Functions of Neurons Resting & Action Potential Synapses

Resting Potential & Action Potential Neurons are responsible for receiving and transmitting signals to and from the brain Majority of nerve cells are not being used or fired until they receive a stimulus The process of having a neuron engage and fire is due to the RESTING POTENTIAL & ACTION POTENTIAL

Resting Potential When a neuron is inactive, it is considered POLARIZED ▫Cytoplasm of a neuron has a negative charge ▫Area outside a neuron has a positive charge ▫Membranes keep the opposite charges separate ▫The difference in charges is called the resting potential

Sodium-Potassium Pump Cell membranes are semi-permeable The sodium-potassium protein pump is responsible for the neuron electrical charge ▫Moves large numbers of ions outside the cell; creates a positive charge outside the neuron ▫Potassium ions are moved into neuron’s cytoplasm ▫There are more Na ions moved outside the cell than K ions being moved inside the cell, so the outside has a positive charge

Action Potential When a stimulus reaches a resting neuron, the signal is transmitted as an impulse called the ACTION POTENTIAL During A.P., ions will cross back and forth over the membrane due to gated ion channels opening and closing This causes a change in electrical current; Na ions rush into the cell, and the neuron becomes positively charged

Action Potential Once the overall charge on the inside of the neuron becomes high enough, the voltage will reach a threshold, and will trigger the action potential (the firing of the neuron) ▫Causes more gated ion channels to open and more Na ions to go through ▫Once charges have switched, neuron becomes depolarized

Depolarization Once cell is depolarized, and the peak voltage has been reached, the gated ion channels will close, and separate K ion channels will open in order to re-polarize cell This will continue until the cell becomes polarized again (negative on the inside)

Reflexes

Reflexes are automatic, unconscious to changes, either inside or outside the body. a.Maintain homeostasis (autonomic reflexes) – heart rate, breathing rate, bp, digestion. b.Carry out the automatic actions of swallowing, sneezing, coughing, vomiting. c.Maintain balance and posture; e.g., spinal reflexes control trunk and limb muscles. d.Brain reflexes involve reflex center in brainstem; e.g., reflexes for eye movement.

Reflex Arc Nerve impulses follow nerve pathways as they travel through the nervous system. The simplest of these pathways, which include only a few neurons, is called the reflex arc. Reflexes whose arc passes through the spinal cord are called spinal reflexes.

Parts of Reflex Arc 1.Receptor – detects stimulus. 1.Description: the receptor end of a particular dendrite or a specialized receptor cell in a sensory organ. 2.Function: sensitive to a specific type of internal or external change. 2.Sensory neuron – conveys the sensory info. to brain or spinal cord. 1. Description: Dendrite, cell body, and axon of a sensory neuron. 2.Function: transmit nerve impulses from the receptor into the brain or spinal cord.

Parts of Reflex Arc 3. Interneuron: relay neurons. Description: dendrite, cell body, and axon of a neuron within the brain or spinal cord. Function: processing center, conducts nerve impulses from the sensory neuron to a motor neuron. 4. Motor neuron: conduct motor output to the periphery. Description: Dendrite, cell body, and axon of a motor neuron. Function: transmits nerve impulse from the brain or spinal cord out to an effecter.

Parts of Reflex Arc Effector: ▫ Description: a muscle or gland. ▫Function: Response to stimulation by the motor neuron and produces the reflex or behavioral action.

Synapses

Meeting point between 2 neurons Neurons do not touch; called a gap junction 1000 trillion synapses are found in the body

Function of a synapse During action potential, the electrical message is sent down the axon to the terminal, and the synapse delivers the message to the dendrite of another neuron

Electrical & Chemical Synapses Nerve cells have 2 main ways to communicate messages: ▫Electrical (sent quickly to most cells) ▫Chemical (selected nerves) ▫Electrical is the quickest because it never has to be converted when going between neurons ▫The A.P. in one neuron will create A.P. in others through the synapse

Chemical Synapses Much more abundant Slower More Selective Uses neurotransmitters (chemical substance) instead of electricity

Sending Messages Pre-Synaptic Neuron (nerve cell sending msg) ▫Transmits msg through pre-synaptic terminal (axon terminal) ▫Axon terminal contains synaptic vesicle  Loaded with thousands of molecules of neurotransmitters Post-Synaptic Neuron (nerve cell receiving msg) ▫Accepts neurotransmitters in the receptor region (on dendrites) ▫They communicate without touching

Synaptic Cleft Area between 2 neurons When A.P. is converted to chemical signal, Ca+ ions move synaptic vesicles to membrane which will release neurotransmitters Synaptic vesicles fuse with cell membrane and neurotransmitters diffuse across synaptic gap and bind to receptor sites Neurotransmitters determine whether the post neuron gets excited or inhibited

Excitatory & Inhibitory Neurotransmitters Excitatory: ▫Depolarize the post synaptic neuron ▫Cause action potential Inhibitory: ▫Hyperpolarize the post synaptic neuron (negative) ▫Message does not get passed ▫Neurotransmitters only bind to postsynaptic neurons for a few milliseconds and then release and go back to pre-synaptic neuron (re-uptake)