Thalamocortical Pathways and Ascending Arousal System PDF

Thalamocortical pathways and ascending arousal system: thalamic nuclei ○ The thalamus within the diencephalon is comprised of groups of nuclei that participate in sensory, motor and limbic functions ○ The thalamus is the gateway to the cerebral cortex because it processes virtually all information t...

Thalamocortical pathways and ascending arousal system: thalamic nuclei ○ The thalamus within the diencephalon is comprised of groups of nuclei that participate in sensory, motor and limbic functions ○ The thalamus is the gateway to the cerebral cortex because it processes virtually all information that reaches the cortex. ○ the thalamus has two major groups of nuclei: those that project diffusely to wide areas of the neocortex ( midline and intralaminar nuclei) And those that project to discrete regions of the neocortex and limbic system specific sensory relay nuclei This group includes the medial and lateral geniculate bodies that relay auditory and visual impulses to the auditory and visual Cortexes, respectively and the ventral posterior lateral and ventral posteromedial nuclei that relays them out of sensory information to the postcentral gyrus ○ The ventral anterior and ventral lateral nuclei receive input from the basal ganglia and the cerebellum and project to the motor cortex. ○ The anterior nuclei receive input from the mammalian bodies and project to the limbic cortex to influence memory and emotion. ○ Most thalamic neurons are excitatory and release glutamate. The thalamus also contains inhibitory neurons in the thalamic reticular nucleus. these neurons release gaba, and unlike many thalamic neurons, 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 The electroencephalogram ○ The term EEG refers To a recording that represents the electrical activity of the brain. The EEG can be recorded with scalp electrodes through the unopened skull. the term electrocorticogram is used for the recording obtained with electrodes on the pile surface of the cortex ○ The EEG recorded from the scalp is a measure of the summation of dendritic postsynaptic potentials rather than action potentials. the dendrites of the cortical neurons are a forest of similarly oriented, densely packed units in The Superficial layers of the cerebral cortex. 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 cell body ○ The cell body dendrite relationship is that of a constantly shifting dipole. ○ Current flow in the dipole produces wavelike potential fluctuations in a volume conductor. ○ When the sum of the dendritic activity is negative relative to the cell body the neuron is depolarized and Hyperexcitable. when it is positive the neuron is hyperpolarized and less excitable Types of Seizures ○ Epilepsy is a condition in which there are recurring, unprovoked seizures that may result from damage to the brain. the seizures represent abnormal, highly synchronous neural activity. ○ epilepsy is a syndrome with multiple causes. in some forms characteristic EEG patterns occur during seizures or between attacks; however abnormalities are often difficult to demonstrate. seizures are divided into partial (focal) seizures and generalized seizures ○ Partial seizures originate in a small group of neurons and can result from head injury, brain infection, stroke, or tumor; but often the causes are unknown. symptoms depend on the seizure focus. They are further subdivided into simple partial seizures (without loss of consciousness)And complex partial seizures (with altered consciousness). An example of a simple partial seizure is localized jerking movements in one hand progressing to clonic movements of the entire arm lasting about 60 to 90 seconds. Auras typically precede the onset of a partial seizure and include abnormal Sensations ○ The time after the seizure until normal neurologic function returns is called the postictal period. ○ Generalized seizures are associated with widespread electrical activity and involve both hemispheres simultaneously. They are further subdivided into convulsive and non-convulsive categories depending on whether tonic or clonic movements occur. Absence seizures formally called petit mal seizures, are a form of non-convulsive generalized seizures characterized by a momentary loss of consciousness. they are associated with 3/s doublets each consisting of a typical spike and wave pattern of activity that lasts about 10 seconds. These are not accompanied by auras Or postictal periods. these spikes and waves are likely generated by low threshold Ttype Ca2+ channels in thalamic neurons Sleep disorders ○ Narcolepsy is a chronic neurologic disorder caused by the brains and ability to regulate sleep wake Cycles normally. the affected individual experience is a sudden loss of voluntary muscle tone cataplexy, and eventual irresistible urge to sleep during daytime, and possibly brief episodes of total paralysis at the beginning or end of sleep ○ Narcolepsy is also characterized by a sudden onset of REM sleep, unlike normal sleep that begins with non-rem, slow wave sleep. ○ Prevalence of narcolepsy ranges from 1 in 600 in Japan, to 1 and 500,000 in Israel, with one in 1,000 Americans being affected. ○ ○ ○ ○ ○ ○ ○ ○ Narcolepsy has a familial incidence strongly associated with a Class 2 antigen of the major histocompatibility complex on chromosome 6 at the hladr2 or hladqw1 locus, implying a genetic susceptibility to narcolepsy. The HLA complexes are interrelated genes that regulate the immune system. Compared to brains from healthy persons, the brains of persons with narcolepsy often contain fewer hypocretin (orexin) producing neurons in the hypothalamus. the HLA complex may increase susceptibility to an immune attack on these neurons, leading to their degeneration Obstructive sleep apnea Osa is the most common cause of daytime sleepiness due to fragmented sleep at night; it affects about 24% of middle-aged men and 9% of women in the United states. breathing seizes for more than 10 seconds during frequent episodes of obstruction of the upper Airway (especially the pharynx) due to a reduction in muscle tone The apnea causes brief arousals from sleep in order to reestablish upper Airway tone. An individual with OSA typically begins to snore soon after falling asleep. The snoring gets progressively louder until it is interrupted by an episode of apnea which is followed by a loud snort and gasp as the individual tries to breathe. OSA is not associated with a reduction in total sleep time, but individuals with OSA experience a much greater time and Stage 1 non-rem sleep (from an average of 10% of Total Sleep to 30 to 50%) and a marked reduction in slow wave sleep (stages 3 and 4 non-rem sleep) the pathophysiology of OSA includes both a reduction in neuromuscular tone at the onset of sleep and a change in the central respiratory drive