Post on 17-Jan-2016
THE NERVOUS SYSTEM: NEURAL
TISSUE
• Neurons
• Neuroglia
Cells in Nervous Tissue
• about half the volume of cells in the CNS• smaller than neurons• 5 to 50 times more numerous• do NOT generate electrical impulses• divide by mitosis• 2 types in PNS
– Schwann cells– Satellite cells
• 4 types in the CNS– Astrocytes– Oligodendrocytes– Microglia– Ependymal cells
Neuroglia (Glia)
• Largest of glial cells• Star shaped with many processes projecting from the cell body• Help form and maintain blood-brain barrier• Provide structural support for neurons• Regulate the chemical/ion environment for generation of nerve impulse• Regulate nutrient & ion concentrations for neuron survival• Take up excess neurotransmitters
Astrocytes
Oligodendrocytes
• Most common glial cell type
• Each forms myelin sheath around the axons of neurons in CNS
• Analogous to Schwann cells of PNS
• Form a supportive network around CNS neurons
Microglia
• Small cells found near blood vessels• Phagocytic role - clear away dead cells• protect CNS from disease through phagocytosis of microbes• migrate to areas of injury where they clear away debris of
injured cells - may also kill healthy cells
• few processes• derived from mesodermal cells that also give rise to monocytesand macrophages
Ependymal Cells
• line ventricles of the brain & central canal of spinal cord
• produce & circulate the cerebrospinal fluid (CSF)
• CSF = colorless liquid that protects the brain and SC against
chemical & physical injuries, carries oxygen, glucose and other necessary chemicals from the blood to neurons and neuroglia
• epithelial cells arranged in asingle layer• range in shape from cuboidalto columnar
Cells of the CNS
• Flat cells surrounding PNS neuronal bodies• hold the cell bodies together to form a ganglion
PNS: Satellite Cells
PNS: Schwann Cells
• produces part of the myelin sheath surrounding an axon in the PNS
• contributes regeneration of PNS axons
Cells of the PNS
Neurons•have the property of electrical excitability - ability to produceaction potentials or nerve impulses in response to stimuli
Representative Neuron
1. cell body or soma -same components of a typical eukaryotic cell
-e.g. nucleus, Golgi, mitochondria-Nissl bodies -rough ER & ribosomes for protein synthesis-cytoskeleton of neurofilaments and microtubules to give neuron it’s shape and to move neurotransmitters to the terminals
http://www.horton.ednet.ns.ca/staff/selig/Activities/nervous/na1.htm
Neurons
2. Cell processes = dendrites (little trees)- the receiving or input portion of the neuron-short, tapering and highly branched-surfaces specialized for contact with other neurons
3. Cell processes = axon• conducts nerve impulses away from cell
body to another neuron
• joins the cell body at a cone-shaped elevation = axon hillock
• nerve impulse arises at a region of the axon hillock = trigger zone
• cytoplasm = axoplasm
• plasma membrane = axolemma
• side branches = collaterals arise from the axon
• axon and collaterals end in fine processes called axon terminals
• swollen tips called synaptic end bulbs contain vesicles filled with neurotransmitters
Classification of Neurons• neurons can be classified based on:
– their shape – e.g. multipolar, bipolar, unipolar
– who identified them – e.g. Purkinje
– function
• Sensory (afferent) neurons– transport sensory information from skin, muscles, joints, sense organs &
viscera to CNS
• Motor (efferent) neurons– send motor nerve impulses to muscles & glands
• Interneurons (association) neurons– connect sensory to motor neurons
– 90% of neurons in the CNS
The Nerve Impulse: Terms to know• membrane potential = electrical voltage difference measured across the
membrane of a cell– results from the build-up of negative ions in the cytosol along the inside of the neuron’s
PM– the outside of the PM becomes more positive– this difference in charge can be measured as potential energy – measured in millivolts
• resting membrane potential = membrane potential of a neuron measured when it is unstimulated– ranges from -40 to -90 mV
The Nerve Impulse: Terms to know
• polarization – change in membrane potential• 1. depolarization – increase in MP away from resting• 2. hyperpolarization – decrease in MP away from resting• 3. repolarization – “return to resting membrane potential”
Ion Channels
• ion channels in the PM of neurons and muscles contributes to their excitability
• when open - ions diffuse down their concentration gradients
• some ion channels are permanently open – non-gated channels
• some ion channels possess gates to open and close them – gated channels
• two types: ligand gated & voltage gated
Ion Channels
2. Gated channels: open and close in response to a stimulusA. voltage-gated: open in response to change in voltage - participate in the AP
B. ligand-gated: open & close in response to particular chemical stimuli (hormone, neurotransmitter, ion)C. mechanically-gated: open with mechanical stimulation
1. Leakage (non-gated) or Resting channels: are always open, contribute to the resting potential
-nerve cells have more K+ than Na+ leakage channels -so K+ leak channels contribute more to resting membrane potential than Na+ leak channels-leaking ions are pumped back to where they belong
• Resting membrane potential is -70mV
• AP triggered when the membrane potential reaches a threshold usually -55 MV
• if the membrane potential exceeds that of threshold Action Potential
Action Potential
• action potential = nerve impulse• takes place in two stages: depolarizing phase (more positive) and repolarizing
phase (more negative - back toward resting potential)• followed by a hyperpolarizing phase or refractory period in which no new AP
can be generated http://www.blackwellpublishing.com/matthews/channel.html
Action Potential
2.
5.
6.
7.
8.9.
10.
• 1. neuron is at resting membrane potential (resting MP)
• 2. neuron binds neurotransmitters via ligand-gated sodium channels
• 3. channels open & Na diffuses into neuron = depolarization
– inside of neuron (i.e. MP) becomes more positive
• 4. if neuron depolarizes enough & reaches Threshold Action Potential (AP)
• 5. 1st stage of AP – opening of voltage-gated Na channels
• large diffusion of Na+ ions into neuron = BIG depolarization
– membrane potential goes from negative to positive
• 6. closing of VGNa channels & opening of voltage-gated K channels
• 7. BIG outflow of potassium through VGK channels = repolarization– inside of neuron (MP) becomes more negative
• 8. closing of VGK channels BUT so much K+ has diffused out – neuron’s MP goes past resting and hyperpolarizes
• 9. neuron is hyperpolarized – no new AP can be generated with a normal stimulus
• 10. all voltage-gated channels closed, Na/K pump “resets” ion distribution to resting situation
1. 3.4.
Continuous versus Saltatory Conduction
• Continuous conduction (unmyelinated fibers)– action potential spreads continuously
over the surface of the axolemma
– as one section of the axon is depolarized, the membrane potential of the next section is depolarized toward threshold
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter45/animations.html#
Saltatory Conduction
• Saltatory conduction-depolarization happens only at Nodes of Ranvier - areas along the axon that are unmyelinated and where there is a high density of voltage-gated ion channels-action potential “jumps” from node to node
http://www.blackwellpublishing.com/matthews/actionp.html
Synapses• Synapse: Site of intercellular communication between 2 neurons or between a
neuron and an effector (e.g. muscle – neuromuscular junction)• Permits communication between neurons and other cells– Initiating neuron = presynaptic neuron– Receiving neuron = postsynaptic neuron
• You can classify a synapse according to:
– 1. the action they produce on the post-synaptic neuron – excitatory or inhibitory
– 2. the mode of communication – chemical vs. electrical
• Electrical
– Direct physical contact between cells required
– Conducted through gap junctions
– Two advantages over chemical synapses• 1. faster communication – almost instantaneous
• 2. synchronization between neurons or muscle fibers– e.g. heart beat
Synapses
Chemical Synapse
http://www.lifesci.ucsb.edu/~mcdougal/neurobehavior/modules_homework/lect3.dcr
• Most common type of synapse– Membranes of pre and postsynaptic
neurons do not touch– Space = Synaptic cleft• Most are axon terminal dendrite• Some are axon terminal axon
Chemical Synapse
http://www.blackwellpublishing.com/matthews/nmj.html
• the AP cannot travel across the cleft – release of neurotransmitters• 1. arrival of action potential in the synaptic end bulb• 2. opening of voltage-gated calcium channels – influx of Ca2+ into end bulb• 3. docking of synaptic vesicles with NTs with plasma membrane – release of NTs into
synaptic cleft• 4. binding of NT to ligand-gated channels – channels open• 5. diffusion of Na+ ions into post-synaptic membrane• 6. depolarization of post-synaptic neuron – if the NT is excitatory• 7. depolarization to threshold Action Potential
• if the neurotransmitter is an inhibitory NT - then the post-synaptic neuron will hyperpolarize rather than depolarize
• NO ACTION POTENTIAL!!!
The Neuromuscular Junction
• the motor neuron’s synaptic terminal is in very close association with the muscle fiber• distance between the bulb and the folded sarcolemma = neuromuscular junction•neurotransmitter released = acetylcholine
https://www.youtube.com/watch?v=7wM5_aUn2qs