – Inferior view (LA-LL)

aVR – Lateral view (LA+LL – RA)

aVL – Lateral view (RA+LL – LA)

aVF – Inferior view (RA+LA – LL )

This diagram is a useful way of understanding the relationships between the leads


Lead viewpoints

Viewpoints of the heart

It’s important to understand which leads represent which part of the heart.

This allows you to localise pathology to a particular heart region.

For example if there is ST elevation in leads V3 and V4 it suggests an anterior myocardial infarction (MI).

You can then combine this with some anatomical knowledge of the heart’s blood supply, to allow you to work out which artery is likely to be affected (e.g left anterior descending artery).

How to read ECG paper

The paper which ECGs are recorded upon is standardised across all hospitals (usually):

  • Each small square represents 0.04 seconds
  • Each large square on the paper represents 0.2 seconds
  • 5 large squares therefore = 1 second
  • 300 large squares = 1 minute

The shape of the ECG waveform

Each individual leads ECG recording is slightly different in shape.

This is due to each lead recording the electrical activity from different directions.

When the electrical activity of the heart travels towards a lead you get a positive deflection.

When the electrical activity travels away from a lead you get a negative deflection.

Electrical activity in the heart flows in many directions at once.

The wave seen on the ECG paper represents the average direction.

The height of the deflection also represents the amount of electricity flowing in that direction.

The lead with the most positive deflection is closest to the direction the heart’s electricity is flowing.

If the R-wave is greater than the S-wave it suggests depolarisation is moving towards that lead.

If the S-wave is greater than the R-waves it  suggests depolarisation is moving away from that lead.

If the R and S-waves are of equal size it means depolarisation is travelling at exactly 90° to that lead.

Cardiac axis explained

The electrical activity of the heart starts at the sinoatrial node then spreads to the atrioventricular (AV) node.

It then spreads down the bundle of His and then Purkinje fibres to cause ventricular contraction.

Whenever the direction of electrical activity is towards a lead you get a positive deflection in that lead.

Whenever the direction of electrical activity is away from a lead you get a negative deflection in that lead.

The cardiac axis gives us an idea of the overall direction of electrical activity when the ventricles are contracting.

Normal cardiac axis

In healthy individuals you would expect the axis to lie between -30° and +90º.

The overall direction of electrical activity is towards leads I,II and III (the yellow arrow below).

As a result you see a positive deflection in all these leads, with lead II showing the most positive deflection as it is the most closely aligned to the overall direction of electrical spread. You would expect to see the most negative deflection in aVR. This is due to aVR looking at the heart in the opposite direction to the overall electrical activity.

Normal Cardiac Axis

Right axis deviation

Right axis deviation (RAD) is usually caused by right ventricular hypertrophy.

In right axis deviation the overall direction of electrical activity is distorted to the right (between +90º and +180º).

Extra heart muscle causes a stronger positive signal to be be picked up by leads looking at the right side of the heart.

This causes the deflection in lead I to become more negative and the deflection in III to be more positive.

RAD is associated with pulmonary conditions as they put strain on the right side of the heart.

It can also be a normal finding in very tall individuals.

Right axis deviation

Left axis deviation

In left axis deviation (LAD) the direction of overall electrical activity becomes distorted to the left (between -30° and -90°).

This causes the deflection in lead I to become more positive and the deflection in III to be more negative.

LAD is usually caused by conduction defects and not by increased mass of the left ventricle.

Left axis deviation

Myelin Sheath

Myelin Sheath
Myelin sheaths are sleeves of fatty tissue that protect your nerve cells. These cells are part of your central nervous system, which carries messages back and forth between your brain and the rest of your body.

If you have multiple sclerosis (MS), a disease that causes your immune system to attack your central nervous system, your myelin sheaths can be damaged. That means your nerves won’t be able to send and receive messages as they should.

Because of this, MS can weaken your muscles, damage your coordination, and, in the worst cases, paralyze you. MS affects about 1 in every 750 people and usually shows up between the ages of 20 and 50. It’s not clear what causes it, and there’s no known cure.

Myelin and Your Nerves

Myelin Sheath

The myelin sheath wraps around the fibers that are the long threadlike part of a nerve cell. The sheath protects these fibers, known as axons, a lot like the insulation around an electrical wire.

When the myelin sheath is healthy, nerve signals are sent and received quickly. But if you have MS, your body’s immune system treats myelin as a threat. It attacks both the myelin and the cells that make it.

When that happens, the nerves inside the sheath can be damaged. That leaves scars on your nerves — known as sclerosis — and that makes it harder for them to carry the messages that tell your body to move.

Myelin Research

A lot of the research into MS is focused on boosting your body’s ability to repair damaged myelin. Scientists are looking into:

  • Ways to prevent the chemical reactions that lead to myelin damage
  • Drugs or experimental treatments that might prevent or fix multiple sclerosis
  • Which antibodies — the disease-fighting proteins your immune system makes when you get sick — attack myelin
  • If stem cells — which can grow into different types of tissues — can be used to reverse the damage caused by MS

Floor or Rhomboid Fossa

Floor or Rhomboid Fossa

Floor of fourth ventricle is formed by dorsal surfaces of pons and upper-half of medulla oblongata. It is called rhomboid fossa because it is rhomboid in outline. The area is outlined superolaterally by superior cerebellar peduncles and inferolaterally by inferior cerebellar peduncles. At the inferior angle, on either side of midline, floor is limited by gracile tubercle and superolateral to it lies cuneate tubercle. Whole area of rhomboid fossa is lined by ependyma, just beneath which lie different areas of gray matter, which are more precisely some cranial nerve nuclei.

Floor of fourth ventricle is divided by a vertically running midline sulcus called median sulcus. Each half of the floor is again subdivided into a medial part called medial eminence and a lateral part called vestibular area by a narrower sulcus limitans. Just above the horizontal line of pontomedullary junction, medial eminence presents a round elevation called facial colliculus. It is so called because, efferent facial nerve fibers from motor nucleus of facial nerve loop around abducens nucleus beneath this bulge. Above the level of facial colliculus, sulcus limitans presents a small depression called superior fovea.

Above the level of superior fovea, sulcus limitans becomes flattened and forms lateral limit of floor of fourth ventricle. This area is bluish gray in color and named locus coeruleus (to be pronouncedceruleus). Beneath this area, the group of neurons, containing melanin pigment, is called substantia ferrugenia. These neurons are rich in noradrenaline (norepinephrine).

Lateral to sulcus limitans, rhomboid fossa presents a wide triangular area known as vestibular area or vestibular triangle. Vestibular nuclei are situated beneath this area.

Just below the level of facial colliculus, fine strands of nerve fibers are found to pass beneath ependyma, in mediolateral direction, from median sulcus across medial eminence towards lateral angle. These are known as stria medullaris. These are efferent fibers from arcunate nucleus present on ventral aspect from pyramid. These fibers initially pass in ventrodorsal direction across whole thickness of medulla oblongata to reach rhomboid fossa, where they bend at right angle and cross the median sulcus to pass horizontally towards lateral angle. Finally the fibers reach opposite half of cerebellum via inferior cerebellar peduncle.

Floor or Rhomboid Fossa

Below the level of stria medullaris, medial eminence presents a triangular area with apex directed downward. This area is known as hypoglossal triangle beneath which lies nucleus of hypoglossal nerve. Lateral to hypoglossal triangle, lower end of sulcus limitans presents a small depression called inferior fovea.
Below inferior fovea, lateral to apical part of hypoglossal triangle, a smaller triangular area is present with the apex directed upward. This is called vagal triangle as beneath this area lies dorsal nucleus of vagus.
Inferolateral to vagal triangle, just above the upper end of central canal of medulla oblongata, anarrow area is called area postrema. This narrow area contains some neurons covered by thickened ependyma.
Area postrema is separated from vagal triangle by a ridge of ependyma called funiculus seperans. Lower angle of floor of fourth ventricle looks like a pen’s nib for which it is known as calamus scriptorius.

Following features are not parts of floor of fourth ventricle, but are closely related to it.

  1. Inferolateral boundary of rhomboid fossa, which is formed by inferior cerebellar peduncle is crossed by tranverse ridge of white matter called tinea.
  2. Tinea from both sides converge inferomedially towards the lower apex of fourth ventricle to form a thin fold called obex. It forms the roof of lower apex of fourth ventricle.

Source: Easy and Interesting Approach to Human Neuroanatomy (Clinically Oriented)

Relations of Brainstem

Relations of Brainstem

Brainstem is the tubular stalk-like part of the brain made up of midbrain, pons and medulla oblongata from above downward. It is so called because it is like stem of a tree. Main mass of the brain, cerebrum with cerebellum rests on the brainstem and through it, is connected to spinal cord below. Long axis of brainstem is oblique, directed downward and backward.

  • Extent: Above, upper end of brainstem (midbrain) is continuous with diencephalon of forebrain.
  • Below: Lower end of brainstem (medulla oblongata) passes out of cranial cavity through foramen magnum to become continuous with spinal cord at the level of upper border of first cervical vertebra.

Relations of Brainstem

  • With cranial cavity: Brainstem lies in posterior cranial fossa of skull and rests on the slope of clivuswhich is formed by posterosuperior surface of basilar parts of sphenoid and occipital bones.
  • With tentorium cerebelli: Tentorium cerebelli is a crescentic horizontal shelf of dura mater of brain lying between posterior part of cerebrum (occipital lobe) and cerebellum. It posseses peripheral convex border. In front of concave anterior border (tentorial notch), brainstem passes downwards. Midbrain is the supratentorial part and, pons with medulla oblongata is the infratentorial part of brainstem lying above and below the tentorium cerebelli respectively.
  • With cerebrum and cerebellum: Cerebrum with thalamus (diencephalon) is above and, cerebellum is behind the brainstem. Ventral compact partof midbrain, composed of bundle of descending fibers connects the brainstem (midbrain) above with cerebrum. It is called cerebral peduncle having right and left identical halves. Cerebellum is connected to midbrain, pons and medulla oblongata of brainstem by three pairs of compact bundle of white matter. These are called superior, middle and inferior cerebellar peduncles respectively.
  • With fourth ventricle of brain: Fourth ventricle is the cavity of hindbrain. It is related anteriorly to pons and medulla oblongata and posteriorly to cerebellum.

Cavity related to brainstem is of different shapes and natures at different level as follows:

  • Midbrain– A narrow linear slit known as aqueduct of Sylvius.
  • Pons and upper part of medulla oblongata: A wide tent shaped space forming cavity of hindbrain called fourth ventricle of brain.
  • Lower part of medulla oblongata: A narrow central canal of medulla continuous below with central canal of spinal cord.


Source: Easy and Interesting Approach to Human Neuroanatomy (Clinically Oriented)

Organization of Internal Structure at Different Level of Brainstem

Organization of Internal Structure at Different Level of Brainstem

Central cavity of brainstem show different characteristics and names at different level. At lower end of medulla it is a narrow canal continuous below with central canal of spinal cord. At the level of pons and upper half of medulla oblongata, it becomes wide to form the cavity of 4th ventricle of brain. At the level of midbrain it is a narrow slit called aqueduct of Sylvius.
Fundamentally, neurons of basal plate are motor and those of alar plate are sensory in function. Throughout the whole length of developing brainstem, initially, many neurons of both basal as well as alar plate will form number of continuous columns of cells which are as follows:
In basal plate (from medial to lateral)

  1. Somatic efferent
  2. Branchial efferent (special visceral efferent)
  3. General visceral efferent.

In alar plate: From medial to lateral in closed part of brainstem, i.e. midbrain and lower end of medulla oblongata and, from lateral to medial in open part, i.e. pons and upper part of medulla oblongata.

  1. Somatic afferent
  2. Branchial afferent (special visceral afferent)
  3. General visceral afferent.

Ultimately, neurons of all these columns will persist in some level and disappear in some level. So they will no longer be present in the form of continuous cell column althrough. These cell groups will form different motor and sensory nuclei of 3rd to 12th (last 10) cranial nerves.

Migration of neurons of alar lamina: Apart from formation of sensory (afferent) nuclei of cranial nerves, neurons of alar plate will migrate from its original position either ventrally or further dorsally to form some other named nuclei in different level of brainstem (described below). This nuclei, as migrated, will intermingle with the components (white matter) of marginal zone.
Derivatives of marginal zone: It is already understood that, marginal zone is composed of processes of nerve cells of mantle zone. These processes will form different groups of bundles of nerve fibers which are basically of following two types:

  1. Vertical: These are either ascending (afferent) or descending (efferent) tracts of nerve fibers connecting spinal cord with various higher centers.
  2. Horizontal: These are fiber bundles connecting various centers of central nervous system with cerebellum in both direction, passing through 3 cerebellar peduncles.

Organization of Internal Structure at Different Level of Brainstem

Migration of cells of alar plate to form various nuclei: As already stated, neurons of alar plate form various sensory neclei of last 10 pairs (3rd–12th) of cranial nerves. Besides, neurons from alar plate migrate either ventrally or further dorsally to form various nuclei in different levels of brainstem as follows.

  1. At the level of lower closed part of medulla oblongata: Cells of alar plate migrate further dorsally on either side of posterior median sulcus to form two nuclei.
    1. Medial: Nucleus gracilis
    2. Lateral: Nucleus cuneatus.
  2. At the level of upper half of medulla oblongata:
    Cells of alar plate migrate ventrally in the peripheral plane of marginal zone in the form of
    following nuclei.

Medulla oblongata at its lower end (at the plane of motor decussation)

Medulla oblongata at its lower end (at the plane of motor decussation)

Structural characteristics

  1. At this level structure of medulla oblongata is almost similar to the structure of spinal cord, with centrally positioned gray matter and peripheral white matter.
  2. Ventral horn of gray matter gets separated from main mass due to decussation of pyramidal tract fibers which pass backwards and laterally to approach lateral white column before passing downwards to the spinal cord.

Structural detail
Gray matter:

  1. Central gray matter is traversed by more dorsally pushed central canal lined by ependyma.
  2. Apex of posterior horn of spinal cord is represented at this level by nucleus of spinal tract of trigeminal nerve. On either side it is directed backwards and laterally with further abduction.
  3. Medial to nucleus of spinal tract of trigeminal nerve, gray matter shows, on either side, two small bulge of gray matter, nucleus gracilis (medial) and nucleus cuneatus (lateral) which receive the fibers of fasciculus gracilis and fasciculus cuneatus respectively, which are the ascending tracts in posterior column of white matter.
  4. Anterior gray horn becomes detached from main mass of gray matter by decussating fibers of corticospinal (pyramidal) tract.

Topographically, cells of anterior horn is a part of gray matter of medulla oblongata. But functionally, these are upwards continuation of cells of anterior horn of upper cervical segments of spinal cord. These cells form following two nuclei.

  1. Supraspinal nucleus of first cervical nerve: It is the upward continuation of anterior horn cells of first cervical segments of spinal cord. Axons of these neurons pass downward and are distributed along the ventral root of first cervical nerve.
  2. Ascending nucleus: It is the upward continuation of spinal nucleus of accessory nerve which is continuous below up to fifth cervical segment of spinal cord. Above it is continuous with nucleus ambiguous.

Medulla oblongata at its lower end (at the plane of motor decussation)

White matter: Pattern of three white columns (funiculi) of spinal cord, namely anterior, lateral and posterior, is grossly maintained.

  1. Anterior column: On either side of ventral median fissure, area of anterior white column mainly presents the bundle of pyramidal tract fibers which shows decussation of fibers at this level. Through anterior column, also traverse tectospinal tract, vestibulospinal tract, anterior spinothalamic tract.
  2. Lateral Column:
    1. Peripherally: Dorsal and ventral spinocerebellar tracts.
    2. Centrally:
    1. Lateral corticospinal tract which is formed at this level after decussation of fibers of pyramid.
    2. At the center of lateral white column, a scattered group of nerve cells intermingled with nerve fibers form brainstem reticular formation.
    3. Lateral spinothalamic tract.
  3. Posterior column: It present upward continuation of fasciculus gracilis and fasciculus cuneatus of posterior white column of spinal cord. As already mentioned earlier, these two tracts will relay in next order of neurons in nucleus gracilis and nucleus cuneatus which are seen to appear at this level of medulla oblongata, ventral to the corresponding tracts.

Source: Easy and Interesting Approach to Human Neuroanatomy (Clinically Oriented) (2014)

Medulla oblongata at its middle

Medulla oblongata at its middle

Structural characteristics

  1. There is no more existance of gray matter area which is homologous to anterior horn.
  2. Gray matter of posterior horn presenting nucleus gracilis, nucleus cuneatus and spinal nucleus of trigmenial nerve gets detached from central gray matter. This detachment is because of the arched fibers arising from nucleus gracilis and nucleus cuneatus which decussate ventrally to form ascending fiber tract which is called medial lemniscus.
  3. Central canal surrounded by central gray matter is pushed more dorsally. Central gray matter presents appearance of cranial nerve nuclei.
  4. It is the plane of medulla oblongata from where upward typical relationship of central gray matter and peripheral white matter of spinal cord is lost. It results intermingling of gray and white matters.

Structural details

  1. On either side of ventral median fissure bulge of pyramid presents sections through descending (efferent) fibers of pyramidal (corticospinal) tract.
  2. Lateral to fibers of pyramid, inferior olivary nucleus starts appearing. It looks like a small irregularwalled sac whose cavity opens backwards and medially. Inferior olivary nucleus is the most prominent part of olivary nuclear complex of human brain. Rudimentary components are dorsal and medial olivary nuclei which together are known as accessory olivary nuclei.
  3. Ascending (afferent) tracts, e.g. dorsal and ventral spinocerebellar tracts, lateral and anterior spinothalamic tracts are found to be in corresponding positions as noticed in previous section of medulla oblongata.
  4. Nucleus gracilis and nucleus cuneatus are seen to be more prominent in this section. These nuclei receive fibers from fasciculus gracilis and fasciculus cuneatus which carry conscious proprioceptive sensation and sense of tactile discrimination from lower and upper halves of body respectively.
  5. Dorsolateral to nucleus cuneatus, a smaller accessory cuneate nucleus is seen. It receives fibers of fasciculus cuneatus which carry same sensations from uppermost part (head-end) of body. Cuneocerebellar tract from this nucleus end in cerebellum as spinocerebellar pathway above T1 spinal cord segment.
  6. Central core of the section presents scattered nerve cells and reticulum (network) of fibers to form brainstem reticular formation.
  7. Posterior gray horn separated from central gray matter is represented by spinal nucleus of trigeminal nerve which is capped on the surface by fibers of sensory root of trigeminal nerve carrying pain and temperature sensation, called spinal tract of trigeminal nerve.

Nucleus gracilis and nucleus cuneatus are the medial and lateral mass of gray matter on either side of posterior median septum. These are also the components of posterior gray horn which are detached from central gray matter.
Reason for separation of spinal nucleus of trigeminal nerve, nucleus gracilis and nucleus cuneatus from central gray matter is due to followingcharacteristic of structure of medulla oblongata at this level.

Medulla oblongata at its middle

Fasciculus gracilis and fasciculus cuneatus are the two ascending tracts of posterior column of spinal cord which carry sense of conscious proprioception and tactile discrimination from lower and upper halves of body respectively. Reaching the medulla oblongata upto this level, fibers of these two tracts relay in corresponding nuclei lying ventrally. Processes of next order of neurons in nucleus gracilis and nucleus cuneatus, before ascending further upwards to relay in thalamus, decussate to cross the midline. During decussation, these fibers presents following three characteristics.

  1. Fibers of both nucleus gracilis and nucleus cuneatus pass forwards arching along the lateral aspect of central gray matter horizontally in a curved fashion that is why they are called internal arcuate fibers.
  2. After decussation, the fibers form a compact bundle just behind the bulge of pyramid, before this compact bundle of fibers ascend upwards to reach thalamus. This bundle is known as medial lemniscus (Plural – Lemnisci).
  3. During formation of medial lemnisci, fibers from nucleus gracilis (carrying sensations from lower half of body) are positioned anterior to the fibers from nucleus cuneatus (carrying sensation from upper half of body).
    Behind medial lemniscus, pass tectospinal tract medial longitudinal fasciculus.

Central gray matter: It encircles the central canal of medulla oblongata which is pushed more posteriorly. It presents following cranial nerve nuclei which are interrelated ventrolaterally.

  1. Hypoglossal nerve nucleus (XII): It is the nucleus of somatic efferent column, lying ventral to central canal of medulla oblongata.
  2. Nucleus ambiguous (IX, X, XI): It

Source: Easy and Interesting Approach to Human Neuroanatomy (Clinically Oriented)