Efferent fibers

In addition to these clearly defined peripheral motor portions of the ANS, large numbers of afferent fibers run from the periphery to integrating centers, including the enteric plexuses in the gut, the autonomic ganglia, and the CNS. Many of the sensory pathways that end in the CNS terminate in the integrating centers of the hypothalamus and medulla and evoke reflex motor activity that is carried to the effector cells by the efferent fibers described previously. There is increasing evidence that some of these sensory fibers also have peripheral motor functions. 

Tire neuronal events that occur within the cerebral cortex are extraordinarily complex and little understood 409 In what way the brain is able to initiate voluntary movement of muscles is obscure. However, it is established that the signals that travel out of the brain down the efferent fibers to the muscles arise from large motor neurons of the motor cortex,410 a region that extends in a band across the brain and adjacent to the sensory cortex (Fig. 30-14). The axons of these cells form the pyramidal tract that carries impulses downward to synapses in the spinal cord and from there to the neuromuscular junctions. These are specialized synapses at which acetycholine is released, carrying the signal to the muscle fibers themselves. Passing over the cell surface and into the... 

Additionally, cardiac function can also be modulated by centrally-located H3-receptors. The intracerebroventricular administration of (R)a-methylhistamine in conscious animals is associated with a marked reduction of the heart rate, an effect which is antagonised by thioperamide (McLeod et al., 1991). The peripheral administration of ipratropium, a muscarinic antagonist which does not cross the blood-brain barrier, prevents such inhibition, demonstrating that the activation of centrally-located histamine H3-receptors leads to an increase in the vagal tone to the heart, rather than to a facilitatory effect on the sympathetic efferent fibers. 

Cardiovascular System. Atropine is sometimes used to block the effects of the vagus nerve (cranial nerve X) on the myocardium. Release of acetylcholine from vagal efferent fibers slows heart rate and the conduction of the cardiac action potential throughout the myocardium. Atropine reverses the effects of excessive vagal discharge and is used to treat the symptomatic bradycardia that may accompany myocardial infarction.4 Atropine may also be useful in treating other cardiac arrhythmias such as atrioventricular nodal block and ventricular asystole. 

The tectorial membrane rests at the top of the organ of Corti, and the basilar membrane forms the base. Two types of hair cells are found along the basilar membrane. There are three rows of outer hair cells and one row of inner hair cells. The outer hair cells form part of the mechanical system of the cochlear partition, while the inner hair cells provide transduction from mechanical motion into neural firing patterns. There are about 30,000 nerve fibers in the human ear. The vast majority are afferent fibers that conduct the inner hair cell neural pulses towards the brain approximately 20 fibers are connected to each of the 1,500 inner hair cells. Approximately 1,800 efferent fibers conduct neural pulses from the brain to the outer hair cells [Pickles, 1988],... 

The autonomic nervous system consists of central connections, visceral afferent fibers, and visceral efferent fibers. The hypothalamus is where the principal integration of the entire autonomic nervous... 

The somatic nervous system is composed of sensory afferents and motor efferents and serves to perceive external states and to modulate appropriate body responses. The autonomic nervous system (ANS), together with the endocrine system, controls the milieu interieur. It adjusts internal organ functions to the changing needs of the organism. The ANS operates largely autonomously, beyond voluntary control, at the subconscious level. Its central components reside in the hypothalamus, brain stem, and spinal cord. The ANS has sympathetic and parasympathetic branches. Both are made up of afferent, mainly in the vagus nerve, and efferent fibers. 

The nerve fibers in the cochlea can be classified as afferent fibers (toward the brain) or efferent fibers (toward the periphery). The afferent fibers that terminate on IHCs constitute approximately 90-95% of all afferent fibers in the cochlea. These afferent fibers originate from type I ganglion cells and are coated with a thick myelin sheath. Each fiber is connected to only one IHC, but each IHC is innervated by 2-10 individual afferent fibers. The principal neurotransmitter released by IHCs, glutamate, activates the afferent fibers in a quantal manner. The remaining 5-10% of afferent fibers are connected to OHCs and are unmyelinated. They originate from type II ganglion cells. Each afferent fiber is highly branched and is connected to 6-100 OHCs. OHCs are innervated mainly by efferent fibers, which are derived from the medial olivocochlear bundle. Efferent fibers release acetylcholine (ACh) as their principal neurotransmitter. Each OHC is in contact with 2-5 efferent synaptic boutons at its base. IHCs receive axodendritic efferent innervation onto afferent fibers from the lateral olivocochlear bundle (reviewed by Raphael and Altschuler, 2003). 

Haines DE, Whitworth RH (1978) Cerebellar cortical efferent fibers of the paraflocculus of tree shrew (Tupaia glis). J. Comp. Neurol, 182, 137-150. 

Klinkhachorn PS, Haines DE, Culberson JL (1984a) Cerebellar cortical efferent fibers in the North American opossum, Didelphis virginiana. 1. The anterior lobe, J. Comp. Neurol, 227, 424-438. 

One of the subdivisions of the SOC, the dorsomedialperiolivary nucleus (DMPO), is a source of efferent fibers that reach the contralateral cochlea in the crossed olivocochlear bundle (COCB). Neurons of the DMPO receive inputs Ifom collaterals of globular bushy cell axons of the contralateral ACVN that project to the MNTB and from octopus cells on both sides. The functional role of the feedback from the DMPO to the cochlea is not well understood. 

Within a bundle, the fibers are packed with interstitial connective tissue (endoneurium). Many bundles constitute a nerve. The outermost part of a nerve is a sheet of connective tissue (epineurium). A nerve bundle usually contains both efferent fibers (motor fibers, signal direction toward periphery) and afferent fibers (receptor signals toward CNS). 

To maintain homeostasis, the continuous transudation of fluid and solutes from the pulmonary capillary bed into the surrounding inteistitium and alveolar space is balanced by lymphatic drainage out of the lung. The lymphatic flow is directed toward the hilum from the pleural surfaces. From lymph nodes in the hilum, the lymph travels to the paratracheal nodes and then eventually into the venous system via the thoracic duct. The lung has nerve fibers from both the vagal nerves parasympathetic) and the sympathetic nerves. The efferent fibers go to the bronchial musculature and the afferents come from the bronchi and alveoli. 

As a major fraction of striatal efferent fibers appear to be GABAergic in nature (Fonnum et at., 1978) and only other neurotransmitters will be... 

The cells most relevant to the present discussion are those forming the photoreceptive neurons. The dendrites of the neurons assume a special shape well suited to photoreception. The synapses transfer the impulse through their connections with dendrites originating from the bipolar and the horizontal cells. The photoreceptor cell is made up of cone or rod, body, and efferent fibers with the synapse. 

---