Download 27: Protection and Support of the Central Nervous System

The Human Body
27: Protection and Support of the Central Nervous System
•
Nervous tissue in the central nervous system is very fragile, so it requires support and
protection. Connective tissue (in the form of meninges), bone, fluid and filtration
work to fulfill these requirements.
Anatomy of the vertebral column and skull:
a. Vertebral column:
Irregular bones called vertebrae form the vertebral column, which provides support
and protection for the spinal cord.
• There are 30 vertebrae in total:
- 7 cervical vertebrae
- 12 thoracic vertebrae
- 5 lumbar vertebrae
- 5 fused sacral vertebrae
- 1 coccygeal vertebra (which also consists of fused bones)
• 31 pairs of spinal nerves branch off the spinal cord, which lies inside these vertebrae.
•
1
www.aptitute.co.nz
© 2009 AptiTute
The Human Body
•
Structure of a typical vertebra:
- Anterior: Vertebral body
- Posterior: Spinous process
- Lateral: Transverse processes
- Central: The vertebral foramen, a gap for the spinal cord
- Nerves exit through inter-vertebral foramina located between the transverse
processes of adjacent vertebrae.
b. Skull:
•
•
The bones that comprise the skull provide support and protection for the brain.
The skull consists of:
- The 8 bones of the cranium
- The 14 bones of the face
- The 6 bones of the ossicles
•
The bones of the cranium (the part of the skull that holds the brain) are separated by
ossified fibrous joints known as ‘sutures’.
•
Names for some bones of the cranium correspond to those for lobes of the brain:
2
www.aptitute.co.nz
© 2009 AptiTute
The Human Body
BONES OF THE CRANIUM
Frontal
•
•
Anterior
Forms forehead
Parietal (2)
•
•
Lateral
One on either side
Temporal (2)
•
•
•
Lateral
One on either side
Form temples
Occipital
•
•
Posterior
Has a large hole for brain
stem
Sphenoid
•
•
Forms back of the orbit
Bat-shaped bone
Ethmoid
•
Makes up floor of nasal
bone
Contains cribriform plate,
with numerous foramina
for olfactory nerves
•
Anatomy of the meninges:
•
•
•
•
The meninges lie between the brain and skull and act as a protective covering for the
brain.
The cerebrum of the brain consists of two ‘hemispheres’ separated by layers of
meninges that lie in the median plane, called the falx cerebri.
Layers of meninges called tentorium cerebelli sit over the cerebellum (located in
the posterior and inferior part of the head) like a tent. Medially, these layers form
flaps that surround the brainstem.
The hemispheres of the cerebellum are separated by meninges called the falx
cerebelli.
3
www.aptitute.co.nz
© 2009 AptiTute
The Human Body
•
The meninges consist of three layers:
a. Dura mater:
•
•
•
•
The dura mater (‘hard mother’) is the outermost layer and the toughest of the three,
it follows the contours of the skull.
Consists of two layers made of dense fibrous connective tissue (DFCT).
The outer layer lines the skull, and the inner layer folds down to form the falx cerebri
and tentorium cerebelli.
Spaces between these layers contain venous blood (blood returning to the heart) and
are known as the ‘venous sinuses’.
b. Arachnoid mater:
•
•
•
•
The arachnoid mater (‘spider-like mother’) is inner to the dura mater and is
characterised by its many spider-like extensions.
These extensions are called arachnoid villi, and lie in between the arachnoid mater
and pia mater. They protrude into the venous sinuses and provide a path for
cerebrospinal fluid to enter the blood.
It stretches over the sulci (grooves) and gyri (bumps) of the brain to form a smooth
layer.
Associated with blood vessels.
The area in between the arachnoid mater and the pia mater is known as the subarachnoid space.
4
www.aptitute.co.nz
© 2009 AptiTute
The Human Body
c. Pia mater:
•
•
•
The pia mater (‘delicate mother’) is the innermost layer of the meninges and adheres
to and follows the contours of the brain. It is transparent and very delicate.
Lines the sulci and gyri of the brain as opposed to bridging over them like the
arachnoid mater.
Contains blood vessels.
Cerebrospinal fluid:
•
•
•
•
•
•
•
•
Cerebrospinal fluid (CSF) is a straw-coloured fluid that bathes and cushions the
brain as well as acting as a shock absorber.
Provides nutrients for the brain and facilitates the removal of waste products.
Is closely monitored by the brain in order to detect and respond to changes in the
internal environment.
Generated in the choroid plexi (found on the ventricles of the brain) by separating
fluid from blood.
The brain has four ventricles: two lateral ventricles, the third ventricle and the
fourth ventricle.
CSF circulates around the spinal cord, and through the sub-arachnoid space of the
brain.
It is reabsorbed into the venous sinus through the arachnoid villi, preventing buildup.
Can be sampled by performing a lumbar puncture (LP) between the L3 and L4
vertebrae, in order to test for meningitis (inflammation of the meninges).
The blood brain barrier (BBB):
•
•
•
Provides a filtration barrier to protect the brain: prevents large molecules from
entering.
Composed of tightly packed endothelial cells supported by astrocytes (a type of glial
cell).
The ‘feet’ of the astrocytes surround the cells of the capillary wall forming a filter.
5
www.aptitute.co.nz
© 2009 AptiTute