TRANSTENTORIAL OR UNCINATE HERNIATION:

 

 

The tentorium cerebelli is an inward fold of the dura matter that separates the cerebrum with the cerebellum from the lateral sides.

Increased cranial pressure in the supra-tentorium (above the tentorium cerebelli) puts pressure on the the insula (between the frontal and temporal lobe) and the insula moves downwards in the notch between the midbrain and tentoriom cerebelli. In the midbrain it damages the following structures from lateral to medial.

The posterior cerebellar artery, the optic tract causing contralateral homonymous hemianopia and the parasympathetic fibers, overstimulates the 3^rd nerve nucleus causing pupillo-constriction. It also pushes the midbrain towards the opposite side, damaging the cortico-spinal fibers of the other side resulting in hemiplegia (paralysis of muscles of the limbs and trunk of one side of the body). The hemiplegia is on the same side as the herniation, although the opposite cortico-spinal tracts are affected because the cortico-spinal tracts of one side innervate the other side of the body. In severe cases the third nerve nucleus may get damaged and there is no innervation to the medial rectus muscle. Thus! The eyeball of the eye on the side of the herniation can not move on the side of herniation.

Kernohans notch is a notch formed in the continuous cerebral peduncle due to damage to some fibers of the peduncle.

In the midline region and paramedian regions of the midbrain arteries and veins rupture causing DURET HEMORAGES.

PAPILLEDEMA:

It is edema in the optic disc in which the optic disc shows a blurred circumference. Due to high intracranial pressure in the subarachnoid space both retinal arteries and veins {that enter across the optic nerve penetrating the dura, arachnoid and pia matter to enter the inside of the eye via the optic disc.} get compressed. When arteries get compressed blood is pushed faster (since blood is moving under high pressure) while when veins are compressed, blood is not allowed to pass through it (since blood is moving under low pressure). Since blood moves faster into the eye through the arteries and cannot be drained by the veins edema or accumulation of fluid takes place in the optic disc. Papilledema may also be caused due to infection.

PTOSIS:
Of the extraoccular muscles of the eye, the superior rectus & superior oblique (collectively called superior tarsal muscles) and the levator palpebrae superioris control the opening and closing of the eyelids.

These muscles are innervated by axons from the 3^rd nerve nuclei and the sympathetic fibers. If these muscles get paralysed due to various reasons, drooping of the eyelids take place, a condition called ptosis.

Disruption of the sympathetic fibers (HORNERS SYNDROME) leads to occulo-sympathetic ptosis.

Disruption of the 3^rd nerve nucleus fibers leads to occulo-sympathetic ptosis.

Neuro-muscular junction disorders such as myasthenia gravis {auto-immune antibodies are produced against the ACH receptors on the post synaptic membrane, blocking them} can also paralyze these muscles leading to occulo-myasthemic ptosis.

DIFFERENCE BETWEEN THE SYMPATHETIC (THORACO-LUMBER OUTFLOW) AND THE PARASYMPATHETIC (CRANIO-SACCRAL OUTFLOW)

 

1.  The sympathetic PREGANGLIONIC CELLBODIES are located in the intermedio-lateral grey horn of the spinal cord at level T1-L2. The parasympathetic PREGANGLIONIC CELLBODIES are located in the cranial nerve nucleus (grey matter of brain) 3, 7, 9, and 10 AND the intermediate grey horn of the spinal cord at level S2-S4. Some may also be seen in the splanchnic nerves.

2.  The sympathetic PREGANGLIONIC AXONS are found in spinal nerves T1-L2 and can also be seen crossing the sympathetic (paravertibral) trunk without synapsing.  The parasympathetic PREGANGLIONIC AXONS are found in cranial nerves 3, 7, 9 and 10 AND sacral nerves of spinal cord S2-S4. They leave their anterior ramus to form PELVIC SPLANCHNIC NERVES. All preganglionic axons are myelinated forming bundles of axons called white rami comunicantes.

3.  The sympathetic POSTGANGLIONIC CELLBODIES are found in either the sympathetic (paravertibral) trunk or the prevertibral plexus (adjacent to the aorta). The parasympathetic POSTGANGLIONIC CELLBODIES are found in the ganglions in the head and neck, cranial nerve autonomic ganglions, and the hypogastric plexus. The pelvic splanchnic nerves innervate the hypogastric plexus.

4.  The POSTGANGLIONIC AXONS of both sympathetic and parasympathetic go to different viscera of the body. Note: the hair follicles are only innervated by the sympathetic postganglionic axons, since they are only contracted, as part of sympathetic response. Bundles of postganglionic axons form the grey rami comunicantes because these axons are non-myelinated.

5.  The primary neurotransmitters released by both sympathetic and parasympathetic preganglionic axons are acetylcholine. The primary neurotransmitter released by postganglionic parasympathetic is also acetylcholine but the ones released by postganglionic sympathetic is NOREPINEPHRINE to all target organs except for the sweat glands to which acetylcholine is released.

Note: acetylcholine released in the synaptic cleft after used is hydrolyzed to acetic acid and choline by enzyme ACETYLCHOLINESTERASE.

6. The sympathetic preganglionic to postganglionic are in the ratio 1:10 while parasympathetic preganglionic to postganglionic are in the ratio of 1:3.

7. The sympathetic postganglionic fibers are longer than the parasympathetic postganglionic because the latter originate from plexus and ganglia around the viscera

 

Note: the preganglionic axons are myelinated, longer and slow conducting B fibers while the post ganglionic fibers are unmyelinated, shorter and slower conducting C fibers.

CLEARING CONCEPTS OF CONUS MEDULARIS, FILUM TERMINALIS AND CAUDA EQUINA.

 

 

In infants the spinal cord extends from the base of the medulla oblongata and terminates between vertibra L4 and L5 of vertebral coloumn. The curved caudal terminal part of the spinal cord is the conus medularis. The pia matter around the conus medularis connects the conus medularis to the innerside of the coccygeal vertibra. In adults the spinal cord ascends in the vertebral column and the conus medularis comes to lie between L2 and L3 vertibra. The pia matter connecting the conus medularis with the coccygeal vertibra now elongates forming the filum terminalis.

Cauda eqina is a collective term given to anterior and posterior roots of the thoracic sacral and lumber nerves.

PHARMACOLOGY OF AUTONOMIC NERVOUS SYSTEM. ALL YOU NEED TO KNOW FROM SNELL NEUROANATOMY.

 

·      Ganglion stimulating agents: bind with nicotinic receptors on post synapting membrane initiating fast EPSP to cause sympathetic or parasympathetic response. Eg: nicotine, lobeline and dimethyl-pipera-zinium.

·      Ganglion inhibiting agents: hexa-methonium and tetra-ethyl-ammonium bind with nicotinic receptors and do not let ACH bind with it. Nicotine in high concentrations bind with nicotinic receptors causing depolarization and maintaining it (depolarization is not followed by repolarization and action potential is not generated). Muscurinic receptors on post synaptic membrane can be blocked by atropine.

·       Stimulation at neuroeffector junction: PHENY-EPHIRINE: alpha receptor stimulator. ALBUTEROL & METAPROTERENOL: beta 2 receptor stimulator causing bonchodilation.

·      Inhibition at neuroeffector junction: alpha receptors can be blocked by phenoxybenzamine while beta receptors can be blocked by propranolol. Reserpin is a drug that does not allow synthesis of norepinephrine inside the postganglionic axon and thus it is not released in the neuroeffector synaptic cleft. {acetylcholine is formed from dopamine inside the postganglionic axon, but dopamine before converting into ACH has to be preserved in a vesicle or else it would be destroyed by monoamine-oxidase (MAO), an enzyme present freely in the cytoplasm of the axon. Reserpin binds with the vesicle and does not allow dopamine uptake and dopamine is destroyed by MAO}. Muscurinic receptors on target organs can be blocked by atropine.

ACETYLCHOLINE AND NOREPINEPHRINE SENSITIVE RECEPTORS ON POSTSYNAPTIC MEMBRANE OF THE PREGANGLIONIC AND POSTGANGLIONIC JUNCTION AND TARGET ORGANS.

 

 

1. On all postsynaptic membranes of the preganglionic and postganglionic junction, cholinergic (acetylcholine sensitive) receptors are present since all preganglionic axons release acetylcholine to the postganglionic axons. There are two types of cholinergic receptors: nicotinic and muscurinic. The nicotinic cause fast action potential while the latter causes slow. The acetylcholine released binds predominantly with the nicotinic receptors on the postganglionic axons causing fast action potential also called fast EXCITORY POST SYNAPTIC POTENTIAL (EPSP). After the fast EPSP has been fired, the acetylcholine may bind with muscurinic receptors on the post synaptic membrane to cause slow EPSP or slow INHIBITORY POST SYNAPTIC POTENTIAL (IPSP).

During EPSP, when ACH (acetylcholine) binds with cholinergic receptors, the Na and Ca channels open causing their influx generating an action potential. During IPSP when ACH binds to muscurinic cholinergic receptors, k channels open causing their efflux resulting in hyperpolarization.

              

2.The target organs posses both adrenergic (noradrenaline sensitive receptors) for the sympathetic response and cholinergic MUSCURINIC receptors for the parasympathetic response. An exception is the receptors on the sweat glands and blood vessels of skeletal muscles which posses cholinergic MUSCURINIC receptors and respond to ACH for SYMPATHETIC response.

 

Note: the muscurinic receptors are found on postsynaptic membrane of the postganglionic axon and the target organs while nicotinic are only found on post synaptic membrane of the post ganglionic axons.

 

Atropine can competitively inhibit muscurinic receptor sensitivity to ACH. 

 

Similarly the axons that release ACH are called cholinergic fibers and those that release Noradrenaline are called adrenergic fibers.

 

There are also 4 types of adrenergic receptors a1 a2, b1 and b2 and are only present on target organs for sympathetic response. A1 is also present on the presynaptic membrane of the neuroeffector junction.

 

All respond to norepinephrine. A1 and a2 on target organs when stimulated cause excitory functions while b1 and b2 cause inhibitory functions. The a1 present on presynaptic membrane is stimulated when there is too much norepinephrine in the synaptic cleft. It inhibits the production of more norepinephrine into the synaptic cleft.

 

All beta receptors cause inhibitory effects (e.g. B2 receptors present on lungs cause bronchodilation) except for the b1 receptors present on myocardium that cause excitory effects.

Note: norepinephrine has a greater effect on alpha receptors than beta receptors.

AUTONOMIC NERVOUS SYSTEM: INTRODUCTION.

AUTONOMIC NERVOUS SYSTEM: INTRODUCTION.

 

The cell bodies of the preganglionic sympathetic axons lie in the intermediolateral grey horn at the level T1-L2 of the spinal cord while the cell bodies of the preganglionic SACRAL parasympathetic axons lie in the intermediate grey horn at the level S2-S4 of the spinal cord.

 

The sympathetic and parasympathetic SACRAL efferents travel in the anterior root of spinal cord, the trunk and the anterior rami. From here it takes different courses.

 

SYMPATHETIC MOTOR PATHWAY

 

1. Each sympathetic efferent leave the anterior ramus and innervate in the sympathetic trunk (paravertibral trunk) at which the preganglionic axons terminate and postganglionic axons begin. The postganglionic axons again enter the same anterior ramus, from where they also go to the posterior ramus to provide both the anterior and posterior body parts. Note that the sympathetic trunks are two, on either side of the spinal cord and lie anterior to the anterior rami of spinal cord so the preganglionic axons from an anterior ramus moves anteriorly innervates the cell body of the postganglionic axon in the sympathetic trunk and then the postganglionic axon again moves posteriorly to join the same anterior ramus (Students Grays. Pg: 44 Fig: 1.45). The sympathetic efferent preganglionic axon may ascend or descend along the sympathetic trunk to innervate a postganglionic cell body at a different level. This postganglionic axon moves posteriorly from the sympathetic trunk to enter the anterior ramus of its own level. The axons also go to the posterior ramus.

2.Some sympathetic efferents after leaving the anterior ramus pass the sympathetic trunk without synapsing and synapse at the prevertibral plexus   (adjacent to the aorta and anterior to the vertebral column, hence the name PRE vertebral), the renal plexus or the adrenal medulla instead. One of the sub divisions of the prevertibral plexus is the celiac plexus. The bundles of preganglionic axons penetrate the diaphragm to synapse at the above mentioned plexus. There are three.

 

·GREATER SPLANCHNIC NERVE: that synapse at the renal plexus, celiac plexus and suprarenal medulla (the suprarenal medulla is a collection of ganglia which release norepinephrine and epinephrine).

 

·LESSER SPLANCHNIC NERVE: that synapse only at the

Renal plexus.

 

·LEAST SPLANCHNIC NERVE: that synapse only at the celiac plexus.

 

The plexus contain a connective tissue capsule, satellite cells, postganglionic cell bodies, efferent and afferent axons (both sympathetic and parasympathetic).

 

The postganglionic axons leaving the renal plexus provide the kidneys and leaving the celiac plexus provides the celiac artery.

 

Note: Each sympathetic trunk has 3 cervical ganglia, 12 thoracic ganglia, 5 lumber and 5 sacral ganglia.

 

PARASYMPATHETIC MOTOR PATHWAY

 

1.The sacral parasympathetic preganglionic efferents after leaving the anterior rami of sacral nerves S2-S4 form the pelvic splanchnic nerves that synapse at the hypogastric plexus from where postganglionic axons innervate the gastro intestinal tract.

2.The cranial parasympathetic preganglionic efferents initiate from the grey matter of the brain stem at level cranial 3, 7, 9 and 10 to form the corresponding cranial nerves. The cranial nerves synapse either at ganglions  in the head and neck or the cranial autonomic nuclei from where postganglionic axons emerge and innervate parts of the head and neck.

 

All efferent preganglioinc axons release acetylcholine.

The parasympathetic postganglionic axons also release acetylcholine but the sympathetic postganglionic mainly release norepinephrine except that to the sweat glands and blood vessels of skeletal muscles, it releases acetylcholine.

 

The parasympathetic and sympathetic postganglionic axons are also seen to release neuropeptide Y, substance P and ATP. These have an inductive effect on the primary neurotransmitters.

 

Apart from the preganglionic and postganglionc neurons, there may be interneurons called small intensely fluorescent cells-SIFs (since they contain catecholamine and shine under light). The SIFs release dopamine.