6.5 Nerves, hormones, and homeostasis

6.5.1 Central nervous system

The nervous system is a network of specialized cells that communicate information about an organism's surroundings and itself. It processes this information and causes reactions in other parts of the body. It is composed of neurons and other specialized cells called glia, that aid in the function of the neurons. The nervous system is divided broadly into two categories: the peripheral nervous system and the central nervous system. Neurons generate and conduct impulses between and within the two systems.
	1.     Central Nervous System (CNS)   a. Structures of the CNS:     -Brain     -Spinal cord   b.    Function:  The CNS coordinates and interprets information to determine the best response    2. Peripheral Nervous System (PNS)   a. Consist of Cranial nerves  (12 pairs) & Spinal nerves: (31 pairs) - Carry nerve impulses to and from the spinal cord to body parts not served by the cranial nerves. Sensory (afferent) nerves bring sensory inputs into the CNS and motor (efferent) nerves takes signals back out.   b. Can be separated into:      1. Somatic nervous system - Carries nerve impulses to the skeletal muscles, joints and skin          2. Autonomic nervous system (ANS) -Carries nerve impulses to the smooth muscles of internal organs and to glands without conscious thought   Two subdivisions of autonomic nervous system :     -Sympathetic system -  Controls "fight or flight" responses. Its neurotransmitter is norepinephrine (or noradrenaline). "Inducible system"       -Parasympathetic system -Controls those responses associated with a relaxed state. Its neurotransmitter is acetylcholine (Ach).
Nervous system animation: http://health.howstuffworks.com/adam-200011.htm

Central Nervous System
	The central nervous system (CNS) is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of multicellular organisms. The CNS contains the majority of the nervous system is contained within the dorsal cavity, with the brain in the cranial cavity and the spinal cord in the spinal cavity. The brain is protected by the skull, while the spinal cord is protected by the vertebrae. Together with the peripheral nervous system it has a fundamental role in the control of behavior. The CNS is covered by the meninges, a three layered protective coat. consisting of: the dura mater, the arachnoid mater, and the pia mater. The primary function of the meninges and of the cerebrospinal fluid is to protect the central nervous system
Peripheral Nervous System
	The peripheral nervous system (PNS) resides or extends outside the central nervous system (CNS). The main function of the PNS is to connect the CNS to the limbs and organs. Unlike the central nervous system, the PNS is not protected by bone or by the blood-brain barrier, leaving it exposed to toxins and mechanical injuries. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system
Neurons
	Neurons are electrically excitable cells in the nervous system that process and transmit information by electrochemical signalling. Neurons are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves. They use electrochemical signals and neurotransmitters to transmit impulses from one neuron to the next. A number of different types of neurons exist: 1. Sensory neurons (Afferent Neurons) respond to touch, sound, light and numerous other stimuli effecting sensory organs and send signals to the CNS, away from the peripheral system. Unlike neurons of the central nervous system, whose inputs come from other neurons, sensory neurons are activated by physical modalities such as light, sound, temperature, chemical stimulation, etc. 2. motor neurons (Efferent Neurons) receive signals from the CNS and carry this to the peripheral system, causing muscle contractions and affecting glands. 3. Interneurons are multipolar neurons that connect neurons to other neurons within the brain and spinal cord. The reside entirely within the CNS

6.5.2 Motor neuron

In vertebrates, the term motor neuron (or motoneuron) classically applies to neurons located in the central nervous system (or CNS) that project their axons outside the CNS and directly or indirectly control muscles. Motor neuron is often associated with efferent neuron, primary neuron, or alpha motor neurons.

 

The soma is the central part of the neuron. It contains the nucleus of the cell, and therefore is where most protein synthesis occurs. The nucleus ranges from 3 to 18 micrometers in diameter.[11] The dendrites of a neuron are cellular extensions with many branches, and metaphorically this overall shape and structure is referred to as a dendritic tree. This is where the majority of input to the neuron occurs. Information outflow (i.e. from dendrites to other neurons) can also occur, but not across chemical synapses; there, the back flow of a nerve impulse is inhibited by the fact that an axon does not possess chemoreceptors and dendrites cannot secrete neurotransmitter chemicals. This unidirectionality of a chemical synapse explains why nerve impulses are conducted only in one direction. The axon is a finer, cable-like projection which can extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length. The axon carries nerve signals away from the soma (and also carries some types of information back to it). Many neurons have only one axon, but this axon may - and usually will - undergo extensive branching, enabling communication with many target cells. The part of the axon where it emerges from the soma is called the axon hillock. Besides being an anatomical structure, the axon hillock is also the part of the neuron that has the greatest density of voltage-dependent sodium channels. This makes it the most easily-excited part of the neuron and the spike initiation zone for the axon: in neurological terms it has the most negative action potential threshold. While the axon and axon hillock are generally involved in information outflow, this region can also receive input from other neurons. The axon terminal contains synapses, specialized structures where neurotransmitter chemicals are released in order to communicate with target neurons.

Axons often have an insulating sheath that lets nerve impulses travel faster. This sheath is made of a fatty substance called myelin, which consists of glial cell membranes wrapped around the axon. The myelin of the neurons in the brain is composed of oligodendrocytes, while that of the neurons in the peripheral nervous system is composed of Schwann cells. The myelin sheath does not cover the entire axon; it leaves small sections uncovered. These small exposed sections are called nodes of Ranvier. They are spaced from 0.2 to 2 millimeters apart. The reason that the myelin sheath speeds up neural conduction is that the action potentials literally jump from one node of Ranvier to the next. In fact, these nodes are the only places where the ion exchanges that generate the action potential can take place. This process is called saltatory conduction (from the Latin saltare, meaning “to jump”), as opposed to the much slower, continuous propagation that occurs in non-myelinated axons.

6.5.3 Nerve impulses

nerve impulse animation: http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html

6.5.4 Resting and action potentials

An action potential is a self-regenerating wave of electrochemical activity that allows nerve cells to carry a signal over a distance. It is the primary electrical signal generated by nerve cells, and arises from changes in the permeability of the nerve cell's axonal membranes to specific ions. Action potentials (also known as nerve impulses or spikes) are pulse-like waves of voltage that travel along several types of cell membranes.^[1] The best-understood example of an action potential is generated on the membrane of the axon of a neuron, but also appears in other types of excitable cells, such as cardiac muscle cells, and even plant cells.

Neuron potentials animations: http://bcs.whfreeman.com/thelifewire/content/chp44/4401s.swf

Studying action potentials animation: http://www.sumanasinc.com/webcontent/animations/content/action_potential.html

Channel gating during action potential animation: http://www.blackwellpublishing.com/matthews/channel.html

Sodium-Potassium Exchange:  http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter45/animations.html#
 

6.5.5 Nerve impulse

Nerve impulse (myelinated vs. nonmyelinated neurons) animation: http://www.blackwellpublishing.com/matthews/actionp.html

Action Potential Propagation in an Unmyelinated Axon: http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter45/animations.html#

6.5.6 Synaptic transmission

Synaptic transmission animation: http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__transmission_across_a_synapse.html

neuromuscular synapse animation: http://www.sumanasinc.com/webcontent/animations/content/synaptictransmission.html

synaptic vesicle release animation: http://www.blackwellpublishing.com/matthews/nmj.html

Comparison of direct and indirect neurotransmitter actions: http://www.blackwellpublishing.com/matthews/neurotrans.html

6.5.7 Endocrine system

The endocrine system is a system of small organs that involve the release of extracellular signaling molecules known as hormones. The endocrine system is instrumental in regulating metabolism, growth, development and puberty, and tissue function and also plays a part in determining mood.^[1] The field of study that deals with disorders of endocrine glands is endocrinology, a branch of the wider field of internal medicine.

Hormones: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/H/Hormones.html

Hormonal communication animation: http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter20/animation__hormonal_communication.html

Endocrine pathology (images): http://library.med.utah.edu/WebPath/ENDOHTML/ENDOIDX.html

Hormone action animations: http://www.wisc-online.com/objects/index_tj.asp?objID=AP13704

6.5.8 Homeostasis

Homeostasis animation: http://health.howstuffworks.com/adam-200092.htm

Site: http://www.biology-online.org/4/1_physiological_homeostasis.htm

6.5.9 Monitoring

Positive & negative feedback animation: http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__positive_and_negative_feedback__quiz_2_.html

6.5.10 Body temperature & homeostasis

One of the most important examples of homeostasis is the regulation of body temperature. Not all animals can do this physiologically. Animals that maintain a fairly constant body temperature (birds and mammals) are called endotherms, while those that have a variable body temperature (all others) are called ectotherms. Endotherms normally maintain their body temperatures at around 35 - 40°C, so are sometimes called warm-blooded animals, but in fact ectothermic animals can also have very warm blood during the day by basking in the sun, or by extended muscle activity 9e.g. bumble bees, tuna). The difference between the two groups is thus that endothermic animals use internal corrective mechanisms, whilst ectotherms use behavioural mechanisms (e.g. lying in the sun when cold, moving into shade when hot). Such mechanisms can be very effective, particularly when coupled with internal mechanisms to ensure that the temperature of the blood going to vital organs (brain, heart) is kept constant. We use both!

In humans, body temperature is controlled by the thermoregulatory centre in the hypothalamus. It receives input from two sets of thermoreceptors: receptors in the hypothalamus itself monitor the temperature of the blood as it passes through the brain (the core temperature), and receptors in the skin (especially on the trunk) monitor the external temperature. Both sets of information are needed so that the body can make appropriate adjustments. The thermoregulatory centre sends impulses to several different effectors to adjust body temperature:

6.5.11 Glucose & homeostasis

Glucose levels animation: http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter20/animation__blood_sugar_regulation_in_diabetics.html

Glucose levels tutorial: http://www.wisc-online.com/objects/index_tj.asp?objID=AP15004

6.5.12 Diabetes

Diabetes mellitus is a syndrome of disordered metabolism, usually due to a combination of hereditary and environmental causes, resulting in abnormally high blood sugar levels (hyperglycemia). Blood glucose levels are controlled by a complex interaction of multiple chemicals and hormones in the body, including the hormone insulin made in the beta cells of the pancreas. Diabetes mellitus refers to the group of diseases that lead to high blood glucose levels due to defects in either insulin secretion or insulin action in the body. Diabetes develops due to a diminished production of insulin (in type 1) or resistance to its effects (in type 2 and gestational). Both lead to hyperglycemia, which largely causes the acute signs of diabetes: excessive urine production, resulting compensatory thirst and increased fluid intake, blurred vision, unexplained weight loss, lethargy, and changes in energy metabolism. All forms of diabetes have been treatable since insulin became medically available in 1921, but there is no cure. The injections by a syringe, insulin pump, or insulin pen deliver insulin, which is a basic treatment of type 1 diabetes. Type 2 is managed with a combination of dietary treatment, exercise, medications and insulin supplementation. Diabetes and its treatments can cause many complications. Acute complications (hypoglycemia, ketoacidosis, or nonketotic hyperosmolar coma) may occur if the disease is not adequately controlled. Serious long-term complications include cardiovascular disease (doubled risk), chronic renal failure, retinal damage (which can lead to blindness), nerve damage (of several kinds), and microvascular damage, which may cause erectile dysfunction and poor wound healing. Poor healing of wounds, particularly of the feet, can lead to gangrene, and possibly to amputation. Adequate treatment of diabetes, as well as increased emphasis on blood pressure control and lifestyle factors (such as not smoking and maintaining a healthy body weight), may improve the risk profile of most of the chronic complications. In the developed world, diabetes is the most significant cause of adult blindness in the non-elderly and the leading cause of non-traumatic amputation in adults, and diabetic nephropathy is the main illness requiring renal dialysis in the United States

Insulin interaction: http://www.abpischools.org.uk/page/modules/hormones/horm6.cfm

Type II diabetes animation: http://www.healthscout.com/animation/1/34/main.html

Agrosy animations: http://www.argosymedical.com/medical_ani_sys/nervous.html