A & P Ch. 12 Nervous Tissue

organs and cells that make up the nervous system

brain, spinal cord, peripheral nerves, ganglia, neurons

3 basic steps the nervous system uses to coordinate tasks

receives information and transmits to CNS, CNS processes information, signals muscle and gland cells

2 main anatomic divisions of the nervous system

central nervous system CNS, peripheral nervous system PNS

brain, spinal cord; performs most decision-making functionsand integration of sensory information.

central nervous system CNS

nerves, ganglia; connects to CNS, carries signals

peripheral nervous system PNS

sensory/afferent (somatic, visceral) motor/efferent (somatic/visceral)

PNS divisions & subdivisions

carries receptor signals to CNS, informs of stimuli

sensory (afferent) division

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

somatic sensory division

carries signals from viscera in thoracic and abdominal cavities, heart, lungs, stomach urinary bladder

visceral sensory division

carries signals from CNS to gland and muscle cells to elicit response/action

motor (efferent) division

carries signals to skeletal muscles producing voluntary movement

somatic motor division

carries signals to glands, cardiac muscle, smooth muscle, operates unconsciously as reflexes

visceral motor division (autonomic nervous system, ANS)

arouse body for action; accelerating hearting heartbeat, increasing respiratory airflow; inhibits digestion, reduces urine production

sympathic division

calming effect, slows heartbeat; stimulates digestion

parasympathetic division

wall of digestive tract, more neurons than spinal cord, enables regions of long digestive tract to communicate, coordinate motility and secretion

enteric plexus

3 physiology properties that allow neurons to communicate with other cells

excitability (respond to stimuli), conductivity (produce electrical signals), secretion (neurotransmitter influencing neighboring cells)

3 functional classes of neurons

sensory/afferent neurons, interneurons, motor/efferent neurons

structural classification of neurons

cell body (neurosoma, perikaryon), neurofibrils (actin filament bundles), chromatophilic substance (rough ER in sark-staining regions)

one axon, multiple dendrites; brain, spinal cord

multipolar neurons

one axon, one dendrite; olfactory nose, certain retina, ear sensory

bipolar neurons

single process leading away from cell body; carry touch and pain signals; spinal cord

unipolar neurons

no axon, multiple dendrites, don’t produce action potentials; brain, retina, adrenal medulla; help in visual processes (contrast perception)

anaxonic neurons

process of neurons that usually receive signals from other neurons

dendrites

mound on cell body from which the axon (nerve fiber) originates

axon hillock

solitary neurite

axon (nerve fiber)

cytoplasm of axon

axoplasm

membrane of axon

axolemma

____ send a receive signals while ____ ____ support them and help them function

neurons; glial cells

protect neurons and help them function, bind neurons together, provide supportive framework for the nervous tissue

neuroglia (glial cells)

bulbous body up to 15 arms, each arm reaches to axon and spirals around it forming the myelin sheath to insulate axon from extracellular fluid & speed up conduction in CNS

oligodendrocytes

resemble cuboidal epithelium lining in brain and spinal cord, no basement membrane, rootlike process penetrate underlying tissue; produce cerebrospinal fluid (CSF) which surrounds CNS and fills its cavities

ependymal cells

small macrophages that wander though CNS to probe from cellular debris or issues, phagocytizes dead tissue, etc., become concentrated in areas damaged by infection

microglia

cover entire brain surface and nonsynaptic neuron regions, starlike shape; form blood-brain barrier, nourish neurons by converting blood glucose to lactate, produce growth factors that stimulate neurons, promotes synapse formation, electronically communicate with neurons, removes K⁺ some neurotransmitters from brain & spinal cord ECF, help regulate ECF composition, form scar tissue

astrocytes

form neurolemma around all PNS axons & myelin around most, aid in damaged nerve regeneration

schwann cells

surround cell bodies of neurons in ganglia, provide electrical insulation & regulate chemical environment of neurons

satellite cells

layers of insulation composed of lipids and proteins around axon formed by Schwann cells in PNS & oligodendrocytes in CNS

myelin sheath

____ axons produce myelin sheaths while ____ axons do not

myelinated; unmyelinated

slow axons sufficient for processes such as secreting stomach acid or dilating pupil in both CNS & PNS

unmyelinated axons

fast axons for motor commands to skeletal muscles and sensory signals for vision & balance in both CNS & PNS

myelinated axons

distal part of axon including Schwann cells degenerate from lack of protein, macrophages clean up tissue debris at and around point of injury

nerve regeneration step 1

some neurons die at this stage; cell body swells, ER breaks up, nucleus moves off center), axon stump sprouts multiple growth processes, distal end continues degeneration (denervation atrophy)

nerve regeneration step 2

Schwann cell neurolemma, endoneurium & basal lamina form regeneration tube, produce cell-adhesion molecules and nerve growth factors

nerve regeneration step 3

regeneration tube guides growing sprout back to original target cells

nerver regeneration step 4

nerve body shrinks and returns to original appearance when synaptic contact is established, reinnervated muscle fibers regrow

neve regeneration step 5

PNS nerves can regenerate if ____ intact and some ____ remains

cell body; neurolemma

which is required for nerve fiber regeneration

endoneurium

CNS nerves can/cannot regenerate

cannot

how long can it take for nerves to regenerate

up to 2 years

charge difference across plasma membrane

resting membrane potential (RMP)

RMP value of unstimulated neuron

-70 mV

to have potential or voltage; separation of electrical charge across cell membrane so that one side is more positive and the other is more negative

polarized

when at rest, high concentrations of K⁺ are located in the ____ fluid

intracellular

when are rest, high concentrations of Na⁺ are located in the ____ fluid

extracellular

what type of transport is responsible for stablishing this gradient

sodium-potassium pumps

what direction would Na⁺ and K⁺ passively diffuse after they are pumped by Na⁺/K⁺ pump if able to

against their concentration gradient

negative membrane potential indicates

interior of cell is more negatively charged than exterior

unmyelinated axons are found

in CNS and PNS

potassium-sodium pumps ____ Na⁺ ion(s) for every ____ K⁺ ion(s)

3; 2

any such case in which the voltage shifts to a less negative value

depolarization

short-range change in voltage where incoming Na⁺ diffuses along inside of plasma membrane creating a wave of depolarization

local potential

graded or vary in magnitude; decremental or get weaker as they spread from point of origin; reversible, if stimulation ceases cation diffuses out of cell; excitatory or inhibitory (less likely to produce action potention)

characteristics of local potentials

a hyperpolarized neuron is

inhibited

a hyperpolarized neuron is caused by

influx of K⁺ ions

rapid voltage change where plasma membrane briefly reverses electrical polarity producing traveling wave of excitation in nerve and muscle cells

action potential

local current arrives at axon hillock; depolarizes membrane

action potential step 1

local potential rises to critical voltage (threshold), about -55mV

action potential step 2

neuron fires; Na⁺ channels open quickly, depolarization spike; K⁺ channels open more slowly

action potential step 3

as voltage peaks K⁺ channels fully open and exit cell repolarizing cell

action potential step 4

more K⁺ leaves cell than amount of Na⁺ entering so membrane voltage drops more negative than original RMP (hyperpolarization)

action potential step 5

if ftimulus depolarizes neuron to threshold, neuron fires at max voltage or doesn’t fire at all

all-or-none law

follow all-or-none law; nondecremental or don’t get weaker with distance;irreversible

characteristics of action potentials

period of resistance to restimulation during action potential and a few seconds after

refractory period

refractory period lasts from start of action potential untel membrane returns to resting potential

absolute refractory period

refractory period that lasts until hyperpolarization ends

relative refractory period