Chapter 6: From Neuronal to Hemodynamic Activity

 

 

Study Questions

 

 

 

 

 

1.   What is the difference between integrative and signaling activity of neurons?

 

Integrative activity is the summation of the various input coming from other neurons. Signals come in through dendritic and somatic connections. These inputs are summed, and this process is called integrative activity. Signaling activity is the transmission of the outcome of the integrative processes to other neurons. Signalling activity is the result of integrative activity. 

 

 

 

 

 

 

2.   Under what conditions is energy required for movement of substances across neuronal membranes?

 

When substance has to move through the sodium-potassium pump, energy is required. This pump pumps out three Na+ (sodium ions) out of the cell, bypassing the cellular membrane, and pumps K+ (potassium ions) back into the cell. This process requires chemical energy found in the bonds of ATP.  Movement through ion  channels only requires sufficient kinetic energy from heat, however the operation of pumps requires cellular sources of energy. The analogy was made to that of a water tower with pipes coming from the bottom. Water is able to move through the pipes with only the potential energy from gravity allowing movement (ion channels), but in order to pump water into the tower, additional energy is needed (sodium potassium pump). This analogy is good for getting a general sense of the procoess, but it does not take into consideration the electric properties of cellular architecture. There is an electrical potential between the inside and outside of the membrane, so this can give rise to certain forces that move substances across the membrane. Therefore, chemical and electrical gradients govern movement across membranes.

 

 

 

 

3.   What are the various types of postsynaptic potentials and which neurotransmitters commonly modulate them?

 

The excitatory postsynaptic potention (EPSP) is the local depoplarization of the postsynaptic cell membrane. The neurotransmitter glutamate opens normally blocked ion channels allowing Na+ to pass through the postsynaptic membrane into the neuron. The surplus of Na+ decreases the electrical potential between the inside and outside of the membrane at the channel location. This process creates a depolarization known as EPSP.

The inhibitory postsynaptic potential (IPSP) is the local hyperpolarization of the neuronal membrane. During this process, aminobutyric acid, or GABA, interact with the receptors to open chlorine or potassium channels. The influx (coming in) of Cl- and efflux (going out) of K+ creates a net increase in the resting potential in the vicinity of these newly opened channels, hence a hyperpolarization. 

 

 

 

 

 

4.   What is ATP? What is it used for?

 

Adenosine Triphosphate (ATP) is a nucleotide that contains three phosphate groups. It is used as an energy source for the sodium potassium pump to restore the asymmetric distribution of Na+ and K+ across the cell membrane and restore the resting membrane potential. This process can again be likened to the water tower analogy. When an action potential occurs, these potentials cause changes in ion concentration that requires energy to restore. The sodium potassium pump removes Na+ from within the cell for every two K+ ions it brings into the cell. Operation of the sodium potassium pump requires energy and ATP supplies this energy. Thus, it is not used directly in ISPS or ESPS processes, but in restoring the resting potential across the membrane of a neuron.

 

 

 

 

5.   What is glycolysis? What steps are present in aerobic glycolysis? What steps are present in anaerobic glycolysis?

 

Glycolysis is the process in which glucose is broken down. This can happen in two different ways:

 

1)Aerobic glycolysis: is this process when oxygen is present. Glycolysis consumes 2 ATP, but produces 4 ATP. When oxygen is present in this cycle, the pyruvate product undergoes a process called the tricarboxylic cycle (TCA cycle). TCA cycle uses oxygen extracted from the hemoglobin in the blood to oxidize pyruvate. The process also uses the electron transfer chain to pass electrons across a series of compounds to release energy that is used by ATP synthase to produce and extra 34 ATP molecules.

2) Anaerobic glcolysis: If oxygen is not present, the process is called anaerobic glycolysis and the pyruvate is reduced to the end product of lactate. This process is not good at producing mass quantities of ATP, but it is about 100 times faster than aerobic glycolysis. Plus, some cells can change lactate back into pyruvate and use it as fuel for the TCA cycle resulting in 34 ATP molecules.

 

 

 

 

 

 

6.   Why is oxygen needed for neuronal activity?

 

Neuronal activity requires energy for synthesis of proteins, the maintenance and turnover of membranes, and axoplasmic transport that require energy. Neurons also require energy to synthesisze, package, and break down neurotransmitters to operate the pumps that restore unequal distributions of ions following ESPS, IPSP, and action potentials. Oxygen is required to create the ATP in aerobic glycolysis that produces the efficient 34 molecules of ATP. The ATP is then used to power the above mentioned processes.

 

 

 

 

 

 

7.   What is the molecule that carries oxygen within the bloodstream?

 

Hemoglobin. Hemoglobin carries about 4 molecules of oxygen per molecule. There are around 280 million hemoglobin molecules in each red blood cell. The deoxygenated hemoglobin also has a job of bringing waste carbon dioxide which is bound to this hemoglobin to the heart which pumps it to the lungs for re-oxygenation.

 

 

 

 

 

 

8.   According to Attwell and Laughlin, which activities consume major portions of the brain’s energy budget?

 

Integrative and signaling activity consume about 75% of the energy use by the brain. The other 25% is used by housekeeping. In a rodent brain, it was seen that 47% of the energy resources in the brain was used to restore the resting potential after an action potential.

 

 

 

 

 

 

9.   What neuroanatomical terms are used to indicate the following directions: 1) front of the brain, 2) back of the brain, 3) top of the brain, 4) bottom of the brain, 5) close to the midline, and 6) close to the sides?

 

1) Anterior/ rostral

2) Posterior/ caudal

3)Superior/ dorsal

4) Inferior/ ventral

5) Medial

6) Lateral

 

 

 

10. What are the three cardinal views of the brain called, and what directions do they represent?

 

Sagital: A slice from caudal to rostral  

Coronal: from dorsal to ventral orthogonal to sagital

Axial: Horizontal plane

 

 

 

 

 

 

11. What are the five primary lobes of the brain?

 

Occipital Lobe - The posterior lobe of the brain that deals with visual processing

Temporal Lobe - The lobe on the ventral surface of the cerebellum that deals with auditory and viual processing, language, memory, and other functions

Parietal Lobe - The posterior-dorsal lobe that is important for spatial processing, cognitive processing, and many other functions.

Frontal Lobe - The most anterior lobe of the cerebellum, which deals with executive processing, motor control, memory, and other functions

Limbic Lobe-  Related to emotional processing and olfactory.

 

 

 

 

 

 

12. What are gyri and sulci?

 

Gyri: raises in the cortical surface

Sulci: indentations in the cortical surface

 

 

 

 

 

 

 

13. What did Roy and Sherrington speculate about the physiological correlates of brain function?

 

They postulated that changes in acticity associated with specific brain functions might result in locally increased blood flow. The main root of their postulation was the there an "automatic mechanism" that would allocate a higher proportion of the blood supply with the variations of functional activity. In essence, there is a mechanism dictates the amount of blood in the brarin based upon brain activity.  Mosso performed an experiment with this notion in mind where he made a metal bed balanced on a fulcrum and tried to get the individual to have brain activity to see if this would unbalance the fulcrum. He claimed positive results, but his conclusions are disregarded today.

 

 

 

 

14. Describe the arterial system that supplies blood to the brain. Trace the flow of blood from the major artery leaving the heart, through the arteries supplying the brain. What are the major cerebral arteries, and what parts of the brain do they support?

 

Oxygen diffuses from the aleoli of the lungs and it binds with hemoglobin in red blood cells. It goes to the heart where it reaches the aorta. The aorta gives rise to large arteries. Each artery branches into smaller arteries called arterioles. Blood is supplied to the brain by two main arterial systems called the left and riht internal carotid arteries and the vertebral/ basilar arteries. The right and left common carotid arteries divide at the neck into external and internal carotid arteries. The external arteries provede blood to the head and face, while the internal arteries provide blood to the brain through an opening in the base of the skull called the foramen lacerum.

 

 

 

The major cerebral arteries are the anterior, middle, and posterior cerebral arteries. The anterior cerebral artery supplies blood to the medial surface of the brain and the head of the caudate. The middle cerebral artery supply the lateral and superior cerebral cortex as well as the remainder of the basal ganglia. The posterior cerebral artery supplies the posterior temporal and occipital cortex.

 

 

 

 

 

 

15. What are the major draining veins for the brain? What path does blood take in going back to the heart?

The extraction of oxygen and glucose from the blood occurs at the surfaces of thin walled vessels called capillaries. After deoxygenation, the homoglibin molecules bind to waste carbon and are carried to venules. Venules collect into veins that eventually return the blood through the vena cava to the right atrium of the heart. Then, it travels to the right ventricle, which pumps the de-oxygenated blood to the lungs. Here, the waste carbon dioxide is released as a gas and oxygen binds to the hemoglobin to start the cycle again.

 

Draining the brain’s circulation uses the left and right jugular veins. These structures exit the skull through the jugular foramen, which join the subclavian vein and eventually the superior vena cava, progressing to the right atrium and ventricle.

 

The jugular veins are fed by the sinus system of the brain. Thus, the sinus system is the primary drainage system of the brain.

 

 

 

16. How does blood flow change throughout the vascular system? How does the brain control blood flow?

 

Bloodflow varies as a function of the blood vessel radius. The flow is proportional to the vessel radius expressed to the fourth power. Small arteries have a high resistance and thus oppose flow. This resistance plays a hand in creating an equilibrium to the pulsing flow being emitted by the heart. As an indirect result of brain activity, blood flow in the active region of the brain increases.

 

 

 

 

 

 

17. What two types of neuroanatomical structures does the word “sinus” describe?

 

1) Long venous channels formed by meningeal coverings that form the primary draining system for the brain.

2) Air filled cavities in the skull.

 

 

 

 

 

 

18. How might neurons/neurotransmitters control blood flow directly?

 

Neuronal activation causes the surrounding arterioles to dilate. This dilation can be as large as a 33% increase that was found in rats by Ngai, Winn, and colleagues. The authors had a sensory stimuli and measured the arteriole dilation increases as a function of neuronal activity. They found that not only did the local arterioles dilate and increase blood velocity, but arterioles upstream from the region they were inspecting dilated as well. This is bad news for the spatial resolution of MRI when using hemodynamic blood response as an indicator of the spatial indicator of neural activity. This experiment showed that a relatively large region around an active neuron has increased blood flow, therefore limiting the spatial resolution that can be achieved using an MRI scanner.