Nervous Tissue

initial segment

part of axon closest to the axon hillock

most impulses arise at junction between axon hillock and initial segment (trigger zone)

action potential occurs here

multipolar neurons

have several dendrites and one axon

most neurons in brain and spinal cord are these kinds of neurons

bipolar neurons

have one main dendrite and one axon

unipolar neurons

have dendrites and one axon that fuse together to form a continuous process that emerges from cell body

epineurium

perineurium

endoneurium

connective tissue layers surrounds neuron

epineurium

dense irregular CT that surrounds the entire nerve

perineurium

dense irregular CT that surrounds fascicles (bundles of axons)

endoneurium

areolar CT that separates and insulates each axon

glial cell properties

support, nurture and protect neurons and maintain the interstitial fluid that bathes them

smaller than neurons, but 5-50 times more numerous

they do not generate or propagate action potentials

can multiply and divide in the mature nervous system

oligodendrocytes

form and maintain myelin sheath around CNS axons

ependymal cells

produce, monitor and assist in circulation of cerebrospinal fluid

myelin sheath

insulates axons and increases the speed of nerve impulses conduction

pumps

move substances against their concentration gradients, requires energy

channels

move a substance down its concentration gradient, passive process

receptive segment

dendrites and cell body

chemically gated channels found here (K+ and Cl-)

initial segment

axon hillock

voltage-gated potassium and sodium channels

conductive segment

length of axon

voltage gated Na+ and K+ channels

transmissive segment

synaptic knobs

voltage-gated calcium channels and calcium pumps

receptive segment

initial segment

conductive segment

transmissive segment

functional segments of neurons

resting membrane potential

the relative difference in charge across the plasma membrane of a resting, excitable cell

resting membrane potential (RMP) of a neuron has a negative value —> -70 millivolts

depolarization

occurs when the inside of a cell becomes more positive than the resting membrane potential (+30mV)

occurs when gated channels open, allowing positively charged Na+ ions into the neuron

repolarization

occurs when the inside of the cell returns to a negative value (-70mV)

occurs when gated channels open, allowing positively charged K+ ions to move out of cell

graded potential

occurs in the receptive segment of the neuron (dendrites, cell body), decreases in intensity

action potential

generated in the initial segment of the axon and propagated along the axon

due to the opening of voltage gated channels if the threshold value (minimum voltage) is reached