A&P1: Module 4 Terms

Nervous System

what are the functions of the nervous system?

receiving sensory input, integrating information, controlling muscles and glands, maintaining homeostasis, and establishing and maintaining mental activity

the brain, brainstem, cerebellum, and spinal cord are part of which division of the nervous system?

central nervous system

the cranial and spinal nerves are part of which division of the nervous system?

peripheral nervous system

what does the sensory functional division of the nervous system do?

conducts action potentials from sensory receptors to the CNS

what does the motor functional division of the nervous system do

conducts action potentials to effector organs, such as muscles and glands

somatic motor division function

stimulates skeletal muscles

autonomic motor division function

stimulates cardiac muscle, smooth muscle and glands

neurons

receive stimuli, conduct action potentials, and transmit signals to other neurons or effector organs

glial cells

supportive cells, do not conduct action potentials; carry out different functions that enhance neuron function and maintain normal conditions within nervous tissue

neuron cell body

contains single nucleus

neuron dendrite

extension(s) from cell body, receives information from other neurons and transmits information to cell body

neuron axon

long cell process that branches at terminal, leaves the cell body at at the axon hillock, conducts sensory signals to CNS and motor signals away from CNS

multipolar neuron

have many dendrites and a single axon; they are most of the neurons within the CNS and nearly all motor neurons

bipolar neuron

a dendrite and an axon; they are in some sensory organs like the retina and olfactory cells in the nasal cavity

pseudo-unipolar neuron

found in the sensory system, have single process extending from the cell body and divides into 2 processes; one process extends to the PNS and the other CNS

neuroglial cells

supporting cells for neurons, more numerous than neurons, undergo mitosis

what is an astrocyte?

a star-shaped neuroglial cell that is the most abundant and forms a blood-brain barrier

what is an ependymal cell?

a neuroglial cell that produces and circulates cerebrospinal fluid (CSF)

what is a microglia?

a neuroglial cell that helps remove bacteria and cell debris from CNS; immune cells of the CNS

what is an oligodendrocytes?

a neuroglial cell that produces myelin sheath in CNS

what is a Schwann cell?

a neuroglial cell that produces myelin sheath in PNS

what are myelin sheaths?

specialized layers that wrap around the axons of some neurons, making them myelinated

which cells form the myelin sheath in the CNS?

oligodendrocytes

which cells form the myelin sheath in the PNS

Schwann cells

what is the primary function of myelin?

it acts as an insulator that prevents almost all ion movement across the cell membrane

where can ion movement occur along a myelinated axon?

at the nodes of ranvier - gaps in the myelin sheath that occur about every mm

how does myelination affect action potential conduction?

it increases the speed and efficiency of action potential generation along the axon

how is nervous tissue organized?

it varies in color due to the abundance or absence of myelinated axons

gray matter

consists of groups of neuron cell bodies and their dendrites, where there is very little myelin

where is gray matter located?

cerebral cortex, spinal cord horns, nuclei and ganglia

white matter

consists of bundles of parallel axons with their myelin sheaths that are white in color, neurons headed to/from the same location will travel together, form tracts in CNS and nerves in PNS

what type of properties do all cells have that are evident at their cell membranes?

electrical properties

in most cells, such as muscles and neurons, what is the charge inside the membrane compared to the outside?

the inside is negatively charges relative to the outside, which is positively charged (polarized)

what exists across the cell membrane due to this charge difference?

a small but measurable voltage

what is the typical resting membrane potential of a neuron?

-70mV

what do leak channels contribute to during the resting membrane potential?

the difference in ion concentrations across the membrane

why do ions flow through ion channels?

because of difference in concentration and charge across the membrane

how does the number of K+ leak channels compare to Na+ leak channels?

there are 50-100 times more K+ leak channels than Na+ leak channels

which ion is the resting membrane more permeable to?

potassium (K+)

what is required to help maintain the resting membrane potential?

the sodium-potassium pump

what does the sodium-potassium pump actively transport?

it moves K+ into the cell and Na+ out of the cell

how much of a typical cell’s ATP is used by the sodium-potassium pump?

about 25% of all ATP in a typical cell

how much of a neuron’s ATP is consumed by the sodium-potassium pump?

about 70% of the ATP in a neuron

what property makes nerve cells excitable?

their ability to change the resting membrane potential in response to stimuli

what happens to the resting membrane potential when a nerve cell is stimulated?

it changes in response to the stimulus

how do nerve cells communicate with other cells?

by changing their membrane potential

what are the changes in membrane potential used by nerve cells to communicate called?

action potentials

what causes gated ion channels to open and generate an action potential?

stimuli that activate gated channels, which are normally closed until opened by specific signals

how does the opening and closing of gated ion channels affect the membrane?

it changes the membrane’s permeability to ions, which can alter the membrane potential

what happens when an action potential is generated?

it travels down the axon of the neuron, transmitting the signal

what happens when a stimulus is applied to a nerve cell?

chemically gated Na+ channels open briefly, allowing Na+ to diffuse into the cell

what charge changes occur inside and outside the cell during local potential?

inside of the cell becomes positive and outside the cell becomes negative

what happens if depolarization is not strong enough?

the Na+ channels close, and the local potential disappears without being conducted along the membrane

what happens when depolarization is large enough?

the local potential reaches a threshold value, triggering voltage-gated Na+ channels to open

where do voltage-gated Na+ channels typically open?

at the axon hillock

how much does membrane permeability to Na+ increase during depolarization?

it increases about 600-fold

what is the result of Na+ rapidly entering the cell during depolarization?

the inside of the cell becomes positively shared, causing a brief reversal of charge to +20 mV

what causes Na+ channels to close during repolarization?

the charge reversal to +20 mV inside the cell

what happens when Na+ channels close?

Na+ stops entering the cell

what ion movement occurs during repolarization?

K+ channels open, allowing K+ to leave the cell

what is the overall result of K+ leaving the cell?

the membrane repolarizes, returning toward its negative resting potential

what happens to the cell membrane at the end of repolarization?

the membrane becomes more negative than the resting potential, a condition called hyper polarization 

how long does hyper polarization last?

it occurs briefly at the end of repolarization

can another action potential begin during hyper polarization?

no, another action potential cannot being during this phase

what helps restore the resting membrane potential after hyperpolarization?

the sodium-potassium pump

all-or-none principle

a concept stating that action potentials either occur fully or not at all - there is no partial action potential

threshold

the critical level of depolarization that must be reached for an action potential to occue

if the threshold is reached…

an action potential occurs and travels the entire length of the axon to the terminal

if the threshold is not reached…

no action potential occurs; the neuron does not fire

action potential conduction

the process by which an action potential travels down a neuron’s cell membrane after being generated

continuous conduction

type of action potential conduction that occurs in unmyelinated axons; it is slower because the impulse moves along the entire membrane

saltatory conduction

type of conduction that occurs in myelinated axons; it is faster because the impulse jumps from node of ranvier to node of ranvier

node of ranvier

gaps between myelin sheaths where ion exchange occurs, allowing the action potential to “jump” during saltatory conduction

neuroneuronal synapse

a junction where the axon of one neuron communicates or interacts with another neuron

presynaptic terminal

the end of the axon of the sending neuron; it releases neurotransmitters into the synaptic cleft

postsynaptic membrane

the membrane of the receiving neuron that contains receptors to bind neurotransmitters released from the presynaptic terminal

synaptic cleft

the tiny gap between the presynaptic terminal and the postsynaptic membrane where neurotransmitters diffuse to transmit the signal

postsynaptic membrane channels

channels on the postsynaptic membrane that open or close when neurotransmitters bind to their receptors, altering ion flow and the cell’s response

effect of neurotransmitter binding

the binding of neurotransmitters to receptors opens or closes chemically gated ion channels, depending on the neurotransmitter type and receptor type

possible responses in the postsynaptic cell

the response can be either stimulation (excitation) or inhibition of an action potential

opening of Na+ channels

causes the postsynaptic cell to become depolarized; if threshold is reached, and action potential occurs

opening of K+ or Cl- channels

causes the postsynaptic cell to become more negative (hyperpolarized), inhibiting an action potential from occurring

neurotransmitters

chemical messengers that transmit signals across a synapse from one neuron to another

common neurotransmitters

the two best-known neurotransmitters are acetylcholine and norepinephrine

duration of neurotransmitters after release

act for a short duration because they do not remain in the synaptic cleft

fate of neurotransmitters after release

either broken down by enzymes in the synaptic cleft or transports back into the presynaptic terminal

acetylcholinesterase

an enzyme that rapidly breaks down acetylcholine in the synaptic cleft

norepinephrine removal

either reabsorbed into the presynaptic terminal or broken down by enzymes

spinal cord

extends from foramen magnum to 2nd lumbar vertebra

cauda equina

group of spinal nerves that travel to the pelvis and lower extremities

gray matter in spinal cord cross section

center of spinal rod, looks like letter H or a butterfly, location of cell bodies

white matter in spinal cord cross section

outer layer of spinal cord, contains myelinated fibers

spinal cord white matter columns

contain ascending and descending tracts

ascending tracts

sensory axons that conduct action potentials toward the brain

descending tracts

motor axons that conduct action potentials away from the brain

dorsal horns (gray matter)

contain sensory axons which synapse with interneurons, axons enter spinal cord through dorsal roots

ventral horns (gray matter)

contain motor neurons, axons travel out through ventral roots

lateral horns (gray matter)

contain autonomic neurons

central canal (gray matter)

fluid filled space in center of cord

spinal nerve

nerve that arises from the union of the dorsal (posterior) and ventral (anterior)  roots of the spinal cord