L3 - Metabolism

Question 1

Explain the factors that drove cortical evolution.

Answer 1

• Blood flow rate to the brain increased faster than brain volume
• Changes in diet: leaves to fruit to vegetables and meat. Human saliva contains more amylase which makes possible the extraction of more usable calories from starchy vegetables – broken down prior to digestion to make it more efficient for the gut to extract nutrients. Humans have 6 copies of salivary amylase genes while primates only have 2.
• Agrarian revolution may have further accelerated development, because it involved heat treatment of food (increasing amount of energy we can obtain from feeding). This increased daily caloric intake while decreasing daily feeding time.

Question 2

Explain how the frontal cortex has evolved relative to older brain regions.

Answer 2

• Fourfold acceleration of metabolic changes in the PFC compared with cerebellum
• Neuronal activity level and energy availability has increased over human evolution
• Increased glia-neuron ratios but amount of energy required per unit volume of frontal cortex has decreased
• Greater metabolic consumption of human neocortical neurons related to the energetic costs of maintaining expansive dendritic arbors and long-range projecting axons of enlarged brains

Question 3

What accounts for the increase in neocortical volume and surface area of human cortex?

Answer 3

• Expansion of progenitor cells in outer sub ventricular zone during development – set skeleton for radial glial cells to guide neurons

Question 4

What process is involved in delivering nutrients to the brain?

Answer 4

• Energy is obtained through aerobic metabolism of glucose carried out by oxidative phosphorylation in mitochondria
• Oxygen and glucose are delivered to the brain through cerebral circulation at a rate of 750 mL / min (15% of cardiac output)

Question 5

What is the BBB? How do nutrients cross?

Answer 5

• BBB: A highly selective, semi-permeable barrier between circulating arterial blood supply and the CNS’s interstitial, extracellular fluid
• Formed by the brain’s endothelial cells that connect and form tight junctions with tightness maintained by blood pressure
• Passive diffusion: Does not require ATP (water, oxygen, some gases, lipid soluble molecules)
• Active transport: Requires ATP (glucose, amino acids)
• Facilitated diffusion: Nutriments are coupled to ions that move across a concentration gradient

Question 6

What is the blood-CSF barrier?

Answer 6

• Formed by epithelium of choroid plexus

- Certain drugs are delivered to the brain by injecting them into CSF

Question 7

Explain the brain’s tradeoff between mobility and mass.

Answer 7

• Carrying cost refers to energy required for providing nutrients and vegetative physiology to brain
• A greater carrying cost is bigger and heavier (more energy and nutrients supplied to the brain). A heavier mass with more infrastructure causes mobility to decrease so at a certain point, the carrying cost isn’t worth it.

Question 8

How does cerebral blood flow differ between gray matter and white matter?

Answer 8

• Baseline CBF in gray matter: 50 - 70 cc blood / 100 g tissue / min
• Baseline CBF in white matter: 25 - 35 cc blood / 100 g tissue / min
• CBF in gray matter is twice of that of white matter because it requires more energy for supporting EPSPs, regulating ionic imbalances, etc.

Question 9

List and describe the 3 regulatory processes of CBF.

Answer 9

1. Cerebral perfusion auto regulation: Cerebral arterioles maintain a constant blood flow with changes in blood pressure. Myogenic regulation is when changes in artery diameter are signalled by stretch receptors in arteriole smooth muscle. Uses negative feedback control to adjust the diameter to keep blood flow constant.
2. Flow-Metabolism coupling: CBF varies with cerebral metabolism. When metabolic demands increase, neurons send control signals to arterioles to supply more blood to them.
3. Neurogenic regulation: Vascular smooth muscle actuators in the arterioles are controlled by sympathetic innervation that receive input from the brainstem (may also receive nitric oxide from parasympathetic fibres). For example, CGRP is a vasodilator released by the trigeminovascular system that causes vessel engorgement and headache.

Question 10

List deficits that occur with blockage of each of the cerebral arteries.

Answer 10

• Posterior cerebral artery: visual cortex, medial aspects of temporal lobe / hippocampus
• Middle cerebral artery: Somatosensory and motor deficits, minus the genitals, feet, legs, and trunk; auditory cortex, Broca’s and Wernicke’s areas (cortex)
• Anterior cerebral artery: Somatosensory and motor deficits (genitals, feet, legs, trunk), Broca’s and Wernicke’s areas (white matter)