Cell Body

The cell body, or soma, consists of a large, round, nucleus 5–10 μm in diameter surrounded by cytoplasm, also called perikaryon (karyon = nucleus). The cytoplasm contains all the usual cell organelles other than centrioles required for the formation of the mitotic spindle associated with cell division. These include free ribosomes, smooth and rough endoplasmic reticulum (ER), mitochondria, and Golgi apparatus. The rough ER is also known as a Nissl substance that stains darkly in the presence of basic dyes called Nissl stains. As in other cells, the rough ER is the major site of protein synthesis destined for insertion into the membrane of the cell or an organelle. Free ribosomes are responsible for the synthesis of proteins destined to remain in the cytoplasm. See Chapter 2 for a summary of the structure and functions of cellular organelles.

Dendrites

The term dendrite is derived from the Greek word for “tree,” and can be easily imagined like branches of a tree originating from the cell body. All the dendrites together make up the dendritic tree. The dendrites on some neurons also have dendritic spines. Dendrites act as the receptive region of the neuron. The combination of the dendritic tree and dendritic spines makes for a large surface area to facilitate this function. As discussed below, neurons can be classified based on dendrites.

Axon

While many of the structures discussed above are common to most cells, the axon is unique to neurons. The axon is specialized to allow an impulse to be transmitted along its length, and thus, carried from one location to another. A long axon is sometimes called a nerve fiber. A bundle of axons within the CNS is called a tract. In the PNS, it is called a nerve.

A neuron has a single axon that originates from the soma in a region called the axon hillock. This is where the nerve impulse originates and is sometimes called the trigger zone. The axon is unique in that it contains no rough ER; few, if any, free ribosomes; and its membrane has a different protein composition from that of the soma.

While the axoplasm (cytoplasm inside the axon) contains neurofibrils, microtubules, lysosomes, mitochondria, and small vesicles, it lacks rough ER and ribosomes. Therefore, protein synthesis does not occur in the axon. Instead, proteins must be synthesized in the soma and transported along the axon.

Axons may extend less than a millimeter or over a meter in length. Axons typically branch, forming collaterals that enable one neuron to communicate with multiple sites. Occasionally, an axon collateral may communicate with the same neuron from which it originated, thus forming a recurrent collateral. The diameter of an axon can range from less than 1 μm to as large as 1 mm. The thicker the axon diameter, the faster the speed of conduction of the nerve impulse down its length.

The nerve fibers of many nerves are covered with a whitish, fatty sheath called myelin. Myelin acts to protect and insulate the axon and most critically increases the speed of conduction of the impulse. Whereas the speed of conduction may be 1 m/s in unmyelinated fibers, it can be 150 m/s in myelinated fibers. The dendrites are always unmyelinated.

In the PNS, myelin is produced by the Schwann cells. The Schwann cell spirals around the axon producing many concentric circles enclosing the axon to create the myelin sheath. During the spiraling process, the nucleus and cytoplasm of the Schwann cell get squeezed into the outer layer of the cell and appear as a bulge on the outer surface. The outer layer is the neurilemma. Occasionally, the Schwann cell does not spiral around the axon but instead encloses many axons at one time in what look like indentations on its surface. Such axons are said to be unmyelinated. Adjacent Schwann cells do not touch one another, but instead form a space called the node of Ranvier, in which the axonal membrane is exposed.

In the CNS, myelin is produced by another type of cell called an oligodendrocyte. Oligodendrocytes are a type of glial, or supportive, cell in the CNS. Instead of spiraling around the axon, the oligodendrocytes form end feet that surround the axon and form the myelin sheath. One oligodendrocyte can thereby myelinate many axons, whereas a Schwann cell myelinates only a single axon. In the CNS, areas containing myelinated fibers are referred to as white matter and generally consist of fiber tracts. Areas containing cell bodies are referred to as gray matter; collections of cell bodies are called nuclei.

The collaterals that branch off the main axon trunk end in a series of fine extensions called telodendria. A collection of telodendria is called a terminal arbor. The telodendria end in a knoblike structure called the axon terminal, terminal bouton, or synaptic knob. Microtubules do not extend into the terminal, but the terminal will typically contain synaptic vesicles. Synaptic vesicles are small membrane‐bound spheres measuring 50 nm in diameter and containing quanta of neurotransmitters. The axon terminal will end at another neuron or effector cell such as an endocrine gland or muscle cell.

Synapse

The region where the axon terminal meets another cell is called the synapse. It consists of a presynaptic membrane, a synaptic cleft, and a postsynaptic membrane. The axon terminal will typically contain synaptic vesicles. The average neuron forms approximately 1000 synaptic junctions.

The synaptic terminal of one neuron can synapse on any part of an adjacent neuron. This can create a variety of synapses such as axo‐axonic, axodendritic, and axosomatic. When a motor neuron synapses on a skeletal muscle cell, it creates a specialized junction called a neuromuscular junction. If the neuron synapses with a gland, it creates a neuroglandular synapse.

There are two types of synapses, electrical and chemical, which have different functional properties (Table 8.1). While chemical synapses are much more common between neurons in the mammalian and avian brain, electrical synapses are common between nonneural cells such as glial cells, epithelial cells, smooth and cardiac muscle cells, liver cells, and some glandular cells.

In electrical synapses, the pre and postsynaptic cells communicate through gap junctions (Fig. 8.4). These junctions provide a channel between the cytoplasm of the adjacent cells. Gap junctions consist of a pair of hemichannels, with one associated with the presynaptic membrane and the other with the postsynaptic membrane. Each hemichannel consists of specialized proteins called connexins. Six connexins combine to form a channel called a connexon through which ions can pass from the cytoplasm of one cell to the cytoplasm of the connected cell. The pore formed within the connexon is about 2 nm in diameter, making it one of the largest known, and it is of sufficient size to allow small organic molecules to pass. It appears that the connexins can alter their configurations to open or close the channel.

Type of Synapse	Distance within Synaptic Cleft	Components of Synapse	Agent of Transmission	Synaptic Delay	Direction of Transmission
Chemical	20–40 nm	Synaptic vesicles, active zone, postsynaptic receptors	Chemical	1–5 ms	Unidirectional
Electrical	3.5 nm	Gap‐junction channels	Electrical	Virtually absent	Bidirectional

The electrical synapse allows an action potential to pass from one cell to another with virtually no delay. This is beneficial when it is necessary for a signal to pass as rapidly as possible to another cell and for that signal to not be modified.

The chemical synapse is characterized by a synaptic cleft 20–50 nm wide (Fig. 8.5). Within the synaptic cleft, a fibrous protein matrix aids adherence between the cells. An electrical signal cannot cross the synaptic cleft, so a chemical substance called a neurotransmitter carries the signal to the postsynaptic cell. The neurotransmitter is stored in the synaptic vesicles. The concentration of neurotransmitters can be 10,000 times higher than in the cytosol. This is accomplished by a counter‐transport system in which a molecule of transmitter enters the synaptic vesicle in exchange for an H⁺ ion (Fig. 8.6).

The presynaptic membrane also expresses pyramid‐shaped integral proteins which project into the cytoplasm. These proteins, and the associated cell membrane, make the active zone. The active zone is the site of neurotransmitter release. The pyramid‐shaped proteins associated with the active zone are believed to be calcium channels.

Classification of Neurons

Neurons can be classified based on several characteristics. These include the structural classifications based on the number of neurites that extend from the soma or the length of the axon, the function, or the neurotransmitter they contain.

Classification Based on Neurite Number

Based on neurite number, neurons can be classified as unipolar, bipolar, pseudounipolar, or multipolar (Fig. 8.7). Unipolar neurons are the simplest nerve cells, and they have a single process. They appear in the autonomic nervous system. Bipolar neurons have an oval‐shaped soma from which two processes emerge: the dendrite and the axon. Many sensory neurons are bipolar, and their examples include those in the retina and olfactory epithelium. Pseudounipolar cells serve as mechanoreceptors that sense touch, pressure, and pain. The pseudounipolar develops embryologically as a bipolar neuron with two processes, but eventually, these two processes fuse into a single axon that emerges from the soma. The axon then divides into two segments, with one going to the periphery and one going to the spinal cord. The predominant type of neuron is the multipolar neuron that has a single axon and usually many dendrites.

Classification Based on Axon Length

Classifications based on axon length include Golgi type I and Golgi type II neurons. Golgi type I neurons have long axons and are considered projection neurons since they carry signals to other sites. An example would be pyramidal cells with cell bodies located in the cerebral cortex and whose axons extend to the spinal cord. Golgi type II neurons have short axons and are involved in local circuits. Examples include stellate or basket neurons in the cerebellum.

Classification Based on Function

Based on function, neurons are classified in the following ways: (1) sensory, or afferent neurons; (2) motor, or efferent neurons; (3) interneurons, or association neurons. Sensory neurons respond to sensory stimuli, and transmit that information to the nervous system, with most information being carried to the CNS. Most sensory neurons are pseudounipolar, and their cell bodies are in the dorsal root ganglion of the spinal nerves. Motor neurons transmit signals from the brain or spinal cord to the muscles or glands. Their cell bodies are in the CNS. Interneurons are the largest class of neurons—constituting 99% of all neurons, including all those neurons that are not sensory or motor. Most are multipolar neurons. This class can be divided into two groups. Relay or projection interneurons have long axons and transmit signals over considerable distances, such as the dorsal columns located in the spinal cord. Local interneurons have short axons and are involved in processing information in localized circuits, such as the horizontal cells in the retina or stellate cells that inhibit Purkinje cells in the cerebellum.

Interneurons, sometimes called association neurons, are the most numerous of all neuronal types. Most are in the brain and spinal cord, but some are found in the autonomic ganglia. They function to distribute sensory information and coordinate motor activity. Interneurons produce patterns of connections such as divergence and convergence (Fig. 8.8). Information coming from a single source and spreading to multiple neurons is known as divergence, whereas input from multiple neurons synapsing on a single interneuron is called convergence.

Classification Based on Neurotransmitter

Finally, neurons can be classified according to the neurotransmitter they release. In the motor neurons, which innervate skeletal muscle, they all release acetylcholine (ACh) and are called cholinergic neurons. Neurons that release serotonin, such as 5‐hydroxytryptamine (5‐HT), are called serotonergic neurons. For example, neurons in the raphe nucleus of the brain stem.

Ependymal Cells

The brain has four fluid‐filled cavities within its borders called cerebroventricles; the spinal cord has a central canal. These cavities are lined with ependymal cells that range in shape from squamous to columnar and may be ciliated. These cells generally form a permeable barrier between the cerebrospinal fluid and tissue. The exception is ependymal cells covering the choroid plexus (a capillary tuft in each cerebroventricle responsible for forming cerebrospinal fluid). These ependymal cells form tight junctions creating a barrier between the blood and brain. The cilia located on ependymal cells help “circulate” or move the cerebrospinal fluid within the ventricular system.

Astrocytes

The star‐shaped astrocytes are the most abundant glial cells. They have radiating processes that expand at the end to create projections that wrap around capillaries. The astrocytes produce signals that cause the endothelial cells lining the brain capillaries to form tight junctions. These tight junctions form the blood–brain barrier that greatly restricts the movement of compounds into the brain. Astrocytes also help guide the migration and connections of new neurons, control the chemical environment around neurons, and help inactivate neurotransmitters that are released into the synapse. Because astrocytes are also connected via gap junctions, they also signal one another, but detailed understanding is lacking.

Oligodendrocytes

Oligodendrocytes have fewer processes than astrocytes and smaller cell bodies. As previously noted, these cells are responsible for producing the myelin sheath found around axons in the CNS. This sheath is formed by the oligodendrocyte wrapping around an axon forming concentric layers, much like wrapping layers of gauze around a cut finger.

Microglia

These are small ovoid cells that have narrow cytoplasmic processes with many branches. These cells originate from mesodermal stem cells related to those that produce monocytes and macrophages. They can migrate within the CNS where they phagocytize microorganisms and dead neurons. Although cells in the peripheral immune system cannot enter the CNS, the microglia seems to partially fill this aspect of their function.

Satellite Cells and Schwann Cells

Also called amphicytes, satellite cells surround neuronal cell bodies in the peripheral ganglia. Their function is unknown. As previously discussed, Schwann cells form the myelin sheath around peripheral axons. Therefore, they are analogous to oligodendrocytes.

The Resting Membrane Potential

Nerve cells and muscle cells have an excitable membrane. When such a cell is not generating an impulse, it is said to be at rest. When at rest, the cytosol has a negative electrical charge relative to the outside of the cell (Fig. 8.9). The potential energy generated by the separation of charge across the membrane is called voltage and is measured in volts or millivolts (1 V = 1000 mV). By placing an electrode both inside and outside the cell, a voltage can be measured between these two sites. That voltage is called the potential difference, or potential. The difference in electrical charge across the membrane of a cell at rest is called the resting membrane potential, and it is typically about −60 to −70 mV.

The inside and outside of the cell are separated by a membrane that acts as a semipermeable membrane. The membrane potential is caused by the unequal distribution of ions across the membrane. For the four most important ions regarding membrane potential, the concentration of Na⁺ and Cl⁻ is higher outside the cell, whereas the concentration of K⁺ and organic anions is higher inside the cell. The organic anions consist mostly of proteins and amino acids. These differences in ion concentrations establish chemical gradients.

The lipid bilayer of the cell membrane acts as an insulator between the interior and exterior of the cell. Ions cannot cross the membrane except by way of ion channels. Within the membrane, there are passive ion channels, sometimes called leak channels, which remain open. These passive channels are most permeable to K⁺ and chloride ions, relatively less permeable to Na⁺, and impermeable to proteins. Because K⁺ is in higher concentrations inside the cell, it tends to diffuse outwardly, down its concentration gradient. The movement of ions to eliminate the potential difference is called a current. How much the membrane restricts the movement of ions is a measure of its resistance. As K⁺ moves outward, an excess of negatively charged anions remains inside the cell, thus establishing an electrochemical gradient. Eventually, the movement of K⁺ outward reaches equilibrium because the outwardly directed chemical gradient is opposed by the inwardly directed electrical gradient. This point is called the ionic equilibrium potential, or equilibrium potential, and is represented by the symbol E_ion.

The equilibrium potential of an ion can be calculated using the Nernst equation:

where E_ion equals ionic equilibrium potential, R equals the gas constant, T equals absolute temperature, z equals the charge on the ion, F equals Faraday’s constant, and [ion] is the concentration of the ion either inside or outside the cell. At body temperature, the Nernst equation for the monovalent ions K⁺, Na⁺, and Cl⁻ can be simplified to

whereas for Ca⁺⁺, a divalent cation, the equation would be

E _ion is very important for understanding the effect an ion has on the membrane potential (Table 8.2).

If the membrane were permeable only to K⁺, the membrane potential would be approximately −80 mV. At this point, the chemical gradient for K⁺ would equal the electrochemical gradient. However, the membrane is permeable to ions other than K⁺. The Nernst equation does not take into consideration the permeability of multiple ions, or that the relative permeability of the various ions is different. If the permeability of the ions is known, the membrane potential can be calculated using the Goldman equation:

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