Ap2 Final Study Guide - Chapter 14 Part 1

- The cranial meninges are continuous w/ the spinal meninges, have the same basic structure, and bear the same names : the outer dura mater, the middle arachnoid mater, and the inner pia mater

- However, the cranial dura mater has two layers; the spinal dura mater only has one

- The periosteal layer (which is external) and the meningeal layer (which is internal)

- The dural layers around the brain are fused together except where they separate to enclose the dural venous sinuses (endothelial-lined venous channels) that drain venous blood from the brain and deliver it into the internal jugular veins

- Three extensions of the dura mater separate parts of the brain :

1. The falx cerebri separates the 2 hemispheres of the cerebrum

2. The falx cerebelli separates the 2 hemispheres of the cerebellum

3. The tentorium cerebelli separates the cerebrum from the cerebellum

- Consists mainly of tight junctions that seal together the endothelial cells of brain blood capillaries and a thick basement membrane that surrounds the capillaries

- The processes of many astrocytes press up against the cap. and secrete chemicals that maintain the permeability characteristics of tight junctions

- A few water-soluble substances, such as glucose, cross the BBB by active transport

- Other substances, such as creatinine, urea, and most ions, cross the BBB very slowly

- Still other substances─proteins and most antibiotic drugs─do not pass at all from the blood into brain tissue

- Lipid-soluble substances, such as oxygen, carbon dioxide, alcohol, and most anesthetic agents, are able to access brain tissue freely

- Trauma, certain toxins, and inflammation can cause a breakdown of the blood-brain barrier

- A clear, colorless liquid composed primarily of water that protects the brain and spinal cord from chemical and physical injuries

- It also carries small amounts of oxygen, glucose, and other needed chemicals from the blood to neurons and neuroglia

- CSF continuously circulates through cavities in and around the brain and spinal cord in the subarachnoid space (the space between the pia mater and arachnoid mater)

- The total volume of CSF is 80-150 mL (3-5oz) in an adult

- There are 4 CSF-filled cavities w/in the brain, called ventricles

- There is one lateral ventricle in each hemisphere of the cerebrum (ventricles 1 & 2)

- Anteriorly, the lateral ventricles are separated by a thin membrane, the septum pellucidum

- The third ventricle is a narrow slitlike cavity along the midline superior to the hypothalamus and between the right and left halves of the thalamus

- The fourth ventricle lies between the brain stem and the cerebellum

1. Mechanical Protection :

• CSF serves as a shock-absorbing medium that protects the delicate tissues of the brain and spinal cord from jolts that would otherwise cause them to hit the bony walls of the cranial cavity and vertebral canal

• CSF also buoys the brain so it "floats" in the cranial cavity

2. Homeostatic Function :

• The pH of the CSF affects pulmonary ventilation and cerebral blood flow, which is important in maintaining homeostatic controls for brain tissue

• CSF also serves as a transport system for polypeptide hormones secreted by hypothalamic neurons that act at remote sites in the brain

3. Circulation :

• CSF is a medium for minor exchange of nutrients and waste products between the blood and adjacent nervous tissue

- The choroid plexuses, networks of blood capillaries in the walls of the ventricles

- Ependymal cells joined by tight junctions cover the capillaries of the choroid plexuses

- The secretory capacity is bidirectional and accounts for continuous production of CSF and transport of metabolites from the nervous tissue back to the blood

- B/c of the tight junctions between ependymal cells, materials entering CSF from choroid capillaries cannot leak between these cells; instead, they must pass through them

- Permits certain substances to enter the CSF but excludes others, protecting the brain and spinal cord from potentially harmful blood-borne substances

- In contrast to the BBB, which is formed mainly by tight junctions of brain capillary endo. cells, the blood-CSF barrier is formed by tight junctions of ependymal cells

- It consists of three structures :

1. Medulla Oblongata

2. Pons

3. Midbrain

- Extending through the brain stem is the reticular formation, a netlike region of interspersed gray and white matter

- Also called the medulla

- Continuous w/ the superior part of the spinal cord; it forms the inferior part of the brain stem

- It begins at the foramen magnum and extends to the inferior border of the pons, a distance of about 3 cm

- The medulla's white matter contains all sensory (ascending) tracts and motor (descending) tracts that extend between the spinal cord and other parts of the brain

- Some of the white matter forms bulges, called pyramids, on the anterior aspect of the medulla, which are formed by the large corticospinal tracts that pass from cerebrum to spinal cord

- Just superior to the junction of the medulla w/ the spinal cord, 90% of the axons in the left pyramid cross to the right side, and 90% of the axons in the right pyramid cross to the left side

- This crossing is called the decussation of pyramids and explains why each side of the brain controls voluntary movements on the opposite side of the body

- Nuclei; Some of these nuclei control vital body functions (Ex- cardiovascular center, medullary rhythmicity area)

- The cardiovascular center regulates the rate and force of the heartbeat and the diameter of blood vessels

- The medullary rhythmicity area of the respiratory center adjusts the basic rhythm of breathing

- The vomiting center of the medulla causes vomiting, the forcible expulsion of the contents of the upper GI tract through the mouth

- The deglutition center of the medulla promotes deglutition (swallowing) of a mass of food that has moved from the oral cavity of the mouth into the pharynx (throat)

- Sneezing involves spasmodic contraction of breathing muscles that forcefully expel air through the nose and mouth

- Coughing involves a long-drawn and deep inhale and then a strong exhale that suddenly sends a blast of air through the upper respiratory passages

- Caused by spasmodic contractions of the diaphragm (a muscle of breathing) that ultimately result in the production of a sharp sound on inhalation

- Oval-shaped swelling just lateral to each pyramid

- W/in the olive is the inferior olivary nucleus, which receives input from the cerebral cortex, red nucleus of the midbrain, and spinal cord

- By influencing cerebellar neuron activity, the inferior olivary nucleus provides instructions that the cerebellum uses to make adjustments to muscle activity as you learn new motor skills

- These nuclei are the right and left gracile nucleus and cuneate nucleus

- Ascending sensory axons of the gracile fasciculus and the cuneate fasciculus, which are 2 tracts in the posterior columns of the spinal cord, form synapses in these nuclei

- The axons ascend to the thalamus in a band of white matter called the medial lemniscus, which extends through the medulla, pons, and midbrain

- The tracts of the posterior columns and the axons of the medial lemniscus are collectively known as the posterior column-medial lemniscus pathway