Cerebrospinal fluid

Cerebrospinal fluid (CSF) is a watery liquid similar in composition to blood plasma. It is
formed in the choroid plexuses and circulates through the ventricles into the subarachnoid
space, where it is returned to the dural venous sinuses by the arachnoid villi. The prime
purpose of the CSF is to support and cushion the brain and help nourish it.  Figure 2–4
illustrates the flow of CSF through the central nervous system.

Major regions of the brain and their functions

The major regions of the brain (Figure 2–5) are the  cerebral hemispheres,  diencephalon,
brain stem and cerebellum

Cerebral hemispheres

The cerebral hemispheres (Figure 2–6), located on the most superior part of the brain, are
separated by the longitudinal fissure. They make up approximately 83% of total brain mass,
and are collectively referred to as the cerebrum. The cerebral cortex constitutes a 2-4 mm
thick grey matter surface layer and, because of its many convolutions, accounts for about
40% of total brain mass. It is responsible for conscious behaviour and contains three
different functional areas: the motor areas,  sensory areas and association areas. Located
internally are the white matter, responsible for communication between cerebral areas and
between the cerebral cortex and lower regions of the CNS, as well as the basal nuclei (or
basal ganglia), involved in controlling muscular movement.
Diencephalon
The  diencephalon is located centrally within the forebrain. It consists of the  thalamus,
hypothalamus and  epithalamus, which together enclose the third ventricle. The thalamus
acts as a grouping and relay station for sensory inputs ascending to the sensory cortex and
association areas. It also mediates motor activities, cortical arousal and memories. The
hypothalamus, by controlling the autonomic (involuntary) nervous system, is responsible
for maintaining the body’s homeostatic balance. Moreover it forms a part of the  limbic
system, the ‘emotional’ brain. The epithalamus consists of the pineal gland and the CSF-
producing choroid plexus.

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Human Skull

Depending on their shape, bones are classified as long, short, flat or irregular. Bones of
different types contain different proportions of the two types of osseous tissue: compact and
spongy bone. While the former has a smooth structure, the latter is composed of small
needle-like or flat pieces of bone called trabeculae, which form a network filled with red or
yellow bone marrow. Most skull bones are flat and consist of two parallel compact bone
surfaces, with a layer of spongy bone sandwiched between. The spongy bone layer of flat
bones (the diploë) predominantly contains red bone marrow and hence has a high concen-
tration of blood.
The skull is a highly complex structure consisting of 22 bones altogether. These can be
divided into two sets, the cranial bones (or cranium) and the facial bones. While the latter
form the framework of the face, the cranial bones form the cranial cavity that encloses and
protects the brain. All bones of the adult skull are firmly connected by sutures. Figure 2–2
shows the most important bones of the skull. The  frontal bone forms the forehead and
contains the  frontal sinuses, which are air filled cells within the bone. Most superior and
lateral aspects of the skull are formed by the parietal bones while the occipital bone forms
the posterior aspects. The base of the occipital bone contains the  foramen magnum, which
is a large hole allowing the inferior part of the brain to connect to the spinal cord.
remaining bones of the cranium are the temporal, sphenoid and ethmoid bones.

Meninges

The meninges (Figure 2–3) are three connective tissue membranes enclosing the brain
the spinal cord. Their functions are to protect the CNS and blood vessels, enclos
venous sinuses, retain the  cerebrospinal fluid, and form partitions within the skull.
outermost meninx is the dura mater, which encloses the arachnoid mater and the inner
pia mater.

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Basic Anatomy and Physiology of the Human Brain

Main Points

The Brain Skin

Human Skull

Brain stem

Blood supply to the brain

Cerebrospinal fluid

The human  nervous system consists of the  central nervous system (CNS) and  peripheral
nervous system (PNS). The former consists of the brain and spinal cord, while the latter
composes the nerves extending to and from the brain and spinal cord. The primary
functions of the nervous system are to monitor, integrate (process) and respond to informa-
tion inside and outside the body. The brain consists of soft, delicate, non-replaceable neural
tissue. It is supported and protected by the surrounding skin, skull, meninges and cerebro-
spinal fluid.

The brain

Ø Human brain weighs about 1.2 to 1.4kg.
Ø It consumes 25% of the body oxygen supply to generate energy;
Ø It is covered by a membrane called the Meninges.
Ø It is enclosed in a bony case called the skull or cranium.
Ø It is made up of 2 types of nervous tissues, (Nerves cells or grey matter and Nerves fibers or white matter.)





Human brain is divided into 3 parts

a.  Fore brain (i.e.  Large, forward part).
b.  Mid brain (i.e. narrow brain).
c.  Hind brain (i.e. rare path).

Fore Brain: - it consist mainly cerebrum, thalamus and hypothalamus.

Midbrain:  links the forebrain to the hindbrain. It controls auditory and  visual reflexes.

Hindbrain: composed of 3 parts 

(A) Cerebellum

(B) Pons varolii

(C) Medulla oblongota

SELF-ASSESSMENT EXERCISE 1

i.  The autonomic nervous system consists of -- and --------
ii. Mention two portions of the hind brain.

Functions of the parts of the brain
Cerebrum controls all the bodies’ voluntary action and
consciousness.
Frontal lobe – seat of intelligence, Memory, Imagination,
thought, judgment, emotional reaction and movement of
skeletal muscles.
. Parietal lobe – Receives and interprets the sensations of pressure,
temperature and position.
Temporal lobe is concerned with hearing, memory and
understanding of speech.  


Integration with other Systems

To function effectively, every cell in the body must communicate with
its neighbors and with cells and tissues in distant portions of the body.
In a few specialized cases, cellular activities are coordinated by the
exchange of ions and molecules from one cell to the next across gap
junctions. This direct communication occurs between cells of the same
type, and the two cells must be in extensive physical contact. The two
cells communicate so closely that they function as single entity. For
example, gap junctions (1) coordinate ciliary movement among
epithelial cells, (2) coordinate the contractions of cardiac muscle cells,
and (3) facilitate the propagation of action potentials from one neuron to
the next at electrical synapses.
Direct communications is highly specialized and relatively rare. Most of
the communications between cells involves the release and receipt of
chemical messages. Each cell continuously “talks” to its neighbors by
releasing chemicals into the extra cellular fluid. These chemicals tell
what their neighbors are doing at any moment; the result is the
coordination of tissue function at the local level. The use of chemical
messengers to transfer information from cell to cell within a single
tissue is called paracrine communications. The chemicals involved are
called paracrine factors, also known as cytokines, or local hormones.
Examples of paracrine factors include the prostaglandins.

SELF-ASSESSMENT EXERCISE B

(I)  What is the function of the temporal lobe? 
(II)  --------------- is an example of paracrine factors.

CONCLUSION

The nervous system includes the neural tissues that make up the
autonomic nervous system and the peripheral nervous system. The
neurons are the basic functional unit. The nervous system and the
endocrine systems are closely linked.

 SUMMARY

In this unit we have learnt the two major anatomical divisions of the
nervous system; (1) the central nervous system (II) peripheral nervous
system; their divisions and various functions.

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The Brain Skin

Human Skull

Brain stem

Blood supply to the brain

Cerebrospinal fluid

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