Nerve Impulse, Neurotransmitters and the Endocrine System

• These study notes are about the nerve impulse, neurotransmitters, and the endocrine system

Neuron Function

• Neurons fire an action potential or nerve impulse
• This electrical event leads to a chemical event
• The release of neurotransmitters is a chemical event that relays messages to other neurons, muscles, or tissue
• The next neuron is either excited or inhibited depending on the message sent by the neurotransmitter

Nerve Impulse Conduction

• The nerve impulse conduction, also know as how a nerve cell "fires", is dependent on, the cell membrane of a neuron having special characteristics
• The primary special characteristic is that it has a unique pattern of selective permeability
• The pattern of permeability changes depending on whether the cell is being stimulated

Resting Potential

• When a nerve cell is not being stimulated, an electrical potential, in voltage, is maintained across the cell membrane
• Inside of the cell is negatively charged relative to the outside
• To maintain this potential, positively charged particles, such as sodium ions (Na+), are pumped out of the cell

Action Potential

• The action potential starts when a section of the cell membrane at the top of the axon allows positive ions to flow rapidly into the inside of the axon
• This causes depolarization
• The inside of the axon then becomes positive relative to the outside for a brief moment
• Once a section of the membrane has depolarized, it begins pumping ++ ions out again, which reestablishes the resting potential
• The action potential is self-propagating down the membrane

Action Potential Properties

• A strong stimulus can trigger more neurons to fire and to fire more often
• Action potential strength or speed is not affected
• Intensity of an action potential remains the same throughout the length of the axon

Threshold

• Threshold is how a neuron "decides" to fire or not
• Each neuron receives excitatory and inhibitory signals from other neurons
• When the excitatory signals minus the inhibitory signals exceed a minimum intensity, or threshold, the neuron fires an action potential

Synapse

• Once in the synaptic gap, neurotransmitters come into contact with postsynaptic dendrites
• Receptor sites on dendrites each have their own shape
• They are activated by specific types of neurotransmitters with a similar shape
• This is called the "lock and key model"

Stopping the Action

• Action must stop once a neurotransmitter has stimulated the receiving neuron
• Enzymes break it down in the synapse, and its component parts are reabsorbed by the sending neuron
• The whole neurotransmitter molecule is reabsorbed by the sending neuron in a process known as "reuptake"

Neurotransmitters

• There are more than 100 known chemicals
• Neurotransmitters can serve three functions:
  • Transfer information from one neuron to another
  • Can be secreted with other neurotransmitters to prolong their action or modify effects in some way
  • Can be released on muscles and glands to affect them
• The same neurotransmitter can have different actions in different parts of the nervous system

Ligands

• Ligands are any neurotransmitter or drug that can bind to a particular receptor
• Agonists mimic neurotransmitters
  • This agonist molecule excites
  • It is similar enough in structure to the neurotransmitter molecule that it mimics its effects on the receiving neuron
  • Morphine mimics the action of endorphins by stimulating receptors in brain areas involved in mood and pain sensations
• Antagonists block neurotransmitters
  • This antagonist molecule inhibits
  • It has a structure similar enough to occupy the receptor site and block its action
  • It is not similar enough stimulate the receptor
  • Curare poisoning paralyzes its victims by blocking ACh receptors involved in muscle movement

Endocrine system

• The endocrine system is a system of glands and ducts located in the brain and throughout the body that secrete chemicals called hormones into the blood
• Communication is slower and less specific than nervous system communication

Important Endocrine Glands

• Hypothalamus sends messages from the brain to the pituitary gland
• Pituitary gland triggers hormone secretion in other glands, secreting growth hormone, gonadotropin releasing hormone, and follicle stimulating hormone
• Thyroid affects metabolic rate
• Pancreas affects the use of glucose
• Ovaries and testes
• Adrenal glands secrete stress hormones that prepare people for emergency action

Study notes on the nerve impulse, neurotransmitters, and the endocrine system. Neurons fire an action potential or nerve impulse, which leads to the release of neurotransmitters. Neurotransmitters relay messages to other neurons, muscles, or tissues, either exciting or inhibiting the next neuron.