1401 L12 Neural Tissue = Neurons + Neuroglia
Aims:
1. Structure & classification of neurons
2. Role of neuroglia
3. Chemical & electrical gradients
4. Stages of action potentials
Introduction:
1. Neural tissue conducts electrical impulses
2. Distributes impulses around the body
3. Majority (98%) found in brain + spinal cord = CNS
4. Two main cell types
i) Neurons
ii) Neuroglia
Structure of Neurons:
1. Large cell body (soma) contains nucleus
2. Slender projection called axon
3. Dendrites branch out of soma - further branches into dendritic spines
4. Axon terminals branch out of axon - ends with synaptic end bulb (synaptic terminal)
Neurons:
1. Mostly no centrioles – cells cannot divide (very important NB)
2. High mitochondrial density (needs lots of ATP)
3. Clumps of RER (rough endoplasmic reticulum) known as Nissl bodies (coarse
granules) Stain grey
4. Neurofilaments & neurotubules replace microfilaments & microtubules
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Structural Classification of Neurons:
1. Multipolar = Two or more dentrites & one axon
2. Bipolar = Distinct dentritic & axon processes with soma between them
3. Unipolar = Fused continuous axon with soma attached
Functional Classification of Neurons:
1. Sensory (afferent) neurons:
i) Relay information from sensory (skin/organs) receptors to CNS
ii) Most Unipolar
iii) Can be very long - Over 1m long from toe to spinal cord
2. Motor (efferent) neurons:
i) Transmit impulses from CNS to effectors organs (muscles & organs)
ii) Multipolar structure
iii) Cell bodies mainly in CNS (brain + spinal cord)
3. Interneurons:
i) Situated between motor & sensory neurons
ii) Act as messengers for impulse signal
iii) Distribute signal through CNS
iv) Make up most of neurons in CNS
v) Most multipolar
Myelin Sheath = Myelinated Axons:
1. Axon covered with Schwann cells
2. Nodes of Ranvier bisect Schwann cells
3. Myelinated axons allow faster (saltatory) conduction compared to non-myelinated
axons
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Conduction Speed: SALTATORY = FASTER = fast bursts between Schwann cells
Activation of Neurons:
1. Neurons extremely excitable & respond to stimuli
2. Once activated impulse runs down axon
3. This is known as action potential
4. Process begins with change in resting potential of neuron
Electricity Basics:
1. Voltage:
i) Measure of potential energy generated by separated charge
ii) Measured in volts (V) or millivolts (mV)
iii) Always measured between two points
iv) Known as potential difference or just potential
2. Current:
i) Flow of electrical charge from one point to another
ii) Depends on voltage & resistance
3. Resistance:
i) Inhibits the flow of current
ii) Insulators have a high resistance
iii) Conductors have low resistance
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Ohm’s Law:
1. Relationship between voltage, current & resistance
Current = Voltage
Resistance
Cell Membrane Ion Channels:
1. To fire a neuron need to create (a charge separation) or potential
2. achieved this by arranging sodium & potassium ions in different concentrations
3. Ion channels open & close to control entry & exit of these ions
Chemical Gradients:
1. Chemical gradients exist across cell membrane
i) Inner (ICF) = high K+ and low Na+
ii) Outer (ECF) = high Na+ and low K+
2. Concentration gradient exists for sodium (Na+) & potassium (K+) across cell
membrane
Sodium-Potassium Pump:
1. Maintains gradient of sodium and potassium ions across membrane
2. Requires energy – ATP
3. For each molecule of ATP:
i) 3 sodium ions pumped out of cell
ii) 2 potassium ions pumped into cell
Electrical Gradient:
1. Electrical gradient exist across cell membrane
i) Inner membrane slightly negative
ii) Outer membrane slightly positive
2. Difference of -70mV
i) Can be -40mV to -90mV
3. This is known as the resting membrane potential
Resting Membrane Potential:
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Changes in Membrane Potential:
1. Cell membrane is dynamic (constantly changing in response to environment)
2. Membrane potential changes as result of ion movement across membrane
3. Three different types of membrane channel for ions
Membrane Channels: Very important (NB)
1. Leakage channels
i) Alternate between open & closed
ii) More potassium (K+) channels than sodium (Na+) channels
2. Voltage-gated channels
i) Open in response to change in membrane potential
3. Ligand-gated channels
i) Open in response to specific chemical stimulus (Ligand = chemical
natural/man-made)
Cell Membrane Ion Channels
Neuron Activation
1. Neurons ‘fire’ when their resting membrane potential is reversed or depolarised
2. This creates an action potential
3. Membrane potential changes from (-70 mV to +30 mV) (100 mV difference)
4. Followed by repolarisation
Action Potential:
1. Initiated by stimulation of dendrites
2. Opens voltage-gated Na+ channels at axon hillock
3. ‘All or nothing’ in function
4. A ‘wave’ of depolarisation runs down axon
Depolarisation:
1. Change in membrane potential by stimuli activates gated Na+ & K+ channels
2. Na+ channels open faster & so more Na+ enters cell
3. Influx of Na+ causes reduction in –ve charge of inner membrane (-70mV to 0mV)
4. Depolarisation in wave-like flow down axon to synaptic terminals (telodendria)
5. Larger diameter axon - increased flow rate
6. Myelinated axon - fastest flow rate – action potential jumps between Nodes of
Ranvier
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Repolarisation:
1. Reducted Na+ influx due to closure of Na+ gated channels
2. Increase in K+ flow out of cell as K+ channels still open
3. Restores –ve charge of inner membrane
4. Hyperpolarisation
i) Increase in –ve charge of inner membrane
ii) Caused by slow K+ channel inactivation
Action Potential Process:
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Synapse:
1. Synapse: a junction where neurons communicate with other cells via
neurotransmitter or electrical current
2. Presynaptic cell - sends message via synaptic terminals on axon
3. Postsynaptic cell - receives message (usually) across synaptic cleft
Neurotransmitters:
1. Chemicals released from neuron that affects another cell’s membrane potential
2. Acetylcholine = Voluntary muscle movement
3. Norepinephrine = Alertness & arousal
4. Dopamine = Pleasure/wanting – associated with addictions
Activation Sequence:
1. Action potential reaches axon terminal & causes calcium (Ca2+) channels to open
2. Ca2+ more conc. (concentrated) in ECF (extracellular fluid)
3. Ca2+ enter axon terminal & trigger exocytosis of synaptic vesicles with
neurotransmitter
4. Neurotransmitter released across synapse cleft & binds to receptors on postsynaptic cell
5. Receptor channels open to allow ions to enter cell
6. Voltage changes due to ion conc
7. When threshold is reached the post-synaptic cell is activated
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Neuroglia:
1. Glia – Gr. meaning ‘glue’
2. Non-neuronal cells in nervous system
3. Provides support, nutritional needs & Myelin
4. Four types of neuroglia:
i) Ependymal cells - line central canal (spinal cord) & ventricles around brain,
secrete cerebrospinal fluid (CSF) & monitor composition of CSF
ii) Astrocytes support endothelial cells in regulating Blood Brain Barrier
iii) Oligodendrocytes: contact exposed surfaces of neurons, help form ‘myelin
sheath’ along axon, myelinated axons permit faster transmission of info
iv) Microglia - help protect CNS via phagocytosis of waste, debris and pathogens
Summary:
1. Neurons & neuroglia make up neural tissue
2. Neurons conduct electrical information
3. Convey sensory or motor data to & from brain or distribute information within brain
4. Rely on action potentials to ‘fire’
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