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