Electrical properties of neurons Flashcards
What are the components of a typical neuron
- dendrites: receive input
- soma: cell body, protein synthesis occurs
- axon: can be short or long; carries information away from the cell
- presynaptic terminal: releases NT, and synapses with other neurons
Types of Neurons
- multipolar
- bipolar
- Pseudounipolar neuron
Multipolar neuron
- Multipolar neurons have many dendrites and a single axon.
- The neuron transmits information from the spinal cord to skeletal muscle (mostly)
- mostly motor neurons
Bipolar neuron
- used for special senses
- taste, smell
Pseudounipolar neuron
- a neuron that transmits information from the periphery into the central nervous system.
- These neurons are unique in having two axons:
~ a peripheral axon that conducts signals from the periphery to the cell body
and a
~central axon that conducts signals into the spinal cord.
Axoplasmic Transport
- the cellular mechanism that transports substances along an axon
- occurs in two directions
Anterograde
- fast
- substances required by the axon travel from soma to presynaptic terminal
- carries information (NT) towrad the presynaptic terminal
- Neurotransmitter produced in cell body
Retrograde
- material travels from presynaptic terminal to soma
- travels at variety of speeds
- gives information about what is going on at the terminal
How can travel speeds vary
- slows with aging and neurodegenerative diseases
Electrical transmission: what makes an electrical potential
- electrical potential exists when the distribution of ions creates a difference in electrical charge on each side of the cell membrane
- neurons function with rapid changes in electrical potential across the cell membrane
Ion channels
- Openings through membranes
- allows ions to pass through membranes
types of ion channels
A. Leak-allow diffusion of a small number of ions through membrane at a slow continuous rate; responsible for maintaining resting membrane potential
B. Ligand-gated—Opens in response to neurotransmitter binding (Chemical=neurotransmitter)
C. Voltage-gated—Opens in response to change in electrical potential– Important in formation of action potentials
D. Mechanically gated—Opens in response to mechanical forces Ex: stretch, touch, pressure, temp change
Electrical potentials in neurons
- how are they created
- what happens when channels in the membrane open?
- created by difference in ion concentration on each side of the cell membrane
- as soon as transmembrane channels open, ions will move through and potential energy will be converted to kinetic energy in the form of electrical current
Types of potentials essential for transmission of information
- resting membrane
- local potentials
- action potential
Resting membrane potential
- the difference in the electrical potential between the interior and exterior of the neuron when its “resting”
- essential to neuron function
- (-70mV) inside the cell
- electrochemical gradient
How is the electrochemical gradient maintained in resting membrane potential
- Passive diffusion of ions through leak channels
- The sodium/potassium (Na+/K+) pump
- Negatively charged molecules (anions) trapped inside the neuron because they are too large to diffuse through the channels
Describe the charges in resting membrane potential as well as the ions involved
- At rest, nerve cells are positively charged on the outside and negatively charged on the inside.
- The differences in charge across the membrane results from the differences in the distribution of positive ions and negative ions across the membrane
- more K+ inside the cell (salty banana)
- Na/K pump: pumps 2 K into the cell for every 3 NA out of the cell
Changes from resting potential
caused by and types of changes
- caused by ion flow through channels
- type types:
1. local potential
2. action potential
Local potential
- initial change in membrane potential
- spreads short distance across the membrane
Categories of local potentials
based on where it comes from
- receptor potential: sensory neurons; generated at sensory receptor (modality-gated and ligand-gated channels)
- synaptic potential: motor neurons; generated at post-synaptic membrane (result of stretch, compression, deformation, or exposure to thermal or chemical agents)
Types of local potential changes
- depolarization: less negative/closer to 0; this needs to occur for an action potential to occur
- Hyper polarization: no action potential, inhibitory; goes further from 0
what are the Local potential summation types
Based on how they come to the post-synpatic terminal
- A single weak input to a cell = slight depolarization
- temporal summation: several inputs that come rapidly one after the other
- spatial summation: several adjacent inputs that occur at the same time
Action potentials
- rapid movement of information over long distances
- Large depolarizing signal: actively propagated along axon by repeated generation of signal
- all-or-none: amplitude does no vary
- freq. of signal changes the movement or sensation
Stages of action potential
- resting potential
- slow depolarization
- fast depolarization
- repolarization
- hyper polarization
Stages of action potential: Resting potential
Voltage gated Na+ and K+ closed
Stages of action potential: slow depolarization
- local potentials summate to depolarize the membrane
voltage-gated Na+ and K+ channels remain closed
Stages of action potential: Fast depolarization
- when the threshold potential is reached, voltage-gated Na+ channels open and Na+ rushes in
- the membrane quickly depolarizes to a positive membrane potential
Stages of action potential: repolarization
- voltage-gated Na+ channels are inactivated
- many voltages-gated K+ channels are OPEN
- K+ exists taking positive charges out of the axon
Stages of action potential: Hyperpolarization
- Voltage-gated K+ channels remain OPEN
- K+ continues to leave the axon, restoring the polarized membrane potential
Saltatory condution
- action potential can jump from one node of Ranvier to another
- Myelination/axon diameter increases speed of action potential
- AP slows at Nodes of Ranvier (charge is stored here in prep for generating an action potential.
Afferent axons
- carry sensory information
- go from the periphery to the spinal cord
efferent axons
- carry motor information
- go from the spinal cord to the periphery
Glial cells
- astrocytes
- oligodendrocytes
- schwann cells
- microglia
Oligodendrocytes
- CNS
- envelop several axons from different neurons
Schwann Cells
- envelop 1 or more axons
- work in PNS
Myelin Sheath
- Glial cells from myelin sheath
- insulates axon - critical for condition of info n the nervous system
- prevents leakage across the axon membrane
- increase speed of action potentials
Astrocytes
- star-shaped
Functions:
- communication with other astrocytes or neurons
- form blood brain barrier
- remove excess potassium, Neurotransmitter, debris
- connect neurons and capillaries
- pathway for migrating neurons especially with development
- “glue” of the nervous system/holds everything together
What is astrocytoma
- form of cancer which originates in a star shaped brain cells (astrocytes) in the cerebrum
Microglia
- phagocytes
- activated after injury, infection, disease
- defending cells
Satellite cells
- glial cells that cover somas in the PNS and regulate extracellular environment
- found only in the dorsal root ganglia, sympathetic ganglia and parasympathetic ganglia
- increasing evidence suggests that they contribute to the pathology of various pain conditions
Neuroinflammation: beneficial
- microglia and astrocytes remove debris, promote myelination and axon regeneration
- microgliaand astrocytes can be/cause the inflammation
Neuroinflammation harmful
- excessive neuroinflammation can cause microglia and astrocytes become excessively activated
- can lead to Alzheimers, Parkinson’s, MS, ALS and post stroke
- may clean up too much stuff not just debris
Clinical application: Gillian barre syndrome
basic picture of what it is
- inflammation and demyelination of PNS
- Schwann cells attacked
- autoimmune
- typically occurs post-virus
clinical application: peripheral neuropathy
what is affected
- Schwann cell demyelination
Clinical application: multiple sclerosis
- CNS demyelination of brain and spinal cord autoimmune
synaptic transmission
Describe the events
- action potential arrives at presynaptic terminal
- presynaptic membrane depolarizes
- vesicles move toward membrane
- vesicles bind with membrane and neurotransmitters are released
- NT crosses synaptic cleft and binds to postsynaptic membrane receptor
- receptor changes shape - membrane channel changes shape and ions enter postsynaptic cell
Post synaptic potentials
- caused by ligand gated ion channels interacting with NT
- local changes in ion concentration across the post-synaptic membrane
- when a NT binds to a receptor that opens an ion channel on post synaptic membrane causes local depolarization or hyperpolarization
EPSP
- excitatory post synaptic potential
- After NT binds with receptor on postsynaptic membrane:
- Local depolarization: EPSP
- Summation of these can lead to an action potential
- Example: At the synapse between a neuron and muscle cell, the neuron releases the neurotransmitter Acetylcholine, there is a Na+ influx into the mm cell , a mechanical contraction of the muscle cell occurs.
IPSP
- Inhibitor post synaptic potential
- Local hyperpolarization:
IPSP—decreases possibility of an action potential - Flow of Cl- makes the cell more negative and therefore inhibits the action potential.
Neurotransmitters
- chemicals that convey information among neurons
Clinical application: myasthenia gravis
- autoimmune disease
- normal amount of Ach released but too few receptors exist for binding
- repetitive use of the muscle leads to increased weakness
- no lock
Clinical application: Lambert Eaton Syndrome
- autoimmune disease
- neuromuscular junction is attacked at pre-synaptic terminal
- decreased AcH released
