Anatomy - Ch 12 Tissues and Cells of the Nervous System

endocrine system

communicates by means of chemical messengers (hormones) secreted into the blood; slow but broad communication

nervous system

employs electrical and chemical means to send messages from cell to cell; fast and accurate communication

1) sense organs receive information 2) CNS processes information 3) CNS issues commands

what are the three basic steps that a nervous system does to carry out its tasks?

the brain and spinal cord (central nervous system)

where do sense organs that receive information about changes in the body and external environment transmit coded messages to?

the CNS relates it to past experiences, and determines an appropriate response

what happens when the CNS process information that it receives from the sense organs?

to muscles and gland cells

where does the CNS issue commands in order for them to carry out a response?

central nervous system

brain and spinal cord enclosed by the cranium and vertebral column; where we integrate information

peripheral nervous system

all the nervous system except the brain and spinal cord; composed of nerves and ganglia

cranial nerves

directly enter or leave the brain

spinal nerves

directly enter or leave the spine

nerve

a bundle of nerve fibers (axons) wrapped in fibrous connective tissue

ganglion

a knot-like swelling in a nerve where neuron cell bodies are concentrated (most located next to spinal cord)

peripheral nervous system

contains sensory and motor divisions each with somatic and visceral subdivisions

sensory (afferent) division

carries signals from receptors to CNS

somatic sensory division

carries signals from receptors in the skin, muscles, bones, and joints

visceral sensory division

carries signals from the viscera (heart, lungs, stomach, and urinary bladder)

motor efferent division

carries signals from CNS to effectors (glands and muscles that carry out the body’s response)

somatic motor division

carries signals to skeletal muscles; output produces voluntary muscular contraction as well as somatic reflexes (involuntary muscle contractions)

visceral motor division (autonomic nervous system)

carries signals to glands, cardiac, and smooth muscle; its involuntary responses are visceral reflexes

sympathetic division and parasympathetic division

two divisions within the visceral motor division (autonomic nervous system)

sympathetic division

tends to arouse body for action (fight, flight, or freeze); accelerating heart beat and respiration, while inhibiting digestive and urinary systems

parasympathetic division

tends to have a calming effect; slows heart rate and breathing; stimulates digestive and urinary systems

excitability, conductivity, and secretion

what are three universal properties of neurons?

excitability

respond to environmental changes called stimuli

conductivity

respond to stimuli by producing electrical signals that are quickly conducted to other cells at distant locations

secretion

when an electrical signal reaches the end of nerve fiber, the cell secretes a chemical neurotransmitter that influences the next cell

sensory (afferent) neurons

detect stimuli and transmit information about them toward the CNS

interneurons

lie entirely within the CNS connecting motor and sensory pathways; they receive signals from many neurons and carry out integrative functions (make decisions on responses); most neurons in the body and cannot be regenerated

motor (efferent) neuron

send signals out to muscles and gland cells (the effectors)

neurosoma

control center of the neuron; has a single, centrally located nucleus with large nucleolus

mitochondria, lysosomes, Golgi complex, inclusions, extensive rough ER (synthesizes proteins excreted from cell), cytoskeleton

what does cytoplasm in a neuron contain?

inclusions

glycogen, lipid droplets, melanin, and lipofuscin pigment (produced when lysosomes digest old organelles)

cytoskeleton in a neuron

has a dense mesh of microtubules and neurofibrils (bundles of actin filaments) that compartmentalizes rough ER into dark-staining chromatophilic substance (Nissl Bodies)

no, therefore they also don’t have centrioles

do neurons go through mitosis?

dendrites

branches that come off the neurosoma; primary site for receiving signals from other neurons; provide precise pathways for the reception and processing of information

more information can be received

what happens when there are more dendrites on a neuron?

axon (nerve fiber)

originates from a mound on the neurosoma called the axon hillock; carries signals away from the body

axon collaterals

branches of axon; axons branches are specialized for rapid conduction of signals to distant points

axoplasm

cytoplasm of axon

axolemma

plasma membrane of axon

one

how many axons per neuron?

myelin sheath (serves as an extra layer of electrical insulation and speeds up signal)

what may enclose an axon?

terminal arborization

extensive complex of fine branches at the distal end of an axon

axon terminal

little swelling that forms a junction (synapse) with the next cell; contains synaptic vesicles full of neurotransmitter

unipolar neuron

single process leading away from neurosoma; has dendrite and axon; sensory cells from skin and organs to spinal cord have these

anaxonic neuron

have many dendrites but no axon so they only receive signals; retina, brain, and adrenal gland have these

multipolar neuron

has one axon and multiple dendrites; used for signal processing and they are the most common neurons in the CNS

bipolar neuron

has one axon and one dendrite; olfactory cells, retina, and inner ear has these

must be transported to the axon and axon terminal in order to repair the axolemma, serve as gated ion channels, enzymes, or neurotransmitters

where do many proteins made in the neurosoma need to be transported to and why?

axonal transport

two-way passage of proteins, organelles, and other material along an axon

anterograde transport

movement down the axon away from neurosoma

retrograde transport

movement up the axon toward the neurosoma

microtubules; motor proteins carry materials while “crawling” along microtubules

what guides materials along the axon?

kinesin

motor proteins in anterograde transport

dynein

motor proteins in retrograde transport

fast anterograde transport and fast retrograde transport

what are the two types of fast axonal transport?

fast anterograde transport

fast axonal transport of organelles, enzymes, synaptic vesicles, and small molecules

fast retrograde transport

fast axonal transport for recycled materials and pathogens (rabies, herpes simplex, tetanus, polio viruses); it is the delay period between infection and symptoms since it takes time for it to be transported up the axon

always anterograde

what direction is slow axonal transport?

slow axonal transport

moves enzymes, cytoskeletal components, and new axoplasm down the axon during repair and regeneration of damaged axons; damaged nerve fibers regenerate at a speed governed by this (governed by simple diffusion, not motor proteins)

protect neurons and help them function; bind neurons together and form framework for nervous tissue; in fetus, guide migrating neurons to their destination; if mature neuron is not in synaptic contact with another neuron, it is covered by glial cells which prevents neurons from touching each other and gives precision to conduction pathways

functions of neuroglia or glial cells

oligodendrocytes

type of neuroglia that forms myelin sheaths in CNS that speed signal conduction; has arm-like processes that wrap around nerve fibers

ependymal cells

type of neuroglia of the CNS that line internal cavities of the brain; secrete and circulate cerebrospinal fluid (CSF); they are cuboidal epithelium with cilia on the apical surface

microglia

type of neuroglia that wanders through the CNS looking for debris and damage; they develop from white blood cells (monocytes) and become concentrated in areas of damage

astrocytes

the most abundant type of glial cell in the CNS, that covers the brain surface and most nonsynaptic regions of neurons in the gray matter

form supportive framework; have extensions that contact blood capillaries and stimulate them to form a seal called the blood-brain barrier; monitor neuron activity and regulate blood flow to match metabolic need; convert glucose to lactate and supply this to neurons; regulate chemical composition of tissue fluid by absorbing excess NT and ions

what are the functions of astrocytes?

schwann cells

type of neuroglia in the PNS that envelopes nerve fibers; it winds repeatedly around a nerve fiber and produces a myelin sheath similar to the ones produced by oligodendrocytes in CNS; they assist in regeneration of damaged fibers

satellite cells

type of neuroglia that surround the neurosomas in ganglia of the PNS; they provide electrical insulation around the neurosoma and regulate the chemical environment of the neurons (protect PNS from bacterial and viral attack)

myelin sheath

insulation around a nerve fiber; formed by oligodendrocytes in CNS and schwann cells in PNS; consists of the plasma membrane of glial cells

myelination

production of the myelin sheath; begins at week 14 of fetal development, proceeds rapidly during infancy, and is completed in late adolescence

dietary fat

what is important to CNS development (myelination)?

they spiral repeatedly around a single nerve fiber and can lay down as many as one hundred layers of membrane with no cytoplasm in between the membranes

how do schwann cells form the myelin sheath in the PNS?

neurilemma

thick, outermost coil of myelin sheath that contains the nucleus and most of its cytoplasm

the basal lamina and a thin layer of fibrous connective tissue called endoneurium

what is external to the neurilemma?

it is anchored to multiple nerve fibers; they cannot migrate around any one of them like schwann cells; instead they must push newer layers of myelin under the older ones, so myelination spirals inward toward nerve fiber

how do oligodendrocytes in the CNS myelinate several nerve fibers in its immediate vicinity?

only nerve fibers in the PNS have neurilemma and endoneurium

do nerve fibers in the CNS or PNS have neurilemma and/or endoneurium?

many

how many schwann cells or oligodendrocytes are needed to cover one nerve fiber?

nodes of ranvier

gap between segments in the myelin sheath

internodes

myelin-covered segments from one gap to the next in the myelin sheath

initial segment

short section of nerve fiber between the axon hillock and the first glial cell in the myelin sheath

trigger zone

the axon hillock and the initial segment in the myelin sheath; plays an important role in initiating a nerve signal

no, many are unmyelinated (they still have schwann cells wrapping around them)

do CNS and PNS fibers have to be myelinated?

1) diameter of the fiber (larger fibers have more surface area and conduct signals more rapidly) 2) presence of absence of myelin (myelin speeds up signal conduction)

what are the two factors that the speed at which a nerve signal travels along the surface of a nerve fiber depend on?

to gastrointestinal tract where speed is less of an issue

where are slow signals from nerve fibers sent to?

to skeletal muscles where speed improves balance and coordinated body movement

where are fast signals from nerve fibers sent to?

if its neurosoma is intact and at least some neurilemma remains

when can regeneration of damaged peripheral nerve fiber occur (not much of this occurs in the CNS)?

1) fiber distal to the injury cannot survive and degenerates (macrophages clean up tissue debris at point of injury and beyond) 2) neurosoma swells, ER breaks up, and nucleus moves off center (due to loss of nerve growth factors from neuron’s target cell) 3) axon stump sprouts multiple growth processes as severed distal end continues to degenerate 4) schwann cells, basal lamina, and neurilemma form a regeneration tube (enables neuron to regrow to original destination and reestablish synaptic contact) 5) once contact is reestablished with original target, the neurosoma shrinks and returns to its original appearance (nucleus returns to normal shape and atrophied muscle fibers regrow)

steps of nerve regeneration

it is not fast, perfect, or always possible; may take 2 years; some nerve fibers connect with the wrong muscle fibers and some die; regeneration of damaged nerve fibers in the CNS cannot occur at all because it can be hard to form the correct connections

what are the issues with nerve regeneration?

nerve growth factor

protein secreted by a gland, muscle, or glial cells and picked up by the axon terminals of neurons

prevent apoptosis (programmed cell death) in growing neurons and enables growing neurons to make contact with their targets

what do nerve growth factors do?

electrophysiology

study of cellular mechanisms for producing electrical potentials and currents; it is the basis for neural communication and muscle contraction

electrical potential

a difference in concentration of charged particles between one point and another; living cells are polarized and have a resting membrane potential; cells have more negative particles on inside of membrane than outside (neurons have about a -70 mV resting membrane potential)

electrical current

a flow of charged particles from one point to another; in the body, currents are movements of ions, such as Na+ or K+, through channels in the plasma membrane; gated channels are opened or closed by various stimuli; enables cell to turn electrical currents on and off

resting membrane potential

what exists because of unequal electrolyte distribution between extracellular fluid and intracellular fluid

1) ions diffuse down their concentration gradient through the membrane 2) plasma membrane is selectively permeable and allows some ions to pass easier than others 3) electrical attraction of cations and anions to each other

what are the three factors that combine to make the resting membrane potential?

potassium ions

what has the greatest influence on resting membrane potential?

potassium

what ion is the plasma membrane most permeable to?

it leaks out until the electrical charge of cytoplasmic anions attract it back in and equilibrium is reached (meaning no more net movement of potassium ions)

when do potassium ions stop leaking out from the plasma membrane?

in the ICF compared to the ECF

where are potassium ions more concentrated (40x more)

cytoplasmic anions

these cannot escape from the plasma membrane due to either size or charge (includes phosphates, sulfates, small organic acids, proteins, ATP, and RNA)