from graded potentials to action potentials

human anatomy and physiology lecture 4

components of the central nervous system

brain and spinal cord

functions of the central nervous system

where sensory and motor signals are integrated and coordinated, where higher functions like consciousness, memory, and emotion manifest

components of the peripheral nervous system

sensory receptors, peripheral nerves, all neural tissue outside the brain and spinal cord

divisions of the peripheral nervous system

afferent (sensory) division and efferent (motor) division

afferent division

communicates incoming sensory signals with sensory neurons

efferent division

communicates outgoing motor signals with motor neurons

types of autonomic signals

sympathetic signals (fight-or-flight) and parasympathetic signals (rest-and-digest)

major functions of the nervous system

• monitor internal and external environments

• integrate sensory information

• coordinate responses from all organ systems

major cell types of the nervous system

neurons and neuroglial cells

neurons

cells specialized for the propagation of electrical signals for communication, do not divide in adults

neuroglial cells

support the neurons (regulating neuronal environment, protecting the neurons from mechanical trauma or infectious threats, providing a physical framework for the neurons), divide as needed

action potentials

electrical impulses transmitted by neurons

cell body of a neuron

contains the nucleus and most organelles

dendrites of a neuron

projections off the cell body that receive signals

axons of a neuron

projections off the cell body that transmit incoming messages away from the cell

Nissl bodies

stained rough endoplasmic reticulum in the cell body of a neuron

grey matter

part of the central nervous system containing the neuronal cell bodies

axon hillock

start of the axon, identified by the absence of Nissl bodies

collaterals

branches off the axon of a neuron

axon termini

branched ends of the axon of a neuron which synapse onto other neurons or effectors

direction of electrical impulses

dendrites → cell body → axon

axolemma

axon plasma membrane

axoplasm

axon cytoplasm

basic pattern of nervous impulse

1. stimulation

2. summation

3. generation

4. propagation

5. transmission

stimulation of a nervous impulse

receptors or neurotransmitters open pores to let ions cross the cytoplasmic membrane, changing the charge/voltage of the neuron

summation of a nervous impulse

ions entering/leaving the neuron accumulate at the axon hillock to make a big enough difference in charge/voltage

generation of a nervous impulse

the activation threshold is reached, an action potential is generated

propagation of a nervous impulse

movement of the action potential along the axon, from the axon hillock to the synaptic terminals

transmission of a nervous impulse

the release/exocytosis of neurotransmitter from the synaptic vesicles in the synaptic terminals of the presynaptic neuron to stimulate the dendrites of the postsynaptic neuron

resting potential

membrane potential of a resting cell

graded potential

temporary, localized change in resting potential caused by stimulus

action potential

electrical impulse produced by the summation of graded potential

simple diffusion

can diffuse through the membrane unaided

facilitated diffusion

large or polar molecules need help of a protein transporter to diffuse across the membrane

diffusion

substrate moves from high concentration to low concentration, does not require energy

primary active transport

uses metabolic energy to move substrate against their gradient (from low concentration to high concentration)

Na⁺/K⁺ pump

pumps 3 Na⁺ ions outside the cell and 2 K⁺ ions inside

resting membrane potential of a neuron

negatively charged (-70mV)

use of membrane potential for sensory signals

sensory signals stimulate sensory neurons by changing their membrane potential

use of membrane potential for motor signals

motor signals cause changes in membrane potential in effectors, causing muscle contraction or exocytosis of glandular cells

leak channel

always open

gated channels

need to be stimulated to be opened

chemically-gated channels

opened once they’re bound to a molecule

ligand

molecule that binds to a chemically-gated channel to open it

mechanically-gated channels

opened by physical forces

voltage-gated channels

opened when the membrane potential reaches a certain value

electrochemical membrane potential

sum of chemical and electrical forces

electric potential

separate charges on either side of membrane result in potential difference, charges want to move to eliminate the potential difference

chemical potential / chemical gradient

concentration gradients of ions

threshold value to produce an action potential

-55mV

depolarization of a neuron

entrance of sodium ions into the cell, causing the membrane potential to increase

repolarization of a neuron

exit of potassium ions from the cell, causing the membrane potential to decrease to resting levels

hyperpolarization of a neuron

while repolarizing, membrane potential reaches -70mV but overshoots to -90mV

closed setting of a voltage-gated Na⁺ channel

prevents ions from moving through the channel, remains closed until threshold is reached

open setting of a voltage-gated Na⁺ channel

allows ions to move through the channel down their concentration gradient

inactivated setting of a voltage-gated Na⁺ channel

prevents ions from moving through the channel, but the channel can’t be re-opened from this position

absolute refractory period

no new action potential can be triggered between when repolarization starts and hyperpolarization ends

relative refractory period

during repolarization when the voltage reaches close to the threshold, voltage-gated Na⁺ channels are reset to closed and can be opened if the threshold is reached again (only able to be initiated by a very large stimulus)

refractory period

from the beginning of an action potential to return to resting state, time in which a neuron will normally not respond to additional stimuli

excitatory neurotransmitters

bind to neurotransmitter receptors and open chemically-gated sodium channels

inhibitory neurotransmitters

bind to neurotransmitter receptors and open chemically-gated potassium channels

result of differing stimulus strength

high intensity results in high frequency rather than amplitude

continuous propagation

propagation in grey matter, only action potentials are propagated

saltatory propagation

propagation in white matter

myelin sheath

created by neuroglial cells to help propagate signal faster by preventing the loss of ions

node of Ranvier

depolarized region between sections of the axon

factors affecting speed of propagation

diameter of axon and degree of myelination

information conducted through large, fast axons

afferent information about things that threaten survival or from the skin, somatic motor commands that prevent injury