NERVES, HORMONES AND HOMEOSTASIS

Topic 6.5

Assessment Statements

6.5.1 State that the nervous system consists of the central nervous system (CNS) and peripheral nerves, and is composed of cells called neurons that can carry rapid electrical impulses.

6.5.2 Draw and label a diagram of the structure of a motor neuron. 6.5.3 State that nerve impulses are conducted from receptors to the CNS by sensory

neurons, within the CNS by relay neurons, and from the CNS to effectors by motor neurons.

6.5.4 Define resting potential and action potential (depolarization and repolarization). 6.5.5 Explain how a nerve impulse passes along a non-myelinated neuron. 6.5.6 Explain the principles of synaptic transmission. 6.5.7 State that the endocrine system consists of glands that release hormones that

are transported in the blood. 6.5.8 State that homeostasis involves maintaining the internal environment between

limits, including blood pH, carbon dioxide concentration, blood glucose concentration, body temperature and water balance.

6.5.9 Explain that homeostasis involves monitoring levels of variables and correcting changes in levels by negative feedback mechanisms.

6.5.10 Explain the control of body temperature, including the transfer of heat in blood, and the roles of the hypothalamus, sweat glands, skin arterioles and shivering.

6.5.11 Explain the control of blood glucose concentration, including the roles of glucagon, insulin and α and β cells in the pancreatic islets.

6.5.12 Distinguish between type I and type II diabetes.

Organization of the human nervous system

Central nervous system (CNS) consists of the brain and spinal cord

CNS receives sensory information from various receptors and then interpret and process that sensory information

Motor response – response initiated by the CNS

Neurons – cells which carry electrical impulses from one point in the body to another Sensory neurons bring info. into the CNS Motor neurons carry response info. to muscles

Peripheral nervous system (PNS) is made up of the sensory and motor neurons

When many individual neurons are grouped together into a single structure, the structure is called a nerve

Peripheral nerves: Spinal – 31 pairs emerge directly from the

spinal cord; mixed sensory and motor nerves Cranial – 12 pairs emerge from an area of the

brain called the brainstem

Motor neuron

nucleus

dendrites

Cell body

axonAction potential

Myelin sheaths made of Schwann cells

Nodes of Ranvier

Synaptic terminals of axon

Nerve impulses are conducted…

From receptors to the CNS by sensory neurons

Within the CNS by relay neurons From the CNS to effectors by motor

neurons

Example

You touch the arm of the person sitting next to you; Touch was accidental and you immediately remove your hand

Your touch began caused a pressure (sensory) receptor to begin an action potential

One of the spinal nerves carried the signal to spinal cord

Spinal cord made use of relay neurons to route the potential in the CNS to the appropriate area for interpretation

Your brain used relay neurons to pass action potential to spinal cord, then spinal nerves, then by way of motor neurons to cause your hand to move

When the action potential reaches the muscle (effector), the motor neuron sends a chemical signal to the muscle which results in a contraction

What is a nerve impulse?

Series of action potentials carried by axons

Axons are surrounded by a membranous structure called the myelin sheath which increase rate at which an action potential passes

Resting potential

The state of being where an area of neuron is ready to send an action potential; area is said to be polarized

Polarization is characterized by the active transport of sodium ions (Na+) and potassium ions (K+)

Vast majority of Na+ are transported out of the axon; majority of K+ are transported into the axon; additionally there are negatively charged ions permanently located in the cytoplasm of the axon

Net result is + outside the axon and - inside

Action potential of non-myelinated sheaths Self-propagating wave of ion movements in and out of the

neuron membrane Movement is not along length of axon, but consists ions

diffusing outside and inside of the axon Requires active transport (protein channels and ATP) to set up

a concentration gradient of both K+ and Na+ Na+ actively pumped out; diffuse in when a channel opens Channel opens for K+ to diffuse out This diffusion is the “impulse” or action potential Nearly instantaneous event which occurs in one area of an

axon and is called depolarization This area then initiates the next area of the axon to open up

the channels for sodium, then potassium and thus the action potential continues down the axon

http://www.blackwellpublishing.com/matthews/channel.html http://bcs.whfreeman.com/thelifewire/content/chp44/4402002

.html

Myelinated fibers

Myelin insulates fiber from the extracellular fluid

Myelin sheath interrupted by nodes of Ranvier

Na+ that enters at the previous node diffused down the fiber under the axolemma

Resistance occurs and the signal becomes weaker the further it goes

Nodes are close together and the signal is just strong enough to open gates and create a new action potential

Return to the resting potential Neurons may send dozens of action

potentials in a short period of time Must wait until the sodium and

potassium ions have been restored to original resting potential

Active transport causes this repolarization

The time that it takes for any one neuron to send an action potential and then repolarize so it can send another is called the refractory period of that neuron

Synaptic transmission

A sensory pathway is unidirectional b/c the sensory neurons of the pathway are lined up so that the terminal end of the axon of the first neuron adjoins the dendrites of the next neuron

1st neuron – presynaptic neuron 2nd neuron – postsynaptic neuron Synapse occurs between neurons

Patterns of synaptic transmission Presynaptic neuron → postsynaptic

neuron Several presynaptic neurons →

postsynaptic neuron Presynaptic neuron → several

postsynaptic neurons

Mechanism of synaptic transmission Far end of axons are swollen

membranous areas called terminal buttons

Within these terminal buttons are many small vesicles filled with a chemical called a neurotransmitter (ex: acetylcholine)

When an action potential reaches the area of the terminal buttons, it initiates the following sequence of events

1. Calcium ions diffuse into the terminal buttons

2. Vesicles containing neurotransmitter fuse with the plasma membrane and release neurotransmitter

3. Neurotransmitter diffuses across the synaptic gap from the presynaptic neuron to the postsynaptic neuron

4. Neurotransmitter binds with a receptor protein on the postsynaptic neuron membrane

5. This binding results in an ion channel opening and sodium ions diffusion in through this channel

6. This initiates the action potential to begin moving down the postsynaptic neuron because it is depolarized

7. Neurotransmitter is degraded by specific enzymes and is released from the receptor protein

8. The ion channel closes to sodium ions

9. Neurotransmitter fragments diffuse back across the synaptic gap to be reassembled in the terminal buttons of the presynaptic neuron

Homeostasis

Maintaining normal limits for physiological variables

Variables include: Blood pH Carbon dioxide concentration Blood glucose concentration Body temperature Water balance within tissues

The physiological changes that bring a value back closer to a set point are called negative feedback mechanisms

Endocrine system

Works cooperatively with the nervous system in order to ensure homeostasis

Consists of numerous glands which produce a wide variety of hormones

Each hormone is transported by the bloodstream from the gland where it is produced to the specific cell types in the body that are influenced by that particular hormone

Control of body temperature Biological thermostat for temperature

control is in the hypothalamus1. You exercise, body temp. rises2. Hypothalamus receives info. from

thermoreceptors in your skin and begins cooling mechanisms

3. Increased activity of sweat glands, evaporative cooling

4. Arterioles in skin dilate filling skin capillaries with blood; heat leaves the skin capillaries by radiation

1. You are in cold air environment2. Hypothalamus receives info. from

thermoreceptors in skin and begins warming mechanisms

3. Constriction of skin arterioles, blood is diverted to deeper organs; less heat is lost as radiation

4. Skeletal muscle stimulated to shiver; results in production of heat

Blood glucose level

Concentration of glucose dissolved in blood plasma

Cells rely on glucose for the process of cell respiration which they are constantly carrying out

Glucose is absorbed into the bloodstream in the capillary beds of the villi of the small intestine and thus increases blood glucose level

Level “see-saws” 24 hrs a day Maintained by negative feedback

mechanism

Route of glucose

1. Intestinal villi2. Capillaries, small venules, veins,

hepatic portal vein3. Liver4. Hepatocytes5. All other blood vessels Hepatocytes are directed to action by

two hormones, insulin and glucagon, which are produced in the pancreas

When blood glucose level goes above the set point… First effect:

Within the pancreas β cells produce the hormone insulin and secretes the insulin which is later absorbed by the bloodstream

Insulin opens protein channels in cells’ plasma membranes allowing glucose to diffuse into the cell by facilitated diffusion

Second effect: When blood relatively high in glucose enters the liver by

the hepatic portal vein, insulin stimulates the hepatocytes to take in the glucose and covert it to glycogen

The glycogen is then stored as granules in the cytoplasm of the hepatocytes. The same effect occurs in muscles.

When blood glucose level goes below the set point… Begins when someone has not eaten for

many hours or exercises vigorously for a long time

α cells of the pancreas produce and secrete the hormone glucagon

Glucagon circulates in the bloodstream and stimulates hydrolysis of the granules of glycogen stored in hepatocytes and muscle cells producing glucose

Glucose enters the bloodstream, increasing blood glucose

Diabetes

Disease characterized by hyperglycemia (high blood sugar)

People who have untreated diabetes have plenty of glucose in their blood, but not in their body cells where it is needed

Uncontrolled diabetes can lead to many serious effects including: Damage to the retina leading to blindness Kidney failure Nerve damage Increased risk of cardiovascular disease Poor wound healing (and possibly gangrene)

Type I diabetes

Caused when the β cells of the pancreas do not produce enough insulin

Can be controlled by the injection of insulin at appropriate times

An autoimmune disease where the body’s own immune system attacks and destroys the β cells of the pancreas so little to no insulin is produced

< 10% of diabetics are this type Most often develops in children or young

adults, but can develop in people of any age

Type II diabetes

Caused by body cell receptors do not respond properly to insulin

Controlled by diet Known as insulin resistance Initially, the pancreas continues to

produce a normal amount of insulin, but this level may decrease after a period of time.

Most common form (90% of diabetics) Associated with genetic history, obesity,

lack of exercise, advanced age, and certain ethnic groups

Diary of a diabetic