The Central Nervous System

The Central Nervous System

Protection
of the Brain

1. Body membranes that cover the brain and spinal cord for
protection and nourishing purposes.

2. Consists of 3 layers: (each layer is a “menix”)

a) Dura mater
(“tough mother
“): 
outermost layer that attaches the brain to cranial bones, and the spinal
cord to vertebrae.  Made up of fibrous
connective tissue that protects the CNS.

b) Arachnoid mater
(“web like mother”):
middle layer made up of thin membranes that
lack blood vessels. It reabsorbs cerebrospinal fluid (CSF).

c) Pia mater
(“gentle mother”):
innermost layer made of thin membranes that
contain capillaries for providing nourishment to the brain and spinal cord.
Forms capillary networks called plexuses which produce CSF.

The Brain

1. The largest organ in the nervous system; composed of
about 100 billion neurons (interestingly, although the neurons contain DNA,
there is no DNA replication or mitosis in the brain, as a result the number of
neurons decreases as a person ages).

2. Divided into 3 main regions:  Cerebrum, Cerebellum, and the Brain Stem.

3. Contains spaces called ventricles where choroid plexuses
of pia mater produce cerebrospinal fluid (CSF), and these ventricles allow CSF
to circulate around the brain and into the spinal cord (through the central
canal).

4. Cerebrum:

a) Cerebral Cortex (outer region) is made of gray matter
(unmyelimated neurons) which contains up 75% of all neurons in the nervous
system, while Cerebral Medulla (inner region) is made of white matter
(myelinated neurons).

b) Consists of left and right hemispheres, created by the longitudinal
fissure at the center of cerebrum, and are connected by the corpus callosum.

c)  Its surface is
marked by ridges called convolutions (gyri) which are separated by grooves called
sulcus (or fissure, if the grooves are deeper).

d) Frontal lobe controls skeletal muscle movement and
intellectual processes.

e) Parietal lobe controls sensations and speech.

f) Temporal lobe controls hearing and memory.

g) Occipital lobe controls vision.

h) Functional regions of cerebral cortex:

Motor areas:
located in frontal lobe, to control voluntary muscles.

Motor speech area
(“Broca’s area”):
located in frontal lobe, to control muscles of
mouth, tongue, and larynx for speech.

 

Frontal eye field:
located in frontal lobs just above the broca’s area, to control muscles of the
eye and eyelid.

Auditory area:
located in temporal lobe, to control hearing. Visual area: located in occipital
lobe, to control visual recognition of objects and combine visual images.

Sensory areas:
located in parietal lobe, to be involved in cu!aneous sensations of
temperature, touch, pressure and pain.

Association areas:
located in all of cerebral cortex, to interconnect sensory and motor functions
of all lobes of the cerebrum.

5.
Cerebellum

• Coordinates and controls muscular movement and muscle
tone.

• Maintains body posture, by working with the equilibrium
receptors in the inner ear.

    New data suggest
that it also functions as the “thesaurus” for speech, finding the
right words to use.

Brain Stem

6. Made of brain tissue at the base of cerebrum, connecting
the cerebrum to the spinal cord.

    Functions largely
for autonomous activities.

    Subdivided into
diencephalon, midbrain, Pons and Medulla Oblongata.

I.
Diencephalon:
consists of Thalamus and Hypothalamus.

 

a) Thalamus- It
is a major relay center to direct nerve impulses from various sources to the
proper destinations.

b) Hypothalamus (an
important area for regulating homeostatic activities, such as hunger, thirst,
sex drive, and even addictions).

Hypothalamus
regulates:

1) Heart rate and arterial blood pressure.

2) Body temp.

3) Water & electrolyte balance.

4) Control of hanger & body weight.

5) Control of movements and glandular secretion of the
stomach & intestines.

6) Production of neurosecretory substances that stimulate
the pituitary gland to release hormones that help regulate growth and influence
reproduction

Other parts of
diencephalons:

1) optic tracts  and
optic chiasma that is farmed by optic nerves 
crossing  over.

2) infundibulum – site of attachment of pituitary gland

3) post. Pituitary gland• hangs from the floor of
hypothalamus

4) mammillary bodies• Relay 
station in olfactory pathway.

5) pineal  gland-  produce melatonin that controls sleep – awake
cycle.

II.
Midbrain:
  serves as a
major cerebral reflex center, and also helps direct CSF from the third
ventricle to fourth ventricle.

III. Pons: contains
at least 2 “respiratory centers” (groups of specialized neurons)
which regulate the duration and depth of breathing.

IV. Medulla
oblongata:
  at the base of
base of brain stem and continuous to become spinal cord. It contains
specialized neurons that form”cardiac centers” (to control heart
rate), “vasomotor centers” (to control blood flow and blood
pressure), and “respiratory centers” (to control respiratory rhythms).

7.
Cerebrospinal fluid (CSF):

• Its chemical composition is similar to blood plasma, since
it is derived from brood plasma.

• Made by choroids plexuses (clusters of capillaries in pia
mater) in cerebral ventricles.

• Circulates in the brain (through the ventricles), spinal
cord (through central canal), and between arachnoid and pia mater layers of
meanings around the central nervous system.

• Re.absorbed constantly by arachnoid mater and drained into
veins.

• Mainly designed to protect CNS by serving as a cushion,
and as a relatively stable solution in maintaining ionic concentrations and
waste removal.

• When too much CSF accumulates in the skull, it can result
in hydrocephalus where the excess volume creates dangerously high pressure
exerting onto brain tissue.

Structural
Development of the Brain

Embryonic vesicle:
Forebrain (Prosencephalon)

a. Anterior portion (Telencephalon) produces Lateral
Ventricles, cerebrum and basal ganglia.

b. Posterior portion (Diencephalon) produces Third
Ventricle, Thalamus, Hypothalamus, post. Pituitary gland, and pineal gland.

Embryonic vesicle: Midbrain
(Mesencephalon)
produces cerebral aqueduct, and midbrain.

Embryonic vesicle:  Hindbrain (Rhombencephalon)

a. Anterior portion (Metencephalon) produces fourth
ventricle, cerebellum, and pons.

b. Posterior portion (Myelencephalon) produces fourth
ventricle, and medulla oblongata.

Spinal cord

• A long nerve cord that begins at the foramen magnum and
ends at the first or second lumbar vertebra.

• Divided into 31 segments (named after the vertebral
regions), each segment gives rise to a pair of spinal nerves (part of the PNS).

• In general, the location of the spinal nerve corresponds
with the location of the effector organ (e.g. cervical nerves connect to muscles
and glands on the head, face, and neck).

• Most spinal nerves form net – works called plexuses.

• C1 to C4 from. Cervical plexus which serves the head,
face, and neck.

• C5 to T1 from brachia! Plexus which serves the shoulder,
arm, and hands.

• T 2 to T11 do not from any plexus.

• T12 to S5 form lumbosacral plexus which serves the lower
body and lower limbs.

• Coccygeal nerves do not form any plexus.

Cross
sectional anatomy of the spinal cord:

Two grooves divide spinal cord into night left halves.

1. Ant. median fissure or groove (deep).

2. Post median sulcus (shallow groove).

    The spinal cord
consist of white matter surrounded by gray matter.

Gray matter resembles a butterfly. The upper and lower wings
of gray matter are called posterior horn and anterior horn respectively.

    Lat. horn is
located between post.  and ant. horns on
either side .

   Central canal
contains CSF.

    The gray matter
further divides the white matter into 3 regains on each side.

a) The anterior column (or Funiculi)

b) The lateral column (or Funiculi)

c) The posterior column (or Funiculi)

Ant. horn mostly house nerve cells bodies of somatic motor
neurons. These send their axons out via the ventral root of the spinal cord to
skeletal muscles.

Afferent fibers carrying impulses from peripheral sensory
receptors form the dorsal root. Their nerve cell bodies are found in an
enlarged area called dorsal root ganglion.

Tracts of the
Spinal Cord

• Tracts: The
nerve tracts of the spinal cord provide a two- way communication system between
the brain and the body.

a) Ascending tract:
conduct sensory impulses to the brain.

b) Descending tract:
conduct motor impulses from the brain to motor neurons reaching muscles,
glands, etc.

    A) Ascending Tracts

1. Fasciculus
gracilis (transmits sensory impulses 
from lower limbs) and Fasciculus Cuneatus (transmits sensory
impulses  from upper limbs):
Located in
posterior funiculi  and conduct sensory
impulses associated with senses of touch, pressure  and body movement  from skin, muscles,  tendons, 
and joints  to the brain.

2. Spinocerebellar tract: conduct impulses (sensory)
required for coordination of muscle movements from lower limb and trunk muscles
to cerebellum.

3. Spinothalamic tract: conduct sensory impulses for pain
and temperature to brain.

    B. Descending Tracts

1.  Corticospinal tract: conducts motor
impulses associated with voluntary movement from the brain to skeletal muscles.

2. Reticulospinal
tract:
conducts motor impulses associated with maintenance of muscle tone and
the activity of sweat glands from the brain.

Reflex Arc

In a spinal reflex arc, a stimulus ( i.e. heat, sharp
objects) has to be detected by a receptor which sends a nerve impulse to the
sensory neuron, which relays the impulse through the dorsal root of the spinal
cord, to the interneuron in the gray matter of spinal cord. The impulse is now
relayed to the motor neuron through the ventral root of the spinal cord, and
finally causes a quick action at the effector organ (muscle or gland) to react
to the stimulus.

Clinical
Terms

• Neurologist:  doctor who specializes in nervous system disorders.

• Concussion:  slight brain injury causing dizzines   & loss of consc . ousness.

• Cerebrovascular
accident:
commonly known as stroke occurs when blood flow to brain is
blocked.

• Alzheimer’s
disease:
progressive disease causing degeneration of brain, loss of memory,
language and disorientation.

• Epilepsy: disorder
of the CNS That is caused by temporary disturbances in normal brain function (impulses)
and is accompanied by seizures and loss of consciousness.

• Parkinson’s disease:
disorder of brain causing pill-rolling movement of fingers tremor, head nodding,
and lack of facial expression.

• Encephalopathy:
any disorder of the brain.

• Neuralgia: sharp
recurring pain associated with a nerve, usually caused by inflammation or
injury.

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