NEETMEDICALGK

The trochlear nerve is one of 12 sets of cranial nerves. It enables movement in the eye's superior oblique muscle. This makes it possible to look down. The nerve also enables you to move your eyes toward your nose or away from it.

The oculomotor nerve is the third cranial nerve (CN III). It allows movement of the eye muscles, constriction of the pupil, focusing the eyes and the position of the upper eyelid. Cranial nerve III works with other cranial nerves to control eye movements and support sensory functioning.

The optic nerve is a bundle of more than 1 million nerve fibers that carry visual messages. You have one connecting the back of each eye (your retina) to your brain. Damage to an optic nerve can cause vision loss. The type of vision loss and how severe it is depends on where the damage occurs.

The olfactory nerve is the first cranial nerve (CN I). It is a sensory nerve that functions for the sense of smell. Olfaction is phylogenetically referred to as the oldest of the senses. It is carried out through special visceral afferent nerve.

Gray's anatomy:Cranial nerves & Pharyngeal arches. 12 cranial nerves

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12 cranial nerves

Author: Jana Vasković MD • Reviewer: Nicola McLaren MSc

Last reviewed: March 03, 2022

Reading time: 23 minutes



Facial nerve (lateral right view)

You know when someone mentions cranial nerves and you roll your eyes all the way back to your midbrain? We know that cranial nerves have always been a challenging subject among anatomy students. So we’re here to make it easier for you.

Cranial nerves anatomy is essential for almost any medical specialty since they control so many body functions, such as rolling your eyes when you’re annoyed by something. So let’s break the stigma of them being hard to understand, and learn this important neuroanatomy topic once and for all.

Types of nerves- Sensory: Olfactory nerve (CN I), optic nerve (CN II), vestibulocochlear nerve (CN VIII)

- Motor: Oculomotor nerve (CN III), trochlear nerve (CN IV), abducens nerve (CN VI), accessory nerve (CN XI), hypoglossal nerve (CN XII).

- Mixed (both): trigeminal nerve (CN V), facial nerve (CN VII), glossopharyngeal nerve (CN IX), vagus nerve (CN X).

Mnemonic (by the numerical order): Some Say Money Matters, But My Brother Says Big Brains Matter Most.NervesOlfactory nerve (CN I), optic nerve (CN II), oculomotor nerve (CN III), trochlear nerve (CN IV), trigeminal nerve (CN V), abducens nerve (CN VI), facial nerve (CN VII), vestibulocochlear nerve (CN VIII), glossopharyngeal nerve (CN IX), vagus nerve (CN X), accessory nerve (CN XI), and hypoglossal nerve (CN XII).

Mnemonics: 

-  Oh, Oh, Oh, To Touch And Feel Very Good Velvet, such-A Heaven

-  On, On, On, They Traveled And Found Voldemort Guarding Very Ancient Horcruxes.

Subdural haematoma

Separation of the arachnoid and dura mater requires little physical

force, which means that damage to small bridging veins in the space

can result in subdural haematoma after even relatively mild head

trauma. In cases of chronic or subacute subdural haematomas, the

accumulation is of relatively low pressure and seldom presents as a

medical emergency; even sizeable accumulations may be tolerated on a chronic basis with mild or no symptoms, and can be surgically

drained through small openings in the skull (burr-holes). In many

cases, there is some predisposing factor, such as cerebral atrophy or

increased size of the underlying subarachnoid space. In cases of acute

subdural haematoma, there is rapid accumulation of blood in the subdural space, typically after severe head trauma, which requires emergency neurosurgical drainage through a large craniotomy or craniectomy.

The distinction between subdural and extradural haematoma on a CT

scan relies on the anatomical features of the clot. Extradural collections

tend to be lentiform in shape, reflecting the pressure required to separate the dura and periosteum. They will not pass deep to any major

dural sinus and cannot extend along the falx cerebri or tentorium

cerebelli. In contrast, acute subdural haematomas tend to be biconcave

in shape and often follow the line of the dura along the falx or tentorium. Gray's anatomy

Cavernous sinus : Gray's anatomy The cavernous sinus is part of the brain's dural venous sinus and contains multiple neuro-vasculatures. It is situated bilaterally to the sella turcica and extends from the superior orbital fissure anteriorly to the petrous part of the temporal bone posteriorly, and is about 1 cm wide and 2 cm long.The cavernous sinus is located on either side of the pituitary fossa and body of the sphenoid bone between the endosteal and meningeal layers of the dura. It spans from the apex of the orbit to the apex of the petrous temporal bone.The cavernous sinus in turn drains to the superior and inferior petrosal sinuses. Both sinuses join the sigmoid sinus, which then becomes the internal jugular vein. The internal jugular vein meets with the subclavian vein to become the left (or right) brachiocephalic vein.The cavernous sinuses are irregularly shaped, trabeculated cavities located at the base of the skull. The cavernous sinuses are the most centrally located of the dural sinuses and lie on either side of the sella turcica.It can be life-threatening. The cavernous sinuses are hollow spaces located under the brain, behind each eye socket. A major blood vessel called the jugular vein carries blood through the cavernous sinuses away from the brain.

ll iabetes mellitus is the third leading cause

lJ of death (after heart disease and cancer) in

many developed countries. lt affects about 2 to

3% of the general population. The complications

of diabetes affect the eye, kidney and nervous

system. Diabetes is a major cause of blindness,

renal failure, amputation, heart attacks and

stroke. (The term diabetes, whenever used, refers

to diabetes mellitus. lt should, however, be

noted that diabetes insipidus is another

disorder characterized by large volumes of

urine excretion due to antidiuretic hormone

deficiency).

Diabetes mellitus is a clinical condition

characterized by increased blood glucose level

(hyperglycemia) due to insufficient or inefficient

(incompetent) insulin. ln other words, insulin is

either not produced in sufficient quantity or

inefficient in its action on the target tissues. As a

consequence, the blood glucose level is elevated

which spills over into urine in diabetes mellitus

(Creek : diabetes-a siphon or running through;

mellitus-sweet).

An important feature of diabetes is that the

body cells are starved of glucose despite

its very high concentration around i.e. scarcity in

plenty. For a comprehensive understanding of

diabetes, the relevant hormones, namely insulin

and glucagon, homeostasis of blood glucose,

besides the biochemical aspects of diabetes, are

discussed in this chaoter.

lnsufin is a polypeptide hormone produced

by the B-cells of islets of Langerhans of

pancreas. lt has profound influence on the

metabolism of carbohydrate, fat and protein.

Insulin is considered as anabolic hormone, as it

promotes the synthesis of glycogen,

triacylglycerols and proteins. This hormone has

been implicated in the development of diabetes

mellitus.

Insulin occupies a special place in the history

of biochemistry as well as medicine. Insulin was

the first hormone to be isolated, purified and synthesized.Clucagon, secreted by a-cells of the pancreas,

opposes the actions of insulin. lt is a polypeptide

hormone composed of 29 amino acids (mol. wt.

3,500) in a single chain. Clucagon is actually

synthesized as proglucagon (mol. wt. 9,000)

which on sequential degradation releases active

glucagon. Unlike insulin, the amino acid

sequence of glucagon is the same in all

mammalian species (so far studied). Clucagon has

a short half-life in plasma i.e. about 5 minutes.

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The secretion of glucagon is stimulated by

Iow blood glucose concentration, amino acids

derived from dietary protein and low levels of

epinephrine. Increased blood glucose level

markedly inhibits glucagon secretion.

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Clucagon influences carbohydrate, lipid and

protein metabolisms. In general, the effects of

this hormone oppose that of insulin.

1. Effects on carbohydrate metabolism :

Glucagon is the most potent hormone that

enhances the blood glucose level (hyperglycemic).

Primarily, glucagon acts on liver to cause

increased synthesis of glucose (gluconeogenesis)

and enhanced degradation of glycogen

(glycogenolysis). The actions of glucagon are

mediated through cyclic AMP (Chapter t3).

2. Effects on lipid metabolism : Clucagon

promotes fatty acid oxidation resulting in

energy production and ketone body synthesis

(ketogenesis).

3. Effects on protein metabolism : Glucagon

increases the amino acid uptake by liver which,

in turn, promotes gluconeogenesis. Thus,

glucagon lowers plasma amino acids.

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Clucagon binds to the specific receptors on

the plasma membrane and acts through the

mediation of cyclic AMP, the second messenger.

The details are given elsewhere Diabetes mellitus is a metabolic dr'sease, more

appropriately adisorder of fuel metabolism. lt is

mainly characterized by hyperglycemia that

leads to several long term complications.

Diabetes mellitus is broadly divided into 2

groups, IDDM & NIDDM.

f nvironment constitutes the non-living (air,

Lwater, land, energy etc.) as well as the living

(biological and social) systems surrounding man.

Environmental biochemistry primarily deals with

the metabolic (biochemical) responses and

adaptations in man (or other organisms) due to

the environmental factors.

A healthy environment is required for a

healthy life which is however, not really possible

or practicable. This is mainly because of the

atmospheric (climatic) changes and

environmental pollution.

Environmental biochemistry is a very vast

subject. The basic concepts regarding the

atmospheric changes and environmental

oollution on humans are dealt with here.

The climatic changes include cold, heat etc. .

The body makes every effort to maintain its

normal temperature (despite cold and

surroundings) for optimal physiological

biochemical functions.

EXPOSURE TO COLD

Short-term exposure to cold causes shivering

(mainly due to skeletal muscle) to produce extra

heat. Heat is generated by the hydrolysis of ATP'

tw,+. g,,:t:t; j,i+'r e "t: 94 r:li:*s;ie

Chronic exposure to cold results in non-

shivering phase which is characterized by

several metabol ic adaPtations.

heat

and

Energy metabolism : Heat generation by a

process called chemical thermogenesis occurs

in non-shivering phase. The foodstuffs undergo

oxidation to generate heat at the expense of

growth and other anabolic processes.

Elevation in BMR, and increased intake oi

foods are observed.

Lipid metabolism : Stored fat (triacylglycerol)

in the adipose tissue is mobilized to supplyfree fatty acids for oxidation and production of

energy. Brown edipose tissue, particularly in

neonatal life, significantly contributes to

thermogenesis.

. Hormonal changes : Thyroxine, a hormone

closely associated with energy metabolism, is

elevated. Further, corticosteroids are increased

on exoosure to cold.

EXPOSURE TO HEAT

There is a continuous generation of heat by

the body due to the ongoing biochemical

processes, referred to as metabolic heat. This

heat has to be exchanged with the environment

to maintain a constant body temperature. On

exposure to heat in surroundings, as happens in

sLrmmer, the body is subjected to an

uncomfortable situation (since temperature of the

surroundings is much higher than that of the

body). However, heat is still lost from the body

through sweating and evaporation. Normally, the

body (thermoregulation) gets acclimatized to

higher temperature within 3-5 days.

Heat stroke : lt is characterized by the failure

of the heat regulatory system (thermoregulation)

of the body. The manifestations of heat stroke

include high body temperature, convulsions,

partial (some times total) loss of consciousness.

In extreme cases, heat stroke may cause

irreversible damage to brain. The treatment for

the heat stroke involves rapid cooling of the

body.

The milder form of heat stroke is referred to

as heat syncope. Although the body temperature

is not raised much in this condition, the blood

pressure falls and the person may collapse

suddenly. Heat syncope is easily reversible.

Environmental pollution may be regarded as

the addition of extraneous (foreign) materials to

air, water or land which adversely affects the

quality of life. Pollution may be caused by

physical, chemical or biological processes.

The term pollutant refers to a substance

which increases in quantity due to human

activity and adversely affects the environment

(e.9. carbon monoxide, sulfur dioxide, lead). A

substance which is not present in nature but

released during human activity is the

contaminant (e.g. methyl isocyanate, DDT,

malathion). A contaminant however, is regarded

as a pollutant when it exerts detrimental effects.

Environmental pollution may be considered in

different ways-industrial pollution; agricultural

pollution; pollution due to gaseous wastes, liquid

wastes and solid wastes.

A cquired immunodeficiency syndrome (AIDS)

Awas first reported in 1981 in'homosexual

men. AIDS is a retroviral disease caused by

human immunodeficiency virus (HlV). The

disease is characterized by immunosuppression,

secondary neoplasma and neurological

manifestations. AIDS is invariably fatal since

there is no cure. In the USA, it is the fourth

leading cause of death in men between the ages

15 to 55 years.

No other disease has attracted as much

aftention as AIDS by the governments, public and

scientists. AIDS has stimulated an unprecedented

amount of biomedical research which led to a

major understanding ofthis deadfy disease within

a short period of time. So rapid is the research on

AIDS (particularly relating to molecular biofogy),

any review is destined to be out of date by the

time it is published!

The isolation of human immunodeficiency

virus (HIV) from lymphocytes of AIDS patients

was independently achieved by Gallo (USA) and

Montagnier (France) in 1984.

Epidemiology

AIDS was first described in USA and this

country has the majority of reported cases. The

prevalence of AIDS has been reported from

almost every country. The number of people

living with HfV worldwide is estimated to be

around 40 million by the end of the year 2005.

(lndia alone has about 5 million persons). At

least 5 million deaths occurred in 2005, due to

AIDS. AIDS is truely a global disease with an

alarming increase in almost every country.

Transmission of HIV : Transmission of AIDS

essentially requires the exchange of body fluids

(semen, vaginal secretions, blood, milk)

containing the virus or virus-infected celfs. There

are three major routes of HIV transmission-

sexual contact, parenteral inoculation, and from

infected mothers to their newborns.

The distribution of risk factors for AIDS trans-

mission are as follows.

Sex between men (homosexuals)

Sex between men and women

- 60"/"

-15%Intravenous drug abusers - 15"/"

Transfusion of blood and blood products - 6%

All others - 4o/o

The predominant methods of HIV

transmission (about 75o/") are through anal or

vaginal intercourse. The risk for the transmission

is much higher with anal than with vaginal

intercourse. The practice of 'needle sharing' is

mainly responsible for the transmission of HIV in

drug abusers. Pediatric AIDS is mostly caused by

vertical transmission (mother to infant).

It should, however, be noted that HIV cannot

be transmitted by casual personal contact in the

household or work place. Further, the

transmission of AIDS from an infected individual

to health personnel attending on him is

extremelv rAre.

Virology of HIV

AIDS is caused by a retrovirus, namely human

immunodeficiency virus (HlY), belonging to

lentivirus family. Retroviruses contain RNA as

the genetic material. On entry into the host cell,

they transcribe DNA which is a complementary

copy of RNA. The DNA, in turn is used, as a

template to produce new viral RNA copies.

Two different forms of HlV, namely HIV-I

and HIV-2 have been isolated from AIDS

patients. HIV-1 is more common, being found in

AIDS patients of USA, Canada, Europe and

Central Africa while HIV-2 is mainly found in

West Africa. Both the viruses are almost similar

except they differ in certain immunological

properties.

HIV-1 is described in some detail.

Structure of HIV : The viruse is spherical with

a diameter of about 110 nm. lt contains a core,

surrounded by a lipid envelop derived from the

host pfasma membrane (Fig.3fl.l). The core of

the HIV has two strands of genomic RNA and

four core proteins, PZq, PtB, reverse tranScriptase

(poolpsr) and endonuclease (p32). Note that the

naming of the proteins is based on the molecular

weight. For instance, a protein with a molecular

weight of 24,0OO is designated as p2,4.

The lipid membrane of the virus is studded

with two glycoproteins Bprzo and gpot. The

surface antigen 8p126 is very important for the

viral infection CD4.