The Central Nervous System
The Central Nervous System
Functional Anatomy of the Brain
· There are four main regions of the brain which include:
Cerebral hemispheres, diencephalons, brain stem and the cerebellum
· The Cerebral Hemispheres: The paired left and right superior parts of the brain and make up more than half the mass of the entire brain! This part of the brain involves conscious behaviors including; speech, memory, emotional responses, and voluntary movement.
o The surface is made of ridges (gyri) and grooves (sulci)
o Fissures (deep grooves) divide the cerebrum into lobes
o The outermost area of the cerebrum is made of gray matter, and called the cerebral cortex. The remaining cerebral hemisphere tissue- the deeper white matter- is
composed of bundles of nerve fibers carrying the impulses to or from the cortex.
§ One large fiber tract (bundle of nerves) called the corpus callosum, connects the right and left cerebral hemispheres
§ The basal nuclei is an internal island of gray matter deep within the cerebral cortex. It helps regulate the voluntary motor activities.
o Surface lobes of the cerebrum include:
§ Frontal lobe
1) Primary motor area- allows us to move our skeletal muscles under conscious control
2) Broca’s area- motor speech area that directs muscles involved in speech, usually only present in left hemisphere.
3) High level intellectual reasoning are also believed to be in the anterior portion of the frontal lobe
§ Parietal lobe
1) Somatic sensory areas- allows you to recognize pain, coldness and touch
- The left side of the sensory cortex interprets impulses from the right side of the body and vice versa.
§ Occipital lobe
Ø Visual interpretation
§ Temporal lobe-
Ø Auditory interpretation
Specialized areas of the Cerebrum:
· The Diencephalon or Interbrain
o Sits on top of the brain stem and is enclosed by the cerebral hemispheres.
o Composed of three main parts: Thalamus, epithalamus, and hypothalamus
§ Thalamus- A relay station for impulses, allows us to recognize a sensation as pleasant or unpleasant, the impulse is eventually sent to the sensory cortex for localization and interpretation of the sensation
§ Hypothalamus- An important part of the autonomic system. Its job is regulating body temperature, water balance and metabolism. Also includes the Limbic system- our emotional brain. The Limbic system is involved in our appetite, thirst, pain, sex, and pleasure centers. The pituitary gland is attached to the hypothalamus.
§ Epithalamus- Forms the roof of the third ventricle. Important parts include the pineal body and choroids plexus (forms cerebrospinal fluid).
· The Brain Stem
o Attaches to the spinal cord.
o There are three parts of the brain stem: midbrain, pons, and medulla oblongata.
§ Midbrain- is mostly composed of nerve fiber tracts. It also contains the cerebral aqueduct which is a tiny canal that connects the third and fourth ventricle. The corpora quadrigemina is dorsally located with rounded protrusions that are reflex centers for vision and hearing.
§ Pons- The bulging part of the brain stem that is composed mostly of nerve fiber tracts. It houses important nuclei that control our breathing.
§ Medulla Oblongata- The lowest part of the brain stem that merges into the spinal cord. It controls important centers that help control blood pressure, heart rate, breathing, swallowing and vomiting.
· Reticular Formation
o Gray matter that runs along the brain stem.
o Involved in motor control of visceral organs. It also contains a special group of neurons
called the reticular activating system that plays a role in consciousness and awake/sleep cycles.
· The cerebellum
· Projects dorsally from under the occipital lobe of the cerebrum. It also contains two
hemispheres with convoluted surfaces.
· Its job is providing involuntary control of body movements (balance).
Protection of the Central Nervous System
· The scalp and skin provide the first layer of protections.
· The skull and vertebral column provide the second layer.
· A layer of connective tissue called the meninges provides the final protection for the CNS. There are three meningeal layers.
1. Dura mater- the outermost layer that is tough and hard.
2. Arachnoid mater- the middle web-like layer
3. Pia Mater- internal layer that clings to the surface of the brain
· Cerebrospinal fluid- the fluid that forms a watery cushion to protect the brain.
o Its composition is similar to blood plasma
o Formed by the choroids plexus
o Circulated in arachnoid space, ventricles, and central canal of the spinal cord
Ventricles and Location of Cerebrospinal Fluid
· Blood Brain Barrier- The barrier that keeps neurons separated from blood borne substances. The brain is dependent upon the constant internal environment more than any other organ in your body.
o The barrier is created by the least permeable capillaries in the body.
o The astrocytes contribute to creating this barrier.
o Exclusion of many substances helps maintain the delicate balance required by the brain. However, the barrier fails to prevent the following substances from entering:
Fats and fat soluble molecules, Respiratory gases, Alcohol, Nicotine, Anesthesia
Traumatic Brain Injuries
· Concussion- a slight brain injury, victim may be dizzy, or lose consciousness briefly but there is no permanent brain damage.
· Contusion- nervous tissue destruction occurs and the tissue does not regenerate. A severe brain stem contusion would cause someone to go into a coma.
· Cerebral Edema (swelling) or hemorrhage (bleeding) - Swelling of bleeding of the brain due to an inflammatory response. This places pressure on brain tissue- this is a serious situation.
· Cerebrovascular Accident (CVA) - commonly called a stroke. It is the result of a ruptured blood vessel supplying a region of the brain. The brain tissue that is supplied with oxygen from that vessel will die. The loss of function depends on the area of the brain affected and the severity of the stroke.
· Alzheimer’s disease- A progressive degenerative disease affecting the brain. Victims experience memory loss, and confusion.
· Parkinson’s disease- A basal nuclei problem resulting from the degeneration of dopamine releasing neurons. Patients experience tremors, a shuffling gait, trouble getting their muscle going, and head nodding.
Spinal Nerves
· There is a pair of spinal nerves at the level of each vertebra for a total of 31 pairs
· Spinal nerves are formed by the combination of the ventral and dorsal roots of the spinal cord
· Spinal nerves are named for the region from which they arise
· Complex networks of nerves called plexuses serve the motor and sensory needs of the limbs.
o Cervical plexus- origin of nerves- C1- C5, control mainly the muscles of the shoulder and neck.
o Brachial plexus- C5 – C8 and T1, control mainly the arm
o Lumbar plexus- L1 – L4, controls the lower abdomen, buttocks, leg and thigh
o Sacral plexus- L4 –L5 and S1 – S4, posterior surface of thigh, and leg, lower leg, foot, and gluteus medius. This plexus includes the sciatic nerve- largest in the body.
Cranial Nerves
· Twelve pairs of cranial nerves extend from the brain to serve the head and neck region. The exception is the vagus nerve which extends into the thorax and abdomen.
· Most cranial nerves are mixed nerves; however three pairs are purely sensory. The optic (vision), olfactory (smell), and vestibulocochlear (balance and hearing). There is a chart on pg. 231-232 that identify each cranial nerve by number, origin, function and how to test the nerve.