Lecture 5: Physiology of Nerve Transmission Flashcards
What are Neurons?
- Receives information and convey it to other neurons
- Receives and transmits information to other cells.
- Adult brain: 100 billion neurons.
What are Glia?
- Cells serve many supporting functions, do not convey information over large distances.
- Many functions, support Neurons in their functions.
- Smaller but more numerous than neurons.
- Many supportive functions.
- Do not transmit information across long distances.
- Astrocytes
- Help synchronize the activity of neurons.
- Remove waste material.
- Microglia
- Remove waste materials and microorganisms (viruses, fungi)
- Oligodendrocytes & Schwann cells
- Build the myelin sheaths.
- Radial Gila
- Guide the migration of neurons and the growth of their axons.
- Differentiate into neurons when development is over.
What is Soma (cell body)?
*Contains nucleus, ribosomes, mitochondria.
What is Dendrites?
*Branching, fibres, receive info via synaptic receptors.
What is Axon?
*Thin fibres of constant diameter, information sender to the neuron.
What is Myelin Sheath?
*Insulates the axon. Nodes of Ranvier: interruptions of the myelin sheath.
What is Presynaptic terminal (end bulb, bouton)?
*Point at which the axon transmits information.
What are Afferent Axons?
*Brings info into a structure.
What are Efferent Axons?
*Carries info away from a structure.
Are sensory neurons Afferent or Efferent?
*Afferent to the rest of the nervous system.
Are Motor Neurons Afferent or Efferent?
*Efferent from the nervous system.
What protects the nervous system?
- Skull (brain), backbone (spinal cord), meninges (membrane surrounding brain and spinal cord).
- Blood brain barrier
- prevents viruses, bacteria, and harmful chemicals from entering the brain.
- Depends on the endothelial cells that forms the walls of the capillaries.
- What is stopped by the barrier?
- Most viruses, bacteria, toxins, large molecules, electrically charged molecules.
- Some useful molecules e.g. fuels and amino acid, the building blocks of proteins.
- What passes the barrier?
- *Passively (no expenditure of energy).
- Small uncharged molecules (o2m co2).
- Water
- Fat- soluble molecules (vitamins A and D, various drugs).
- *Activity (expenditure of energy)
- glucose
- Amino Acid
- Certain Vitamins
How is information transmitted in the nervous system?
- Neurons conduct information via electric impulses.
* Speed
What is resting potential?
- At rest, neurons are more negative inside than outside.
- Voltage difference called the resting potential
- Typical resting potential - 7 millivolts.
- Results from an unequal distribution of negatively and positively charged particles.
- *Negatively charged
- Chloride ions (CI)
- Proteins.
- *Positively charged.
- Sodium ions (Na)
- Potassium ions (k)
What 4 interacting factors maintain resting potential?
1) Concentration gradients
* Ions are in random motion.
* Move form areas of high concentration to areas of low concentration.
2) Electrical Gradient
* Accumulation of changes repels like charges and attracts unlike charges.
3) Membrane permeability at rest
* Cl and K pass
* Na+ passes with difficulty.
4) Sodium - potassium pump
* Transports Na+ out and K in
* Requires energy.
What is action potential?
- The response of a neuron to stimulation above a certain threshold.
- Resting potential prepares the neuron to respond quickly to a stimulus with an AP.
- Stimulation results in depolarization - the neuron becomes less negative.
- Stimulation that results in a depolarization beyond the threshold of excitation results in a sudden, massive depolarization.
- After the depolarization the potential returns to slightly below the RP and back to the RP value (repolarization).
- Subthreshold stimulation produces depolarization proportional to the strength of stimulation.
- Stimulation above threshold produces always a very similar AP.
- All or now law: amplitude and velocity of an AP are independent of the initiating stimulus.
What is the molecular basis of the action potential?
- Na+ enters the cell.
- K+ leaves the cell
- Na+ K+ pump restores the original ion distribution.
- Immediate after AP, membrane area cannot produce another AP.
- Refractory period.
- Absolute refractory period (1ms) - No AP can be produced.
- Relative refractory period (2-4ms) - Stronger than usual stimulus can initiate as AP.
- Mechanisms:
- Na+ channels closed during the absolute refractory period, but returning to normal during the relative refractory period.
- K+ flowing out of the cells.
What is Neural Coding?
*Firing an action potential is all a neuron can do.
*1/10 system.
*How can the nervous system code for the complexity of our experience?
-Different depending on experience.
- Basic principles
~ Frequency
~ Changes in the population of neurons involved.
What is the Propagation of the Action Potential?
- Starts at the axon hillock.
- Propagates along the axon.
- Mechanisms
- slight depolarisation of adjacent areas of the membrane causes area to reach its threshold.
- AP is generated.
- Direction of propagation determined by refractory period.
- Speed depends on Axon diameter
- thin: up to 1m/s
- thick: up to 10m/s
- myelin sheath increases speed up to 100m/s
- crucial: nodes of ranvier.
- Saltatory conduction: AP jumps from node to node: increases speed & saves energy.
What is the concept of the synapse?
- Late 1800s Ramon y Cajal: existence of a gap separating one neuron from another.
- Early 1900s Charles Scott Sherrington: communication between one neuron and the next differs from communication along a single axon.
- Speed of a reflex and delayed transmission at synapse.
- Side at which the information flows from one neuron to another.
- Small gap between neurons.
- 3 main parts
- presynaptic
- Synaptic cleft or gap
- Postsynaptic neuron.
What are the events at a synapse?
*AP arrives at the presynaptic terminal
*Ca2+ enters, neurotransmitters attach to receptor and alter activity of the postsynaptic neuron.
*Neurotransmitters attach to receptor and alter the activity of the postsynaptic neuron.
Neurotransmitters separate from the receptors.
*Neurotransmitters are taken up by the presynaptic cell or diffuse away.
What is the activation of postsynaptic receptors?
- Neurotransmitters attach to postsynaptic receptors.
- Key - lock system: specific receptor for specific neurotransmitters.
- Ionotropic receptors:
- effect fast and short.
- Neurotransmitter open/closes channels directly.
- Glutamate, Gaba
- Relevant for vision, hearing (quickly changing information).
- Metabotropic receptors
- Effects slow and long
- Neurotransmitters activates G - protein within the cell.
- G - protein binds to ion channels or stimulate synthesis of second messengers.
- Relevant for hunger, thirst, fear, anger (long term changes).
What is the inactivation and reuptake of neurotransmitters?
- Break down by enzymes
- Detachment from receptor without break down.
- Neurotransmitters taken up and reused by presynaptic neuron (reuptake).
What is Temporal and Spatial summation?
- Neurons conduct information through action potentials
- Neurons communicate through synapse.
- One synapse does not initiate an action potential
- Post synaptic potentials are graded potentials (depolarisation below threshold, the more stimulation, the larger the change).
- Graded depolarisation can be depolarising (excitatory) or hyperpolarizing (inhibitory).
- The trick: action potential can be initiated by temporal or spatial summation of excitatory postsynaptic potential (EPSPs)?
What does temporal summation effect?
- summation of EPSPs from one source
- Effects depends on:
- size of EPSPs
- Time between EPSP
- Threshold of postsynaptic neuron.
What does spatial summation effect?
- Summation of several simultaneous EPSPs from separate locations.
- Effects depends on:
- Size of EPSPs
- Time between EPSP (simultaneous)
- Threshold of postsynaptic neurons.
