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Muscle contraction without a change in muscle length is known as isometric contraction. The lady is performing a plank exercise which is an isometric exercise to strengthen the abdominal muscles.

Muscles can generate force through isotonic contraction or isometric contraction:

• Isotonic contraction involves maintaining constant tension as the muscle changes length. It can be concentric or eccentric.
• Concentric contraction involves shortening of the muscle. Example: biceps and brachialis contracting while lifting a weight by elbow flexion.
• Eccentric contraction involves lengthening of the muscle. Example: biceps and brachialis lengthening while lowering the weight by elbow extension.
• Isometric contraction involves force generation in the muscle while maintaining constant length.
• Example: Holding a crouched position or doing a wall-sit exercise.

Fenestrated capillaries are absent in the lungs.

Fenestrated capillaries have gaps between endothelial cells. They are present in certain tissues that allow the passage of larger molecules such as the renal glomeruli, endocrine glands, intestinal mucosa, and pancreas.

The vasa vasorum (Latin: vessels of the vessels) is a network of small blood vessels that supply tunica adventitia and the outer part of tunica media of large blood vessels.

Blood supply of arteries:

• Small arteries- The vessel wall gets nourishment by diffusion from the blood circulating in the vessel itself. When the vessel wall thickness is more than 1 mm, simple diffusion from the lumen becomes inadequate.
• Larger arteries- They have their own vascular supply within the adventitia called vasa vasorum. It supplies the adventitia and the outer part of the tunica media. The intima and the

inner part of the media of large vessels are nourished directly by diffusion from the luminal blood.

The given image shows a transverse section of the aorta showing vasa vasorum and the layers as follows:

• Tunica intima
• Tunica media
• Tunica adventitia
• Vasa vasorum

Circulation through the anastomosis is called collateral circulation.

Precapillary or postcapillary communication between neighbouring vessels is called anastomoses. Anastomoses are of the following types:

• Arterial anastomoses are communications between the arteries or branches of arteries. e.g., palmar arch, plantar arch, the circle of Willis.
• Venous anastomoses are communications between the veins or tributaries of veins. e.g., dorsal venous arches of the hand and foot.
• Arteriovenous anastomosis (shunt) are communications between an artery and a vein. e.g., av shunts found in the skin of nose, lips, ears, mucosa of nose and intestines, and erectile tissue.

The image below is a graphical representation of superficial and deep palmar arch.

Umbilical arteries carry deoxygenated blood from the heart to the placenta during the fetal period.

Arteries carry oxygenated blood away from the heart, with the exception of the pulmonary and umbilical arteries that carry deoxygenated blood.

Muscular arterioles contribute to the major part of peripheral resistance to blood flow.

The smallest arteries of the muscular type are called arterioles. They have a tunica media that is relatively thicker than the tunica adventitia. Arterioles cause the largest drop in blood pressure before the blood flows into the tissue capillary beds. They are densely innervated by sympathetic fibres. The contraction of arterioles is mainly regulated by local vasoactive and metabolic factors, but central mechanisms also play a role.

The following image shows a micrograph of a muscular arteriole:

All the other arteries except the posterior cerebral artery are end arteries.

Arteries whose terminal branches do not anastomose with branches from neighbouring arteries are called end arteries or terminal arteries. They can be of two types:

• Anatomic end arteries (or true end arteries) - do not have anastomoses. e.g., central artery of retina, internal auditory artery (labyrinthine artery of the internal ear).
• Functional end arteries - have anastomoses but they are insufficient to provide adequate blood supply when the main vessel is occluded. e.g., segmental arteries supplying the kidney.

The testicular veins have valves.

Valves within the veins maintain a unidirectional flow of blood, even against gravity. Congenital absence of valves in testicular veins may be a predisposing factor for the development of varicocele. The inferior vena cava also has a valve (the Eustachian valve) to direct the flow of oxygenated blood towards the foramen ovale to escape the pulmonary circulation. After birth, with the establishment of the pulmonary circulation and the increase in systemic vascular resistance driving the closure of foramen ovale, the valve has no function and tends to regress.

The valves are absent in the following veins:

• Veins less than 2 mm in diameter
• Venae cavae
• Hepatic, renal, uterine, ovarian, cerebral, spinal, pulmonary, and umbilical veins.
• The following image is a histological section showing the valve within a vein:

Sinusoids are absent in parotid glands.

Sinusoids are large, irregular, vascular spaces with true discontinuities in their walls. This allows close contact between blood and parenchyma of the surrounding organ.

Sinusoids are present in:

• Liver
• Spleen
• Bone
• Suprarenal glands
• Adenohypophysis
• Parathyroid glands
• Carotid body

Compared to their corresponding arteries, the veins have a much lesser muscular and elastic content. This explains the venous pressures being lower than the arterial pressures.

The following image shows a graphical illustration of the structure of the vessels (a amp; b) and a micrograph of the cross-sections(c):

Comparison of arteries and v eins

Arteries

Carry oxygenated blood away from the heart except for pul monary artery

Thick-walled,highly muscular except arteries of cranium a nd vertebral column

Narrow lumen Valves absent

High-pressure flow Reddish in colour

Veins

Carry deoxygenated blood to wards the heart except for pu lmonary veins

Thin-walled

Wide lumen

Valves present to maintain u nidirectional blood flow

Low-pressure flow Bluish in colour

The thoracic and right lymphatic ducts are vessels of the lymphatic system which is referred to as the drainage system of coarse type.

The lymphatic system helps remove larger particles such as proteins and particulate matter from the tissue fluid. Therefore, they are considered as the drainage system of coarse type.

The venous system is considered as the drainage system of fine type.

The lymph nodes have both afferent and efferent lymphatics.

• The afferent lymphatics enter the lymph node at different parts of its periphery.
• The efferent lymphatics exit at the hilum.

The following image shows a schematic representation of a lymph node:

The tonsils, thymus, and the spleen have no afferent lymphatics.

The right upper limb is drained by the right lymphatic duct, not the thoracic duct.

The filtered lymph passes through larger lymphatics and eventually collect into two large trunks, the thoracic duct and right lymphatic duct, which in turn drain into the brachiocephalic veins.

• Thoracic duct - drains both lower limbs, left upper limb, abdomen, left halves of thorax, and left side of the head and neck regions (areas shaded green in the image).
• Right lymphatic duct - drains right upper limb, and right halves of thorax, and right side of the head and neck regions (areas shaded blue in the image).

Injury to the thoracic duct is a rare complication of traumatic chest tube insertion.

Lymph capillaries are wider than blood capillaries.

Lymph capillaries begin blindly (tips are closed) in the tissue spaces and form intricate networks. The endothelial wall is thin and is permeable to substances of much greater molecular size. They have relatively lower pressure than blood capillaries.

Lymph capillaries

Colourless, difficult to observ e

Blind (closed at the tip)

Wider than blood capillaries

Wall consist of thin endotheli um and poorly developed bas ement membrane

Contain colourless lymph Have relatively low pressure

Absorb tissue fluid from inter cellular spaces

Blood capillaries Reddish, easy to observe

Joined to arterioles at one en d and to venules at another e nd

Narrower than lymph capilla ries

Wall consist of normal endot helium and basement membr ane

Contain red blood

Have relatively high pressure

Add tissue fluid to intercellul ar spaces

There are 8 pairs of spinal nerves in the cervical region.

The cerebrospinal nervous system includes 12 pairs of cranial nerves and 31 pairs of spinal nerves. The 31 pairs of spinal nerves are distributed as follows:

• Cervical nerves - 8 pairs
• Thoracic nerves-12 pairs
• Lumbar nerves-5 pairs
• Sacral nerves-5 pairs
• Coccygeal nerves-1 pair

Biploar neurons are not seen in spinal ganglia. Spinal ganglia consist of pseudounipolar neurons. Neurons are classified based on the number of processes as:

• Unipolar, e.g., mesencephalic nucleus
• Pseudo-unipolar, e.g., sensory ganglia or spinal ganglia.
• Bipolar, e.g., spiral and vestibular ganglia, and bipolar neurons of the retina.
• Multipolar, e.g., neurons in cerebrum and cerebellum.

The reflex arc consists of an effector, usually a muscle and not an affector.

A reflex arc is the basic functional unit of the nervous system which can perform an integrated neural activity.

A mono-synaptic reflex arc is made up of:

• Receptor, e.g., skin
• Sensory or afferent neuron
• Motor or efferent neuron
• Effector, e.g., muscle.

Microglial cells do not originate from the ectoderm but are derived from the mesoderm. Microglia are possibly derived from the circulating monocytes which migrate into the CNS during the late fetal and early postnatal life.

Neuroglial cells found in the parenchyma of the brain and spinal cord are broadly classified as follows:

• Macroglia cells of ectodermal origin (neuroectoderm) including the astrocytes, oligodendrocytes, and glioblasts.
• Microglia cells of mesodermal origin.

Capillaries with fenestrations are not part of the blood-brain barrier. The blood-brain barrier is formed by:

• Capillary endothelium with tight junctions (no fenestrations).
• Basement membrane of the endothelium.
• End feet of astrocytes covering the capillary walls.

The barrier permits a selective passage of blood constituents to the nervous tissue.

Nerve plexuses are formed only by the ventral rami.

Nerve plexuses contain afferent and efferent fibers that arise from the ventral rami of spinal nerves. The spinal nerve emerges from the intervertebral foramen and divides into a ventral and a dorsal ramus. All nerve plexuses are formed only from the ventral rami and not from the dorsal rami.

List of spinal plexuses:

• Cervical plexus
• Brachial plexus
• Lumbar plexus
• Sacral plexus
• Coccygeal plexus

Dry skin is not expected to see due to sympathetic stimulation.

The sympathetic nervous system (SNS) arises from the lateral horn of T1 to L2 segments of the spinal cord (thoracolumbar outflow). Sympathetic stimulation causes the following effects:

• Sweating due to secretomotor supply to sweat glands - Sudomotor
• Erection of hair due to contraction the arrector pili muscle - Pilomotor
• Raised blood pressure due to vasoconstriction - Vasomotor
• Dilated pupils
• Dry mouth
• Tachycardia

Common examples of sympathomimetic drugs include phenylephrine, albuterol, cocaine and amphetamines.

The correct order of the given layers from deep to superficial is stratum basale, stratum spinosum, stratum granulosum, stratum corneum.

Most of the body is covered by thin skin that has 4 layers as follows (superficial to deep):

• stratum corneum
• stratum granulosum
• stratum spinosum
• stratum basale

Some parts of the body such as the palms, soles and flexor surfaces of digits have thick skin

(glabrous skin) with 5 layers as follows (superficial to deep):

• stratum corneum
• stratum lucidum
• stratum granulosum
• stratum spinosum
• stratum basale

Mnemonic for skin layers (superficial to deep): Come Let's Get Some Beer (Corneum, Lucidum, Granulosum, Spinosum, and Basale).

The following image shows an illustration of layers of epidermis:

Non-nucleated cells are present in the stratum corneum layer of the epidermis. The cells are devoid of both a nucleus and membranous organelles.

The cells in this layer contain densely arranged keratin filaments within the cytoplasmic matrix providing a barrier function that prevents the entry of microbes from the environment and leakage of fluids from the body.

The number of melanocytes does not affect the colour of human skin.

The racial differences in skin colour is determined by:

• Melanocyte morphology and activity.
• Number and size of melanosomes in melanocytes.
• The number of melanosomes individually dispersed within keratinocytes.

Melanin pigmentation is influenced by constitutive (genetically determined) and facultative factors (reversible changes due to environmental factors like UV-rays, chemicals, and hormones).

The given clinical scenario of an itchy, asymmetrical pigmented lesion on the face with

an irregular border(image) that rapidly grew in size, points towards a diagnosis of malignant melanoma arising from the melanocytes. These cells are present in the stratum basale layer of the epidermis.

Melanocytes are derived from neural crest cells. They produce melanin which is packed into organelles called melanosomes and transferred via finger-like processes called dendrites to the keratinocytes.