nerve impulse Flashcards
(8 cards)
resting membrane
The plasma membrane of a resting, or inactive, neuron is polarized, which means that there are fewer positive ions sitting on the inner face of the neuron’s plasma membrane than there are on its outer face (Figure 7.9 ). The major positive ions inside the cell are potassium , whereas the major positive ions outside the cell are sodium . The polarized membrane is more permeable to than to at rest, maintaining a more negative inside (fewer positive ions) compared to outside, as ions exit the cell. This maintains the inactive, resting state of the neuron.
two major functions of neurons
Neurons have two major functional properties: irritability, the ability to respond to a stimulus by producing a nerve impulse, and conductivity, the ability to transmit the impulse to other neurons, muscles, or glands.
stimuli of neurons
Many different types of stimuli excite neurons to become active and generate an impulse. For example, light excites the eye receptors, sound excites some of the ear receptors, and pressure excites some cutaneous receptors of the skin. However, most neurons in the body are excited by neurotransmitter chemicals released by other neurons, as we will describe shortly.
depolarization
Regardless of the stimulus, the result is always the same—the permeability properties of the cell’s plasma membrane change for a very brief period. Normally, sodium ions cannot diffuse through the plasma membrane to any great extent, but when the neuron is adequately stimulated, the “gates” of sodium channels in the membrane open. Because sodium is in much higher concentration outside the cell, it then “floods” (by diffusion) into the neuron. This inward rush of sodium ions changes the polarity of the neuron’s membrane at that site, an event called depolarization (Figure 7.9 ). Locally, the inside is now more positive, and the outside is less positive, a local electrical situation called a graded potential. However, if the stimulus is strong enough and the sodium influx is great enough, the local depolarization (graded potential) activates the neuron to initiate and transmit a long-distance signal called an action potential, or a nerve impulse. The nerve impulse is an all-or-nothing response, like starting a car. It is either propagated (conducted, or sent) over the entire axon , or it doesn’t happen at all. The nerve impulse never goes partway along an axon’s length, nor does it die out with distance, as do graded potentials.
repolarization
Almost immediately after sodium ions rush into the neuron during depolarization, the membrane’s permeability changes again: it becomes impermeable to sodium ions but permeable to potassium ions. This allows potassium ions (K⁺) to rapidly diffuse out of the neuron into the surrounding interstitial fluid.
This outflow of positive K⁺ ions helps to restore the electrical conditions of the membrane back to its polarized (resting) state — a process called repolarization.
After repolarization, the sodium-potassium pump (Na⁺/K⁺-ATPase) uses ATP (cellular energy) to restore the original ionic concentrations by actively pumping excess sodium ions out of the cell and bringing potassium ions back in.
Until repolarization and ionic balance are restored, the neuron is in a refractory period and cannot conduct another impulse.
Once this sequence begins, it propagates along the entire length of the neuronal membrane, allowing the nerve impulse to travel.
mylanaited fibers
In unmyelinated fibers, the nerve impulse moves as a continuous wave along the entire length of the axon membrane. The depolarization spreads step-by-step along every bit of the membrane, which takes more time.
In myelinated fibers, the axon is wrapped in a fatty myelin sheath that insulates much of the membrane, preventing electrical current from leaking out.
Because of this insulation, the nerve impulse cannot flow across the membrane where the myelin is present. Instead, the impulse “jumps” from one unmyelinated gap (called a node of Ranvier) to the next.
This jumping dramatically increases the speed of impulse conduction and is called saltatory conduction (from the Latin saltare, meaning “to leap or dance”).
what stops a nerve impulse
Factors That Impair Nerve Impulse Conduction:
Chemical blockers (e.g., sedatives and anesthetics):
These substances alter the membrane’s permeability to ions, especially sodium ions (Na⁺).
Since sodium entry is crucial for generating an action potential, blocking sodium channels prevents nerve impulses from occurring.
This is how anesthetics numb sensation during medical procedures.
Cold and continuous pressure:
These can reduce blood flow to neurons by interrupting circulation.
Without adequate blood supply, neurons don’t get enough oxygen and nutrients, which impairs their function.
That’s why your fingers go numb when holding ice or your foot “falls asleep” when you sit on it.
When the cold or pressure is removed, blood flow returns, nerve impulses start again, and you feel that tingly, prickly sensation as the nerves “wake up.”
synapse transmitter
Neuronal Conductivity: How One Neuron Communicates with the Next
So far, we’ve talked about irritability—how a neuron responds to a stimulus by generating an electrical impulse (action potential). But how does that impulse actually pass to the next neuron or effector cell (like a muscle or gland)?
The key is: the electrical impulse doesn’t jump across the synapse.
Instead, the signal is passed chemically, using neurotransmitters.
Step-by-Step: What Happens at the Synapse
The action potential travels down the axon and reaches the axon terminal.
This change in voltage opens calcium (Ca²⁺) channels in the terminal membrane.
Calcium ions enter the terminal, triggering synaptic vesicles to fuse with the membrane.
These vesicles release neurotransmitter molecules into the synaptic cleft by exocytosis.
The neurotransmitter diffuses across the cleft and binds to receptors on the membrane of the next neuron (or effector cell).
If enough neurotransmitter binds, it triggers:
Sodium channels to open
Sodium influx
Local depolarization (graded potential)
If strong enough, this can trigger a new action potential in the receiving neuron.
Signal Ends Quickly
To keep communication fast and precise, the neurotransmitter’s effect is brief. It is quickly removed by one of these methods:
Diffusion away from the cleft
Reuptake into the axon terminal
Breakdown by enzymes (e.g., acetylcholinesterase for acetylcholine)
This ensures that each nerve impulse only affects the next cell for a fraction of a second—shorter than the blink of an eye.
