Sensory Mechanisms and Neuron Synapses Flashcards
What are sensations
- are triggered by sensory stimuli
- travel to brain as action potentials (APs) via sensory pathways
- awareness of sensory stimuli
What are three problems with sensory mechanisms that the body must overcome
Conversion of stimulus energy into a neuronal signal
Encoding information about stimulus
Interpretation of information
What are the three parts of information processing
- Information processing
- sensory input
- external and internal information from sensory receptors
- Integration
- interpretation of input
- association of input with responses
- Motor output
- signals from integration centre(s) to effector cells
- sensory input
What do you call neurons that travel to / from the CNS
- Afferent neurons: from periphery to the central nervous system (CNS)
- efferent neurons: from CNS to the periphery
What is the solution to the problem of converting stimulus energy into a neuronal signal
Problem: conversion of stimulus energy into a neuronal signal
Solution: Transduction
Stimulus → sensory receptors → change in receptor membrane permeability → receptor potential (graded change in membrane potential)
Amplification (e.g. by activation of a cascade) and sensory adaptation (continued stimulation → lowers responsiveness)
Transmission:
If receptor = sensory neuron: it conducts APs to CNS
if receptor ≠ sensory neuron: sensory neuron generates APs
how does chemoreception work
Sugar molecule activates sweet receptor → G protein → Phospholipase C → PIP2 → IP3 (second messenger) → IP3-dated calcium channel → sodium channel → sodium is taken in
Perceptions of taste and smell are usually interrelated
what are the two ways a receptor can exist
A receptor can be an afferent neuron, or it can regulate an afferent neuron through neurotransmitter release in the receptor cell to the afferent neuron
What are the four solutions to the problem of Encoding information about stimulus
Problem: Encoding information about stimulus
Solution: Type of stimulus
Type of activated receptor
Solution: Intensity
Number of activated receptors
Frequency of action potentials
Solution: Location
location of activated receptors
timing of receptor activation (for sound and smell)
Solution: duration
Patter of action potentials
what are the receptor types
- chemoreceptors
- mechanoreceptors
- thermoreceptors
- nociceptors
- electromagnetic receptors
what are the solutions to the problem of interpretation of information
Problem: Interpretation of information
Solution: Process and integrate sensory information, starting in the sensory pathways and culminating in the brain
Solution: Hierarchical and parallel processing of information
Solution: different parts of the brain process different perceptions
Solution: incorporation of information from different modalities in higher association centres
what is perception
- ability to discriminate various aspects of the stimulus
- meaningful interpretation of sensory data
what did Luigi Galvani say/do
Luigi Galvani (~1750): The role of electricity in nerves was first observed in dissected frog muscle, led him to propose the theory of animal electricity.
What did Alessandro Volta think
Alessandro Volta: electrical current is generated by contact between different metals.
what did Alessandro Volta do
Dissimilar metals create electricity, which merely stimulates the frog muscle.
That animal tissue creates electricity is non-sense! (According to Alessandro Volta) (wrong)
led to creation of batteries.
what did the 1794 experiment reveal
The 1794 experiment: when the surface of sections of the nerve touches the muscle the leg contracts.
what did the 1797 experiment reveal
The 1797 experiment: when the surface of a section of the right sciatic nerve touches the intact surface of the left sciatic nerve, both legs contract
what did Galvani say
According to Galvani … in the animal, there is a particular machine capable of generating such disequilibrium, and it will be convenient to refer to this form of electricity as to animal electricity
what did Carlo Matteucci do
Only 40 years after Galvani did Carlo Matteucci, a physicist at Pisa ~ convincingly demonstrated the existence of animal electricity using a ‘galvanometer’
what is membrane potential
Every cell has voltage (difference in electrical charge) across its plasma membrane called a membrane potential
what is resting potential
The resting potential is the membrane potential of a neuron not sending signals
what is an action potential
Changes in membrane potential is used for transmitting information across large distances
This brief, all or none signal is called the action potential
what is the state at resting potential
- K+ selective ion channels are the main (but not only) channels that are open at rest
- Neurons maintain a certain concentration gradient across their membranes which is different for each ion. E.g. KCl is higher inside and lower outside
- K+ diffuses out, down its concentration gradient
- Negative charges (Cl-) builds up along the inner membrane creating an opposing electrical force
- At equilibrium, both electrical and chemical forces are balanced (Equilibrium potential)
How does Na+/K+ ATPase maintain the Na+/k+ gradients
- Pumps 3 Na+ out, 2 K+ in per 1 ATP
- Energy use: ~20-30% of body’s resting energy
- Brain demand: ~60% of neuronal ATP
- Discovery: Jens Skou (1957, Nobel Prize 1997) in crab nerves
- Maintains resting potential. Essential for nerves, muscles, heart
- Drug target: Digoxin (heart medication) blocks it and Oubain (a plant that makes arrow poison in Somalia) targets it
What is the Nernst Equation what what does it tell us
Ex(mV)=RT/zF *ln([x]out/[x]in)=61.5log([x]out/[x]in)/z
At T = 37ºC you can reduce RT/F to 61.5 mV when converting ln to log form; still need to account for z (valence)
Given: [K+]in = 120 mM, [K+]out = 4.5 mM
Ek=61.5log(4.5/120)=61.5log(0.0375)=-88mV
Typical resting membrane potential for neurons ~ -80mV to -65 mV
How does K+ drive Vm
Driving Force: (Vm-Ek+)
At -78mV, it is depolarized and K+ tries to flow out: DF = (-78mV)-(-88mV) = + 10 mV
At -88 mV, it is at equilibrium and K+ tries to flow equally in and out: DF = (-88mV) - (-88mV) = 0
At -98 mV, it is hyper polarized and K+ tries to flow in: DF = (-98mV) - (-88mV) = -10 mV
Depolarized = more positive than resting potential
Hyper polarized = more negative than resting potential
How does ion flow change membrane potential
- Flow of ions (i.e. current) cause the membrane potential to change
- Changes that push the membrane potential above rest is called depolarization
- Changes that cause the membrane to move below the neurones resting membrane potential is called a hyper polarization
- Each Ion has its own equilibrium potential (EK = -90mV; ENa = + 40 mV)
how does a membrane and channel work in regards to current
Current through channels depends on resistance
V = I * R
I = V / R
The membrane acts as a capacitor, stores a charge when it can’t go through the insulating hydrophobic region
Channels act as a resistor
what is the Time constant
Time Constant (t):
- the time taken for cell voltage to reach approximately 63% of its final value.
- t = resistance * Capacitance
What are Graded potentials
- Graded potentials are changes in polarization where the magnitude of the change varies with the strength of the stimulus (analogue signals)
- Graded signals cannot be propagated through great distances
How does hyper polarization occur
- When positive ions move out of the cell (or negative ions move into the cell), the membrane becomes hyper polarized
- E.g. hyper polarization occurs if K+ channels open (Ek = -80mV) or Cl- channels open Ecl = -75 mV
How does depolarization occur
- When positive ions move into the cell (or negative ions move out), the membrane becomes depolarized
- For example, depolarization occurs if Na+ channels open and Na+ diffuses into the cell
how do action potentials work
- A depolarization above a certain threshold results in a massive change in membrane voltage called an action potential (AP).
- APs have a constant magnitude, are all-or-none, and may transmit signals over longer distances.
- They arise because some ion channels are voltage-gated, opening or closing when the membrane potential passes distinct levels.
what is neuron structure
- Most neurons have dendrites, highly branched extensions that receive signals from other neurons
- Signals are graded and funnelled to the soma (cell body), where most of a neuron’s organelles reside.
- The soma extends into a cone-shaped structure called the axon hillock. This is the site of AP generation
- The axon is typically a much longer extensions that can transmit ATPs
What are the steps of an action potential
APs rely on voltage-gated Na+ and K+ channels with special properties
- Resting state: At the resting membrane potential (RMP), most voltage-gated sodium (Na+) and potassium (K+) channels are closed.
- Slow depolarization: Bring the cell to threshold where Voltage-gated Na+ channels are activated
- Depolarization: A positive feedback loop ensures a rapid rising phase of the action potential
- Re-polarization: Voltage-gated Na+ channels inactivate; and slower voltage-gated K+ channels open, and K+ flows out of the cell, leading to the falling phase of the AP
- Hyper polarization: Cell goes to EK
- Return to rest
What is a refractory period
Since the membrane permeability to K+ is higher than at rest, the membrane potential undershoots the RMP
Hyperpolarization helps reset Na+ channels, enabling them to recover from inactivation
In this phase neurons are refractory, and cannot initiate a second AP
As voltage-gated K+ channels close the resting potential is restored
Why do APs only travel in one direction
- Where an action potential is generated, at the axon hillock, an electrical current spreads and depolarizes the neighbouring region of the axon membrane, causing the cycle to repeat.
- Inactivated Na+ channels behind the zone of depolarization prevent the action potentials from traveling backwards
- Therefore APs travel in only one direction: towards the synaptic terminals (also may back propagate to soma/dendrites)
what do myelin insulators do
- The speed of an action potential increase with the axon’s diameter
- In vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increase.
- Myeline sheaths are made by glia-oligodendrocytes in the CNS and Schwann cells in the PNS
what is saltatory conduction
- Action potentials are formed only at nodes of Ranvier, gaps in the myeline sheath were voltage gated Na+ channels are found
- Action potentials in myelinated axons jump between nodes of Ranvier in a process called saltatory conduction.
how do you code stimulus intensity
A gentle stimulus causes a low frequency of action potentials per receptor
A higher stimulus causes a high frequency of action potentials per receptor
what is the Enigma machine
The Enigma machine is an encryption device developed an used in the early to mid 20th century to protect commercial, diplomatic, and military communication. It was employed extensively during WW2 in all branches of the german military
Alan Turing devised a machine to break the German Enigma code during WW2 that helped end the war early
