Nervous Tissue

neurons

electrically excitable cells that transmit nerve impulses. structure = body, axon, dendrites. characteristics = long-lived, amitotic, High metabolic rate. function is nerve impulse transmission

neuroglia

cells that surround neurons and promote proper functioning

5 neuroglia cells

1. astrocytes (CNS)

2. microglia (CNS)

3. ependymal cells (CNS)

4. oligodendrocytes (CNS)

5. schwann cells (PNS)

astrocytes

connects neurons and capillaries

microglia

substitues for immune cells which are prevented from entering CNS ( uses phagocytosis)

ependymal cells

line the cavities of brain and spinal cord, involved in production and circulation of cerebrospinal fluid

oligodendrocytes

form myelin sheaths in CNS

schwann cells

form myelin sheaths in PNS

nerve cell body

contains nucleus and organelles. axon hillock = cone-shaped area that generates nerve impulses (action potentials)

dendrites

receptive regions of the neuron, receive imput from sensory receptors and transmit toward the axon hillock as graded potentials

axons structure

long axons = nerve fibers
telodendria = branched end of an axon
boutons = contain nuerotransmittiers

axons function

transmit action potentials and secrete neurotransmitters when the action potential reaches the bouton

myelin sheaths structure

white, segmented sheath around most long axons

function of myelin sheath

increase speed of nerve impulse transmission

nodes of ranvier (neurofibril node)

gaps in the myelin sheath between adjacent Schwann cells

saltatory conduction

along a myelinated axon depolarization occurs only at the nodes of ranvier. action potentials jump from one node to the next = rapid conduction

connective tissue wrapping

1. endoneurium = innermost, outside every nerve

2. perineurium = outside of small bundles

3. epineurium = surrounds outside of the entire nerve

electrical current in the body

• flow of electrolytes through ion channels in plasma membrane

• at rest Na+ is higher outside the cell; K+ higher inside.

• plasma membrane normally prevents flow of electrolytes but when ion channels open they are free to flow in/out

ion channels

1. passive channels = always open, electrolytes leak in/out

2. chemically-gates channels = open when specific neurotransmitter binds

3. voltage-gated channels = open/close in response to membrane potential; tend to be very specific

Operation of a Voltage-Gated Ion Channel

closed when the intracellular enviroment is negative and open when the intracellular environment is positive

what happens when gated channels open

electrolytes move quickly down their electrical and concentration gradients

depolarization

inside of membrane becomes less negative

repolarization

membrane returns to its resting membrane potential

hyperpolarization

inside of the membrane becomes more negative

important membrane potential values

1. resting membrane potential = -70mV

2. threshold for generating action potential = -50mV

3. membrane potential during transmission of an action potential = +30mV

how membrane potential changes

movement of sodium and potassium ions across the plasma membrane (through the ion channels)

graded potentials

short-lived, local changes in membrane potential and is generated on the dendrites and cell body which causes a decrease in strength over distance. distance traveled depends on the strength of the stimulus

action potentials (nerve impulses)

electrical signals transmitted along the entire length of the axon. brief reversal of membrane potential occurs sequentially along entire length of the axon. do not decrease in strength over distance “all or none”

4 phases of action potential

1. resting state

2. depolarization

3. repolarization

4. sodium/potassium pump

resting state of action potential

sodium and potassium channels closed, RMP = -70mV, sodium trapped outside, potassium trapped inside

depolarization of action potential

• threshold to generate action potential is reached(-50mV)

• voltage-gated sodium ion channels open and sodium rushes in, while the potassium channels remain closed.

• location where nerve impulse is transmitted

repolarization of action potential

sodium channels close and sodium becomes trapped inside the cell but voltage-gated potassium ion channels open and potassium exits the cell

Na+/K+ pump

3 Na+ out and 2 K+ in and it returns sodium and potassium to resting locations

refractory periods

occurs when the neuron is unresponsive or less responsive to normal stimuli. neuron is either:

1. transmitting a nerve impulse

2. not completely repolarized

absolute refractory period

occurs during depolarization and part of repolarization when the neuron cannot generate an action potential no matter the strength of stimulus. this ensures each action potential is a separate event

relative refractory period

occurs during hyperpolarization where the threshold is elevated but a very strong stimulus can generate action potential

synapses

junction that facilitates transmission from one neuron to another neuron and an effector cell

chemical transmission across synapse

1. action potential reaches bouton and stored neurotransmitters are released into the synapse

2. diffuse across synapse

3. bind to receptors on an effector cell (or another neuron)

4. binding opens chemically-gated ion channels

enzymatic degradation

broken down by enzymes

reuptake

moved back into vesicle in presynaptic neuron and is reused