13 Functions and channels of the nervous system

Sensation. Sensation refers to receiving information about the environment, either what is happening outside (ie: heat from the sun) or inside the body (ie: heat from muscle activity). These sensations are known as stimuli (singular = stimulus) and different sensory receptors are responsible for detecting different stimuli. Sensory information travels towards the CNS through the PNS nerves in the specific division known as the afferent (sensory) branch of the PNS. When information arises from sensory receptors in the skin, skeletal muscles, or joints, it is transmitted to the CNS using somatic sensory neurons; when information arises from sensory receptors in the blood vessels or internal organs, it is transmitted to the CNS using visceral sensory neurons.

Integration. Stimuli that are detected by sensory structures are communicated to the nervous system where information is processed. In the CNS, information from some stimuli is compared with, or integrated with, information from other stimuli or memories of previous stimuli. Then, a motor neuron is activated to initiate a response from the effector organ. This process during which sensory information is processed and a motor response generated is called integration (Figure 13.2).

Response. The nervous system produces a response in effector organs (such as muscles or glands) due to the sensory stimuli. The motor (efferent) branch of the PNS carries signals away from the CNS to the effector organs. When the effector organ is a skeletal muscle, the neuron carrying the information is called a somatic motor neuron; when the effector organ is cardiac or smooth muscle or glandular tissue, the neuron carrying the information is called an autonomic motor neuron. Voluntary responses are governed by somatic motor neurons and involuntary responses are governed by autonomic motor neurons, which are discussed in the next section.

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 process is the action potential. An action potential is a predictable change in membrane potential that occurs due to the opening and closing of voltage gated ion channels on the cell membrane.

Electrically Active Cell MembranesMost cells in the body make use of charged particles (ions) to create electrochemical charge across the cell membrane. In a prior chapter, we described how muscle cells contract based on the movement of ions across the cell membrane. For skeletal muscles to contract, due to excitation–contraction coupling, they require input from a neuron. Both muscle and nerve cells make use of a cell membrane that is specialized for signal conduction 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. 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, cannot pass through the cell membrane without assistance (Figure 13.3). Specific transmembrane channel proteins permit charged ions to move across the membrane. Several passive transport channels, as well as active transport pumps, are necessary to generate a transmembrane potential, and an action potential. Of special interest is the carrier protein referred to as the sodium/potassium pump that uses energy to move sodium ions (Na⁺) out of a cell and potassium ions (K⁺) into a cell, thus regulating ion concentration on both sides of the cell membrane.

The sodium/potassium pump requires energy in the form of adenosine triphosphate (ATP), so it is also referred to as an ATPase pump. As was explained in the cell chapter, the concentration of Na⁺ is higher outside the cell than inside, and the concentration of K⁺ is higher inside the cell than outside. Therefore, this pump works against the concentration gradients for sodium and potassium ions, which is why it requires energy. The Na⁺/K⁺ ATPase pump maintains these important ion concentration gradients.

Ion channels are pores that allow specific charged particles to cross the membrane in response to an existing electrochemical gradient. Proteins are capable of spanning the cell membrane, including its hydrophobic core, and can interact with charged ions because of the varied properties of amino acids found within specific regions of the protein channel. Hydrophobic amino acids are found in the regions that are adjacent to the hydrocarbon tails of the phospholipids, where as hydrophilic amino acids are exposed to the fluid environments of the extracellular fluid and cytosol. Additionally, ions will interact with the hydrophilic amino acids, which will be selective for the charge of the ion. Channels for cations (positive ions) will have negatively charged side chains in the pore. Channels for anions (negative ions) will have positively charged side chains in the pore. The diameter of the channel’s pore also impacts the specific ions that can pass through.  Some ion channels are selective for charge but not necessarily for size. These nonspecific channels allow cations—particularly Na⁺, K⁺, and Ca²⁺—to cross the membrane, but exclude anions.

Some ion channels do not allow ions to freely diffuse across the membrane, but are gated instead. A ligand-gated channel opens because a molecule, or ligand, binds to the extracellular region of the channel (Figure 13.4). These channels are usually found on the dendrites of a cell.

A mechanically-gated channel opens because of a physical distortion of the cell membrane. Many channels associated with the sense of touch are mechanically-gated. For example, as pressure is applied to the skin, mechanically-gated channels on the subcutaneous receptors open and allow ions to enter (Figure 13.5).

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 and reaches a value specific to the channel, it opens and allows ions to cross the membrane (Figure 13.6).

A leak 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. Leak channels contribute to the resting transmembrane voltage of the excitable membrane (Figure 13.7). Voltage-gated and leak channels are generally found along the axon and are involved in maintaining membrane potential, and moving an action potential forward.