The Nerve Impulse

The Nerve Impulse The plasma membrane contains a variety of ion channels. Some are open – passive or leakage channels Some are active or gated allowing certain molecules to change the shape and pass through. (open when neurotransmitter binds)

An electrochemical gradient creates an electrical current when ion channels open and ions diffuse across the plasma membrane.

chemical gradients – passive, ions move from areas of high concentration to areas of lower concentration

2. electrical gradients – ions move towards areas of opposite charge

Polarized Membrane + + + + + + + + + + - - - - - - - - - - - - - A neuron is at its resting potential when it is inactive. The plasma membrane is positive on the outside compared to the inside. The membrane is said to be polarized. Positive outside + + + + + + + + + + - - - - - - - - - - - - - axon Negative inside Positive outside Polarized Membrane

This resting potential is due to: active transport of Na+ ions to the exterior of the cell - sodium pump

2. reciprocal transfer of K+ ions to the interior - potassium pump

“leaking” of K+ ions to the exterior (membrane more permeable to K+) this creates a deficit of positive charge in interior

The electrochemical change that occurs across the membrane surface of a neuron is dependent upon an unequal distribution of ions between the outside and the inside of the plasma membrane.

When a stimulus occurs, a rapid inflow of Na+ changes the interior locally - negative to positive. The membrane becomes depolarized.

A halt of the inflow of Na+ and a rapid outflow of K+ return the interior negative - repolarization. This “action potential” or nerve impulse sweeps along a nerve fiber.

Absolute Refractory Period during the time of one impulse traveling down the neuron (depolarization), a second impulse cannot be sent

All or Nothing Phenomenon Stimulus is strong enough to produce an action potential - threshold stimulus produces an “all the way” action potential No such thing as a little stimulus producing a weak response.

When the nervous system perceives a strong stimulus, the nerve impulses are generated often. Two impulses cannot travel down a neuron. The absolute refractory period is the time between the completion of one sweep and the beginning of another.

What Factors Can Affect the Nerve Impulse?

Axons of large diameter carry impulses faster. Myelinated axons carry impulses faster.

Impulse is able to jump from node to node = faster impulse

Alcohol, sedatives, and anesthetics block nerve impulses by reducing the membrane’s permeability to Na+.

Cold or pressure that cuts off circulation will reduce nutrients and oxygen and impair impulses. (cold causes vessels to narrow)

The Synapse

The Synapse Synapses are junctions between neurons or nervous and muscle cells (or other effectors). * Most occur between an axon and a dendrite.

presynaptic neurons are “senders” postsynaptic neurons are “receivers” postsynaptic cell either a neuron or an effector cell (muscle or gland cell) Postsynaptic Neuron Presynaptic Neuron

Two Kinds of Synapses 1. electrical synapses - allow flow of ions between neurons found in: fetal tissue cardiac muscle smooth muscle allows sequential and rhythmic excitations

2. chemical synapses - release and receive chemical neurotransmitters which open and close ion channels

Neurotransmitters regulate many body activities ex: sleep, hunger, memory, anger, joy over 100 have been identified

Neuron Circuits

Circuits Different patterns of synaptic connections are called circuits. 1. divergent – amplify impulse

2. convergent – concentrates impulse

Neural Processing The Reflex Arc

Neural Processing - The Reflex Arc Reflexes are rapid, automatic responses to stimuli. ex: pull away from pain gag blink cough sneeze

The reflex arc is a serial processing pattern consisting of:

1. receptor - site of stimulus action Stimulus at distal end of neuron Skin Receptor (a)

2. sensory neuron - transmits to CNS Stimulus at distal end of neuron Skin Receptor Sensory neuron (a)

3. integration center - within CNS connects sensory neuron to motor neuron Stimulus at distal end of neuron Skin Spinal cord (in cross section) Interneuron Receptor Sensory neuron Integration center (a)

4. motor neuron - transmits to effector organ Stimulus at distal end of neuron Skin Spinal cord (in cross section) Interneuron Receptor Sensory neuron Motor neuron Integration center (a)

5. effector - muscle fiber or gland cell that responds to impulse Stimulus at distal end of neuron Skin Spinal cord (in cross section) Interneuron Receptor Effector Sensory neuron Motor neuron Integration center (a)

The reflex arc is a serial processing pattern consisting of: receptor - site of stimulus action sensory neuron - transmits to CNS integration center - within CNS; connects sensory neuron to motor neuron motor neuron - transmits to effector organ effector - muscle fiber or gland cell that responds to impulse