This effect of descending inhibition can be shown by electrically stimulating the periaqueductal grey area of the midbrain. Consider skin , where a routine section of epidermis reveals almost everything interesting about the size, shape and growth sequence of epidermal cells. What is the Autonomic Nervous System? Epidermal Merkel cells are mechanosensory cells that tune mammalian touch receptors. Relatively few nerve cell bodies occur peripherally, in the ganglia small clusters of nerve cells of sympathetic and parasympathetic nervous systems.
Mack 1 2 Jing Feng 1 3 Timothy M. Whelan 1 2 Haixia Niu 1 2 Changxiong J. Variations in the detailed appearance "cytoarchitecture" of the several cortical layers, as described a century ago by K.
Brodmann , formed the original basis for recognizing regional differentiation of the cortex " Brodmann's areas ". Now, of course, this cytoarchitectural differentiation is known to correspond with functional localization in the cortex.
See WebPath for cortical changes associated with Alzheimer's disease. The cortex of the cerebellum consists of three very well-defined layers. The most prominent nerve cells are Purkinje cells , whose cell bodies all lie in a discrete layer. The inner granular layer is packed with nuclei of vastly many cerebellar granule cells. These are among the smallest and most numerous neurons in the body. The Purkinje cell layer contains large cell bodies of Purkinje cells , the sole output cells for the cortex.
The outer molecular layer consists principally of the dendrites of Purkinje cells and the axons of granule cells. The odd name "molecular layer" derives from the fine texture of this layer, due to its composition largely of dendrites and fine axon terminals. Nuclei in this layer belong mostly to glial cells. The pattern of connections among various axons and dendrites in the cerebellum is extremely elegant and regular, and has been described in extensive detail.
Any thorough neuro text e. Both the paravertebral ganglia of the sympathetic nervous system and the scattered ganglia of the parasympathetic nervous system consist of small clusters of nerve cell bodies. Parasympathetic ganglia may turn up in sections of various visceral organs, where they can be recognized by the classic appearance of nerve cell bodies. Like other "pieces" of the nervous system, peripheral nerves are a part of a functioning, highly organized whole. Each "piece" must be understood in relation to the rest of the system.
Examples of peripheral nerves are often fairly easy to find in sections of the skin. Larger nerves also often run in parallel with blood vessels.
Peripheral nerves consist of axons bundled together within an epineurium connective tissue sheath. Peripheral nerves are only meaningful in relation to their connections. All of the axons which travel along peripheral nerves begin and end somewhere else. Motor axons originate with cell bodies in the spinal cord's ventral horn or in the brainstem's motor nuclei or in peripheral sympathetic or parasympathetic ganglia. Motor axons terminate at muscles including smooth muscle along blood vessels or glands.
Somatosensory axons begin with a peripheral receptor e. These sensory axons then travel toward their cell bodies in a dorsal root ganglion or trigeminal ganglion , and finally terminate at synapses within the spinal cord or brain stem. Note that somatosensory axons are an exception to the rule that axons always conduct impulses away from the cell body. All the cellular nuclei which are obviously visible within a peripheral nerve belong not to nerve cells but to Schwann cells or to fibroblasts.
All three are eosinophilic, and all contain scattered, elongated nuclei. Several features may be used to distinguish nerves from smooth muscle or other fibrous tissue.
Note that the texture of peripheral nerves can differ from site to site, depending on axon size and especially on the proportion of myelinated to unmyelinated axons. Nerves in the tongue, with many large myelinated axons, are much more obvious than are autonomic nerves in Auerbach's plexus of the gut, where most axons are smaller and unmyelinated. In peripheral nerve cross sections stained for myelin, the myelin is generally visible as a dark or black frame around each pale myelinated axon.
The typical round shape is often distorted by tissue preparation. In longitudinal sections containing large myelinated axons, nodes of Ranvier can be easily seen where the myelin appears to be "pinched".
Seldom can a single axon be followed throughout an entire internode i. Nevertheless, the length of each internode can be estimated by measuring the total length of all axons visible in a field of view and dividing by the number of nodes that appear.
Less-than-ideal fixation also often distorts the relationship, so the axon may not be centered within the halo. Many details of peripheral nerves cannot be well-appreciated by light microscopy. For electron micrographs of peripheral nerves, see the online Electron Microscopic Atlas of Mammalian Tissues the text is in German, but most figure labels can be deciphered fairly easily. For sensory receptors in skin, see skin innervation.
For sensory receptors associated with muscle, see muscle innervation. The organization of the central nervous system is based upon interconnections across varying distances among billions of individual nerve cells. The basic principle of neural organization is quite straightforward.
Nervous tissue consists of nerve cells communicating with other nerve cells. This simple yet fundamental concept can easily become lost in the forest of details presented in standard textbooks. Here, then, is a brief guide to nervous tissue, including the classification and nomenclature of nerve cells.
Each nerve cell has a cell body in one place and an axon which travels some distance to synapse with the cell bodies and dendrites of other neurons. The microscopic appearances of gray matter and white matter may be conveniently contrasted in a section of spinal cord.
Various stains have various effects on gray matter. Note that a popular neuroanatomical stain Weigert's , used to highlight different brain regions, colors myelin black. Thus, paradoxically, in many pictures of the brain, white matter appears black while gray matter appears pale. Where cell bodies and dendrites are common, the gross color of fixed dead brain tissue is gray. Hence we have the term gray matte r. Note that gray matter is not just a place where cell bodies and dendrites happen to be.
Gray matter is the cell bodies and dendrites. Note that gray matter necessarily contains both the beginnings and endings of axons, even though the greater portion of many axons' length is contained within the fiber tracts of white matter. Gray matter is gray not because it lacks myelin, but because it contains lots of other stuff besides myelinated axons.
Axons from many different neurons often gather together in large numbers at some distance from their cell bodies. In such regions, the relatively large amount of myelin confers a white color, hence, white matter. Myelin is largely fat, which is white in both living and fixed condition. Note that although white matter consists of myelinated axons and unmyelinated axons as well , myelinated axons are not excluded from gray matter. Myelinated axons must begin and end somewhere, and that place is with cell bodies and dendrites of gray matter.
Gray matter just has a lot of other stuff in it besides myelinated axons. Also note that in many neuroanatomical images, white matter has been stained black. Sensory neurons convey sensory information into the central nervous system. Primary sensory neurons receive their information directly through sense receptors rather than dendrites. Second, third and higher order sensory neurons relay information to sequentially higher levels in the brain.
Motor neurons or motoneurons convey information out from the central nervous system to muscles or glands. Lower motor neurons , located in the ventral horn of the spinal cord or in motor nuclei of the brainstem, send their motor axons out peripheral nerves. Upper motor neurons , pyramidal cells located in the motor cortex , relay information to the lower motor neurons.
All other neurons are interneurons. They interconnect neurons with other neurons. Nearly all the nerve cells in the central nervous system are interneurons. Their axons arise in one region of the CNS where the cell body resides and end somewhere else sometimes several other places. Second, third and higher order sensory neurons can be considered as ascending interneurons; upper motor neurons can be considered as descending interneurons.
Information from primary sensory neurons does not reach the highest levels the cerebral cortex directly. Rather it is relayed at least twice once in the spinal cord or brain stem , again in the thalamus.
At each relay, incoming afferent , presynaptic axons terminate by synapsing onto the dendrites of the next neurons in the series. The outgoing axons of these neurons then relay the information to the next level. At each relay site, some information processing and distribution can occur, so the information can be altered as it travels upward. Similarly, muscle commands are relayed downward from motor cortex and other motor centers to the " final common pathway ", the lower motor neurons of cranial nerve nuclei and the anterior horn of the spinal cord.
Because each relay occurs at synapses onto dendrites and cell bodies of the next neurons in the pathway, each relay is associated with gray matter. Conversely, every gray matter region nucleus or cortex is associated with relaying information from one set of axons the afferent axons that enter the region in question to another the efferent axons that leave the region. Sometimes it is sufficient just to know the beginning and ending points of an entire pathway.
Other times knowing how far the neurons of each relay extend will be necessary to determine the site or effects of a lesion. All gray matter regions of the brain, both cortex and nuclei, are associated with afferent "input" and efferent "output" axons. Afferent axons enter the region from somewhere else i. Efferent axons arise from cell bodies within the region and leave the region to go somewhere else.
Thus every long-distance axon is both efferent with respect to its source, the location of its cell body and afferent with respect to its destination. The terms "afferent" and "efferent" are relational terms. Neither can be used precisely without specifying a region of reference e.
These terms may often correlate with " afferent " and " efferent ", at least when the reference is high, like cortex. In fact, "afferent" and "efferent" are sometimes used as synonyms of "ascending" and "descending", respectively. But they also have a relational meaning, defined above. But both sensory and motor information can be passed up, down, and sidewise, so these words should not be carelessly interchanged.
Gray matter typically contains both many short-axon neurons and a smaller number of long-axon neurons. The largest and most conspicuous cell bodies in a particular region of gray matter are sometimes referred to as the principal cells of that region.
These cells generally have very long axons which leave the local region to go elsewhere, usually traveling within some white matter fiber tract. The axons of these projection neurons may extend for appreciable distances, from a few centimeters to well over a meter. Long-axon neurons are responsible for communicating with other brain regions.
Every parcel of gray matter has a class of long-axon neurons; otherwise information would come in but never go out. The axons of a region's long-axon neurons are by definition identical with the region's efferent axons. The study of neuroanatomy is largely the study of the axonal projections of long-axon cells. The axons of the short-axon neurons do not leave the immediate neighborhood.
Short-axon neurons are also called intrinsic neurons or local interneurons. In most regions, long-axon cells are much better understood than intrinsic cells. Long axons provide opportunity for researchers to record and interfere with neuronal output. Cells with short axons are much more difficult to manipulate. Neurohistology is burdened by a profusion of names for different neuronal cell types.
Every nerve cell can be classified according to its place within the general organization of nervous tissue above. But every nerve cell also belongs to a unique population with a particular role in the information processing of the brain.
Unlike most other basic cell types in the body e. Neurons in one region are structurally and functionally different from those in other regions, with different sources of input, different destinations for output, different patterns of dendritic branching, different neurotransmitters, etc.
The result is a tremendous abundance of nerve cell types, with specific names for each. A few examples follow. Pyramidal cells are the efferent long-axon cells of the cerebral cortex. The name refers to the shape of the cell body as seen in standard sections perpendicular to the cortical surface. The apex of the pyramid points toward the cortical surface.
A large apical dendrite extends further upward toward that surface, while other dendrites arise from the corners and sides of the pyramid. The axon extends down into white matter the internal capsule from the base of the pyramid. This text-book material relating to information or to third party information, products and services , is provided 'as is'. All company names, product names logos included here may be registered trademarks or service marks of their respective owners.
Autonomic Nervous System - Introduction The organs of our body viscera , such as the heart, intestines and stomach, are regulated by a branch of the nervous system known as the autonomic nervous system. The autonomic nervous system is part of the peripheral nervous system and controls the function of many muscles, glands and organs within the body. We are usually quite unaware of the functioning of our autonomic system because it functions in a reflexive and involuntary manner. For example, we are not aware when our blood vessels change size, and we are usually unaware when our hearts speed up or slow down.
What is the Autonomic Nervous System? ANS neurons are responsible for regulating the secretions of certain glands i. The ANS helps to maintain homeostasis internal stability and balance through the coordination of various activities such as hormone secretion, circulation, respiration, digestion and excretion. The ANS is always "on" and functioning unconsciously, so we are unaware of the important tasks it is performing every waking and sleeping minute of every day.
Acetylcholine Ach is a chemical messenger that binds nicotinic acetylcholine receptors to the postsynaptic neurons postsynaptic neurons release norepinephrine NE in response to this stimulus prolonged activation of this stimulus response can trigger the release of adrenaline from the adrenal glands specifically the adrenal medulla once released, NE and adrenaline bind to adrenergic receptors on various tissues, resulting in the characteristic effects of "fight-or-flight" The following effects are seen as a result of activation of adrenergic receptors: However, it may be more correct to say that the SNS and the PNS have a complementary relationship, rather than one of opposition.
The most common are norepinephrine NE and acetylcholine Ach. All presynaptic neurons use Ach as a neurotransmitter. Ach is also released by some sympathetic postsynaptic neurons and all parasympathetic postsynaptic neurons.
In addition to neurotransmitters, certain vasoactive substances are released by postsynaptic automatic neurons, which bind to receptors on target cells and influence the target organ. How does the SNS mediate its action? In the sympathetic nervous system, catecholamines norephinephrine, epinephrine act on specific receptors located on the cell surface of the target organs. These receptors are called adrenergic receptors.
Alpha 1 receptors exert their effect on smooth muscle, mainly by constriction. Effects may include constriction of arteries and veins, decreased motility within the GI gastrointestinal tract, and constriction of the pupil.
Alpa1 receptors are usually located postsynaptically. Alpha 2 receptors bind both epinephrine and norepinephrine, thus reducing the effect of alpha 1 receptors to a certain extent. However, alpha 2 receptors have several specific effects of their own, including vasoconstriction. Effects may include coronary artery constriction, constriction of smooth muscle, constriction of veins, decreased intestinal motility and inhibition of insulin release.
Beta 1 receptors exert their effect mostly on the heart, causing an increase in cardiac output, increased contractility and increased cardiac conduction, leading to an increase in heart rate. There is also stimulation of the salivary glands. Beta 2 receptors exert their effect mostly on the skeletal and cardiac muscles.
Increased contraction speed and mass of muscles, as well as dilation of blood vessels occurs. Receptors are stimulated by circulating neurotransmitters catecholamines. How does the PNS mediate its action? As mentioned, acetylcholine is the primary neurotransmitter of the PNS. Acetylcholine acts on cholinergic receptors known as muscarinic and nicotinic receptors.
Muscarinic receptors exert their effect on the heart. There are two main muscarinic receptors: M2 receptors- acted on by acetylcholine, M2 receptors are located in the heart; stimulation of these receptors causes the heart to slow decreased heart rate and contractility and an increase in refractoriness. M3 receptors- located throughout the body; activation causes increased synthesis of nitric oxide, which results in relaxation of cardiac smooth muscle cells.
How is the autonomic nervous system organized? As previously discussed, the autonomic nervous system is subdivided into two separate divisions: It is important to understand how these two systems function in order to determine how they each affect the body, keeping in mind that both systems work in synergy to maintain homeostasis within the body. Both the sympathetic and parasympathetic nerves release neurotransmitters, primarily norepinephrine and epinephrine for the sympathetic nervous system, and acetylcholine for the parasympathetic nervous system.
These neurotransmitters also called catecholamines relay the nerve signals across the gaps synapses created when the nerve connects to other nerves, cells or organs. The neurotransmitters then attach to either sympathetic receptor sites or parasympathetic receptor sites on the target organ to exert their effect.
This is a simplified version of how the autonomic nervous system functions. How is the autonomic nervous system controlled? The ANS is not under conscious control. There are several centers which play a role in control of the ANS: Limbic system- the limbic system is composed of the hypothalamus, the amydala, the hippocampus, and other nearby areas. These structures lie on both sides of the thalamus, just under the cerebrum. Hypothalamus- the cells that drive the ANS are located in the lateral medulla.
The hypothalamus projects to this area, which includes the parasympathetic vagal nuclei, and also to a group of cells which lead to the sympathetic system in the spinal cord. By interacting with these systems, the hypothalamus controls digestion, heart rate, sweating and other functions.
Brain stem- the brainstem acts as the link between the spinal cord and the cerebrum. Sensory and motor neurons travel through the brainstem, conveying messages between the brain and spinal cord. The brainstem controls many autonomic functions of the PNS, including respiration, heart rate and blood pressure. Spinal cord- two chains of ganglia are located on either side of the spinal cord.
The outer chains form the parasympathetic nervous system, while the chains closest to the spinal cord form the sympathetic element. What are some receptors of the autonomic nervous system? Sensory neuron dendrites are sensory receptors that are highly specialized, receiving specific types of stimuli. We do not consciously sense impulses from these receptors except perhaps pain. There are numerous sensory receptors: Photoreceptors- respond to light Thermoreceptors- respond to alterations in temperature Mechanoreceptors- respond to stretch and pressure blood pressure or touch Chemoreceptors- respond to changes in internal body chemistry i.
In this way, visceral motor neurons can be said to indirectly innervate smooth muscles of arteries and cardiac muscle. In addition, autonomic motor neurons can continue to function even if their nerve supply is damaged, albeit to a lesser extent.
Where are the autonomic nervous system neurons located? The ANS is essentially comprised of two types of neurons connected in a series. The nucleus of the first neuron is located in the central nervous system.
SNS neurons begin at the thoracic and lumbar areas of the spinal cord, PNS neurons begin at the cranial nerves and sacral spinal cord. The first neuron's axons are located in the autonomic ganglia. In terms of the second neuron, its nucleus is located in the autonomic ganglia, while the axons of the second neuron are located in the target tissue.
The two types of giant neurons communicate using acetylcholine. Sympathetic Parasympathetic Function To defend the body against attack Healing, regeneration and nourishing the body Overall Effect Catabolic breaks down the body Anabolic builds up the body Organs and Glands It Activates The brain, muscles, the insulin pancreas, and the thyroid and adrenal glands The liver, kidneys, enzyme pancreas, spleen, stomach, small intestines and colon Hormones and Substances It Increases Insulin, cortisol and the thyroid hormones Parathyroid hormone, pancreatic enzymes, bile and other digestive enzymes Body Functions It Activates Raises blood pressure and blood sugar, and increases heat production Activates digestion, elimination and the immune system Psychological Qualities Fear, guilt, sadness, anger, willfulness, and aggressiveness.
The sympathetic branch mediates this expenditure while the parasympathetic branch serves a restorative function. The sympathetic nervous system causes a speeding up of bodily functions i. The ANS affects changes in the body that are meant to be temporary; in other words, the body should return to its baseline state. It is natural that there should be brief excursions from the homeostatic baseline, but the return to baseline should occur in a timely manner. When one system is persistently activated increased tone , health may be adversely affected.