Cerebral Circulation and Perfusion PDF

**Cerebral Circulation and Perfusion** 1. **Cerebral Perfusion vs. Other Circulatory Loops** - The brain requires a **specialized circulation** due to its high metabolic activity (\~750 ml/min). - Cerebral circulation is mediated through the **Circle of Willis** and i...

**Cerebral Circulation and Perfusion** 1. **Cerebral Perfusion vs. Other Circulatory Loops** - The brain requires a **specialized circulation** due to its high metabolic activity (\~750 ml/min). - Cerebral circulation is mediated through the **Circle of Willis** and its branches. - Any interruption in blood flow can cause **ischemia**, leading to dire consequences. 2. **Blood-Brain Barrier (BBB)** - Protects the brain by regulating the entry of substances from the bloodstream into the brain tissue. - **Components of the BBB**: - **Tight junctions** between endothelial cells, involving proteins like **claudins** and **occludins**. - **Enhanced vesicular transport**, including both adsorptive and receptor-mediated transport. - **Astrocyte foot processes** surrounding the capillaries. - **Metabolic barrier** created by enzymes like **DOPA decarboxylase** and **MAO**. - Enhanced vesicular transport system (both adsorptive and receptor mediated) with a complex transcytosis process (A) - Specialized carrier mediated transport (highly specific) - **Challenges for Drug Delivery**: While essential for protection, the BBB complicates the delivery of therapeutic agents to the brain. - **Special Areas Without BBB**: Some regions, such as the **subfornical organ** and **median eminence**, lack the BBB to perform specific functions. - **Development**: The BBB is not fully developed at birth, impacting neonatal brain protection. - Blood chemistry is variable! eg NE from adrenal gland, bacteria, amino acids, etc. & these can all alter nerve function - Erhlick first described the unusual cerebral circulation → BBB. - **Cerebrospinal Fluid (CSF)** 1. **Composition and Role** - CSF is clear with a **specific gravity of 1005**, low in proteins and leucocytes, but contains ions like **Na+** and **K+**. - Provides **buoyancy** to the brain, reducing its effective weight to \~50g. - Acts as a cushion for mechanical protection and serves a role similar to the **lymphatic system**. - Secreted by the **choroid plexuses** at \~720 ml/day and is reabsorbed into the venous system via **arachnoid villi**. 2. **Ventricular System** - **CSF circulates** through the **ventricles** (lateral, third, fourth), the **subarachnoid space**, and the **spinal cord**. - The volume of CSF is approximately **150 ml**, with a pressure range of **70--180 mmCSF** (as measured by a manometer). - CSF circulation removes waste products, paralleling its role in infection detection (e.g., through **lumbar puncture** for CSF sampling). **Hydrocephalus** - **Etiology**: Occurs due to the imbalance between CSF secretion and absorption, leading to excess fluid in the brain. - Can be either **communicating** (CSF can still circulate) or **non-communicating** (blockage in the CSF flow). **Clinical Procedures** 1. **Lumbar Puncture**: - Performed between the **L3-L4** or **L4-L5** vertebrae. - Used to sample CSF, measure CSF pressure, or deliver drugs (**intrathecal injection**). - CSF usually accessed between L3-L4 or L4-L5 spinous processes **Key Points on BBB and Drug Administration** - The **BBB** prevents entry of neurotransmitters like **dopamine**. Thus, in treating Parkinson\'s disease, **L-DOPA** (a precursor) is administered along with **carbidopa** to inhibit DOPA decarboxylase outside the brain, ensuring dopamine production inside the brain.