Anatomy Week Eight

In this week we learned about the Endocrine system.  The nervous system and the endocrine systems work together to coordinate the functions of all the body systems.  At the synapses, nerve impulses trigger the release of the mediator molecules that are called neurotransmitters.  The endocrine system, as mentioned before, controls body activities by releasing these mediator molecules that are called hormones.  The means of control of these two systems are very different.

The hormone is a molecule that is released in one part of the body but regulates the activity of cells in other parts of the body as well.  The majority of the hormones enter the interstitial fluid and then the bloodstream.  As the blood circulates, it delivers these to the cells throughout the body.  Like neurotransmitters, hormones exert their effects by binding to receptors on or in their target cells.  There are, however, several mediator molecules that act as both hormones and neurotransmitters.  An important example of this is norepinephrine which is released by as a neurotransmitter by sympathetic postganglionic neurons, and also as a hormone by cells of the adrenal medullae.

The responses produced by the endocrine are often much slower than those of the nervous system.  Some hormones act within seconds, but most take several minutes or more to cause a response in the body.

There are two main kinds of glands in the body.  There are exocrine glands and endocrine glands.  The exocrine glands secrete their products into ducts that carry the secretions into body cavities, the lumen of an organ, or to the outer surface of the body.  Some examples of exocrine glands are the sebaceous glands, sudoriferous glands, mucous glands, and digestive glands.  

Endocrine glands in contrast secrete their products or hormones into the interstitial fluid surrounding the secretory cells, rather into ducts.   These hormones diffuse into capillaries and blood carries them to the target cells throughout the body.  The majority of hormones are required in very little dosages so the circulating levels are typically very low.  The glands that are endocrine are pituitary, thyroid, parathyroid, and pineal glands.  There are other glands that do not function exclusively as endocrine glands, but they do contain cells that are hormones.  These are the hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver,small intestine, skin, heart, adipose tissue, and placenta.  These and the endocrine glands act as the endocrine system.  

Without these hormones coursing through our blood, normal everyday functions of the body would not happen.  Something as simple as your body being able to sweat when you are overheating to cool you down, would not be able to happen without the endocrine system in place.  

Anatomy Week Four

We also talked about the diversity of structures of neurons in our bodies.  Neurons vary in sizes and shape.  They also range in different diameters.  The dendritic part of the neurons may also vary in branching styles depending on where they are in relation to where they are in the nervous tissue.  Some lack an axon, and others may have very short axons.  There are some axons that are incredibly long!  They can be as long as a person is tall, extending from a persons toe all the way to the lower part of the brain.  There are three main types neurons that may be classified.  These are multipolar neurons, bipolar neurons, and unipolar neurons.  

Multipolar neurons are the most common type of neuron found in the brain and spinal cord.  They have several dendrites and one axon.  

Bipolar neurons are typically found in the retina of the eye, the inner ear, and the olfactory part of the brain.  These have one main dendrite and one axon.

Unipolar neurons, however, are a little more complicated.  These are sensory neurons that begin in the embryo as bipolar neurons.  The axon and the dendrite fuse into one process that divides into two branches a short distance from the cell body during development.  The two branches that divided have the characteristic structures and functions of an axon.  They are also long and cylindrical processes that propagate action potentials.  The axon branches of these, however, that extend into the periphery has dendrites at its distal tip, whereas the axon branch that extends into the CNS end at synaptic bulbs.  The dendrites of the unipolar neurons monitor a sensory stimulus such as a touch or stretching.  The impulses from this point then propagate toward the synaptic end bulbs.

These structures of the neurons are important in the body, and there are many chemical and electrical factors that get these messages and stimuli sent along through the Central and Peripheral nervous system.  Without being able to send these electrical or chemical messages from the receiving dendrites to the terminal axon, down the axon, to the terminal bulbs, and across the synaptic cleft to the next neuron or effector cell, we wouldn't be able to do something as simple as blinking or lifting a finger.  It is a highly complex process and there are many outside factors that effect or determine how these messages get sent.

Anatomy Week Three

So In this week of class we really got into the structure and diversity of neurons.  We learned that nervous tissue contains two types of cells, and these are neurons and neuroglia.  The neurons carry out a lot of the unique functions of the system like the sensing, thinking, remembering, controlling muscle activity, and the regulation of glandular secretions.  They also have the property of electrical excitability.  Electrical excitability is the ability to produce action potentials or impulses in response to stimuli.  Once these action potentials arise, they propagate from one point to the next along the neuron.  The neuroglia are responsible for supporting, nourishing, protecting the neurons, and maintaining homeostasis in the interstitial fluid that bathes them.  

The neurons have three main parts that comprise them structurally.  They have a cell body, dendrites, and an axon.  the cell body is what contains the nucleus and is surrounded by cytoplasm.  In the cytoplasm are the typical organelles such as mitochondria, lysosomes, and the Golgi apparatus complex.  Neurons, however, cannot divide though because they lack the centrioles that are essential in mitosis.  What makes them really stand out is the fact that they contain very prominent clusters of endoplasmic reticulum that are called Nissl bodies.  This is where the protein synthesis occurs in the neuron.  The Nissl bodies are what produce newly synthesized proteins that are used to replace cellular components as material for growth for new neurons, and to regenerate damaged axons.  

There are two types of extensions that come off the neuron, and these are multiple dendrites and a single axon.  The dendrites are the portions of the neuron that receive the messages or input.  These structures are usually short, tapering, and highly branched.  They kind of form a tree like array of these processes that extend from the cell body.

The axon extends from the cell body as well, and is usually kind of the opposite end of the dendrites.  This is where the neuron propagates nerve impulses toward another neuron, muscle fiber, or gland cell.  The axon itself looks like a long, thin, cylindrical projection that joins the cell body at a cone-shaped elevation called the axon hillock.  Sometimes small branches, called axon collaterals, form at right angles to the axon.  All the axons end by dividing into many fine processes called the axon terminals.  These form little bulb-like structures called synaptic end bulbs.  These are the sites of the communication between the neuron and another neuron or an effector cell.  

The structures of the neurons are extremely important, because are where the physiological aspects of the Central and Peripheral nervous system take place.  There are millions of neurons in the body, and they are constantly sending messages to one another or to receptor cells to eventually have your body do something, like move your arm, or even to blink.  It it constantly happening in your body, and sometimes you can control it, and with some things you have no control and these neurons are constantly sending messages to do things you have no control over!      

Anatomy Week Two

The first week we learned about the nervous system and how it was divided into 2 main systems, the central nervous system and the peripheral nervous system.  In this week we learned how they work together to convey information.  The peripheral nervous system is further divided into the autonomic nervous system and the somatic nervous system.  

The somatic nervous system consists of the sensory neurons that are responsible for conveying the information from somatic receptors in the head, body wall, and limbs, and also from the receptors for the special senses of vision, hearing, taste, and smell to the CNS.  The somatic nervous system also includes the motor neurons that conduct impulses from the CNS to only skeletal muscles.  The action of these motor responses can be controlled, so these are considered voluntary.  

The autonomic nervous system consists of sensory neurons and motor neurons.  The sensory neurons convey information to the CNS from autonomic sensory receptors that are located primarily in the visceral organs such as the stomach and lungs.  The motor neurons conduct nerve impulses from the CNS to smooth muscle, cardiac muscle and glands.  Because its motor responses are not normally under conscious control, the actions of the autonomic nervous systems are considered involuntary.  The ANS (autonomic nervous system) is made up of three divisions: the sympathetic, parasympathetic, and the enteric.  

With a few exceptions, effectors are innervated by both the sympathetic and parasympathetic divisions, usually with opposing actions.  An example of this would be that the sympathetic neurons work to increase heart rate, and the parasympathetic neurons work to slow the heart rate down.  The Enteric division consists of enteric plexuses that extend the length of the gastrointestinal tract.  The enteric sensory neurons monitor the chemical changes within the GI tract and the stretching of its walls.  The enteric motor neurons monitor and govern the contraction of the of the smooth muscle that lines the GI tract, the secretions of the GI tract organs (such as the acid secretion by the stomach), and the activity of the GI tract endocrine cells.  The Enteric plexuses also communicate with the CNS by the sympathetic system and parasympathetic neurons, although many of the neurons in the enteric plexuses function independently of the CNS to some extent.  The enteric plexuses are sometimes thought of as the 'brain' of the gut, and there are there are over 100 million sensory, motor, and interneurons which is as many as the spinal cord. 

So all these things are happening in your body every second of our lives and we don't even know its happening.  Our  nervous systems are extremely complex and we never really stop to think about all the processes that go on in it even when you do something as simple as touching something.  Our bodies being able to perceive external stimuli as well as making sure that all our bodily functions are running smoothly is simply amazing.   

       

Anatomy Week One

The first week in anatomy class we covered the nervous system.  We learned that there are two main divisions of the nervous systems, and these are the central nervous system and the peripheral nervous system.  The central nervous system consists of the brain and the spinal cord, and the peripheral nervous system includes all the nervous tissue outside the CNS.  In the peripheral nervous system the nerves are classified by their origin, for example the cranial nerves emerge from the base of the brain.  The nervous system is responsible for all of one's perceptions, behavior, memories, and movements.  The diverse actions of the nervous system can be divided into 3 main groups: the sensory function, the integrative function, and the motor function.  

The sensory function has sensory receptors that can detect internal stimuli.  Some examples of this are when the body can sense an increase in blood acidity and external stimuli such as a feeling a raindrop hitting your skin.  The sensory or afferent neurons carry these messages through cranial and spinal nerves into the brain and the spinal cord to be processed.

The Integrative function of the nervous system integrates or processes the sensory information received from afferent neurons by analyzing it, storing it, and making decisions for appropriate responses to that stimuli.  Interneurons are some of the many neurons involved in this process and these have axons that extend only for a short distance within the CNS and contact nearby neurons in the brain, spinal cord, or a ganglion.  The majority of the neurons in the body are the interneurons.  

The Motor function of the nervous system involves responding to to integration decisions that are made by the interneurons after being processed.  The Neurons that serve this function are called motor neurons or they can also be called efferent neurons.  These carry the information from the brain and the spinal cord through cranial and spinal nerves.  The cells and organs that are contacted by these neurons are called effectors.  Some examples of these are muscle fibers and glandular cells.  

Without even realizing it, our nervous system is constantly working, even in our sleep.  It is responsible for making sure that everything in our body is running smoothly.  It is the key factor in maintaining the bodies homeostatic balance.  It helps to make sure that things like our body temperature, blood acidity, heart rate, blood pressure, etc. are all in the ranges that they are supposed to be in.  If for some reason our body starts to go out of the normal ranges in maintaining homeostasis, it is responsible for detecting these changes, sending it to the integration centers in the brain, and then processing the proper course of action to correct the problem.  The nervous system is also responsible for processing external stimuli such as the rain drop falling onto one's skin, burning a finger, or falling down and scraping a knee.