Topic 6.5

 Central Nervous System (CNS) consists of brain and spinal cord  These receive information from receptors, process the information and then initiate a motor response  Neurons are the cells of the nervous system  Sensory neurons – carry information to the CNS  Motor neurons – carry the response to muscles

 Sensory and motor neurons make up the peripheral nervous system (PNS)  A cluster of neurons make up a nerve  There are 2 categories of peripheral nerves:  Spinal nerves – 31 pair, some sensory, some motor  Cranial nerves – 12 pair emerging from the brainstem

 Autonomic nervous system  Nerves from internal receptors  Nerves to smooth (involuntary) muscle  Somatic nervous system  Motor neurons to skeletal muscles  Sensory neurons from sense organs

 Sensory neurons have long axons and transmit nerve impulses from sensory receptors all over the body to the central nervous system.  Motor neurons also have long axons and transmit nerve impulses from the central nervous system to effectors (muscles and glands) all over the body.  Interneurons AKA relay neurons are much smaller cells, with many interconnections.

 A neuron at rest has a net positive charge outside the axon and a net negative charge inside the axon. This condition is resting potential.  This electric potential difference is maintained by the sodium/potassium pumps found in the plasma membrane. This pump requires ATP to operate.  3 sodium ions are pumped out of the membrane while two potassium ions are pumped in. This makes the cell more positive on the outside than on the inside.

 Remember, the resting potential is maintained by active transport, Na and K are being pumped across the membrane against their concentration gradient.  First Na ion channels open, Na enters the cell. This is depolarization.  Next K ion channels open, K leaves the cell.This is repolarizes the membrane but Na and K ions are not in the correct place to conduct the next action potential.  The Na/K pump restores ions to their proper condition..

 The time required for a neuron to send a action potential then repolarize so it can send another action potential  Lasts about 2 ms  This ensures that the action potential can move in one direction and keeps action potentials from catching up to each other

 actionpotential.swf actionpotential.swf  This is the hotlink animation mentioned in your book (not so great!) /channel.html /channel.html

 The space between 2 adjacent neurons is called a synapse  The neuron before the synapse is the presynaptic neuron and the one after the synapse is the postsynaptic neuron  See page 178 figure 6.15 for 3 patterns of synaptic transmission  An action potential cannot cross the synapse. Instead neurotransmitters carry the impulse from one neuron to the next.

 An action potential reaches the end of the presynaptic neuron. Voltage gated Ca channels open and Ca diffuses into the neuron.  The Ca ions cause vesicles of neurotransmitter to fuse with the presynaptic membrane and release neurotransmitter into the synapse (an example of exocytosis).  The neurotransmitter diffuses across the synapse

 The neurotransmitters bind to neuroreceptors on the postsynaptic membrane. This causes Na ion channels to open and Na diffuses into the cell  This depolarization of the postsynaptic membrane begins a new action potential  The neurotransmitter is broken down by enzymes and is released from the neuroreceptor

 The breakdown prevents the synapse from being permanently on  Components of the neurotransmitter are taken up by the presynaptic neuron (endocytosis) and used to synthesize more neurotransmitter

 The endocrine system consists of glands that release hormones directly into the bloodstream  The hormone travels in the blood to a target tissue with receptors specific to that hormone  Once the target tissue (effector) is reached a response takes place

 Homeostasis involves maintaining the bodies internal environment between specific limits  Some levels which must be maintained:  Blood pH  Carbon dioxide concentration  Blood glucose levels  Body temperatures  Water/osmotic balance within tissues

 Homeostasis involves monitoring levels of variables and correcting changes in levels by negative feedback.  The nervous system and endocrine system work together to maintain homeostasis  The autonomic nervous system is responsible for many or our homeostatic mechanisms

An example of negative feedback!

 Steroids – ex) estrogen These hormones are synthesized from cholesterol so are classified as lipids.  H.1.3 (mode of action) As lipids, steroids can enter cells easily then bind to a receptor.  The hormone/receptor complex enters the nucleus and controls transcription of genes

 Example – insulin. These hormones are proteins.  H.1.3 (mode of action)They bind to receptor proteins on the outer surface of a cell membrane.  Once the hormone is joined to the receptor a secondary messenger molecure is triggered to act in the cytoplasm of the cell.

 Example – thyroxin. Thyroxin increases the metabolic rate

 The pituitary is often referred to as the master gland because its 2 lobes (anterior and posterior) produce many different hormones.  The pituitary is under the control of the hypothalamus

 Neurosecretory cells have their dendrites and cell bodies in the hypothalamus and their axons extend into the posterior pituitary.  The hypothalamus produces hormones then they move down the axons into the posterior pituitary where they are secreted  Oxytocin and ADH (antidiruetic hormone) are produced this way

 The hypothalamus produces secretions then absorbs them via capillary beds. These capillary beds join and form a portal vein which extends into the anterior pituitary.  Many of these hypothalamus secretions are releasing hormones such as gonadotrophin releasing hormone (GnRH). GnRH causes the anterior pituitary to secrete both follicle stimulating hormone (FSH) and luteinizing hormone (LH). The target of these hormones are the ovaries and testes…more later on this.

 The hypothalamus is sensitive to changes in concentration of blood plasma  Neurosecretory cells in the hypothalamus synthesize ADH and transport this along the axon of their nerves for storage in their synaptic knob endings in the posterior lobe of the hypothalamus.

 Cells in the hypothalamus which are sensitive to plasma concentrations stimulate the neurosecretory cells to transmit impulses to their storage regions in the posterior lobe of the hypothalamus  ADH is released and acts on the kidneys distal convoluted and collecting tubules by increasing water reabsorption.  If no ADH is secreted the collecting duct is impermeable to water and water is secreted in urine

 An area of the brain called the hypothalamus is responsible

 Sensors are in hypothalamus  Effectors are in the skin and muscle  Normal body temperature, 37.8 degrees C.

 Cooling mechanisms include increased sweat gland activity (evaporative cooling) and dilation of arterioles in skin (heat radiation)  Warming mechanisms include constriction of skin arterioles and stimulation of skeletal muscles to begin shivering (generates heat). Erector pili muscles cause hair to stand up, trapping heated air near skin (not effective in humans!)

 Blood arriving at the liver via the hepatic portal vein fluctuates a great deal in glucose concentration  Liver cells called hepatocytes help maintain blood sugar levels under the direction of two hormones secreted by the pancreas, glucagon and insulin.  These two hormones are antagonistic – have opposite effects of blood sugar levels.

 Low blood sugar is detected by the pancreas. This causes  (alpha) cells in pancreatic islets to secrete the hormone glucagon  Glucagon travels in blood to receptors in liver. It causes the liver to breakdown glycogen stored in hepatocytes and release glucose to the bloodstream.

 High blood glucose levels stimulate  (beta) cells in pancreas to release insulin.  Insulin travels to receptors in liver which causes hepatocytes to take in glucose and convert it to glycogen. The glycogen is stored in the cells in granules. This also happens in muscle cells.  Insulin also triggers body cells to open protein channels which allow the facilitated diffusion of glucose into the cells

a) Low glucose concentration is detected by the pancreas. b) Alpha cells in the pancreatic islets secret glucagon. c)Glucagon flows through the blood to receptors on liver cells. d)Liver responds by adding glucose to blood stream. h) High blood glucose levels stimulate the beta pancreatic cells a) Beta pancreatic cells secrete insulin. f)Insulin flows through the blood to the receptors on liver cells. g)Insulin stimulates the liver to remove blood glucose and store it as glycogen

 Type I – an autoimmune disease where the body’s immune system destroys the  cellss of the pancreas. Little insulin is produced. Less than 10% of diabetics are this type.  Type II – the body’s cells no longer respond to insulin (insulin resistance). This type of diabetes is associated with genetic history, obesity, lack of exercise and advanced age.