Ch 39 - Neurons and Synapses

What are the functions of the nervous system? (5)

detects changes in the external and internal environment

center of mental activity - thought, decision making, learning and memory

coordinates with endocrine system to maintain homeostasis

responds to sensory information by issuing motor responses

body’s communication system

what is included in the PNS?

all neuronal tissue outside the CNS. cranial and spinal nerves

describe the pathway of information processing

stimulus → sensory receptors → afferent neurons (sensory) → integration center (CNS) → efferent neurons (motor) → effector (muscles and glands)

what is contained in the cytoplasm?

nucleus, mitochondria, RER (nissl’s bodies), neurofibrils

sensory - (afferent/efferent) neurons

afferent

motor - (afferent/efferent) neurons

efferent

what are the functional classes of neurons?

afferent (sensory), efferent (motor), interneurons

Describe the process of myelination

schwann cell first surrounds a portion of the axon within a groove of its cytoplasm → schwann rotates → myelin wound around the axon in multiple layers

What is the role of the myelin sheath in propagation of neural impulse?

myelin insulates the axon to speed up conduction. in between sheaths are gaps for the action potentials to jump across

What is the importance of nodes of ranvier?

allows for saltatory conduction

what is a synapse onto a muscle called?

a neuromuscular junction

where is the action potential (electric current) converted into a chemical signal (neurotransmitter)?

at the synapse

describe an electrical synapse

ions can flow from one cell to another via gap junctions. plasma membranes are in direct contact. rapid signal conduction

where are electrical synapses located?

retina, cardiac muscle

describe a chemical synapse

presynaptic and postsynaptic neurons separated by a synaptic cleft. Neurotransmitter release, diffuses across the cleft and binds to ligand gated ion channels at the postsynaptic cell membrane, generating an electrical impulse there.

list the event occurring at the chemical synapse

Arrival of AP, opening of voltage gated Ca²⁺ channels → Ca²⁺ increase, high intracellular concentration → binding of Ca2+ to proteins that attach vesicles to the membrane → vesicles fuse and release → Ca²⁺pumped back out by active transport → removal of NT by enzymatic degradation

AchE

breaks down Ach into acetate and choline

What factors contribute to the membrane potential?

difference in the ionic concentration between the cytosol and the extracellular fluid

what is the resting membrane potential of a human neuron?

-70mV

describe the relationship between ions and resting membrane potential.

ion channels keep the concentration of ions on each side of the membrane different

Na⁺ concentration is higher ___ the cell

outside

K⁺ concentration is higher ___ the cell

inside

what is the role of leaky ion channels and sodium-potassium channels in maintaining the resting membrane potential?

leaky ion channels are passive channels that allow ions to diffuse along the concentration gradient to maintain the resting membrane potential.

Na/K channels maintain the concentration gradients of sodium and potassium ions on either side of the membrane

Na/K pumps allow ___ sodium ions ___ and ___ potassium ions ___

3 sodium ions out, 2 potassium ions in

the role of ions in resting membrane potential

1. K+ diffuse down their concentration gradient (out of cell) via leaky channels. loss of K+ results in a negative charge on the inner plasma membrane face

2. K+ also move into the cell because they are attracted to the negative charge established on the inner plasma membrane face

3. a negative membrane potential is established when the movement of K+ out of the cell equals the movement of K+ into the cell. Now, the concentration gradient promoting K+ exit opposes the electrical gradient for K+ entry

cells respond to stimuli by changing the ____

resting membrane potential

passive vs active ion channels

passive ion channels - leaky channels → always open

active ion channels - gated channels → open or close in response to specific stimuli

describe the 3 types of active ion channels

chemically-gated or ligand-gated: respond to binding of a ligand

voltage-gated: respond to changes to voltage or potential difference

mechanically-gated: respond to changes in mechanical pressure or vibration

graded potential vs action potential

graded - a temporary and localized change in the resting membrane potential

action - a propagated electrical potential along the surface of the axon. does not diminish as it moves from the source

describe each phase of an action potential

depolarization - any shift from the resting membrane potential to a more positive potential (opening of voltage gated Na channels)

repolarization - to normal resting potential after depolarization

hyperpolarization - any shift from resting to more negative

continuous propagation of AP

occurs along unmyelinated axons, in a series of small steps

saltatory propagation of AP

occurs along myelinated axons, in leaps from node to node. action potential occurs only at nodes of ranvier. faster, require less energy

endogenous chemicals that allow neurons to communicate with each other throughout the body

neurotransmitters

principal excitatory neurotransmitter in the brain

Glutamate

major inhibitory neurotransmitters

GABA (gamma-aminobutyric acid) and glycine

released at neuromuscular junction

acetylcholine

what is NO

nitric oxide, a gaseous neurotransmitter

what is an EPSP?

excitatory postsynaptic potential. results from NT binding causing gated Na+ channels to open and cause depolarization.

what is an IPSP?

inhibitory postsynaptic potential. results from neurotransmitter binding to voltage-gated potassium/chloride channels and causing hyperpolarization

summation of ESPS

the process of individual EPSPs combining to integrate the effects of all graded potentials by summation. can be temporal or spatial

temporal summation

same location, different times

spatial summation

same time, different locations