Anatomy of ieurons and Glia

  • Nervous system contains 2 kinds of cells:
    1. ieurons: receive info and transmit it to other cells
    2. Glia: many functions
  • Brain is composed of individual cells; small gap that separates tips of one neuron’s fibers from the surface of next neuron (~ 100 billion neurons) The Structure of an Animal Cell
  • Neurons have a lot in common with the rest of the body cells.
  • Membrane (Plasma membrane): surface of a cell; separates inside and outside of cell; 2 layers of fat molecules (phospholipids). Most molecules can’t cross membrane, some specific ion channels exist for important molecules.
  • All animal cells have a nucleus – except for mammalian red blood cells
  • Mitochondrion: performs metabolic activities, provides energy for cell, requires fuel and oxygen to function
  • Ribosomes: cell synthesizes new protein molecules; provides building material for cell and facilitate chemical reactions
  • Endoplasmic reticulum: network of thin filaments that transport newly synthesized proteins to other locations The Structure of a ieuron
  • Shape of a neuron can vary; they have long branching extensions  Larger neurons have 4 components:
    1. Dendrites
    2. Cell body
    3. Axon
    4. Presynaptic terminals
  • Tiny neurons lack axons, and some well-defined dendrites

Motor neuron: soma in spinal cord, receives excitation from other neurons through the dendrites and conducts impulses along axon to muscle

  • Sensory neuron: specialized at 1 end to be highly sensitive to certain stimulation
  • Dendrites: branching fibers, get more narrow at end; surface lined up with specialized synaptic receptors that receive info from other neurons. Some contain dendritic spines, outgrowths that increase SA for synapses
  • Cell body: contains nucleus, ribosomes, mitochondria. Covered with synapses on its surface in many neurons
  • Axon: fibre of constant diameter, sends info, convey impulse toward other neurons
  • Myelin sheath: insulates axons, interruptions called iodes of Ranvier.
  • Presynaptic terminal: Axon has many branches swelling at the tip; where axon releases chemicals that cross through the junction  A neuron can have many dendrites, but only one axon.
  • Afferent axon: bring info into a structure
  • Efferent axon: carries info away from structure
  • Every sensory neuron is afferent to rest of NS and every motor neuron is an efferent from NS. A neuron is efferent from one and afferent to another structure.
  • Interneuron/intrinsic neuron: if a cell’s dendrites and axon are entirely contained within a single structure Variations Among ieurons
  • Vary in size, shape and function; shape determines connections with other neurons and determines its function/contributions to the nervous system. More branching = connect with more targets Glia (ieuroglia)
  • Don’t transmit over long distances, smaller and higher in number than neurons  Several types of glia:
    1. Astrocytes: (star-shaped) wrap around presynaptic terminal of group of functionally related axons. Take up ions released by axons, release them back to axons  helps synchronize activity of axons  send messages in waves. Remove waste material created when neurons die, control blood flow to each brain area; during periods of heightened activity in some brain areas  dilate blood vessels so more nutrients travel to that area

 

  1. Microglia: small cells; remove waste materials, viruses, fungi, other microorganisms; function like immune system
  2. Oligodendrocytes: in brain and spinal cord
  3. Schwann cells: periphery of body; build myelin sheaths that surrounds and insulate vertebrate axons
  4. Radial glia: guide migration of neurons, axons, and dendrites during embryonic development. Most will differentiate into neurons or other glia The Blood-Brain Barrier
  • Brain needs nutrients from the blood, most chemicals can’t cross the blood to brain

Why we need a blood-brain barrier

  • When a virus invades a cell, cell will extrude virus particles through membrane so that the immune system can find them.
  • When immune system cells identify a virus, they kill it and cell that contains it
  • To minimize risk of brain damage  body builds a wall along sides of brain’s blood vessels  keeps out viruses, bacteria and harmful chemicals
  • Some viruses that invade brain can lead to death, some killed by glia, but a virus that enters the NS will stay with you for life How the Blood-Brain Barrier Works
  • Depends on endothelial cells that form walls of capillaries
  • Outside brain, cells separated by small gaps; in brain, they are joined close so nothing passes. Barrier keeps out useful and harmful chemicals  For brain to function  body needs mechanism to get chemicals across  Mechanisms are:
    1. Small uncharged molecules cross freelyo Special protein channels in wall of endothelial cells (water)
    2. Molecules that dissolve in fats of membrane cross passively
    3. Active transport: protein-mediated process, expends energy to pump chemicals from the blood to the brain
  • Essential to health; people with Alzheimer’s or similar conditions, endothelial cells lining brain’s blood vessels shrink and harmful chemicals enter brain iourishment in Vertebrate ieurons
  • Glucose  vertebrate neurons depend entirely on it
  • Metabolic pathway requires O2; brain uses 20% of O2 consumed

Glucose is only nutrient that crosses the blood-brain barrier after infancy; except for ketones

  • Glucose deficiency is rare (only inability to use); to use glucose  body needs vitamin B1 (Thiamine)
  • Korsakoff’s syndrome  death of neurons by prolonged thiamine deficiency

(alcoholism)  severe memory impairments  

 

THE iERVE IMPULSE

  • Axon doesn’t conduct electrical impulse, it regenerates impulse at each point; impulse travels without weakening; axons transmit info at moderate speeds; properties of impulse conduction in axon adapted to exact needs of info transfer in NS
  • In vision, brain needs to know whether one stimulus began slightly before/after another one unlike touch sense

The Resting Potential of the ieuron

  • Neuronal messages develop from disturbances of the resting potential
  • All parts of membrane covered by membrane composed of 2 layers of phospholipid molecule; among phospholipids are protein molecules, so chemicals can pass.
  • Structure flexibility and firmness, controls flow of chemicals
  • Electrical gradient (polarization): difference in electrical charge between inside and outside of cell; neuron inside membrane has –ve charge compared to outside
  • Resting potential: difference in voltage in a resting neuron
  • Measure resting potential by inserting microelectrode into the cell body; a reference electrode outside the cell completes the circuit; typical level -70mV Forces Acting on Sodium and Potassium Ions
  • If charged ions could cross membrane freely  would depolarize
  • Selectively permeable: membrane; chemicals pass freely  O2, CO2, H2O through channels; large or electrically charged ions don’t cross; Na/K/Cl cross through membrane channels
  • When membrane is at rest, Na channels are closed, K almost closed
  • ia-K pump: transports 3 Na ions out of cell, drawing in 2 K ions in cell
  • Active transport that requires energy. Na more concentrated outside, K inside
  • Effective due to selective permeability of membrane
  • Selective permeability prevents Na to come back in; some K leak out

(carrying +ve charge)  increases electrical gradient

  • When neuron at rest 2 forces push Na into the cell
    1. Electrical gradient: Na+ wants to enter -ve cell; Na +ve charged and inside –ve charged
    2. Concentration gradient (difference in distribution of ions across a membrane); Na higher outside cell  wants to come in, but Na channels closed when cell is at rest, so no Na+ enters the cell
  • K+ electrically wants to move in, but K more concentrated inside cell  gradient drives it out; if K channels open, K would leave in small quantities. Na/K pump pulls more K into cell
  • Negative anions inside the cell are responsible for membrane’s polarization (Cl-) Why a Resting Potential?
  • Prepares neuron to respond rapidly
  • Exciting neuron opens channels that allow Na to enter cell fast (as membrane maintained concentration gradients for Na already) The Action Potential
  • Action potential: messages sent by axons
  • When a membrane is at rest, there is a –ve potential inside cell
  • Hyperpolarization: further increase of –ve charge; increased polarization; stimulation ends  charge returns to original resting level
  • Depolarization: neuron reduces polarization toward 0
  • Threshold of excitation: produces massive depolarization of membrane; opens Na+ channels, Na+ floods into cell; rapid depolarization  reversal = action potential that peaks at +30mV
  • Sub-threshold stimulation: small response proportional to the current

Molecular Basis of the Action Potential

  1. Start  Na+ mostly outside, K mostly inside
  2. When membrane depolarized  Na/K channels open 3. At peak of AP  Na channels close
  • Membrane has many channels that can open or close
  • Voltage-gated channels: regulates Na/K; permeability depends on voltage difference across the membrane,

At rest, Na channels are closed, K channels are almost closed

  • Opening of K channels does little difference because electrical and concentration gradient almost balanced; Na channels make a big difference, as both gradients allow Na to enter
  • When depolarization reaches threshold, Na channels open wide  Na enters cell rapidly until there is a reversed polarization  Na channels shut
  • When many Na ions cross membrane, inside of the cell is slightly +ve  K driven out of the cell  temporary hyperpolarization; membrane returns to resting potential; inside has more Na and less K; Na/K pump restores the correct gradient (excessive Na build up toxic to cell)
  • Local anaesthetic attach to Na channels, prevent Na from entering cell; stop APs The All-or-ione Law
  • AP starts in an axon  propagates without loss along an axon
  • Once started, “back-propagate” to cell body and dendrites  don’t conduct AP’s in same way as axons, but passively register electrical event
  • When voltage across axon membrane reaches threshold, voltage-gated Na channels open, Na depolarizes membrane 
  • All AP are equal in intensity and velocity, all-or-none law: amplitude and velocity of AP independent of intensity of triggering stimulus.
  • AP may vary between neurons
  • More frequent APs (NOT more intense AP)  greater intensity of stimulus The Refractory Period
  • While potential returning from peak towards rest  still above threshold
  • Refractory Period: Right after AP, resists production of new AP (Na channels are closed, K flowing outside the cell at a fast rate)
  • Absolute refractory period: membrane can’t produce AP, regardless of stimulation
  • Relative refractory period: stronger than usual stimulus  initiates AP

 Propagation of the Action Potential

  • In motor neuron, AP starts at axon hillock  where Na enters axon. This spot is temporarily +ve compared to other areas on axon  +ve ions flow to nearby regions, slightly depolarizing next area  area reaches threshold, opens Na+ channels  AP regenerates at that point
  • AP near center of axon doesn’t trigger another AP in areas it passed already because those areas are in refractory period

 

Area of axon reaches threshold  Na + K channels open  K channels little affect at 1st  Na channels opening causes Na to rush into cell  +ve charges flow down axon, open nearby Na channels  at peak of AP, Na channels close (depolarization)  K channels open  K ions flow out (regular potential)  K channels close

 The Myelin Sheath and Saltatory Conduction

  • Increasing diameter of an axon  faster conduction/velocity
  • Myelin: insulating material composed of fat and protein
  • Myelinated axons: those covered with myelin sheath; interrupted periodically with Nodes of Ranvier
  • After AP occurs at node, Na+ enters axon, push ions to next node  regenerate AP
  • Saltatory conduction: jumping of AP from node to node; rapid conduction of impulses, conserves energy Local ieurons
  • Axons produce AP
  • Neurons without axons exchange info with only close neighbors  local neurons
  • Don’t follow all-or-none law because no axons; after receiving info from other neurons, it has a graded potential: membrane potential that varies in magnitude in proportion to intensity of stimulus
  • Change in membrane potential conducted to adjacent areas of cell, weaken with time; areas of cell contact other neurons, then excite/inhibit other neurons through synapses