Ch. 12 Part 1: Neurons and Neuroglia

Two types of neuroglia produce myelin sheaths

Schwann cells (in the PNS) and oligodendrocytes (in the CNS)

neuroglia of PNS

Schwann cells and satellite cells

PNS

all nervous tissue outside the CNS; divided into the SNS, ANS, and ENS

neuroglia of the CNS

astrocytes, oligodendrocytes, microglia, and ependymal cells

varicosities

axon terminals that exhibit string of swollen bumps

The processes of astrocytes make contact with

blood capillaries, neurons, and the pia mater (a thin membrane around the brain and spinal cord).

gliomas

brain tumors derived from glia

schwann cells

encircle PNS axons. Like oligodendrocytes, they form the myelin sheath around axons; participate in axon regeneration

Enteric plexuses

extensive networks of neurons located in the walls of organs of the gastrointestinal tract. The neurons of these plexuses help regulate the digestive system

ture or false: neurons undergo mitotic division

false

retrograde

fast axonal transport (backward) direction moves membrane vesicles and other cellular materials from the axon terminals to the cell body to be degraded or recycled. Substances that enter the neuron at the axon terminals are also moved to the cell body by fast retrograde transport. These substances include trophic chemicals such as nerve growth factor and harmful agents such as tetanus toxin and the viruses that cause rabies, herpes simplex, and polio.

anterogade

fast axonal transport (forward) direction moves organelles and synaptic vesicles from the cell body to the axon terminals.

satellite cells

flat cells surround the cell bodies of neurons of PNS ganglia, regulate the exchanges of materials between neuronal cell bodies and interstitial fluid

When an axon is injured, the neurolemma aids regeneration by

forming a regeneration tube that guides and stimulates regrowth of the axon

Processes of oligodendrocytes are responsible for

forming and maintaining the myelin sheath around CNS axons

pyramidal cells

found in the cerebral cortex of the brain, which have pyramid-shaped cell bodies

nerve fiber

general term for any neuronal process (extension) that emerges from the cell body of a neuron

fibrous astrocytes

have many long unbranched processes and are located mainly in white matter (also described shortly)

protoplasmic astrocytes

have many short branching processes and are found in gray matter (described shortly)

Bipolar neurons

have one main dendrite and one axon. They are found in the retina of the eye, the inner ear, and the olfactory area of the brain

part of the axon closest to the axon hillock

initial segment

ependymal cells location

line the ventricles of the brain and central canal of the spinal cord (spaces filled with cerebrospinal fluid, which protects and nourishes the brain and spinal cord)

The trigger zone for nerve impulses in a unipolar neuron is at

the junction of the dendrites and axon

synaptic end bulbs

tips of some axon terminals swell into bulb-shaped structures

microglial cells function as

phagocytes. Like tissue macrophages, they remove cellular debris formed during normal development of the nervous system and phagocytize microbes and damaged nervous tissue.

ependymal cells function

produce, possibly monitor, and assist in the circulation of cerebrospinal fluid. They also form the blood-cerebrospinal fluid barrier

Newly synthesized proteins produced by Nissl bodies are used to

replace cellular components, as material for growth of neurons, and to regenerate damaged axons in the PNS

axon collaterals

side branches along the length of an axon

ganglia

small masses of nervous tissue, consisting primarily of neuron cell bodies, that are located outside of the brain and spinal cord

astrocytes

star-shaped cells have many processes and are the largest and most numerous of the neuroglia.

motor part of ANS consists of

sympathetic division and the parasympathetic division

what stores nts

synaptic vesicles, contained by both bulbs and varicosities

multipolar neurons

usually have several dendrites and one axon. Most neurons in the brain and spinal cord are of this type, as well as all motor neurons (described shortly)

ANS consists of

(1) sensory neurons that convey information to the CNS from autonomic sensory receptors, located primarily in visceral organs such as the stomach and lungs, and (2) motor neurons that conduct nerve impulses from the CNS to smooth muscle, cardiac muscle, and glands. Because its motor responses are not normally under conscious control, the action of the ANS is involuntary.

SNS consists of

(1) sensory neurons that convey information to the CNS from somatic receptors in the head, body wall, and limbs and from receptors for the special senses of vision, hearing, taste, and smell, and (2) motor neurons that conduct impulses from the CNS to skeletal muscles only. Because these motor responses can be consciously controlled, the action of this part of the PNS is voluntary.

astrocyte functions

1. Astrocytes contain microfilaments that give them considerable strength, which enables them to support neurons. 2. Processes of astrocytes wrapped around blood capillaries isolate neurons of the CNS from various potentially harmful substances in blood by secreting chemicals that maintain the unique selective permeability characteristics of the endothelial cells of the capillaries. In effect, the endothelial cells create a blood-brain barrier, which restricts the movement of substances between the blood and interstitial fluid of the CNS. 3. In the embryo, astrocytes secrete chemicals that appear to regulate the growth, migration, and interconnection among neurons in the brain. 4. Astrocytes help to maintain the appropriate chemical environment for the generation of nerve impulses. For example, they regulate the concentration of important ions such as K+; take up excess neurotransmitters; and serve as a conduit for the passage of nutrients and other substances between blood capillaries and neurons. 5. Astrocytes may also play a role in learning and memory by influencing the formation of neural synapses

neuron functional classifications

1. Sensory or afferent neurons either contain sensory receptors at their distal ends (dendrites); unipolar 2. Motor or efferent neurons convey action potentials away from the CNS to effectors (muscles and glands) in the periphery (PNS) through cranial or spinal nerves; multipolar 3. Interneurons association neurons are mainly located within the CNS between sensory and motor neurons; multipolar

nervous system functions

1. sensory: detect internal stimuli 2. integrative: processes sensory information by analyzing it and making decisions for appropriate responses 3. motor: elicit an appropriate motor response by activating effectors

nodes of Ranvier

Gaps in the myelin sheath that , appear at intervals along the axon; Each Schwann cell wraps one axon segment between two nodes

neuroglia vs. neurons

Neuroglia are smaller cells but they greatly outnumber neurons, perhaps by as much as 25 times. Neuroglia support, nourish, and protect neurons, and maintain the interstitial fluid that bathes them. Unlike neurons, neuroglia continue to divide throughout an individual's lifetime. Both neurons and neuroglia differ structurally depending on whether they are located in the central nervous system or the peripheral nervous system.

dendrite cytoplasm contains

Nissl bodies, mitochondria, and other organelles

neurolemma

The outer peripheral, nucleated cytoplasmic layer of the Schwann cell. Also called sheath of Schwann

Unipolar neurons

Unipolar neurons have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body ; begin in the embryo as bipolar neurons

tract

a bundle of axons that is located in the CNS. Tracts interconnect neurons in the spinal cord and brain.

lipofuscin

a pigment that occurs as clumps of yellowish brown granules in the cytoplasm; product of neuronal lysosomes that accumulates as the neuron ages, but does not seem to harm the neuron

each schwann cell myelinates

a single axon

fast axonal trasport

capable of moving materials a distance of 200-400 mm per day, uses proteins that function as "motors" to move materials along the surfaces of microtubules of the neuron's cytoskeleton. Fast axonal transport moves materials in both directions—away from and toward the cell body

Sensory neurons of the ENS monitor

chemical changes within the GI tract as well as the stretching of its walls. Enteric motor neurons govern contractions of GI tract smooth muscle to propel food through the GI tract, secretions of GI tract organs (such as acid from the stomach), and activities of GI tract endocrine cells, which secrete hormones.

white matter

composed primarily of myelinated axons

axon hillock

cone-shaped elevation at which the axon joins to the cell body

neuron cell body

contains a nucleus surrounded by cytoplasm that includes typical cellular organelles such as lysosomes, mitochondria, and a Golgi complex

grey matter

contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia. It appears grayish, rather than white, because the Nissl bodies impart a gray color and there is little or no myelin in these areas.

slow axonal transport

conveys axoplasm in one direction only—from the cell body toward the axon terminals. Slow axonal transport supplies new axoplasm to developing or regenerating axons and replenishes axoplasm in growing and mature axons; moves materials about 1-5 mm per day

ependymal cells

cuboidal to columnar cells arranged in a single layer that possess microvilli and cilia

axoplasm

cytoplasm of an axon

axolemma

membrane that surrounds axoplasm

axon contains

mitochondria, microtubules, and neurofibrils

trigger zone

nerve impulses arise at the junction of the axon hillock and the initial segment

Components of the PNS

nerves, ganglia, enteric plexuses, and sensory receptors

cytoskeleton includes

neurofibrils composed of bundles of intermediate filaments that provide the cell shape and support, and microtubules, which assist in moving materials between the cell body and axon.

Purkinje cells

neuron in the cerebellum

Nissl bodies

neuronal cell bodies contain free ribosomes and prominent clusters of rough endoplasmic reticulum,