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Golgi cells are inhibitoryinterneurons that inhibit the granule cells’ effects onPurkinje cells.The Somatic Motor SystemExcitatory endings85Golgi (inner stellate) cell (inhibitory)Inhibitory endingsGranule cells (excitatory)Parallel fibers (axons of granule cells)MolecularlayerParallel fibers (cut)Purkinje cells (inhibitory)Purkinje cell layerDendrites of Purkinje cellOuter stellate cell (inhibitory)GranularlayerBasket cell(inhibitory)WhitematterPurkinje cell axonClimbing fiber(excitatory)GlomeruliMossy fibers (excitatory)Axon varicose of locus coeruleus (noradrenergic)Purkinje cell axonClimbing fiber (excitatory)To deep cerebellar nucleiFigure 6.8 Cerebellar Neuronal Circuitry The cerebellar cortex is composed of three layers.
Alloutput of the cortex is carried by inhibitory axons of the Purkinje cells. Their cell bodies are located in thePurkinje cell layer, and their dendritic arbors are in the outer molecular layer. The cortex receives inputthrough climbing fibers, which project from the medullary inferior olive, and mossy fibers, which project fromthe spinocerebellar, vestibulocerebellar, and pontocerebellar tracts.
Mossy fibers form excitatory synapseswith dendrites of granule cells in the granular layer. The excitatory axons of granule cells reach the molecularlayer, where they form parallel fibers that course through and synapse with dendritic arbors of many Purkinjecells. Climbing fibers form excitatory synapses directly with dendrites of Purkinje cells in the molecular layer.Various other interneurons and their actions are illustrated above and discussed in the text.■■The middle Purkinje cell layer.
Purkinje cells areGABA-ergic (utilize the neurotransmitter gammaamino butyric acid) and are the only efferent output ofthe cerebellar cortex. They receive input from variousaxons in the molecular layer and send inhibitory signalsto the deep cerebellar nuclei and lateral vestibularnuclei.The outer molecular layer, which contains basketcells and stellate cells, and the dendritic arbors of Purkinje cells (extending from the Purkinje cell layer).Parallel fibers, containing axons from the granule cellsof the granular cell layer, form excitatory synapses withdendrites of Purkinje, basket, and stellate cells. Basketcells and stellate cells are interneurons that inhibitPurkinje cells through the parallel fibers.
Climbingfibers, projecting from the medullary inferior olive,form excitatory synapses with dendrites of Purkinjecells.Thus, the inhibitory cerebellar cortical output of the Purkinjecells is regulated by excitatory input from climbing fibers andmossy fibers (via the granule cells) and inhibitory input fromvarious interneurons of the cortex.Purkinje cells in the cerebellar cortex are stimulated bygranule cells and climbing fibers; their activity is modulated by various cerebellar interneurons.
When stimulated, Purkinje cells release the inhibitory neurotransmitter GABA atsynapses within deep cerebellar nuclei, regulating motion andposture.86The Nervous System and MuscleBasal GangliaThe basal ganglia are deep nuclei of the cerebral hemispheres,and like the cerebellum, have an accessory role to the motorcortex, regulating motor activity to produce smooth movement and maintain posture.
They consist of the:■■■caudate nucleusputamenglobus pallidusThe caudate nucleus and putamen are referred to as the striatum, based on their striated appearance. The substantia nigraCLINICAL CORRELATEParkinson’s DiseaseDiseases of the basal ganglia produce a variety of deficits in motoractivity and control of posture.
Parkinson’s disease is a progressiveneurodegenerative disorder characterized by tremor of the handsand arms, rigidity, shuffling gait, and bradykinesia (slow move-NORMALDopamineof the midbrain and subthalamic nucleus of the diencephalonare functionally associated with, and sometimes classified as,components of the basal ganglia.The input to the basal ganglia is from the motor cortex; theoutput to the cortex is through the thalamus. A complex seriesof interactions between components of the basal ganglia andassociated structures leads to this output, which then regulatesthe level of excitation in the motor cortex.
These interactionsform an indirect pathway that inhibits cortical excitation anda direct pathway that is excitatory. The balance of these opposing pathways is responsible for coordinating smooth movement and maintenance of posture.ment). This disorder is caused by loss of dopaminergic cells of thesubstantia nigra normally projecting to the striatum (caudatenucleus and putamen), affecting both the direct and indirect pathways of the basal ganglia. L-dopa, a dopamine precursor, is effective in treating some but not all Parkinsonian patients.PARKINSON’SDISEASEDecreaseddopamineLewybodySubstantia nigra showsmarked loss of neurons andpigment.
Residual neuronsmay exhibit Lewy bodies.The Substantia Nigra in Parkinson’s Disease Dopamine production by the substantia nigra isreduced in Parkinson’s disease as a result of degeneration of dopaminergic neurons. Lewy bodies areobserved in the remaining neurons of the substantia nigra of some Parkinson’s disease patients. Theseinclusions are caused by cellular accumulation of the protein alpha-synuclein.87CHAPTER7The Autonomic Nervous SystemKnowledge of the role of the autonomic nervous system iscritical for understanding the function of any of the majororgan systems.
It is the primary mechanism for the involuntary control and coordinated activity of smooth muscle ofvisceral organs, cardiac muscle, and glands and is essential formost homeostatic processes. Within the brain, sensory information is integrated and activity of the autonomic nervoussystem is modulated to orchestrate this involuntary control ofphysiological processes.The fight-or-flight response was originally described in1915 by Walter Canon, who also invented the term“homeostasis.” The fight-or-flight response can be characterized as the physiological response to acute stress, in which generalized sympathetic activation occurs, resulting in effects suchas tachycardia, bronchial dilation, mydriasis (dilation of thepupils), vasoconstriction in much of the body, piloerection, andinhibition of gastrointestinal motility.
It has long been appreciated that the acute stress responses also involves activation ofthe hypothalamic-pituitary-adrenocortical endocrine axis(Section 7).ORGANIZATION AND GENERAL FUNCTIONS OFTHE AUTONOMIC NERVOUS SYSTEMThe two divisions of the autonomic nervous system are thesympathetic nervous system (SNS) and the parasympatheticnervous system (PNS) (the enteric nervous system of the gastrointestinal tract is sometimes considered part of the autonomic nervous system as well; it is discussed in Chapter 21).In many cases, the SNS and PNS have opposing actions onvarious organs and processes, and regulation of bodily functions often involves reciprocal actions of the two divisions.For example, heart rate is elevated by SNS activity anddecreased by PNS activity.As a generalization, the SNS is said to mediate stress responses,such as the classic fight-or-flight response, and the PNS mediates “vegetative” responses, such as digestion.
The fight-orflight response is a generalized reaction to extreme fear, stress,or physical activity and results in a patterned response inmany organ systems. The response includes elevated heartrate, cardiac output, and blood pressure, as well as bronchialdilation, mydriasis (dilation of pupils), and sweating. Althoughthe sympathetic nervous system often responds in such patterned manners, the parasympathetic may produce moreselective effects, for example during the sexual response.The autonomic nervous system has as its central componentsthe hypothalamus, the brainstem, and the spinal cord; peripherally, it consists of sympathetic and parasympathetic nerves.Areas within the hypothalamus and brainstem regulate andcoordinate various processes through the autonomic nervoussystem, including, for example, temperature regulation,responses to thirst and hunger, micturition, respiration, andcardiovascular function.
This regulation is in response tosensory input and occurs through the reciprocal regulation ofthe SNS and PNS.Peripherally, axons of preganglionic neurons of the SNS andthe PNS emerge from the spinal cord and synapse with postganglionic neurons at sympathetic and parasympatheticganglia, respectively, where in both cases, acetylcholine is theneurotransmitter, acting at nicotinic receptors on postganglionic neurons (Fig. 7.1). Postganglionic neurons then sendmotor axons to effector organs and tissues. The catecholamine norepinephrine is released by postganglionic sympathetic axons and acts at adrenergic receptors of effectororgans. One exception is the postganglionic axons innervatingsweat glands, which release acetylcholine.
Furthermore, theadrenal medulla functions as part of the SNS. Preganglionicaxons of the SNS extend to the adrenal gland, where theystimulate chromaffin cells of the adrenal medulla to releaseepinephrine (and to a lesser degree norepinephrine) into thebloodstream. Notably, in addition to the catecholamines(norepinephrine and epinephrine), a number of adrenergiccotransmitters are released by some sympathetic postganglionic nerves, including neuropeptide Y, ATP (adenosinetriphosphate), and substance P, among others. In the PNS,acetylcholine, acting at muscarinic receptors, is the postganglionic neurotransmitter. These and other aspects of the twodivisions of the autonomic nervous system are compared inTable 7.1 and illustrated in Figures 7.2 and 7.3.
Actions of theautonomic nervous system in various organ systems andtissues are listed in Table 7.2, along with the receptor typesinvolved.ParotidglandGlossopharyngealnerve (IX)Medulla oblongataInternal carotid nerveLarynxTracheaBronchiLungsVagus nerve (X)HeartCervicalsympatheticgangliaStriated muscleSweat glandsWhite ramuscommunicansGray ramuscommunicansThoracic partof spinal cordCeliacganglionSuperiormesentericganglionHair folliclesPeripheral arteriesVisceral arteriesUpper lumbar partof spinal cord (L1–L2 [3])Gastrointestinal tractSuprarenalglandInferiormesenteric ganglionPelvic splanchnic nervesUrinary bladderSacral part of spinal cordC Cholinergic synapsesA Adrenergic synapsesSympathetic fibersPresynapticPostsynapticParasympathetic fibersPresynapticPostsynapticUrethraProstateSomatic fibersAntidromic conductionFigure 7.1 Cholinergic and Adrenergic Synapses: Schema Preganglionic neurons of the autonomic nervous system emerge from thespinal cord and synapse at autonomic ganglia.
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