Chapter 6 Human Physiology

Structures & Functions of Neural Tissue

Overview

• Central Nervous System: Brain and spinal cord
• Peripheral Nervous System: Nervous structures that aren’t the brain and spinal cord
• The functional unit of the nervous system is the neuron

The Neuron

• The neuron is made up of the cell body, debdrites, and axon
  • 
• Soma: The cell body of the neuron that includes the nucleus and organelles
• Dendrites: The spikes of a neuron that increase receiving surface
• Axon: The main stem of a neuron that carries output “message” to target cells
  • Initial segment: The beginning of the stem of the neuron, part of the axon
  • Axon collateral branches: The extra branching on the axon of a neuron
  • Axon terminals: The ending “base” of the axon of the neuron; also valled varicosities
• Some axons are myelinated
  • Muelination: 20 to 200 layers of modified plasma membrane, formed by glial cells
  • Glial cells: Non-neuronal cells (i.e. not nerves) of the brain and nervous system that provide support (physical and metabolic) with a capacity for cell division
  • Oligodendrocytes: The glial cells of the CNS
  • Schwann Cells: The glial cells of the PNS
  • Nodes of Ranvier: specialized axonal segments that lack myelin, allowing the saltatory conduction of action potentials
  • Myelination occurs to insulate the axons (prevent ion leakage) and speed up conduction velocity

Functional Classes

• Afferent neurons: Neurons that relay sensory information to the CNS
• Efferent neurons: Neurons that relay commands away fron the CNS
• Interneurons: Neurons that connect afferent and efferent neurons
  • Interneurons are all part of the CNS.
  • 99% of all neurons are interneurons

The Synapse

• Synapse: The junction between two neurons
• The synapse releases neurotransmitters
• Presynaptic neuron: The neuron that releases a neurotransmitter
• Postynaptic neuron: The neuron that receives a neurotransmitter

Neural Support & Maintenance

• 
• Axon damage & regeneration
  • In the PNS, Schwann cells can help with axon regeneration
  • In the CNS, there is no significant regeneration.

Membrane Potentials

Overview

• Membrane potential: Charge inside the cell relative to the extracellular fluid

Resting Membrane Potential

• A chemical gradient exists for certain diffusible ions, which causes charge to move across the cell membrane
  • Sodium and Potassium (& also chloride and calcium)
  • Some ions can move more than other ions
• Ion permeabilities can differ, allowing some ions to have bigger effects on membrane potential than others
• The [Na+] gradient tends to create membrane potential of +60 mV
• The [K+] gradient tends to create membrane potential of -90 mV
• The actual resting membrane potential is about -70mV
• Sodium
  • Sodium is in much higher concentrations outside of the cell because of ion pumps
  • Equilium is reached when the concentration gradient pushing sodium in is equal to the electrical gradient that pushes sodium out
  • +60 mV equilibrium potential for sodium
• Potassium
  • Potassium is in much higher concentrations inside of the cell because of ion pumps
  • Equilium is reached when the concentration gradient pushing out potassium is equal to the electrical gradient that pushes sodium in
  • -90 mV equilibrium potential for potassium
  • More leak channels for potassium than sodium, so overall charge is more influenced by potassium

Graded Potentials

• At rest, a membrane potential is polarized (is negative at resting)
• Graded potential: Local change in membrane potential due to the opening/closing of ion channels, takes place in the dendrites and the cell body (recieving parts)
• A graded potential can cause either…
  • Depolarization, or excitatory post-synaptic potential, is when the graded potential moves the overall potential closer toward 0 from the initial -90
  • Sodium and calcium are likely depolarizing ions
  • Hyperpolarization, or inhibitory post-synaptic potential, is when the graded potential moves the overall potential further away from zero (more negative)
  • Potassium and chloride are likely hyperpolarizing ions
• The size of the potential change is proportional to the size of the stimulus
  • Bigger stimulus = opens/closes more ion channels
• Spatial summation: many graded potentials located close together on a neuron
• Temporal summation: Multiple, repeated graded potentials coming from the same source over a short period of time

Action Potentials

• The action potential is the long-distance communication mechanism of the body, and is a large, rapid change in membrane potential
• The purpose of most graded potentials is to influence action potentials
• If the sum of the graded potentials reaches a threshold potential at the initial segment, axons will generate axon potentials
• Action potentials are always the same magnitude regardless of the size of initiating stumuli (it’s all or none!)
• Action potentials can propagate along an excitable membrane wihtout decreasing in size
  • An action potential depolarizes the membranes around it to the threshold, which sets off new action potentials
  • So it’s not a local response that moves along, it’s more of a flame travelling across gunpowder
  • Backwards propagation is prevented by the refractory period
  • Velocity of action potential propogation depends on axon diameter (larger = faster) and myelination (more = less conduction)
• Voltage-gated ion channels are required for an excitable membrane (aka a membrane that can do an action potential)
• Refactory periods: Times during which it is difficult or impossible to fire a second action potential at a point on the neuron
  • Absolute refactory period: The period from the firing of the action potential to repolarization
  • Relative refractory period: The period from repolarization to the end of after-hyperpolarization