Oral Questions

2018-2019 Questionscore

Basic cells of nervous system

The nervous system is composed of basic cells that are essential for its functionality.

Glial cells and their position in the nervous system

Glial cells play different roles: structural, nutritional, and preventive roles. There are six types of glial cells, four in the CNS and two in the PNS. In the CNS, there are astrocytes, oligodendrocytes, ependymal, and microglia. In the PNS, there are Schwann and satellite cells. Astrocytes prevent the entering of undesirable substances in the brain via blood vessels, which they constrain by forming the Blood Brain Barrier. Furthermore, they help oligodendrocytes perform better and are also important for the nutrition and structure of the nervous system. Oligodendrocytes make myelin sheets around the axon in the CNS, while Schwann cells play the same role in the PNS. Ependymal cells line the cavities of the brain and spinal cord, regulating the exchange of several substances between the cerebrospinal fluid and the nervous tissue, making them important for the structure of the nervous system. Microglia are phagocytes, engulfing bacteria to defend the nervous system. Lastly, satellite cells in the PNS support cell bodies with a structural function, similar to Schwann cells but located around the cell bodies.

Neuron structure

Neurons are formed by a soma, one or more dendrites, and an axon. The soma contains the nucleus of the cell and keeps the cell alive. The dendrites are tree-like fibers that collect information and send it to the soma. The axon transmits the information to the axon terminal, reaching subsequent cells, neurons, muscles, or glands.

Unipolar, bipolar, and multipolar neurons

Structurally, neurons are characterized by the number of processes (dendrites and axons) extending from the soma. They can be unipolar, bipolar, or multipolar. Unipolar neurons have a single process which divides into two branches: one to the CNS and the other to the PNS; they are generally sensory neurons. Bipolar neurons have two processes: a dendrite and an axon; they are generally sensory neurons found in sensory areas such as the ears, eyes, or nose. Multipolar neurons have an axon and many dendrites; 99% of them are in the CNS, and most are motor neurons.

Resting membrane potential and action potential

Since a neuron is a living cell, it has a plasma membrane impermeable to ions, but in the membrane, there are Sodium (Na) and Potassium (K) channels that permit the in and out movement of ions. The resting membrane potential is between -60 and -70 mV, reached when the electrical gradient (ions charge) is balanced by the diffusion gradient (according to Fick’s law). In these particular conditions, Sodium channels are closed, and there is no net movement of K ions. It represents an injection of energy capable of opening Sodium channels. An injection of current pushes the membrane voltage toward more positive (depolarization) or negative (hyperpolarization) voltages, in a proportional way. If the membrane potential reaches the threshold (around -40 mV), the action potential is generated. The action potential is an all-or-none event; the amplitude of the action potential does not vary as a function of the size of the preceding stimulus.

Why the amplitude of the action potential is always the same along the axon?

The amplitude of the action potential is always the same along the axon because it is an all-or-none event, so it does not decrease in an exponential way like the current in a wire. Being an all-or-none event, to be generated, it does not need the whole amplitude of the previous potential, but the next generated potential will have the same amplitude as the previous one, thanks to the all-or-none principle.

Pre and post-synaptic potential

Electrical synapses have direct physical contact and enable the bidirectional passage of currents: the presynaptic action potential propagates to the postsynaptic cell, while the membrane resting potential of the postsynaptic cell simultaneously propagates to the presynaptic cell. Chemical synapses involve neurotransmitters: the energy coming from the action potential opens Calcium channels, allowing Ca ions to pass through synapses and release information. Some Ca ions are re-absorbed by the presynaptic cell, but it is necessary that at least one ion reaches the postsynaptic cell to transmit the information, because when it attaches to a receptor, the receptor opens Sodium channels. Postsynaptic potentials last between 10 and hundreds of milliseconds. Presynaptic potential duration is 2 ms and is biphasic, while postsynaptic potential duration is more than 10 ms and is monophasic (only depolarization).

Draw the action potential given by a certain stimulus

The action potential can be divided into six phases:

• Resting (-60 mV, K channels opened)
• Rising (-60 mV to -40 mV, Na channels are opening)
• Overshoot (positive membrane voltage, Na channels start to close)
• Falling (membrane potential returns to resting value)
• Undershoot (voltage is less than resting voltage because some K channels are still opened)
• Recovery (-60 mV, delayed closing of K channels)

Action potential time is 1 millisecond.

Why the action potential is an all-or-none event?

Action potential is an all-or-none event because it is either generated or not. If the stimulus is sufficient to push the membrane potential past the firing threshold for the neuron, an action potential is generated, and it is independent of the stimulus. Otherwise, it is not generated. From a chemical perspective, action potential is an all-or-none event because it is generated only if Sodium channels open, and they close only when the membrane potential has overcome the zero value.

Temporal and spatial summation

Summation of postsynaptic potential can be temporal or spatial. Temporal summation means that some close (in time) impulses can generate an action potential because the potential has no time to decrease. Spatial summation means that some impulses placed in different positions at the same time can generate an action potential. Impulses can be excitatory (EPSP, depolarizing) or inhibitory (IPSP, hyperpolarizing), and the number of useful impulses is the difference between these two types (#EPSP - #IPSP).

Nervous system organization; brain main regions and relative functions, Wernicke's area

The nervous system coordinates all the activities of the body's cells thanks to the rapid communication achieved by its highly specialized nerve cells. The CNS includes all the nerves contained within the brain and the spinal cord, while the PNS includes all the nerves that enter or exit from the brain and spinal cord. The CNS integrates the information, analyzes the sensory input, and makes decisions about an appropriate response. The PNS collects information about the external and internal environment (sensory) and carries out (motor) the instructions resulting from integration by impulses sent to various effector organs. The cerebral cortex can be divided into four lobes: frontal, parietal, temporal, and occipital. The frontal lobe is the association area (thinking), the parietal lobe is the somatosensory area, the temporal lobe is the audition area, and the occipital lobe is the visualization area. The cerebral cortex is also divided by 52 Broadman or brain areas (BA), according to cytoarchitectonic or functional features. Wernicke's area is in the temporal lobe, corresponding to BA 22 and controlling syntax.

• Action: Execution – 6 / 1,2,3,4,7,8,40; Imagination – 6,40 / 7,9,10,46; Inhibition – 6 / 7,9,10,37,40,46; Observation – 6 / 7,10,19,21,37,40,44,45
• Language: Language – 19 / 9,18,37,39,47; Orthography – 6,7,9,40 / 7,8,19,37,39; Phonology – 6,9 / 7,37,40,46; Semantic – 6,9,18,37 / 19,40,47; Syntactical – 9,10,46 / 18,19,22,38; Execution of speech – 6 / 1,2,3,4
• Working memory: 6,9 / 7,10,40; Cognitive reasoning – 6,7,9,40 / 8,10,19

Decussation

The decussation is the point where the nerves cross from one side of the brain to the other, and it is why the left hemisphere controls the right arm movements, for example. It is the down-terminal part of the hindbrain.

What kind of brain plasticity exist and their characteristics

There are two levels of observation: cellular changes due to learning, and large-scale changes that involve cortical remapping in response to injury. Thus, the two kinds of brain plasticity are structural and functional. Structural plasticity involves new synaptic connections and new nerve cells growing, thereby generating new neural networks. Ramon y Cajal suggested that cerebral gymnastics could lead to the development of new dendritic processes and axonal collaterals beyond what is normally observed, forcing the establishment of new and more extensive intracortical connections. Functional plasticity refers to changes in the strength of existing synapses. The Hebbian hypothesis suggests that when an axon of cell A is near enough to excite cell B or repeatedly takes part in firing it, growth or metabolic changes occur in one or both cells, increasing A's efficiency in firing B. This process is summarized by the phrase: "fire together, wire together," indicating that two cells or systems of cells repeatedly active at the same time will become associated, so activity in one facilitates the activity in the other.

Hebbian theory

The Hebb hypothesis suggests that when an axon of cell A is near enough to excite cell B or repeatedly takes part in firing it, some growth or metabolic change takes place in one or both cells such that A's efficiency in firing B is increased. This process is summarized by the phrase: "fire together, wire together." Thus, two cells or systems of cells repeatedly active at the same time tend to become associated, so that activity in one cell facilitates activity in the other.

Core II

Advantages and disadvantages of non-invasive techniques

Non-invasive techniques include EEG, MEG, and fMRI.

EEG advantages are: it measures directly the neuronal activity; it has a very high temporal resolution (on the order of milliseconds); it is portable; it is cheaper than other techniques; it does not aggravate claustrophobia.

EEG disadvantages are: it has low spatial resolution on the scalp, requiring intense interpretation.