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Chemical synapses are slower than electrical ones but are also far more flexible. This valuable flexibility is the foundation of all learning. On either side of the synapse, the axon and dendrite have evolved specialized molecules to perform their respective tasks. Related questions Is aggression learned or innate? What is self-efficacy? What is the rationale for using adoption studies and twin studies in learning about genetic Why are twin studies used to understand genetic contributions to human behavior?

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How do twin and adoption studies help us differentiate between the influences of nature and nurture? You have learned about this type of signaling before, with respect to the interaction of nerves and muscles at the neuromuscular junction. The voltage at which such a signal is generated is called the threshold , and the resulting electrical signal is called an action potential.

In this example, the action potential travels—a process known as propagation —along the axon from the axon hillock to the axon terminals and into the synaptic end bulbs. When this signal reaches the end bulbs, it causes the release of a signaling molecule called a neurotransmitter. The neurotransmitter diffuses across the short distance of the synapse and binds to a receptor protein of the target neuron. When the molecular signal binds to the receptor, the cell membrane of the target neuron changes its electrical state and a new graded potential begins.

If that graded potential is strong enough to reach threshold, the second neuron generates an action potential. The target of this neuron is another neuron in the thalamus of the brain, the part of the CNS that acts as a relay for sensory information.

At another synapse, neurotransmitter is released and binds to its receptor. The thalamus then sends the sensory information to the cerebral cortex , the outermost layer of gray matter in the brain, where conscious perception of that water temperature begins. Finally, a plan is developed about what to do, whether that is to turn the temperature up, turn the whole shower off and go back to bed, or step into the shower. To do any of these things, the cerebral cortex has to send a command out to your body to move muscles Figure 8.

All of this occurs in a fraction of a second, but this story is the basis of how the nervous system functions. The functions of the nervous system—sensation, integration, and response—depend on the functions of the neurons underlying these pathways. To understand how neurons are able to communicate, it is necessary to describe the role of an excitable membrane in generating these signals. The basis of this communication is the action potential, which demonstrates how changes in the membrane can constitute a signal.

Looking at the way these signals work in more variable circumstances involves a look at graded potentials, which will be covered in the next section. Most cells in the body make use of charged particles, ions, to build up a charge across the cell membrane. Previously, this was shown to be a part of how muscle cells work. For skeletal muscles to contract there must be input from a neuron.

Both of the cells make use of the cell membrane to regulate ion movement between the extracellular fluid and cytosol. As you learned in the chapter on cells, the cell membrane is primarily responsible for regulating what can cross the membrane and what stays on only one side. The cell membrane is a phospholipid bilayer, so only substances that can pass directly through the hydrophobic core can diffuse through unaided.

Charged particles, which are hydrophilic by definition, cannot pass through the cell membrane without assistance Figure 8.

Transmembrane proteins, specifically channel proteins, make this possible. That means that this pump is moving the ions against the concentration gradients for sodium and potassium, which is why it requires energy. In fact, the pump basically maintains those concentration gradients. Ion channels do not always freely allow ions to diffuse across the membrane. They are opened by certain events, meaning the channels are gated. Channels can be categorized on the basis of how they are gated.

Although these classes of ion channels are found primarily in cells of nervous or muscular tissue, they also can be found in cells of epithelial and connective tissues. A ligand-gated channel opens because a signaling molecule, a ligand, binds to the extracellular region of the channel and opens the gated channel Figure 8.

A voltage-gated channel is a channel that responds to changes in the electrical properties of the membrane in which it is embedded. Normally, the inner portion of the membrane is at a negative voltage. When that voltage becomes less negative, the channel begins to allow ions to cross the membrane Figure 8. A leakage channel is randomly gated, meaning that it opens and closes at random, hence the reference to leaking. There is no actual event that opens the channel; instead, it has an intrinsic rate of switching between the open and closed states.

Leakage channels contribute to the resting transmembrane voltage of the excitable membrane Figure 8. Neurons do the same thing, but they pump out positively charged sodium ions. In addition, they pump in positively charged potassium ions potash to the gardeners out there!! Thus there is a high concentration of sodium ions present outside the neuron, and a high concentration of potassium ions inside. The neuronal membrane also contains specialised proteins called channels , which form pores in the membrane that are selectively permeable to particular ions.

Thus sodium channels allow sodium ions through the membrane while potassium channels allow potassium ions through. OK, so far so good. Now, under resting conditions, the potassium channel is more permeable to potassium ions than the sodium channel is to sodium ions. So there is a slow outward leak of potassium ions that is larger than the inward leak of sodium ions. This means that the membrane has a charge on the inside face that is negative relative to the outside, as more positively charged ions flow out of the neuron than flow in.



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