The Brain

Overview of the Brain Contains over 100 billion neurons 3 major regions based on embryologic development: forebrain (cerebrum and diencephalon), midbrain, and hindbrain (pons, medulla oblongata, and cerebellum) Midbrain and hindbrain can also be referred to as the brain stem

Protective Coverings Protected by the cranium, CSF, and the meninges Meninges are formed from 3 distinct layers From superficial to deep: the dura mater, arachnoid, and pia mater Between arachnoid and pia mater is the subarachnoid space which is filled with CSF

CSF and Ventricles of Brain CSF is clear and colorless Functions as a liquid cushion for CNS and helps to nourish the brain and removes waste materials Much is contained within four ventricles which are continuous with the central canal of the spinal cord Has a lateral ventricle in each hemisphere

The third ventricle is located in the center of the diencephalon The fourth ventricle is located between the cerebellum and the medulla oblongata Each lateral ventricle is connected to the third ventricle by a channel called the foramen of Monro The third is connected to the fourth by a channel called the cerebral aqueduct

CSF is formed within ventricles by filtration of blood plasma through dense networks of capillaries in jelly-like structures; each is called a choroid plexus and is part of the blood-brain barrier This structure is selectively permeable and allows in substances such as water, glucose, and oxygen while blocking cells, bacteria, and many proteins. Pressure formed by this production causes CSF to circulate Returns to bloodstream by arachnoid villi

The Cerebrum Largest structure in the brain; varied and complex Functions in conscious thought, memory, and learning Also receives and interprets sensation and initiates many motor responses Divided into right and left portions called hemispheres

Has many prominent wrinkles of foldings, called convolutions, which increase ratio of surface area to volume (gyri project up, sulci form grooves) Deep grooves are called fissures; longitudinal fissure separates right from left while transverse fissure separates cerebrum from cerebellum Four lobes in each hemisphere named for the cranial bones above them: frontal lobe, parietal lobe, occipital lobe and temporal lobe

External surface of cerebrum is gray in color because it is made up of unmyelinated fibers, called the cerebral cortex which is only 2mm thick The cortex contains billions of cell bodies and associated synapses; source of integrative function of brain Most of the brain is made of white matter (myelinated) and forms connections with other parts of CNS

White matter extends from one hemisphere to the other through a structure called the corpus callosum Embedded within the white matter are several masses of gray matter called basal ganglia These serve as relay stations for motor impulses originating in the cerebral cortex en route to the spinal cord and control large, unconscious movements of skeletal muscles such as swinging arms while walking

Diencephalon Located inferior to the corpus callosum Composed of the larger thalamus (principal relay station for sensory impulses and sensations related to survival: pain, temperature, touch, pressure) and the smaller hypothalamus (controls involuntary activities that have direct effect on homeostasis: digestion, respiration, heart rate)

Hypothalamus Small in size, but a giant in function Intermediary between nervous and endocrine system by stimulating that system’s master organ, the pituitary gland Regulates food and water intake, waking and sleeping patterns Associated w/ emotions (rage, aggression, sex drives, and symptoms of emotional stress such as crying)

Midbrain Located between the diencephalon and pons Anterior portion contains bundles of nerve fibers called cerebral peduncles which provide a major connection for nerve pathways between the cerebrum and cerebellum Posterior contains corpora quadrigemina which serve as reflex centers for rapid eye, head, and trunk movements

Pons Rounded bulge inferior to the midbrain made of white matter and embedded w/ scattered masses of gray matter Major function is to serve as a bridge between spinal cord and brain and parts of the brain with each other Some nuclei also help to regulate breathing rhythm

Medulla Oblongata Lies below the pons, extends to the spinal cord Composed of white matter on the outside w/ gray matter on the inside White matter contains ascending and descending nerve tracts On the posterior side, an enlargement known as the pyramids is where descending nerve tracts cross so that the left side of the brain controls the right side of the body and vice versa

The gray matter of the medulla contains reflex centers that play a role in visceral reflexes, consciousness, and arousal Some of its more important reflex centers include the cardiac center which regulates heart rate, the vasomotor center which regulates blood pressure by controlling diameter of blood vessels, and the respiratory center which regulates breathing depth and rhythm

Cerebellum Posterior portion of the hindbrain; divided into two hemispheres connected by a structure called the vermis Has a cortex, like the cerebrum White matter forms a distinctive tree-like pattern called the arbor vitae Contains 3 paired bundles of myelinated fibers called cerebellar peduncles

Cerebellar peduncles carry information from sensory organs in muscles, joints, and inner ear to enable the brain to keep track of the location of body parts (spatial awareness) and maintain balance Also works to coordinate nerve impulses so that movement is smooth and helps to maintain posture