Human Physiology Nerves Homeostasis and Hormones Maintaining Homeostasis

Human Physiology Nerves, Homeostasis and Hormones

Maintaining Homeostasis �The nervous system maintains homeostasis by: • Receptor or sensor monitors the level of a variable • Coordinating centre (CNS) regulates level of the variable • Effector structures bring about the changes directed by the coordinating center, to maintain the level of the variable.

Maintaining Homeostasis �The response can be carried out by a nervous response, which is a nerve impulse (Nervous System) �The response can be carried out by the release of a hormone, acting on organs in the body (Endocrine System)

Maintaining Homeostasis �Nervous System consists of: • Central Nervous System (CNS) consisting of the brain and spinal cord • Peripheral nerves, called neurons. �Their function is to transport messages in the form of electrical impulses to specific sites. • Breathing Rate is controlled by the Nervous System • Thermoregulation is controlled by the Nervous System and Endocrine System

Maintaining Homeostasis �Endocrine System consists of: • endocrine glands �produce hormones to the blood (ex. Adrenal glands on the top of the kidneys. ) �do not release their product into a duct, like exocrine glands (in the digestive system). �considered ductless glands. They secrete their hormones into the blood, which transports it around the body. �Hormones act on organs, when they come in contact with target cells

Nervous System and Impulse �Nervous System • Central Nervous System �Brain and Spinal Cord �Peripheral Nervous System • Voluntary Nerves (somatic) • Autonomic Nerves (visceral)

Nervous System and Impulse �Motor Neuron • Consists of: �axon �Schwann cells, which provide a multi-layered lipid and protein coating called a myelin sheath �nodes of Ranvier. �The axon terminates at a motor end plate, or axon terminal

Motor Neuron

Nervous System and Impulse � Typical Nervous System Pathway • Starts off with a stimulus • Creates an action potential that flows from the sensory neurons to relay neurons to the brain, which interprets the stimulus • The brain sends a response through relay neurons to a motor neuron, which ends in an effector organ, like a muscle, or endocrine gland

Nerve Impulse Resting Potential (RMP) �Nerve is at rest �Maintains a more positive charge on the outside and a more negative on the inside �Nerve is said to be polarized �Charge of -70 m. V �Maintained by greater concentration of Na+ outside the cell compared to K+ and Cl- on the inside and the fact the membrane is more permeable to K+, causing it to leak out, maintaining a negative charge on the inside and positive charge on the outside

Nerve Impulse

Nerve Impulse �Action Potential Caused by a stimulus Stimulus causes depolarization to occur Neuron repolarizes All or None Principle is followed and every action potential is the same size, following the same pattern • Size of stimulus determines how many neurons are stimulated, to carry the message • •

�Progression of Action Potential • Stimulus causes the membrane sodium pores to • • • open Sodium pores allow sodium ions to flow in, reducing the charge on the inside This is called depolarization Na+ ions continue to move in by diffusion Once 40 m. V is reached, sodium pores close Neuron cannot conduct another impulse until it resets (repolarizes) Potassium channels open and K+ ions flow out, reducing positive charge on the inside of the axon

• • This is called repolarization Once axon is polarized, the potassium pores close Ion are in wrong place, so the have to be reset Done by Na+/K+ Pump (Active Transport) � Summary of Action Potential (Impulse) • The action potential is the time of depolarization (1 msec). • The refractory period is the time taken for repolarization. �Refactory period is divided into the absolute refractory state (1 msec), followed by the relative refractory state (up to 10 msec. )

Nerve Impulse – Action Potential

Myelinated vs. Non-Myelinated Axon �Myelinated nerves conduct faster than non -myelinated nerves �At nodes of Ranvier, the sodium channels are present �When impulse travels, it travels from node to node, jumping, conducting faster �Called saltatory conduction

Synaptic Transmission �Like wires, there are points that join neuron to neuron, neuron to cell body, neuron to effector organ �Connection points are called synapses �Two types of synapses: • Electrical • Chemical

Synaptic Transmission �Conduction across the synapse is achieved by a neurotransmitter �Depolarization in the pre-synaptic bulb releases Ca+2, which stimulates the release of neurotransmitter �When neurotransmitter flows across the synaptic cleft, caused depolariztion of the post-synaptic bulb, to continue impulse

Synaptic Transmission �Types of Synapses • Excitatory • Inhibitory �Some • • neurotransmitters Acetylcholine is a common neurotransmitter Noradrenaline Dopamine Serotonin

Synaptic Transmission

Everyday Applications �Local Anesthetics �Poisons

Examples of Homeostasis using the CNS and Endocrine System �Thermoregulation – CNS and Endocrine �Blood Glucose Regulation – Endocrine �All work using a principle of Negative Feedback • control of a process by which, an increase or decrease away from the standard or “normal” condition results in a reversal back to the standard condition.

�Process of Negative Feedback • Sensors are required to measure the current conditions. • The sensors need to pass on the information to a centre, which knows the desired value (the norm) and compares the current situation to the norm. • If the two are not the same, the centre activates a mechanism to bring the current value closer to the norm. �Condition is always reversed �Example – Thermostat in your house

Thermoregulation �Normal Body Temperature – 36 -37 o. C �Controlled by the hypothalamus in the brain �Sensed by the surface skin receptors (Shell Temperature) �Senses the temperature of the blood as it flows from skin surface to core/brain

� If • • you are too hot: � If • • • you are too cold: � vasodilation sweating decreased metabolism (endocrine) behaviour adaptations (last case) vasoconstriction **shivering increased metabolism (endocrine) fluffing of hair or feathers thickening of brown fat or blubber Some organisms have special hair structure – polar bear hair absorbs UV light

Blood Glucose Regulation �Done by Endocrine System �Normal levels 5 mmol / L (dm 3) �Controlled by insulin and glucagon, hormones secreted by the pancreas, from an area called the Islets of Langerhans �Pancreas has chemoreceptors that sense the osmotic pressure of the blood, looking at blood glucose concentration

�If your blood glucose levels are too high: • the cells in the islets will secrete insulin. • Insulin is a protein hormone that acts on the muscle cells and liver • Muscle cells absorb glucose, and the muscle cells and hepatocytes (liver cells) convert glucose into glycogen. • Excess sugar goes to adipose tissue (fat tissue), and glucose is converted to fat in the presence of insulin. • Blood sugar levels drop

�If your blood glucose levels are too low: • cells release glucagon • Glucagon is a protein hormone and is secreted into the blood. • Target cells are in the liver. • Hepatocytes (cells in the liver) will respond to glucagon’s presence by converting glycogen to glucose and releasing it into the blood. The can also convert amino acids into glucose (indirectly). • Blood sugar levels rise

Application �Diabetes • Type II