Brain and Special Senses

Rostral

Toward the forehead (front)

Caudal

Toward the spinal cord (back)

Brain Weight

About 1600g (3.5lbs) in men and 1450g in women; it is only 2% of adult body weight but recieves 15% of the blood (750mL/min)

Cerebrum

Major portion of the brain; 83% of brin volume; cerebral hemispheres, gyri and sulci, longitudinal fissure, corpus callosum

Cerebellum

Major portion of the brain; contains 50% of the neurons, second largest brain region; located in posterior cranial fossa; in charge of balance/coordination

Brainstem

Major portion of the brain; portion of the brain that remains if the cerebrum and cerebellum are removed; diencephalon, midbrain, pons, medulla oblongata

Pons

Coordinates both sides of the body

Medulla Oblongata

Controls breathing and heartrate

Gray Matter

The seat of neurosomas, dendrites, and synapses (cell bodies); dull color due to little myelin; forms surface layer (cortex) over cerebrum and cerebellum; forms nuclei deep within brain

White matter

Composed of tracts or bundles of axons that connects one part of the brain to another or to the spinal cord; lies deep to cortical gray matter, opposite relationship in the spinal cord; pearly white color from myelin (fat) around nerve fibers

Embryonic Development

Nervous system develops from ectoderm (outermost tissue layer of the embryo); Early in 3rd week dorsal midline of embryo thickens to form neural plate; as thickening progresses, neural plate sinks and its edges thicken, forms neural groove with raised neural fold on each, neural folds fuse creating a hollow neural tube by day 26; lumen of neural tube becomes fluid-filled space that will later be ventricles of brain and central canal of spinal cord (figure 14.3) NOT FINISHED

Meninges

3 connective tissue membranes that envelop the brain; they lie between the nervous tissue and bone; dura mater, arachnoid mater, and pia mater; they protect the brain and provide structural framework for its arteries and veins (protects from bacterial infection)

Meningitis

Inflammation of the meninges; serious disease between 3 months and 2 years of age; caused by bacterial or viral invasion of the CNS by way of the nose and throat; pia mater and arachnoid are most often affected; can cause swelling of the brain, enlargement of the ventricles, and hemorrhage; signs: high fever, stiff neck, drowsiness, and intense headache, may progress to coma then death within hours of onset; diagnosed by examining the CSF obtained by lumbar puncture (spinal tap)

Ventricles

4 internal chambers within the brain; 2 lateral ventricles (one in each hemisphere), 3rd ventricle and 4th ventricle

Interventricular Foramen

Tiny pore that connects the lateral ventricles to the 3rd ventricle

Third Ventricle

Narrow medial space beneath corpus callosum; cerebral aqueduct runs through midbrain and connects third and fourth ventricle

Fourth Ventricle

Small triangular chamber between pons and cerebellum; connects to central canal that runs through spinal cord

Choroid Plexus

Spongy mass of blood capillaries on the floor of each ventricle; produces and moves cerebrospinal fluid

Ependyma

A type of neuroglia that lines ventricles and covers choroid plexus; produces and moves cerebrospinal fluid; these cells modify the filtrate, so CSF has more sodium and chloride than plasma, but less potassium, calcium, glucose, and very little protein

Cerebrospinal Fluid (CSF)

Clear colorless liquid that fills the ventricles and canals of CNS; only purpose is to keep neurons moist; comes from choroid plexus; the brain produces and absorbs 500 mL/day (100 to 160 mL normally present at one time, 40% formed in subarachnoid space external to brain, 30% by the general ependymal lining of the brain ventricles, 30% by the choroid plexuses); production begins with filtration of blood plasma through capillaries of the brain

CSF Flow

Continuously through and around the CNS; driven by its own pressure, beating of ependymal cilia, and pulsations of the brain produced by each heartbeat; CSF secreted in lateral ventricles flows through intervertebral foramina into third ventricle, then down the cerebral aqueduct into the fourth ventricle; third and fourth ventricles add more CSF along the way; small amount of CSF fills central canal of spinal cord; all CSF ultimately escapes through three pores (median aperture and two lateral apertures); leads into subarachnoid space of brain and spinal cord surface; CSF is reabsorbed by arachnoid granulations (cauliflower-shaped extension of the arachnoid meninx, protrude through dura mater into superior sagittal sinus, CSF penetrates the walls of the villi and mixes with the blood in the sinus)

CSF Buoyancy

Allows brain to attain considerable size without being impaired by its own weight; if its rested heavily on the floor of cranium, the pressure would kill the nervous tissue

CSF Protection

Protects the brain from striking the cranium when the head is jolted; shaken child syndrome and concussions do occur from severe jolting

CSF Chemical Stability

Flow of CSF rinses away metabolic wastes from nervous tissue and homeostatically regulates its chemical environment

Blood Supply

Neurons have a high demand for ATP, and therefore, oxygen and glucose, so a constant supply of the blood is critical; a 10-second interruption of blood flow may cause loss of consciousness, a 1 to 2 minute interruption can cause significant impairment of neural function, going 4 minutes without blood causes irreversible brain damage, stroke (CVA)

Brain Barrier System

Regulates what substance can get from bloodstream into tissue fluid of the brain; blood is crucial but it can also contain harmful agents (viruses); two points of entry must be guarded (blood capillaries throughout the brain tissue, capillaries of the choroid plexus)

Blood-Brain Barrier

Protects blood capillaries throughout brain tissue (protects brain from blood); consists of tight junctions between endothelial cells that form the capillary walls; Astrocytes reach out and contact capillaries with their perivascular feet; anything leaving the blood must pass through the cells, and not between them; endothelial cells can exclude harmful substances from passing to the brain tissue while allowing necessary ones to pass

Astrocytes

Reach out and contact capillaries with their perivascular feet; they induce the endothelial cells to form tight junctions that completely seal off gaps between them; they’re exposed to toxins in blood before it reaches the neurons

Medulla Oblongata

Comes from the embryonic myelencephalon; this is where sensory and motor signals/neurons cross over

Reticular Formation

Loose network of nuclei extending throughout the medulla, pons, and midbrain; contains cardiac, vasomotor, and respiratory centers; allows you to focus/pay attention; contains additional nuclei concerned with sleep, respiration, and posture

Pons

Anterior bulge in brainstem, rostral to medulla; develops from metencephalon; connects both sides of the body to produce an action; plays a part in respiratory action

Cerebellar Peduncles

Thick stalks on posterior pons that connect it (and the midbrain) to the cerebellum; is the ascending sensory tracts, descending motor tracts, and pathways in and out of cerebellum

Cranial Nerves V, VI, VII, and VIII

Sensory roles: hearing, equilibrium, taste, facial sensations

Motor roles: eye movement, facial expressions, chewing, swallowing, urination, and secretion of saliva and tears

Midbrain

Becomes a mature brain structure from the mesencephalon; short segment of brainstem that connects hindbrain to forebrain; surrounded by central gray matter involved in controlling pain; contains cerebral aqueduct, continuations of medial lemniscus and reticular formation, and motor nuclei of two cranial nerves that control eye movements (III and IV); it has no specific function because it contains a lot of structures/does a lot of functions)

Tegmentum

Dominated by red nucleus; pink color due to high density of blood vessels; connections go to and from cerebellum for motor control

Substantia Nigra

Produces dopamine; black nucleus pigmented with melanin; motor center that relays inhibitory signals to thalamus and basal nuclei preventing unwanted body movement; degeneration of neurons (dopamine) leads to tremors of Parkinson disease

Cerebral Crus

Bundle of nerve fibers that connect cerebrum to pons; carries corticospinal tracts

Sleep and Consciousness

Reticular formation plays a central role in consciousness, alertness and sleep; injury here can result in irreversible coma; reticular activating system modulates activity in cerebral cortex so that it ignores repetitive, inconsequential stimuli