Human physiology chapter 7. Section 1 NEURONS AND SUPPORTING CELLS (Fox) Fox, Stuart. Human Physiology, 14th Edition. McGraw-Hill Higher Education, 20150320. VitalBook file.

anterograde transport / kinesin

Axonal transport may occur from the cell body to the axon and dendrites. This direction is called anterograde transport, and involves molecular motors of kinesin proteins that move cargo along the microtubules of the cytoskeleton. kinesin motors move synaptic vesi- cles, mitochondria, and ion channels from the cell body through the axon.

Bipolar neurons ( structural classification )

Bipolar neurons have two processes, one at either end; this type is found in the retina of the eye.

retrograde transport

By contrast, axonal transport in the opposite direction— that is, along the axon and dendrites toward the cell body—is known as retrograde transport. This involves the motor pro- tein dynein and its activator, dynactin. These molecular motors carry lysosomes, autophagosomes, endosomes, and various molecules along microtubules of the cytoskeleton toward the cell body of the neuron. Retrograde transport can also be responsible for movement of herpes virus, rabies virus, and tetanus toxin from the nerve terminals into cell bodies.

Ganglia (PNS)

Cell bodies in the PNS usually occur in clusters called ganglia.

Dendrites

Dendrites are thin, branched processes that extend from the cytoplasm of the cell body. Dendrites provide a receptive area that transmits graded electrochemical impulses to the cell body.

nodes of Ranvier ( Myelin Sheath in PNS )

Each Schwann cell wraps only about a millimeter of axon, leaving gaps of exposed axon between the adjacent Schwann cells. These gaps in the myelin sheath are known as the nodes of Ranvier.

Parkinson's disease

In the treatment of Parkinson's disease, for example, patients who need a chemical called dopamine in the brain are often given a precursor molecule called levodopa (l-dopa) because l-dopa can cross the blood-brain barrier but dopamine cannot.

Nissl bodies

Nissl bodies are composed of large stacks of rough endoplasmic reticulum that are needed for the synthesis of membrane proteins.

Nogo ( CNS )

Proteins called Nogo, produced predominantly by oligoden- drocytes, inhibit axon regeneration in the CNS.

Pseudounipolar neurons ( structural classification )

Pseudounipolar neurons have a single short process that branches like a T to form a pair of longer processes. Sensory neurons are pseudounipolar—one of the branched processes receives sensory stimuli and produces nerve impulses; the other delivers these impulses to synapses within the brain or spinal cord.

Regeneration of a cut axon

When an axon in a peripheral nerve is cut, the distal portion of the axon that was severed from the cell body degenerates and is phago- cytosed by Schwann cells. The Schwann cells, surrounded by the basement membrane, then form a regeneration tube. central axons have a much more limited ability to regenerate than peripheral axons. Regeneration of CNS axons is prevented, in part, by inhibitory proteins in the membranes of the myelin sheaths. Also, regen- eration of CNS axons is prevented by a glial scar that eventu- ally forms from astrocytes. This glial scar physically blocks axon regeneration and induces the production of inhibitory proteins. apoptosis = cell suicide

Blood-Brain Barrier

all of the endothelial cells of brain capillaries are joined together by tight junctions. molecules within brain capillaries must be moved through the endothelial cells by diffusion and active transport, as well as by endocytosis and exocytosis. This feature of brain capillaries imposes a very selective blood-brain barrier. plasma glucose can pass into the brain using specialized carrier proteins known as GLUT1. The GLUT1 glucose carriers, found in most brain regions, are always present; they do not require insulin stimulation like the GLUT4 carriers in skeletal muscles . Regulatory molecules from astrocytes also stimulate the endothelial cells to produce carrier proteins, ion channels, and enzymes that destroy potentially toxic molecules.

Neurons three principal regions.

(1) a cell body, (2) dendrites, and (3) an axon. Dendrites and axons can be referred to generically as processes, or extensions from the cell body.

There are two types of neuroglial cells in the peripheral nervous system

1. Schwann cells (also called neurolemmocytes), which form myelin sheaths around peripheral axons; and 2. satellite cells, or ganglionic gliocytes, which support neu- ron cell bodies within the ganglia of the PNS.

There are four types of neuroglial cells in the central nervous system

1. oligodendrocytes, which form myelin sheaths around axons of the CNS; 2. microglia, which migrate through the CNS and phagocy- tose foreign and degenerated material; 3. astrocytes, which help to regulate the external environ- ment of neurons in the CNS; and 4. ependymal cells, which are epithelial cells that line the ventricles (cavities) of the brain and the central canal of the spinal cord.

CHECKPOINTS

1a. Draw a neuron, label its parts, and describe the functions of these parts. 1b. Distinguish between sensory neurons, motor neurons, and association neurons in terms of structure, location, and function. 2a. DescribethestructureofthesheathofSchwann,or neurilemma, and explain how it promotes nerve regeneration. Explain how a myelin sheath is formed in the PNS. 2b. Explain how myelin sheaths are formed in the CNS. How does the presence or absence of myelin sheaths in the CNS determine the color of this tissue? 3. Explain what is meant by the blood-brainbarrier.Describe its structure and discuss its clinical significance.

Tract

A bundle of axons in the CNS is called a tract.

Nerve

A nerve is a bundle of axons located outside the CNS.

neurilemma, or sheath of Schwann

All axons in the PNS (myelinated and unmyelinated) are sur- rounded by a continuous living sheath of Schwann cells, known as the neurilemma, or sheath of Schwann. The axons of the CNS, by contrast, lack a neurilemma (Schwann cells are found only in the PNS).

Why astrocytes can be classifies as excitable ? / neuron-glia crosstalk

Although astrocytes do not produce action potentials (impulses), they can be classified as excitable because they respond to stimulation by transient changes in their intracellular Ca2+ concentration. Action potentials in neurons can provoke a rise in Ca2+ within a localized region of an astrocyte, which in turn stimulates the release of ATP and other gliotransmitters that affect the synaptic transmis- sion of neurons. This has been referred to as neuron-glia crosstalk.

Myelin Sheath in PNS

In the PNS, it is formed by Schwann cells.

Microglial activation

Infection, trauma, or any altered state can lead to microglial activation, in which the cells become amoeboid in shape and are transformed into phagocytic, motile cells. They follow chemokines (chemical attractants, including ATP) to the site of the infec- tion or damage, where they may proliferate by cell division. They can kill exogenous pathogens; remove damaged dendrites, axon terminals, myelin, and other debris within the CNS; and release anti-inflammatory chemicals.

How microglia of CNS is unique. Their difference from macrophages.

Microglia of the CNS are unique among neuroglial cells in that they derive from cells that were produced in the embryonic yolk sac and migrated into the developing neural tube. Although microglia are considered to be myeloid cells (related to cells derived from bone marrow), they differ from macrophages in the meninges (connective tissue coverings of the CNS) and elsewhere in the body, which derive from monocytes that originate in the bone marrow. Microglia in the healthy CNS have a small cell body and many fine processes that are constantly waving and surveying their extracellular environment, and seem to participate in maintaining healthy neuronal and synaptic function.

Mixed nerves

Most nerves are composed of both motor and sensory fibers and are thus called mixed nerves. Some of the cranial nerves, however, contain sensory fibers only. These are the nerves that serve the special senses of sight, hearing, taste, and smell.

multipolar neurons ( structural classification )

Multipolar neurons, the most common type, have several dendrites and one axon extending from the cell body; motor neurons are good examples of this type.

Neurons

Neurons are classified functionally and structurally; the various types of supporting cells perform specialized functions.

Classification of neurons and nerves ( functional classification )

Neurons may be classified according to their function or structure. The functional classification is based on the direction in which they conduct impulses.

Sensory / motor / interneurons ( functional classification )

Sensory, or afferent, neurons conduct impulses from sensory receptors into the CNS. Motor, or efferent, neurons conduct impulses out of the CNS to effector organs (muscles and glands). Association neurons, or interneurons, are located entirely within the CNS and serve the associative, or integrative, functions of the nervous system.

supporting cells

Supporting cells aid the functions of neurons and are about five times more abundant than neurons. supporting cells are collectively called neuroglia, or simply glial cells (from the Middle Greek glia 5 glue). Unlike neurons, which do not divide mitotically (except for particular neural stem cells; chapter 8, section 8.1), glial cells are able to divide by mitosis. This helps to explain why brain tumors in adults are usually composed of glial cells rather than of neurons.

Regeneration of axons ( PNS )

Surprisingly, Schwann cells in the PNS also produce myelin proteins that can inhibit axon regeneration. However, after axon injury in the PNS, the fragments of old myelin are rapidly removed (through phagocytosis) by Schwann cells and macrophages. Also, quickly after injury the Schwann cells stop producing the inhibitory proteins. The rapid changes in Schwann cell function following injury (fig. 7.9) create an environment conducive to axon regeneration in the PNS.

Axon / axon hillock / axon collaterals / axonal transport

The axon is a longer process that conducts impulses, called action potentials (section 7.2), away from the cell body. The origin of the axon near the cell body is an expanded region called the axon hillock. Toward their ends, axons can produce up to 200 or more branches called axon collaterals, and each of these can divide to synapse with many other neurons. In this way, a single CNS axon may synapse with as many as 30,000 to 60,000 other neurons. Because axons can be quite long, special mechanisms are required to transport organelles and proteins from the cell body to the axon terminals. This axonal transport is energy-dependent and is often divided into a fast component and two slow components.

autonomic neurons ( functional classification )

The cell bodies of the autonomic neurons that innervate these organs are located outside the CNS in autonomic ganglia (fig. 7.3). There are two subdivisions of autonomic neurons: sympathetic and parasym- pathetic. Autonomic motor neurons, together with their central control centers, constitute the autonomic nervous system.

Nuclei

The cell bodies within the CNS are frequently clustered into groups called nuclei (not to be confused with the nucleus of a cell).

Cell body of neuron

The cell body is the enlarged portion of the neuron that contains the nucleus. It is the "nutritional center" of the neuron where macromolecules are produced. The cell body and larger dendrites (but not axons) contain Nissl bodies, which are seen as dark-staining granules under the microscope.

Fast and slow components

The fast component (at 200 to 400 mm/day) mainly transports membranous vesicles (important for synaptic transmission, as dis- cussed in section 7.3). One slow component (at 0.2 to 1 mm/day) transports microfilaments and microtubules of the cytoskeleton, while the other slow component (at 2 to 8 mm/day) transports over 200 different proteins, including those critical for synaptic function. The slow components appear to transport their cargo in fast bursts with frequent pauses, so that the overall rate of transport is much slower than that occurring in the fast component.

Neurons and supporting cells.

The nervous system is composed of only two principal types of cells—neurons and supporting cells. Neurons are the basic structural and functional units of the nervous system.

CNS / PNS

The nervous system is divided into the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system (PNS), which includes the cranial nerves arising from the brain and the spinal nerves arising from the spinal cord.

structural classification

The structural classification of neurons is based on the number of processes that extend from the cell body of the neuron.

Two types of motor neurons ( functional classification )

There are two types of motor neurons: somatic and autonomic. Somatic motor neurons are responsible for both reflex and voluntary control of skeletal muscles. Autonomic motor neurons innervate (send axons to) the involuntary effectors— smooth muscle, cardiac muscle, and glands.

Neuron activity

They are specialized to respond to physical and chemical stimuli, conduct electrochemical impulses, and release chemical regulators. Through these activities, neurons enable the perception of sensory stimuli, learning, memory, and the control of muscles and glands. Most neurons cannot divide by mito- sis, although many can regenerate .

myelin sheath

in the CNS, it is formed by oligodendrocytes. Myelinated axons conduct impulses more rapidly than those that are unmyelinated.

Functions of Astrocytes

most abundant of the glial cells in the CNS. 1. Astrocytes take up K+ from the extracellular fluid. 2. Astrocytes take up some neurotransmitters released from the axon terminals of neurons. ( For example, the neurotransmitter glutamate is taken into astrocytes and transformed into glutamine. The glutamine is then released back to the neurons, which can use it to reform the neurotransmitter glutamate. ) 3. The astrocyte end-feet surrounding blood capillaries take up glucose from the blood. 4. Astrocytes release lactate, which aids neuron function. 5. Astrocytes appear to be needed for synapse formation, maturation, and maintenance. 6. Astrocytes regulate neurogenesis in the adult brain. 7. Astrocytes secrete glial-derived neurotrophic factor (GDNF). 8. Astrocytes induce the formation of the blood-brain barrier. 9. Astrocytes release transmitter chemicals that can stimulate or inhibit neurons.

Myelin Sheath in CNS, white matter/grey matter

myelin sheaths of the CNS are formed by oligodendrocytes. Unlike a Schwann cell, which forms a myelin sheath around only one axon, each oligodendrocyte has extensions, like the tentacles of an octopus, that form myelin sheaths around several axons (fig. 7.8). The myelin sheaths around axons of the CNS give this tissue a white color; areas of the CNS that contain a high concentration of axons thus form the white matter. The gray matter of the CNS is composed of high concentrations of cell bodies and dendrites, which lack myelin sheaths.

Myelinated vs unmyelinated ( Myelin Sheath in PNS )

myelinated axons of the PNS are surrounded by a living sheath of Schwann cells, or neurilemma. Unmyelinated axons are also surrounded by a neurilemma, but they differ from myelinated axons in that they lack the multiple wrappings of Schwann cell plasma membrane that compose the myelin sheath.

Neurotrophins

nerve growth factor (NGF), a regulatory molecule produced by neurons that promotes the survival and growth of sympathetic and sensory neurons in the developing fetal brain. NGF was the first of the neurotrophins to be discovered. Neurotrophins have important functions in the adult nervous system. NGF is required for the maintenance of sympathetic ganglia, and neurotrophins are required for mature sensory neurons to regenerate after injury. Similarly, GDNF ( Glial cell-derived neurotrophic )promotes the survival of dopaminergic neurons (those that use dopamine as a neurotransmitter) and spinal motor neurons. Neurotrophins regulate the survival and differentiation of adult neural stem cells (chapter 3, section 3.3; and chapter 8, section 8.1) in parts of the brain involved in learning and memory. These roles include the growth of dendrites and axons, the formation of synapses, and synaptic changes during learning.

Neuroglial Cells

supporting cells of the nervous system are derived from the same embryonic tissue layer (ectoderm) that produces neurons. The term neuroglia (or glia) traditionally refers to the supporting cells of the CNS, but in current usage the supporting cells of the PNS are often also called glial cells.