Nervous System

The nervous system is an organ system that contains a network of specialized cells

called neurons. This is the master controlling and communicating system of the body. It

coordinates the action of an animal and transmits signals between the different parts of the

body. Every thought, movement and emotions reflect the activity of the nervous system.

Functions of the NERVOUS SYSTEM

1. To monitor changes that takes place inside and outside the body. The nervous system

utilizes the million sensory receptors to carry out this function. Any changes or stimuli

occurring are noted by the nervous system and the gathered data is now called a sensory

input.

2. Another important function of the nervous system is to process and interpret the sensory

input or gathered data. It is the working of this system to make decision about what should

be done at each moment. This is the process known as INTEGRATION.

3. As the nervous system has reached a decision of what response and appropriate action to be

done in response to the stimuli, it then effects a response by activating muscles or glands

through motor output.

The nervous system consists of two parts, shown in Figure 1:

The central nervous system (CNS) consists of the brain and spinal cord, which occupy

the cavity and act as the integrating and command centers of the nervous system.

They interpret incoming sensory information and issue instructions based on past

experience and current conditions.

The peripheral nervous system (PNS) consists of nerves outside the CNS.

Nerves of the PNS are classified in three ways. First, PNS nerves are classified by how

they are connected to the CNS. Cranial nerves originate from or terminate in the brain, while

spinal nerves originate from or terminate at the spinal cord.

Second, nerves of the PNS are classified by the direction of nerve propagation. Sensory

( afferent) neurons transmit impulses from skin and other sensory organs or from various

places within the body to the CNS. Motor ( efferent) neurons transmit impulses from the CNS to

effectors (muscles or glands).

Third, motor neurons are further classified according to the effectors they target. The somatic

nervous system (SNS) directs the contraction of skeletal muscles. The autonomic nervous

system (ANS) controls the activities of organs, glands, and various involuntary muscles, such

as cardiac and smooth muscles.

The autonomic nervous system has two divisions:

The sympathetic nervous system is involved in the stimulation of activities that prepare

the body for action, such as increasing the heart rate, increasing the release of sugar

from the liver into the blood, and other activities generally considered as fight-or-flight

responses (responses that serve to fight off or retreat from danger).

The parasympathetic nervous system activates tranquil functions, such as stimulating

the secretion of saliva or digestive enzymes into the stomach and small intestine.

Generally, both sympathetic and parasympathetic systems target the same organs,

but often work antagonistically. For example, the sympathetic system accelerates the

heartbeat, while the parasympathetic system slows the heartbeat. Each system is

stimulated as is appropriate to maintain homeostasis.

Figure 1. Two parts of the nervous system.

Motor Neuron

Term Function(s) Structure(s)

Axon   A nerve fibre: a single process

extending from the cell body of a

neurone and carrying nerve

impulses away from it.

Dendrite Carries nerve impulses from

adjacent neurons into the cell

body.

One of the shorter branching

processes of the cell body of a

neurone. All dendrites have

synaptic knobs at the ends, which

are the "connections" to adjoining

nerves.

Mixed Nerves Contain both motor and sensory

nerve fibres running to/from a

particular region of the body.

(Examples include most large

nerves such as the brachial

nerves, and all the spinal nerves.)

 

Motor Neurone =

"Efferent Neurone"

Transmitting impulses (electrical

signals for communication within

the body) from the Central Nervous

System (which is sometimes

referred to by the abreviation:

CNS, and) which consists of the

brain and spinal cord, to muscles &

glands elsewhere in the body.

Myelin Sheath Three key functions of Myelin

Sheath are:

A complex material formed of

protein and phospholipid (fat) that

is laid down as a sheath around

the axons of certain neurons.Protection of the nerve fibre.

Insulation of the nerve fibre.

Increases the rate of transmission

of nerve impulses.

Nerve A nerve is a bundle containing

100s to 1000s of axons (thread-like

conductors) plus the associated

connective tissue and blood

vessels.

Neurilemma The sheath of the axon of a nerve

fibre. The neurilemma of a

medullated fibre contains myelin

laid down by Schwann cells.

Neurone A cell specialized to transmit

electrical nerve impulses and so

carry information from one part of

the body to another.

"Neurone" = "Nerve Cell"

Each neurone has an enlarged

portion the 

cell body (perikaryon), containing

the nucleus; from the body extend

several processes (dendrites)

through which impulses enter from

their branches. A longer process,

the nerve fibre (axon), extends

outwards and carries impulses

away from the cell body. This is

normally unbranched except at the

nerve ending. 

The point of contact of one

neurone with another is known as

a synapse.

Nodes of Ranvier Key functions of Nodes of Ranvier

include:

Gaps that occur at regular intervals

in the myelin sheath of medullated

nerve fibres, between adjacent

Schwann cells.Allowing nutrients and waste

products to enter/leave the

neurone.

Allowing nerve impulses to move

along the neurone through a

process of 

de-polarisation and re-polarisation

of the nerve membrane.

Relay Neurone Located within the Central Nervous

System (CNS), relay neurons

transmit the electrical impulses

generated by the stimuli to other

nerves.

Schwann Cells Schwann cells are the cells that lay

down the protective myelin sheath

around the axon of medullated

nerve fibers.

Each Schwann cell protects one

length of axon, around which it

twists as it grows - enveloping the

axon.

Sensory Neurone Transmit impulses inwards from

sense organs to the Central

Nervous System (CNS).

Synapse Reaching a synapse, an impulse The minute gap across which

causes the release of a

neurotransmitter, which diffuses

across the gap and triggers an

electrical impulse in the next

neurone. (Some brain cells have

more than 15,000 synapses.)

nerve impulses pass from one

neurone to the next, at the end of a

nerve fibre.

Classification:

Bipolar neurons: small neurons with two distinct processes; a dendritic process and an

axon extending from the cell body. They are CNS neurons specific for transmitting

information from specialized sensory systems: sight, smell and hearing.

Unipolar neurons: large neurons with the cell body lying to one side of the continuous

dendritic process and axon.

Multipolar neurons: large neurons with several dendrites and a single axon extending

from the cell body.

Grey and white matter: Grey matter consisting of unmyelinated neurons is the processing area

of the CNS. White matter – located in the inner cortex and surrounding grey matter in the spinal

cord - provide pathways of communication between grey matter.

Glial Cells

CNS Glial Cell Types: there are 4 types of glial cells

1. Astrocytes - Regulates the chemical microenvironment surrounding neurons,

2. Oligodendrocytes - Myelinate central nervous system axons,

3. Microglia - Migrating phagocytic cells resembling immune cells that remove waste,

debris, and pathogens and

4. Ependymal cells - Columnar cells that line the ventricals of the brain and the spinal

canal in the spinal cord.

PNS supporting cells: 2 types

1. Schwann cells - myelin sheaths around nerve fibers, and

2. Satellite cells - act as protective and cushioning cells.

PERIPHERAL NERVOUS SYSTEM

The PNS is the communication network between the CNS and the rest of the body.

Organization and function: The peripheral nervous system (PNS) includes all neural tissue

excluding the brain and the spinal cord.

Proprioception: involve sensors that keep track of where the body is in space.The five senses:

The sensory nervous system includes sensory organs, which receive information from the

environment, and sends it to the CNS.

Skin: detects temperature, touch, and painful stimuli. Three separate kinds of nerves

detect sensation on the skin

o Mechanoreceptors: Detect pressure and tension on the skin

o Thermoreceptors: Detect the temperature of the

stimulus

o Nociceptors: Detect painful stimuli.

Nose: detects aromatic molecules. Thousands of chemicals can be detected by our

olfactory and taste receptors and sorted into “pleasant, toxic, etc.”

Tongue: taste buds detect salty, bitter, sweet, and sour information.

Ears: detect sound waves with mechanical receptors. Fluctuations in air pressure move

a membrane attached to hair cells in the Organ of Corti. These motions open ion

channels in neurons, sending the signal to the CNS.

Eyes: detect photons or light. The retina is the neural portion of the eye. Photons (light)

activate receptors on the retina and the signal is transported to the CNS via the optic

nerve.

Motor nervous system

Spinal Nerve Anatomy: There are 31 nerves exiting the spinal cord, dorsal connections

bring sensory information to the CNS, ventral motor connections send commands to the

periphery.

Reflexes: For painful stimuli, involuntary withdrawal (like a hand from a flame) occurs

without input from the brain. This very simple nervous pathway is called a reflex arc.

Autonomic nervous system: directly controls automatic body functions (involuntary

movements). The autonomic system has two opposing parts: the sympathetic and

parasympathetic nervous systems.

Transmission of Nerve Impulses

The transmission of a nerve impulse along a neuron from one end to the other occurs as a

result of electrical changes across the membrane of the neuron. The membrane of an

unstimulated neuron is polarized—that is, there is a difference in electrical charge between the

outside and inside of the membrane. The inside is negative with respect to the outside.

Polarization is established by maintaining an excess of sodium ions (Na+) on the outside and an

excess of potassium ions (K+) on the inside. A certain amount of Na+ and K+ is always leaking

across the membrane through leakage channels, but Na+/K+ pumps in the membrane actively

restore the ions to the appropriate side.

The main contribution to the resting membrane potential (a polarized nerve) is the difference in

permeability of the resting membrane to potassium ions versus sodium ions. The resting

membrane is much more permeable to potassium ions than to sodium ions resulting in slightly

more net potassium ion diffusion (from the inside of the neuron to the outside) than sodium ion

diffusion (from the outside of the neuron to the inside) causing the slight difference in polarity

right along the membrane of the axon.

Other ions, such as large, negatively charged proteins and nucleic acids, reside within the cell. It

is these large, negatively charged ions that contribute to the overall negative charge on the

inside of the cell membrane as compared to the outside.

In addition to crossing the membrane through leakage channels, ions may cross through gated

channels. Gated channels open in response to neurotransmitters, changes in membrane

potential, or other stimuli.

The following events characterize the transmission of a nerve impulse (see Figure 1):

Resting potential. The resting potential describes the unstimulated, polarized state of a

neuron (at about –70 millivolts).

Graded potential. A graded potential is a change in the resting potential of the plasma

membrane in the response to a stimulus. A graded potential occurs when the stimulus

causes Na+ or K+ gated channels to open. If Na+channels open, positive sodium ions

enter, and the membrane depolarizes (becomes more positive). If the stimulus opens

K+ channels, then positive potassium ions exit across the membrane and the

membrane hyperpolarizes (becomes more negative). A graded potential is a local

event that does not travel far from its origin. Graded potentials occur in cell bodies and

dendrites. Light, heat, mechanical pressure, and chemicals, such as neurotransmitters,

are examples of stimuli that may generate a graded potential (depending upon the

neuron).

Figure 1. Events that characterize the transmission of a nerve impulse.

The following four steps describe the initiation of an impulse to the “resetting” of a neuron to

prepare for a second stimulation:

1. Action potential. Unlike a graded potential, an action potential is capable of traveling

long distances. If a depolarizing graded potential is sufficiently large, Na+ channels in

the trigger zone open. In response, Na+ on the outside of the membrane becomes

depolarized (as in a graded potential). If the stimulus is strong enough—that is, if it is

above a certain threshold level—additional Na+gates open, increasing the flow of

Na+ even more, causing an action potential, or complete depolarization (from –70 to

about +30 millivolts). This in turn stimulates neighboring Na+ gates, farther down the

axon, to open. In this manner, the action potential travels down the length of the axon

as opened Na+ gates stimulate neighboring Na+ gates to open. The action potential is

an all-or-nothing event: When the stimulus fails to produce depolarization that exceeds

the threshold value, no action potential results, but when threshold potential is

exceeded, complete depolarization occurs.

2. Repolarization. In response to the inflow of Na+, K+ channels open, this time allowing

K+ on the inside to rush out of the cell. The movement of K+ out of the cell causes

repolarization by restoring the original membrane polarization. Unlike the resting

potential, however, in repolarization the K+ are on the outside and the Na+ are on the

inside. Soon after the K+ gates open, the Na+gates close.

3. Hyperpolarization. By the time the K+ channels close, more K+ have moved out of the

cell than is actually necessary to establish the original polarized potential. Thus, the

membrane becomes hyperpolarized (about –80 millivolts).

4. Refractory period. With the passage of the action potential, the cell membrane is in an

unusual state of affairs. The membrane is polarized, but the Na+ and K+ are on the

wrong sides of the membrane. During this refractory period, the axon will not respond

to a new stimulus. To reestablish the original distribution of these ions, the Na+ and

K+ are returned to their resting potential location by Na+/K+ pumps in the cell

membrane. Once these ions are completely returned to their resting potential location,

the neuron is ready for another stimulus.

CENTRAL NERVOUS SYSTEM

The anatomy of the brain is complex due its intricate structure and function. This amazing organ

acts as a control center by receiving, interpreting, and directing sensory information throughout

the body. There are three major divisions of the brain. They are the forebrain, the midbrain, and

the hindbrain.

Anatomy of the Brain: Brain Divisions

The forebrain ( Prosencephalon )is responsible for a variety of functions including

receiving and processing sensory information, thinking, perceiving, producing and

understanding language, and controlling motor function.

There are two major divisions of forebrain:

The diencephalon contains structures such as the thalamus and hypothalamus which

are responsible for such functions as motor control, relaying sensory information, and controlling

autonomic functions.

The telencephalon contains the largest part of the brain, the cerebrum. Most of the

actual information processing in the brain takes place in the cerebral cortex. 

The midbrain (Mesencephalon) and the hindbrain together make up the brainstem. The

midbrain is the portion of the brainstem that connects the hindbrain and the forebrain. This

region of the brain is involved in auditory and visual responses as well as motor function.

The hindbrain (Rhombencephalon )extends from the spinal cord and is composed of

the metencephalon and myelencephalon. The metencephalon contains structures such as

the pons and cerebellum. These regions assists in maintaining balance and equilibrium,

movement coordination, and the conduction of sensory information. The myelencephalon is

composed of the medulla oblongata which is responsible for controlling such autonomic

functions as breathing, heart rate, and digestion. 

Anatomy of the Brain: Structures

The brain contains various structures that have a multitude of functions. Below is a list of major

structures of the brain and some of their functions.

Basal Ganglia

Involved in cognition and voluntary movement

Diseases related to damages of this area are Parkinson's and Huntington's

o Parkinson's disease: Nerves in a central area of the brain degenerate slowly,

causing problems with movement and coordination. A tremor of the hands is a

common early sign.

o Huntington's disease: An inherited nerve disorder that affects the brain. Dementia

and difficulty controlling movements (chorea) are its symptoms.

Brainstem

Relays information between the peripheral nerves and spinal cord to the upper parts of the

brain

Consists of the midbrain, medulla oblongata, and the pons.

Broca's Area

Speech production

Understanding language

Central Sulcus (Fissure of Rolando)

Deep grove that separates the parietal and frontal lobes

Cerebellum

Controls movement coordination

Maintains balance and equilibrium

Cerebral Cortex

Outer portion (1.5mm to 5mm) of the cerebrum

Receives and processes sensory information

Divided into cerebral cortex lobes

Cerebral Cortex Lobes

Frontal Lobes 

is concerned with higher intellectual functions, such as abstract thought and reason, speech

(Broca's area in the left hemisphere only), olfaction, and emotion. Voluntary movement is

controlled in the precentral gyrus (the primary motor area); involved with decision-making,

problem solving, and planning.

The primary motor cortex is the most posterior part of the precentral gyrus. The primary

motor cortex on one side controls all moving parts on the contralateral side of the body

(shown on a spatial map called a homunculus); 90% of motor fibers from each hemisphere

cross the midline in the brain stem. Thus, damage to the motor cortex of one hemisphere

causes weakness or paralysis mainly on the contralateral side of the body.

The medial frontal cortex (sometimes called the medial prefrontal area) is important in

arousal and motivation. If lesions in this area are large and extend to the most anterior part

of the cortex (frontal pole), patients sometimes become abulic (apathetic, inattentive, and

markedly slow to respond).

The orbital frontal cortex (sometimes called the orbital prefrontal area) helps modulate social

behaviors. Patients with orbital frontal lesions can become emotionally labile, indifferent to

the implications of their actions, or both. They may be alternately euphoric, facetious, vulgar,

and indifferent to social nuances. Bilateral acute trauma to this area may make patients

boisterously talkative, restless, and socially intrusive. With aging and in many types of

dementia, disinhibition and abnormal behaviors can develop; these changes probably result

from degeneration of the frontal lobe, particularly the orbital frontal cortex.

The left posteroinferior frontal cortex (sometimes called Broca's area or the posteroinferior

prefrontal area) controls expressive language function. Lesions in this area cause expressive

aphasia (impaired expression of words).

The dorsolateral frontal cortex (sometimes called the dorsolateral prefrontal area)

manipulates very recently acquired information—a function called working memory. Lesions

in this area can impair the ability to retain information and process it in real time (eg, to spell

words backwards or to alternate between letters and numbers sequentially).

Occipital Lobes

is responsible for interpretation and processing of visual stimuli from the optic nerves, and

association of these stimuli with other nervous inputs and memories ;involved with vision and

color recognition.

The occipital lobes contain the primary visual cortex and visual association areas. Lesions in

the primary visual cortex lead to a form of central blindness called Anton's syndrome; patients

become unable to recognize objects by sight and are generally unaware of their deficits.

Seizures in the occipital lobe can cause visual hallucinations, often consisting of lines or

meshes of color superimposed on the contralateral visual field.

Parietal Lobes 

is dedicated to sensory awareness, particularly in the postcentral gyrus (the primary

sensory area). It is also concerns with abstract reasoning, language interpretation and

formation of a mental egocentric map of the surrounding area; receives and processes

sensory information.

Parts of the midparietal lobe of the dominant hemisphere are involved in abilities such as

calculation, writing, left-right orientation, and finger recognition. Lesions in the angular gyrus

can cause deficits in writing, calculating, left-right disorientation, and finger-naming

(Gerstmann's syndrome).

The nondominant parietal lobe integrates the contralateral side of the body with its

environment, enabling people to be aware of this environmental space, and is important for

abilities such as drawing. Acute injury to the nondominant parietal lobe may cause neglect of

the contralateral side (usually the left), resulting in decreased awareness of that part of the

body, its environment, and any associated injury to that side (anosognosia). For example,

patients with large right parietal lesions may deny the existence of left-sided paralysis.

Patients with smaller lesions may lose the ability to do learned motor tasks (eg, dressing,

other well-learned activities)—a spatial-manual deficit called apraxia.

Temporal Lobes

is concerned with emotional development and formation, and also contains the auditory

area responsible for processing and discrimination of sound. It is also the area thought to be

responsible for the formation and processing of memories.; involved with emotional

responses, memory, and speech.

Patients with epileptogenic foci in the medial limbic-emotional parts of the temporal lobe

commonly have complex partial seizures, characterized by uncontrollable feelings and

autonomic, cognitive, or emotional dysfunction. Occasionally, such patients have personality

changes, characterized by humorlessness, philosophic religiosity, and obsessiveness.

Cerebrum

Largest portion of the brain

Consists of folded bulges called gyri that create deep furrows

Corpus Callosum

Thick band of fibers that connects the left and right brain hemispheres

Cranial Nerves

Twelve pairs of nerves that originate in the brain, exit the skull, and lead to the head, neck

and torso

Fissure of Sylvius (Lateral Sulcus)

Deep grove that separates the parietal and temporal lobes

Limbic System Structures

Amygdala - involved in emotional responses, hormonal secretions, and memory 

Cingulate Gyrus - a fold in the brain involved with sensory input concerning emotions and the

regulation of aggressive behavior 

Fornix - an arching, fibrous band of nerve fibers that connect the hippocampus to the

hypothalamus 

Hippocampus - sends memories out to the appropriate part of the cerebral hemisphere for

long-term storage and retrievs them when necessary 

Hypothalamus - directs a multitude of important functions such as body temperature, hunger,

and homeostasis 

Olfactory Cortex - receives sensory information from the olfactory bulb and is involved in the

identification of odors 

Thalamus - mass of grey matter cells that relay sensory signals to and from the spinal cord

and the cerebrum

Medulla Oblongata - Lower part of the brainstem that helps to control autonomic functions

Meninges - membranes that cover and protect the brain and spinal cord

Olfactory Bulb - Bulb-shaped end of the olfactory lobe and involved in the sense of smell

Pineal Gland - Endocrine gland involved in biological rhythms and secretes the hormone

melatonin

Pituitary Gland - endocrine gland involved in homeostasis and regulates other endocrine

glands

Pons - Relays sensory information between the cerebrum and cerebellum

Reticular Formation - Nerve fibers located inside the brainstem and regulates awareness and

sleep

Substantia Nigra - Helps to control voluntary movement and regualtes mood

Tectum -The dorsal region of the mesencephalon (mid brain)

Tegmentum - The ventral region of the mesencephalon (mid brain).

Ventricular System - connecting system of internal brain cavities filled with cerebrospinal fluid

Aqueduct of Sylvius - canal that is located between the third ventricle and the fourth ventricle 

Choroid Plexus - produces cerebrospinal fluid 

Fourth Ventricle - canal that runs between the pons, medulla oblongata, and the cerebellum 

Lateral Ventricle - largest of the ventricles and located in both brain hemispheres 

Third Ventricle - provides a pathway for cerebrospinal fluid to flow

Wernicke's Area - Region of the brain where spoken language is understood

Cerebrospinal fluid 

Cerebrospinal fluid (CSF), clear, colourless liquid that fills and surrounds the brain and

the spinal cord and provides a mechanical barrier against shock. Formed primarily in

the ventricles of the brain, the cerebrospinal fluid supports the brain and provides lubrication

between surrounding bones and the brain and spinal cord. When an individual suffers a head

injury, the fluid acts as a cushion, dulling the force by distributing its impact. The fluid helps to

maintain pressure within the cranium at a constant level. An increase in the volume of blood or

brain tissue results in a corresponding decrease in the fluid. Conversely, if there is a decrease in

the volume of matter within the cranium, as occurs in atrophy of the brain, the CSF compensates

with an increase in volume. The fluid also transports metabolic waste products, antibodies,

chemicals, and pathological products of disease away from the brain and spinal-cord tissue into

the bloodstream. CSF is slightly alkaline and is about 99 percent water. There are about 100 to

150 ml of CSF in the normal adult human body.

CSF CIRCULATION:

CSF is produced in the brain by modified ependymal cells in the choroid plexus (approx.

50-70%) and the remainder is formed around blood vessels and along ventricular walls.

CSF Circulation

lateral ventricles--> foramen of Monro third ventricle --> aqueduct of Sylvius --> fourth

ventricle --> foramina of Magendie and Luschka --> subarachnoid space over brain and

spinal cord --> reabsorption into venous sinus blood via arachnoid granulations.

The fluid flows through the interventricular foramen (of Monro) into the third ventricle, is

augmented by fluid formed by the choroid plexus of this ventricle, and passes through the

cerebral aqueduct (of Sylvius) to the fourth ventricle, which also possesses a choroid plexus.

The CSF from all theses sources , as well as any formed in the central canal of the spinal cord,

escapes from the fourth ventricle into the subarachnoid space through the median aperture (of

Magendie) and lateral aperture (of Luschka).

The CSF then circulates through the freely communicating subaracchnoid cisterns at the

base of the brain. From the cisterns, most of the CSF is directed upward over the cerebral

hemispheres and smaller amounts pass downward around the spinal cord.

Normal values typically range as follows:

Pressure: 70 - 180 mm H20

Appearance: clear, colorless

CSF total protein: 15 - 60 mg/100 mL

Gamma globulin: 3 - 12% of the total protein

CSF glucose: 50 - 80 mg/100 mL (or greater than 2/3 of blood sugar level)

CSF cell count: 0 - 5 white blood cells (all mononuclear), and no red blood cells

Chloride: 110 - 125 mEq/L

Note: mg/mL = milligrams per milliliter; mEq/L = milliequivalents per liter

Note: Normal value ranges may vary slightly among different laboratories.

Blood Brain Barrier

Neurons of the brain and spinal cord are protected from many chemical damage and

biological substances by "blood brain barrier", interposed between the blood and the CSF by the

endothelial cells of the capillaries and the choroid plexus. This is clinically important because

some drugs cannot penetrate the barrier. This protective device has many elements, ranging

from junctions between endothelial cells in the capillaries of the brain, restricting permeability of

larger molecules to neuroglia. Large blood vessels penetrating the brain tissue are lined with an

inner layer of endothelium reinforced by fibromuscular tissue.

ABNORMALITIES OF CSF

Leukemic cells in the

CSF

Blood: Blood may be spilled into the CSF by accidental puncture of a leptomeningeal vein

during entry of the LP needle. Such blood stains the fluid that is drawn initially and clears

gradually. If it does not clear, blood indicates subarachnoid hemorrhage. Erythrocytes from

subarachnoid hemorrhage are cleared in 3 to 7 days. A few neutrophils and mononuclear cells

may also be present as a result of meningeal irritation. Xanthochromia (blonde color) of the CSF

following subarachnoid hemorrhage is due to oxyhemoglobin which appears in 4 to 6 hours and

bilirubin which appears in two days. Xanthochromia may also be seen with hemorrhagic infarcts,

brain tumors, and jaundice.

Increased inflammatory cells (pleocytosis) may be caused by infectious and noninfectious

processes. Polymorphonuclear pleocytosis indicates acute suppurative

meningitis. Mononuclear cells are seen in viral infections (meningoencephalitis, aseptic

meningitis), syphilis, neuroborreliosis, tuberculous meningitis, multiple sclerosis, brain abscess

and brain tumors.

Tumor cells indicate dissemination of metastatic or primary brain tumors in the subarachnoid

space. The most common among the latter is medulloblastoma. They can be best detected by

cytological examination. A mononuclear inflammatory reaction is often seen in addition to the

tumor cells.

Increased protein: In bacterial meningitis, CSF protein may rise to 500 mg/dl. A more

moderate increase (150-200 mg/dl) occurs in inflammatory diseases of meninges (meningitis,

encephalitis), intracranial tumors, subarachnoid hemorrhage, and cerebral infarction. A more

severe increase occurs in the Guillain-Barré syndrome and acoustic and spinal schwannoma.

In multiple sclerosis, CSF protein is normal or mildly increased, but there is often an

elevation of IgG in CSF, but not in serum, expressed as an elevation of the CSF IgG/albumin

index (normally 10:1). In addition, 90% of MS patients have oligoclonal IgG bands in the CSF.

Oligoclonal bands are also seen occasionally in some chronic CNS infections

. The type of oligoclonal bands is constant for each MS patient throughout the course of

the disease. Oligoclonal bands occur in the CSF only (not in the serum). These quantitative and

qualitative CSF changes indicate that in MS, there is intrathecal immunoglobulin production. In

addition, the CSF in MS often contains myelin fragments and myelin basic protein (MBP). MBP

can be detected by radioimmunoassay. MBP is not specific for MS. It can appear in any

condition causing brain necrosis, including infarcts.

Low glucose in CSF is seen in suppurative, tuberculous and fungal infections, sarcoidosis, and

meningeal dissemination of tumors. Glucose is consumed by leukocytes and tumor cells.

Spinal Cord

Even though the brain controls the activities of the whole body, it only extends down to

the top of the neck. Below that, the spinal cord carries messages between the brain and your

body. Your face has a direct connection to the brainstem, so it is independent of your spinal

cord.

The spinal cord looks like a long rope about the width of your little finger. It runs from the

base of your brain down to the lower part of your back, and it is fragile. Spinal cord injury [SCI]

can lead to loss of movement and feeling. It can also affect how your brain controls your internal

organs. When your spinal cord is injured, parts of your body below the level of the injury are

affected.

The spinal cord is protected by your

backbones. The backbones are 29 small

bones stacked on top of each other. These bones

are called vertebrae. To allow your back to bend

and to lessen jarring, each vertebra is cushioned

from the next by disks. Disks are made of a

spongy material and act like shock absorbers.

Ligaments hold the vertebrae together and allow your

neck and back to twist and bend.

The Spinal Column

Each vertebra has a hole in it—a hard, bony

tunnel through which the spinal cord passes. This is

the spinal canal. It protects the spinal cord from

damage. The vertebrae and disks, held together by

ligaments, are called the spinal column

Has four sections. The top is the cervical

section, which is your neck. The next down is the

thoracic section, which extends from your lower neck

to your lower ribs. The lumbar section is your lower

back, and the sacral section is your tailbone. Your

sacral section is really only one bone, with five nerve

pairs coming out through holes in it.

T

The cervical section contains eight pairs of nerves and seven vertebrae. The nerves

numbered C1 through C7 exit from the spine above the corresponding numbered vertebrae, and

the C8 nerve pair exits between the C7 and T1 bones on each side. For the thoracic and lumbar

sections, each of the numbered nerves lies below the corresponding numbered vertebra. There

are 12 thoracic nerves and 5 lumbar nerves on each side.

At the lower end of spinal cord [below the second lumbar vertebra], the nerves travel

long distances before they exit the spine. This is because the spinal cord itself ends much

higher, at about the level of the L1 vertebra. The lower lumbar and sacral nerves look like a

horse’s tail inside the spinal column. In fact, this area is known as the cauda equina, which

means “horse’s tail” in Latin.

How The Spinal Cord Functions

The spinal cord is the communicating link between the spinal nerves and the brain. The

long nerve fibers inside the spinal cord are called the upper motor neurons [UMNs]. They run

between the brain and the spinal nerves. The spinal nerves branch out from the spinal cord into

the tissues of your body. Spinal nerves are called lower motor neurons [LMNs]

In movement, the brain sends messages through the spinal cord [UMNs] to the spinal

nerves [LMNs]. The LMNs then carry these messages to the muscles to coordinate movements,

such as walking. In this way, the brain controls movement.

Spinal Nerves

In sensation, nerves in your body collect information

and send it up the spinal cord to the rain. This allows

you to be aware of feelings, such as heat, cold, touch,

or pain.

You may wonder how the spinal cord keeps these

messages from getting confused, with all the running

back and forth between brain and body. The motor

nerves and the sensory nerves carry messages in

different nerve fibers. Within the spinal cord, the nerve

fibers are combined into groups called spinal tracts.

Each tract carries messages one way, either up for sensation or down for voluntary movement.

They are similar to  the lanes on a freeway.

What Is a Spinal Nerve and What Does It Do?

Each spinal nerve has two main parts. One part carries information related to movement from

the spinal cord to the muscles. It is called a motor nerve. Each motor nerve connects to a

specific muscle. Each level of the spinal cord causes movement in a certain group of muscles.

Spinal Tracts for Nerves

The other part of the spinal nerve carries messages of feeling, such as heat and cold, from the

body to the spinal cord. It is called a sensory nerve. Different types of  sensation or feeling are

carried up the spinal cord to the brain. These include pain, touch, heat, cold, vibration, pressure,

and knowing where a body part is located in space without looking at it.

Each sensory nerve collects information about feelings from a given body part or area of skin.

Each skin area is called a dermatome and matches a specific spinal cord level. Try to identify

areas where you have normal sensation and where you do not.

Spinal Cord Injury

Many types of injuries and diseases can cause spinal cord injury or dysfunction. If the

space for the spinal cord [spinal canal] becomes narrowed, the spinal cord can become injured.

This can happen when bones in your back or neck are broken, or when ligaments are torn and

the vertebrae move in different directions.

Gunshot wounds, stab wounds, or fragments from explosions can directly damage the

cord without much breaking of the bones. Infections and tumors near the spine can compress

the spinal cord. Sometimes, arthritis can affect the bones and slowly compress the cord. Finally,

the blood supply to the spinal cord can be blocked, causing part of the spinal cord to die. [This is

similar to how a stroke affects the brain.]

Damage to your spinal cord can cause changes in your movement, feeling, bladder

control, or other bodily functions. The changes depend on where and how badly your spinal

cord was injured. The main problem is that the connection between your brain and your body

below the injury is impaired or broken.

A numbering system is used to name levels of injury. It’s the same as the system used to name

bone and nerve levels in your back. A spinal cord injury is named for the lowest level of the

spinal cord that still functions the way it did before the injury. It is important to your rehabilitation

that you know your level of injury and how it affects your body. The level of spinal cord injury is

not always the same level as where the spine was injured. When the spinal cord injury is at a

cervical level, it is called “tetraplegia” or “quadriplegia.” When it is at a lower level [thoracic,

lumbar, or sacral], it is called “paraplegia.” Most of the nerve supply to the arm and hand comes

from cervical nerve roots. This means that people with tetraplegia have some numbness or

weakness in their arms or hands. Paraplegia does not affect the arms or hands.

Complete and Incomplete SCI

If there is no voluntary movement [spasms don’t count—they are involuntary] and no

feeling below your spinal cord injury level, you have a complete injury. If you have some feeling

or voluntary movement below your injury, you have an incomplete injury. This happens when

there is only partial damage to your spinal cord; that is, some nerve fibers are still working

across your spinal cord injury site.

UMN and LMN Injuries

Earlier in this section, we discussed the difference between upper motor neurons

[UMNs] and lower motor neurons [LMNs]. A complete injury cuts or affects all the UMNs running

down the spinal cord. This disrupts the connection between the brain and the parts of the body

below the injury. However, the LMNs below your spinal cord injury are not damaged. Because

LMNs carry reflex actions, the reflexes below the level of injury are still working.

So the LMNs are still carrying out reflex actions below the level of injury, but this may

cause a problem. In reflexes, the brain normally controls how much your nerves react. In a UMN

injury, messages from the brain can’t get past the point of injury, so the LMNs act by

themselves, which may cause reflexes without limit. One example is spasticity, which is

uncontrolled movement of your arms or legs.

LMN injuries are a different story. This kind of injury is usually at the lower tip of the

spinal cord [the cauda equina]. The cauda equina is made up entirely of LMNs, so damage to it

impairs reflex actions, although other UMNs and LMNs above the injury are still working.

Spasticity does not occur with LMN injuries because the muscles no longer have any nerve

contact to stimulate them.

Stated simply, a UMN injury is one in which the UMN pathway is broken, the LMNs

below the injury are intact, and spasticity usually occurs.

Upper Motor Neuron Injury

An LMN injury, usually at the cauda equina, blocks nerve

activity in muscles controlled below the injury, and no spasticity

develops. It is important to know which kind of injury you have

because that will determine how it is managed.

Lower Motor Neuron Injury

No. Name

Sensory,

motor,

or both

Origin Nuclei Function

I OlfactoryPurely

sensoryTelencephalon

Anterior

olfactory

nucleus

Transmits the sense of

smell from the nasal cavity.[1] Located in olfactory

foramina in thecribriform

plate of ethmoid.

II OpticPurely

sensoryDiencephalon

[Lateral

geniculate

nucleus][2]

Transmits visual signals

from the retina of the eye

to the brain.[3] Located in

the optic canal.

III Oculomot

or

Mainly motor Anterior aspect

of Midbrain

Oculomotor

nucleu

s,Edinger-

Westphal

nucleus

Innervates the levator

palpebrae

superioris, superior

rectus, medial

rectus, inferior rectus,

and inferior oblique, which

collectively perform most

eye movements. Also

innervates thesphincter

pupillae and the muscles

of the ciliary body. Located

in the superior orbital

fissure.

IV Trochlear Mainly motorDorsal aspect of

Midbrain

Trochlear

nucleus

Innervates the superior

oblique muscle, which

depresses, rotates

laterally, and intorts the

eyeball. Located in

the superior orbital fissure.

VTrigemina

l

Both sensory

and motorPons

Principal

sensory

trigeminal

nucleus,Spinal

trigeminal

nucleu

s,Mesencephali

c trigeminal

nucleu

s,Trigeminal

motor nucleus

Receives sensation from

the face and innervates

the muscles of mastication.

Located in thesuperior

orbital fissure (ophthalmic

nerve - V1), foramen

rotundum (maxillary

nerve - V2), andforamen

ovale (mandibular nerve -

V3).

VI Abducens Mainly motor

Nuclei lying

under the floor

of the fourth

ventricle Pons

Abducens

nucleus

Innervates the lateral

rectus, which abducts the

eye. Located in

the superior orbital fissure.

VII FacialBoth sensory

and motor

Pon

s(cerebelloponti

ne angle) above

olive

Facial

nucleu

s,Solitary

nucleu

s,Superior

salivary

nucleus

Provides motor innervation

to the muscles of facial

expression, posterior belly

of the digastric muscle,

and stapedius muscle.

Also receives the special

sense of taste from

the anterior 2/3 of the

tongue and

provides secretomotor inne

rvation to the salivary

glands (except parotid)

and the lacrimal gland.

Located in and runs

through the internal

acoustic canal to the facial

canal and exits at

the stylomastoid foramen.

VIII Acoustic

or

Vestibulo

cochlear(

or auditor

y-

vestibular

nerve o

Mostly

sensory

Lateral to CN VII

(cerebellopontin

e angle)

Vestibular

nuclei,Cochlear

nuclei

Senses sound, rotation,

and gravity (essential for

balance and movement).

More specifically, the

vestibular branch carries

impulses for equilibrium

and the cochlear branch

carries impulses for

racoustic

nerve)

hearing. Located in

the internal acoustic canal.

IXGlossoph

aryngeal

Both sensory

and motorMedulla

Nucleus

ambiguu

s,Inferior

salivary

nucleus, Solitar

y nucleus

Receives taste from the

posterior 1/3 of the tongue,

provides secretomotor

innervation to theparotid

gland, and provides motor

innervation to

the stylopharyngeus.

Some sensation is also

relayed to the brain from

the palatine tonsils.

Located in the jugular

foramen.

X Vagus Both sensory

and motor

Posterolateral

sulcus

of Medulla

Nucleus

ambiguu

s,Dorsal motor

vagal

nucleus, Solitar

y nucleus

Supplies branchiomotor in

nervation to most laryngeal

and pharyngeal muscles

(except

thestylopharyngeus, which

is innervated by the

glossopharyngeal). Also

providesparasympathetic fi

bers to nearly all thoracic

and abdominal viscera

down to the splenic

flexure. Receives the

special sense of taste from

the epiglottis. A major

function: controls muscles

for voice and resonance

and the soft palate.

Symptoms of

damage: dysphagi

a(swallowing

problems), velopharyngeal

insufficiency. Located in

the jugular foramen.

XI

Accessor

y or

spinal-

accessor

y(or crani

al

accessory

nerveor s

pinal

accessory

nerve)

Mainly motorCranial and

Spinal Roots

Nucleus

ambiguu

s,Spinal

accessory

nucleus

Controls the

sternocleidomastoid and

trapezius muscles, and

overlaps with functions of

the vagus nerve (CN X).

Symptoms of damage:

inability to shrug, weak

head movement. Located

in the jugular foramen.

XIIHypoglos

salMainly motor Medulla

Hypoglossal

nucleus

Provides motor innervation

to the muscles of the

tongue (except for

the palatoglossus, which is

innervated by the vagus

nerve) and other glossal

muscles. Important for

swallowing (bolus

formation) and speech

articulation. Located in

the hypoglossal canal.