Lecture 5 ANS CSF KL Revision 3

7/24/2019 Lecture 5 ANS CSF KL Revision 3

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Lecture 5: ANS; CSF

The Autonomic Nervous System (ANS)

Objectives:

1.

Compare the somatic and autonomic nervous systems relative to effectors, efferent pathways

and neurotransmitter released.

2.

Explain the anatomical and functional significance of the sympathetic and parasympathetic

ganglia; identify the location of the pre- and postganglionic sympathetic and parasympathetic

neurons.

3.

Compare and contrast the general functions of the parasympathetic and sympathetic divisions

of the ANS

1. Compare the somatic and autonomic nervous systems relative to effectors, efferent pathways and

neurotransmitter released.

The peripheral nervous system is divided functionally into: somatic (voluntary) and autonomic

(involuntary) systems. The somatic division is responsible for delivering voluntary signals to skeletal

muscle and the autonomic division regulates the activity of glands, cardiac and smooth muscles. Both

systems travel using peripheral nerves to reach their respective destinations. The bodies of somatic

motor neurons are usually located inside the CNS (brain or spinal cord) and their axons travel long

distances to reach the effector organ and deliver a voluntary message. The autonomic nervous system

pathwayconsists oftwo motor neurons: one inside the CNS (called preganglionic) and one outside

the CNS (called postganglionic). Pre-ganglionic neurons synapse with postganglionic neurons in

autonomic ganglia located outside the CNS in the periphery.

The autonomic nervous systemis subdivided into sympathetic and parasympathetic divisions based on

location, pathway and desired response in the body.

Image Courtesy: Pasedena.edu


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In the sympathetic pathway, the bodies of preganglionic neurons originate in the lateral gray horns of

the spinal cord from vertebral levels T1-L2. These neurons have short axons which exit the cord using

the ventral root of spinal nerves and soon after synapse in autonomic ganglia located outside the CNS.

In the parasympathetic pathway, the bodies of preganglionic neurons originate in the brainstem or

sacral division of the spinal cord. These neurons have long axons which exit the CNS at only specificlevels: Cranial nerves III, VII, IX, X and spinal nerves S2-4. They synapse in parasympathetic ganglia

located very near or directly inside target organs.

Neurotransmitters of the ANS

There are two main neurotransmitters acting on target organs in the autonomic nervous system:

Acetylcholine (ACH) and Epinephrine (EPI). Please note that ACH is also a primary neurotransmitter of

the somatic nervous system, however the effect and targets for ACH are different in the autonomic

system. In the somatic nervous system, ACH causes contraction of skeletal muscles by binding at the

neuromuscular junction. In the autonomic nervous system, ACH binding causes a wide variety of

responses discussed in objective #3 of this chapter.

ACHAcetylcholine is released by:

All preganglionic fibers

-

Both sympathetic and parasympathetic

All postganglionic parasympathetic fiberswill

release ACH. This leads to the activation of

nicotinic ACH receptors on peripheral targets.

Epinephrine and Norepinephrine (NE) is

released by:

The majority postganglionic sympatheticfibers

acting on glands, smooth muscle and cardiac

muscle.

ACH


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Exception:

Postganglionic sympatheticfibers will release

ACH instead of NE in two cases:

-sweat glands and smooth muscle surrounding

hair follicles. This leads to activation of

muscarinic receptors.

All nerve fibers that release ACH are referred to

as cholinergic

The adrenal medullawill also release NE and

Epinephrine into the bloodstream when

stimulated by pre-ganglionic sympathetic fibers.

All nerve fibers that release NE/EPI are referred

to as adrenergic

Receptors

The response of the

receptor on the post-

synaptic cell depends on the

nature of the receptor, not

the neurotransmitter. There

are two distinct types of

receptors that bind

acetylcholine: Muscarinic

and Nicotinic.

Muscarinic Acetylcholine

receptorsare activated by

the binding of ACH or a

water soluble toxin called:

Muscarine. This is a water-

soluble toxin derived from

the mushroom:Amanita

muscaria. Muscarine has

the capacity to cause substantial activation of the parasympathetic nervous system, resulting in

convulsions and even death. Muscarinic receptors are involved in a large number of physiological

functions including: decreasing the heart rate and inducing contraction of smooth muscles. Muscarine

receptors can be blocked by atropine and scopolamine.

There are 5 subtypes of muscarinic receptors, based on pharmacologic activity (M1-M5). These

receptors are located in two distinct areas:

1.

Post-synaptically at the parasympatheticjunction (will either increase or decrease the activity of

effector cells)

2.

Post-synaptic sympathetic stimulationof sweat glands: postganglionic sympathetic neurons

innervating sweat glands will release ACH at the neuro-effector junction. Activation will result in

increased sweating.


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Nicotinic receptorsare characterized by their interaction with nicotine. These receptors are located

post-synaptically in all autonomic gangliaand at the neuromuscular junction. These receptors can be

blocked by the plant toxin curare and some snake venoms. Competitive binding of these toxins will

lead to weakness of skeletal muscles and eventual death due to paralysis of the diaphragm. In

myasthenia gravis, nicotinic receptors are destroyed by antibodies which will result in progressive

muscle weakness and eventual paralysis.

2. Explain the anatomical and functional significance of the sympathetic and parasympathetic ganglia;

identify the location of the pre- and postganglionic sympathetic and parasympathetic neurons.

The Sympathetic Division Thoraco-lumbar outflow.

Pre-ganglionic myelinated axons originateinside the lateral grey horns of the spinal cord from segments

T1-L2, hence referred to as a thoraco-lumbar outflow. These axons leave the spinal cord via ventral

roots of spinal nerves and synapse in peripheral ganglia. Anatomically, there are three kinds of

sympathetic ganglia: paravertebral, pre-vertebral and the suprarenal medulla. The paravertebral

ganglia are located beside the vertebral column linked together in a chain sequence; for this reason

they are commonly called the sympathetic trunk. These chains extend along the entire length of the

vertebral column from cervical spine to the coccyx. The cell bodies of post-ganglionic sympathetic

neurons are located inside these peripheral ganglia.

Axons of preganglionic sympathetic neurons enter paravertebral ganglia via small nerve bridges called:

White Rami Communicantes; once inside the ganglion, the preganglionic axon will synapse. Axons from

post-ganglionic sympathetic neurons will exit the paravertebral ganglia to re-join spinal nerves via nerve

bridges known as: Gray Rami Communicantes.

There are a few possible routesthat preganglionic sympathetic axons can take when they enter the

sympathetic chain:

Image courtesy: University of Western Ontario


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1.

Axon will enter the chain and synapse within the sympathetic chain ganglion at the same level

that the nerve emerges off the spinal cord (shown in the picture on previous page).

2.

Axon will ascend or descend using the chain to travel to sympathetic chain ganglia which are

responsible for providing sympathetic innervation to the head/neck or genitourinary system.

3.

Axon will not synapse in sympathetic chain but instead travel far distances to synapse in pre-

vertebral ganglion located close to target tissue.

Pre-vertebral gangliaare situated anterior to the vertebral column and aorta. They are usually solitary

structures located between the target organ and the vertebral column. These ganglia are named after

the big branches of the abdominal aorta: Celiac, Superior Mesenteric and Inferior Mesenteric. Pre-

synaptic sympathetic fibers that are involved in innervation of abdominal viscera will pass through the

sympathetic chain at several levels and synapse in the three pre-vertebral ganglia mentioned above.

This nerve arrangement is referred to as the Thoracic and Lumbar Splanchnic nerves(plexuses that are

responsible for innervating thoracic and abdominal viscera).

Alternatively, the post-ganglionic sympathetic neurons that innervate the viscera of the thoracic cavity

will exit from the sympathetic chain at several levels and travel together as part of the cardiopulmonarysplanchnic plexus to reach the target organs such as the heart and lungs.

The suprarenal (adrenal) glands are located above the kidneys and act as specialized sympathetic

ganglia because theyreceive direct innervation from pre-ganglionic sympathetic fibers. In this case,

there are no post-ganglionic sympathetic fibers, instead the medullary cells of the adrenal glands release

neurotransmitter directly into the blood flow, causing widespread systemic sympathetic response.

The Parasympathetic Division Cranial Sacral Outflow

Preganglionic parasympathetic neurons originate in two distinct locations: grey matter of brain stem and

sacral spinal cord. The preganglionic axons exit the brainstem via cranial nerves III, VII, IX and X as wellas spinal cord levels S2-4.

Cranial nerves containing parasympathetic outflow will have long preganglionicneurons that travel to

peripheral ganglia that are located close to or within target organs. The postganglionic fibers will travel

short distances to influence target organs for the parasympathetic system.

Cranial nerves III, VII, and IX provide parasympathetic innervation to the head, while CN X travels long

distances to provide stimulation to the majority of thoracic and abdominal viscera (making up approx.

85% of the parasympathetic outflow). The chart on the following page indicates the target organ that

are associated with the parasympathetic outflow from the cranial nerves. You will learn more about

these pathways when you study cranial nerves in lecture 9.

The preganglionic parasympathetic fibers from S2-4 travel as pelvic splanchnic nerves and synapse in

intrinsic ganglia located in pelvic organs. Overall the parasympathetic system supplies organs and

glands of the head, thorax, abdomen and pelvis, while it does not reach the body walls or limbs.


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CRANIAL NERVE TARGET ORGAN

CN III CILIARY MUSCLE OF EYE

CN VII LACRIMAL GLAND

SUBMANDIBULAR AND SUBLINGUAL GLANDSCN IX PAROTID GLAND

CN X DIRECT TARGET ORGAN STIMULATION


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3. Compare and contrast the general functions of the parasympathetic and sympathetic divisions of

the ANS.

The autonomic nervous system regulates the activity of glands, cardiac and smooth muscles. The two

divisions of the ANS innervate the same structures and have coordinated effects in order to provide

constant involuntary modulation of organs and tissues. In general, the sympathetic division is catabolicfight or flight and the parasympathetic division is anabolic rest and digest.

TARGET SYMPATHETIC Division PARASYMPATHETIC Division

EYES Pupillary dilation Pupillary constriction

GLANDS (Tear duct and Salivary) Minimal change Increased gland secretion

LUNGS Bronchodilation Bronchoconstriction

HEART Increase heart rate and

contractile force

Decrease heart rate

GI SYSTEM Decreases peristalsis Increases peristalsis to promote

digestion

LIVER Encourages glycogenolysis inliver, inhibits gall bladder

Stimulates bile production,stimulates pancreas secretion

URINARY Inhibits urination Stimulates urination/defecation

Clinical Information: The Pupillary reflex

The pupillary light reflex is an important clinical tool used

to evaluate the function of the brain stem in a comatose

patient. It is also one of the brain stem reflexes tested in

the determination of brain death. Pupillary anomalies

could be indicators of critical or life threateningconditions. Pupillary constriction and dilation may also

depend on integrity of sympathetic (CN V1) and

parasympathetic (CN III) pathways.

Post-Ganglionic Sympathetic fibers from the superior

cervical ganglion innervate blood vessels and eye muscles

by using the nerve pathway of the opthalamic division of

the trigeminal nerve (CN V1). Eye muscles controlled by

CN V include: dilator pupillae and superior tarsal muscle

that elevates the upper eyelid. Interruption of the

sympathetic pathway to the eye will produce pupillary

constriction (miosis) and eyelid droop (ptosis).


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Cerebrospinal Fluid (CSF)

Objectives:

1.

Identify the meninges and the contents of the spaces around the spinal cord and around the

brain.

2.

Describe the formation of cerebrospinal fluid and follow its circulatory pathways: choroid

plexus, ventricles, apertures, central canal, subarachnoid space, arachnoid villi and sinuses.

3.

Discuss congenital abnormalities and clinical applications related to CSF.

1. Identify the meninges and the contents of the spaces around the spinal cord and around the brain.

There are several layers of protection around the brain and spinal cord: bone, connective tissue and

fluid. The connective tissue layer is referred to as the meningeal layer. There are three meningeal

layers that wrap, isolate and protect the entire CNS: Dura mater, Arachnoid mater and Pia Mater.

In the craniumthe dura mater

consists of two layers:outer

periosteal layer which is fused to

the periosteum of the skull bone

and the internal meningeal layer

which lies adjacent to the

arachnoid below. The inner layer of

dura mater forms dural reflections

(infoldings)that divide the cranial

cavity into compartments and act

as seat belts supporting parts of

the brain: falx cerebri, tentorium

cerebelli. Travelling within the

dural folds there are large cavities known as dural venous sinuses. These sinuses are filled with carbon

dioxide rich blood that drains from the brain into the internal jugular vein. Around the brain and spinal

cord the Dura mater is separated from the Arachnoid mater by a potential space: the subdural space. In

a normal healthy brain this space is not real butinstead the pressure of the cerebrospinal fluid (CSF)

usually presses the arachnoid mater against the dura. However, if injury is sustained then blood may fill

this space causing a subdural hematoma. In a dry cadaver the arachnoid will easily fall away from the

dura mater because they are not attached to each other normally. The pressure of the CSF only makes

them appear to be in contact in a living patient.

The middle meningeal layer: the arachnoid materis a delicate avascular membrane composed of

fibrous and elastic membranes resembling a spider web. The arachnoid mater has small extensions or

protrusions called: Arachnoid granulations(or arachnoid villi). These extensions allow cerebrospinal


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fluid (CSF) to exit the sub-arachnoid space and enter the dural venous sinuses. Once the CSF is drained

into the venous sinus, it can be transported out of the brain by the venous system.

The pia materis the thin, delicate, transparent layer that is tightly adhered to the surface of the brain

and spinal cord. It is difficult to remove the pia without damaging the underlying tissue.

The subarachnoid space is the space between the pia and arachnoid mater and it is normally filled with

cerebrospinal fluid that cushions and nourishes neural tissue in both brain and spinal cord.

The internal meningeal layerof dura mater exits the skull via the foramen magnum and together with

the arachnoid, forms a loose sac-like outer-covering known as the spinaldural sac around the spinal

cord. This loose outer covering is separated from the vertebral column by the epidural (extradural)

space which is filled with adipose tissue and has a rich blood supply. The dural sac is anchored to the

periosteum of the skull at the foremen magnum and to the coccyx.

In the spinal cord, there are extensions of pia mater that anchor the cord inside the dural sac called

denticulate ligaments.These ligaments leave the

cord in pairs between the

origin of the dorsal and

ventral roots. The filum

terminaleis another

extension of pia mater that

functions to anchor the cord

and the dural sac to the

coccyx.

Note: Spinal cord is shorter

than vertebral canal (ending

at L1/2 disc in adults)

Identify: conus medullaris,

cauda equineand filum

terminale (image to the

left).


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In this image, you can see the dural sac which

has been opened to reveal the denticulate

ligaments found between the spinal nerve

roots.

The dural sac extends inferior to the end of the

spinal cord termination, surrounding the cauda

equina and filum terminale.

2. Describe the formation of cerebrospinal fluid and follow its circulatory pathways: choroid plexus,

ventricles, apertures, central canal, subarachnoid space, arachnoid villi and sinuses.

Cerebrospinal fluid (CSF)is a clear liquid that is

functionally similar to blood. It carries nutrient,

gases and other important chemicals. However,

normally is does not contain RBC, has very little

WBC and has a low concentration of proteins. It

also has a different ion concentration when

compared to blood. CSF constantly circulates

around the brain and spinal cord via the

subarachnoid space. CSF acts as a shock absorber

and mechanically protects the delicate tissue of the

brain and spinal cord. Essentially the brain and

spinal cord float in CSF inside the cranial and

spinal cavities. CSF also helps to maintain

homeostasis and provides a healthy chemical

environment for precise neuronal signaling. Minor

changes in the ion composition would disturb the electrical status of the neuronal cell membrane and

influence the generation of action potentials. Lastly, CSF plays a role in the exchange of nutrients and

wastes produced inside the CNS.

In addition to the subarachnoid space, CSF circulates inside cavities found deep in brain tissue. These

cavities are known as ventricles. There are four ventricles that are filled with CSF: two lateral ventricles,

the third ventricle and the fourth ventricle.


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CSF is produced by a capillary network in the ventricles known as the choroid plexusat a rate of 500ml

per day. The total amount of CSF in one person is normally 130-150 ml.

Flow of CSF through the CNS:

Once it arrives at the fourth ventricle, CSF will drain into the central canal of the spinal cord and also exit

into the subarachnoid space via three apertures: 1 median and 2 lateral. Once CSF has circulated in the

subarachnoid space, it will drain into the venous blood flow via the arachnoid granulations (arachnoid

villi). These are extensions of the arachnoid mater which allow CSF to drain from the subarachnoid

space to the dural venous sinus: Superior Sagittal Sinus. This sinus located within the dural folds of the

falx cerebri.

Central Canal of Spinal cord

Fourth Ventricle

Cerebral Aqueduct

Third Ventricle

Interventricular Foramina

Lateral Ventricles


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3. Discuss congenital abnormalities and clinical applications related to CSF.

Hydrocephalus

A condition caused by the excessive

accumulation of CSF in the skull or cranium.

Normal flow and absorption through the

cranium is dependent on proper CSF

pressure in the head. A build-up of CSF often

causes a dangerous increase in intracranial

pressure. The combination of CSF build up

and the subsequent increase in pressure can

stress the brain tissue and produce a

characteristic set of signs and symptoms.

Hydrocephalus is categorized as either

communicating or non-communicating based on the cause of CSF buildup.

Communicating Hydrocephalus Non-Communicating Hydrocephalus

Also called non-obstructive hydrocephalus as

the CSF can flow freely through the ventricular

spaces

Caused by disruption of CSF uptake into

subarachnoid space

CONGENTIAL CAUSES:

Cytomegalovirus, Rubella, Toxoplasmosis,

Hemorrhaging as a result of birth trauma

ACQUIRED CAUSES:

Prior infection, Meningitis,

Subarachnoid Hemorrhage,

Cerebral Aneurysm

Also called obstructive hydrocephalus as the

flow of CSF has been blocked at some point in the

ventricular pathway.

CONGENITAL CAUSES:

Aqueductal stenosis

Stenosis of aperture

ACQUIRED CAUSES:

Brain tumorCyst / Abscesses

Brain trauma

The characteristic symptom seen in infants is enlargement of the head; the open sutures allow the

infants skull to expand to accommodate the excess CSF. In older children and adults, the skull bones

are fused so intracranial pressure increases and compresses brain tissue. Characteristic symptoms of

hydrocephalus in adults include: severe headache, nausea, dizziness, poor coordination, sun setting


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eyesand blurred vision. The rapid increase in intracranial

pressure will actually push the eyes downward producing a

sun-setting eye appearance.

If hydrocephalus develops, the CSF must be drained from the

ventricles as soon as possible. The most common procedure is

known as a hydrocephalus shunt. The purpose of this

procedure is to relieve the pressure on the brain and redirect

fluids.

A short term solution is an external ventricular drain (EVD) also

known as a venticulostomy catheter. This procedure places a

catheter in the ventricle and the fluid is drained into a vial by

the bedside. A long term solution for chronic hydrocephalus is

the ventriculoperitoneal shunt: where fluid is drained to the

abdomen and reabsorbed.

Cerebral Trauma

Fractures of the cranial base can cause rupture of the dura mater and leakage of CSF from inside the

cranial cavity to the outside. Some specific examples include: CSF Otorrhea is leakage of CSF from the

external acoustic meatus. Results from afracture of themiddle cranial fossa. CSF Rhinorrheais

leakage of CSF through the nose which results from a fracture of the anterior cranial fossa specifically

involving the ethmoid.

Intracranial hemorrhagesare also a result of cerebral trauma. Blood can escape into the spaces

between the meningeal layers, producing a characteristic appearance on CT and MRI. These

hemorrhages are classified based on their location within the meningeal layers: epidural, subdural and

subarachnoid.

Hemorrhage Origin Development:

Epidural

Disc Shape on

CT scan

Arterial Usually follows trauma (ie/ fracture of skull bone) that causes tearin

of meningeal arteries.

Blood accumulates between the layers of dura and the cranial bone

Leads to brief loss of consciousness followed by a lucid interval,

deterioration of brain function leading to coma

Occlusion of the bleeding vessels and removal of the blood is needed

as an emergency procedure

Subdural

Crescent Shape

on CT scan

Venous Often occur with cerebral contusions which cause tearing of the

bridging veins between dura and arachnoid.

Blood slowly seeps into potential space

Bleeding is typically not as extensive as epidural

Symptoms appear slowly over a period of days post injury (headache

confusion, dizziness, weakness, lethargy)


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Subarachnoid

Blood seen

pooling in

subarachnoid

space on CT

Arterial Results from a ruptured arterial aneurysm

Blood flows into subarachnoid space

Characteristic sign is sudden severe thunderclap headache, severe

nausea and vomiting, pain around the orbit, and dizziness

A secondary complication is often non-obstructive hydrocephalus du

to the presence of blood in the subarachnoid space

Spinal tap or Lumbar Puncture This procedure is performed to

obtain the sample of CSF from the subarachnoid space. The

needle is inserted between L3 and L4, traveling through: skin,

ligaments, epidural space, dura mater, subdural space and

arachnoid to reach the CSF. Reasons to perform a spinal tap

include: sampling CSF for infections, measuring the pressure of

CSF checking for bleeding in subarachnoid space. In an adult

the spinal cord ends at L1/2, therefore the procedure isperformed below the level of the spinal cord to avoid serious

injury.

An epidural blockis a procedure in which an anesthetic

agent is injected into the epidural space through a needle

that is passing between lumbar vertebrae or through the

sacral canal. An epidural block inhibits the sensory signalscoming via peripheral nerves before they can enter the

central nervous system. Unlike a spinal tap, an epidural

injection can be performed at any level in the spinal

column to achieve segmental anesthesia.

BLUE BOXES:in M&A pp. 508-509 Occlusion of Cerebral Veins and Dural venous Sinuses. Metastasis of

Tumor cells to Dural Sinuses, Fractures of the cranial base, Dural Origin of Headaches. pp 511 Head

injuires and Intracranial Hemorrhage; pp 514 Cisternal Puncture, Hydrocephalus, Leakage of CSF.

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Lecture 5 ANS CSF KL Revision 3