Human Physiology Nerves, Homeostasis and Hormones

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Human PhysiologyNerves, Homeostasis and Hormones
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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.

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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)

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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

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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

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Nervous System and Impulse

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

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Nervous System and Impulse

• Motor Neuron
• Consists of
• axon
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• 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

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Motor Neuron
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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

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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 mV
• 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

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Nerve Impulse
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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

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• 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 mV 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

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• 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.)

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Nerve Impulse Action Potential
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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

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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

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Synaptic Transmission

• Conduction across the synapse is achieved by a
  neurotransmitter
• Depolarization in the pre-synaptic bulb releases
  Ca2, which stimulates the release of
  neurotransmitter
• When neurotransmitter flows across the synaptic
  cleft, caused depolariztion of the post-synaptic
  bulb, to continue impulse

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Synaptic Transmission

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

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Synaptic Transmission
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Everyday Applications

• Local Anesthetics
• Poisons

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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.

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• 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

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Thermoregulation

• Normal Body Temperature 36-37oC
• 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

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• If you are too hot
• vasodilation
• sweating
• decreased metabolism (endocrine)
• behaviour adaptations (last case)
• If you are too cold
• 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

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Blood Glucose Regulation

• Done by Endocrine System
• Normal levels 5 mmol / L (dm3)
• 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

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• 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

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• 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
  glucagons 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

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Application

• Diabetes
• Type I
• Type II