Sueño y Vigilia

Question 0

Núcleos Talámicos

Answer 0

• The nuclei that project to wide regions of the neocortex are the midline and intralaminar nuclei.
• The nuclei that project to specific areas include the specific sensory relay nuclei and the nuclei concerned with efferent control mechanisms. The specific sensory relay nuclei include:

1. the medial and lateral geniculate bodies, which relay auditory and visual impulses to the auditory and visual cortices; and
2. the ventral posterior lateral (VPL) and ventral posteromedial, which relay somatosensory information to the postcentral gyrus.
3. The ventral anterior and ventral lateral nuclei are concerned with motor function. They receive input from the basal ganglia and the cerebellum and project to the motor cortex.
4. The anterior nuclei receive afferents from the mamillary bodies and project to the limbic cortex, which may be involved in memory and emotion.

• Most of the thalamic nuclei described are excitatory neurons that release glutamate.
• The thalamus also contains inhibitory neurons in the thalamic reticular nucleus. These neurons release GABA, and unlike the other thalamic neurons just described, their axons do not project to the cortex. Rather, they are thalamic interneurons and modulate the responses of other thalamic neurons to input coming from the cortex.

Question 1

Tipos y funciones de células en el neocortex

Answer 1

Pyramidal neurons are the only projection neurons of the cortex, and they are excitatory neurons that release glutamate at their terminals. Layers II-VI

The other cortical cell types are local circuit neurons (interneurons) which have been classified based on their shape, pattern of projection, and neurotransmitter. Inhibitory interneurons (basket cells and chandelier cells) release GABA as their neurotransmitter.

• Basket cells have long axonal endings that surround the soma of pyramidal neurons; they account for most inhibitory synapses on the pyramidal soma and dendrites.
• Chandelier cells are a powerful source of inhibition of pyramidal neurons because they have axonal endings that terminate exclusively on the initial segment of the pyramidal cell axon. Their terminal boutons form short vertical rows that resemble candlesticks, thus accounting for their name.
• Spiny stellate cells are excitatory interneurons that release glutamate as a neurotransmitter.

Question 2

Distribución de las prolongaciones de las neuronas del neocortex

Answer 2

• The neocortex is generally arranged in six layers . The most common neuronal type is the pyramidal cell with an extensive vertical dendritic tree that may reach to the cortical surface.
• Their cell bodies can be found in all cortical layers except layer I.
• The axons of these cells usually give off recurrent collaterals that turn back and synapse on the superficial portions of the dendritic trees.
• Afferents from the specific nuclei of the thalamus terminate primarily in cortical layer IV, whereas the nonspecific afferents are distributed to layers I–IV.
• Basket, chandelier and spiny stellate cells are located primarily in layer IV and are a major recipient of sensory information arising from the thalamus; they are an example of a multipolar neuron (Chapter 4) with local dendritic and axonal arborizations.

Question 3

Aferencia y eferencia de la formación reticular

Answer 3

• The reticular activating system is a complex polysynaptic pathway arising from the brain stem reticular formation with
• projections to the intralaminar and reticular nuclei of the thalamus which, in turn, project diffusely and nonspecifically to wide regions of the cortex.
• Collaterals funnel into it not only from the long ascending sensory tracts but also from the trigeminal, auditory, visual, and olfactory systems.
• The complexity of the neuron net and the degree of convergence in it abolish modality specificity, and most reticular neurons are activated with equal facility by different sensory stimuli.
• The system is therefore nonspecific, whereas the classic sensory pathways are specific in that the fibers in them are activated by only one type of sensory stimulation.

Question 4

Tipos de registro de electroencefalogramas

Answer 4

• Bipolar records show fluctuations in the potential difference between two cortical electrodes;
• unipolar records show the potential difference between a cortical electrode and a theoretically indifferent electrode on some part of the body distant from the cortex.

Question 5

¿Cómo se explica la existencia de dipolos corticales?

Answer 5

Cortical Dipoles

The EEG recorded from the scalp is a measure of the summation of dendritic postsynaptic potentials rather than action potentials

Propagated potentials can be generated in dendrites. In addition, recurrent axon collaterals end on dendrites in the superficial layers.

As excitatory and inhibitory endings on the dendrites of each cell become active, current flows into and out of these current sinks and sources from the rest of the dendritic processes and the cell body. The cell–dendrite relationship is therefore that of a constantly shifting dipole. Current flow in this dipole produces wave-like potential fluctuations in a volume conductor (Figure 15–4).

When the sum of the dendritic activity is negative relative to the cell, the cell is depolarized and hyperexcitable; when it is positive, the cell is hyperpolarized and less excitable.

The cerebellar cortex and the hippocampus are two other parts of the central nervous system (CNS) where many complex, parallel dendritic processes are located subpially over a layer of cells. In both areas, characteristic rhythmic fluctuations occur in surface potential similar to that observed in the cortical EEG.

Question 6

Alteraciones de las ondas cerebrales según nivel de atención

Answer 6

Alpha rhythm In adult humans who are awake but at rest with the mind wandering and the eyes closed, the most prominent component of the EEG is a fairly regular pattern of waves at a frequency of 8–13 Hz and amplitude of 50–100 microV when recorded from the scalp. It is most marked in the parietal and occipital lobes and is associated with decreased levels of attention.

When attention is focused on something, the alpha rhythm is replaced by an irregular 13–30 Hz low-voltage activity, the beta rhythm (Figure 15–6). This phenomenon is called alpha block and can be produced by any form of sensory stimulation or mental concentration, such as solving arithmetic problems. Another term for this phenomenon is the arousal or alerting response, because it is correlated with the aroused, alert state. It has also been called desynchronization, because it represents breaking up of the obviously synchronized neural activity necessary to produce regular waves. However, the rapid EEG activity seen in the alert state is also synchronized, but at a higher rate. Therefore, the term desynchronization is misleading.

Gamma oscillations at 30–80 Hz are often seen when an individual is aroused and focuses attention on something. This is often replaced by irregular fast activity as the individual initiates motor activity in response to the stimulus.