Physiology 1, Fall 2008, LPC

Chapter 8 - Neurons: Cellular and Network Properties

I-Organization

            CNS = brain & spinal cord
            PNS = afferent & efferent neurons
                        Efferent
                                    Somatic motor neurons -> skeletal muscles
                                    Autonomic neurons
                                                Sympathetic
                                                Parasympathetic
                                                Neurons of enteric nervous system

II- Cell

            Neuron – dendrites, cell body, axon hillock, axon, presynaptic axon terminal
            Synapse – where signal is transmitted from cell to cell

Presynaptic axon terminal, synaptic cleft, postsynaptic dendrite, postsynaptic neuron

A. Cell body – responsible for keeping cell alive & manufacturing essential proteins

B. Dendrites – thin, branched processes that receive incoming messages

C. Axons – carry outgoing signals to target

Originate at axon hillock
End in axon terminals
Neurotransmitters made in cell body, transported to terminal via fast axonal transport
Neurotransmitters released by exocytosis. Synaptic vesicle transported back to cell body for recycling or digestion

D. Glia Cells

Schwann cells (PNS) & oligodendrocyte (CNS) -> myelin sheath

Satellite cells (PNS) & Astrocytes (CNS) -> support

Astrocytes – blood-brain barrier, neurotrophic factors, take up K+ & neurotransmitters

Ependymal cells – -> barriers between compartments, source of neural stem cells

Microglia – immune function

 

III – Electrical Signals in Neurons
           
            Resting membrane potential depends on combination of:

1. concentration gradient of each ion
2. membrane permeability for each ion

K+, Na+, Cl- are relevant ions
Permeability to Na+ low at rest, increases during depolarization
Permeability to K+ increases soon after depolarization -> hyperpolarize neuron
Very few ions move -> change membrane potential

Gated Channels control ion permeability of neuron
Four types – N+, K+, Ca++, Cl-
Three types of stimuli:

1. mechanically gated – sensory, ie, stretch, pressure, etc
2. chemically gated – respond to neurotransmitters, intracellular molecules, etc
3. voltage-gated – respond to changes in membrane potential

Various thresholds
Activation = channel opening
Inactivation = channel closing

Current – flow of electrical charge

K+ = usually out of cell
Na+, Cl-, Ca++ usually flow into cell

           Graded Potential
                        Potential directly proportional to strength of triggering event
                        Travel or short distance

Lose strength as travel through cell
            Current leak
            Cytoplasmic resistance to signal

Subthreshold graded potential -> insufficient signal at trigger zone to
produce action potential

Superthreshold graded potential -> high enough potential at trigger zone to produce action potential

            Action Potential

1. Stimulus depolarizes membrane
2. Fast Na+ channels open, Na+ enters cell
3. Slower K+ channels open
4. Rapid Na+ entry depolarizes cell
5. Slow Na+ channels close
6. K+ leaves cell, -> hyperpolarization
7. K+ channels close K+ leak slows
8. Cell returns to resting membrane potential

Refractory Period – resistance to firing again. Action potentials cannot overlap or travel backwards.

1. Absolute - second action potential cannot be triggered until Na+ channel gates reset.
2. Relative – second action potential can occur if receive a stronger-than-normal depolarizing graded potential.

Stimulus intensity transmitted by frequency of action potential firing, more frequent action potential -> more neurotransmitter release

Conduction through axon

1. Na+ entry to cell -> relative negative outside membrane & positive in cytoplasm
2. Current flows toward negative areas
3. Current cannot go backwards in axon because of absolute refractory period
4. Strength of action potential does not diminish because of continuous entry of Na+ down ax

        Speed of axon conduction

1. larger axons conduct more quickly
2. myelination -> faster conduction

Saltatory conduction- action potential passes through axon by jumping down axon at Nodes of Ranvier.

1. myelin sheath prevents ion flow
2. Na+ channels only in Nodes

Variety of chemicals can alter electrical activity

1. neurotoxins bind to & block Na+ channels
2. resting membrane potential mainly determined by K+ concentratio

1. hyperkalemia -> higher resting membrane potential and causes cells to fire with smaller potential input

2. hypokalemia -> hyperpolarization, need more stimulus to trigger action potential

 

IV – Cell-to–Cell Communication

            Synapses
                        1. axon terminal of presynaptic cell
                        2. synaptic cleft
                        3. membrane of postsynaptic cell (usually a nerve or muscle cell)

Electrical synapses – pass electrical signal directly from cytoplasm of one cell to another through gap junctions

            Chemical synapses – neurotransmitters carry information from one cell to another
                        1. action potential depolarizes membrane at terminal
                        2. Ca++ enters axon terminal
                        3. exocytosis of synaptic vesicle containing neurotransmitter
                        4. neurotransmitter binding initiates a response in postsynaptic cell

            Neurotransmitters – 7 classes (only 2 below)
                        1. Acetylcholine

                                    a. made in axon terminal
                                    b. attaches to cholinergic receptor
                                    c. rapidly broken down by enzyme
                                    d. choline goes back into axon terminal, -> more acetylcholine

                                    Receptors

Nicotinic – affects skeletal muscles, autonomic neurons, CNS. Work via cation channels, -> easier for postsynaptic cell to fire
Muscarinic – affects smooth & cardiac muscle, endocrine & exocrine glands, CNS. Coupled to G proteins, -> 2nd messenger system, -> open ion channels OR modifies existing proteins or regulates synthesis of new proteins

                        2.  Norepinephrine
                                    made by adrenal medulla

receptors in smooth & cardiac muscle, endocrine & exocrine glands, CNS
            a. alpha & beta receptors
            b. work via G proteins -> different 2nd messenger systems

            Duration of Postsynaptic Response
                        Fast if use ion movement
                                    Excitatory if depolarizing
                                    Inhibitory if hyperpolarizing

Slow if neurotransmitter binds to G protein-coupled receptors & -> 2nd messenger system

            Termination of neurotransmitter activity
                        1. reuptake by axon terminal (norepinephrine) or go to glial cell
                        2. enzymes inactivate neurotransmitter (acetylcholine)
                        3. diffuse away from synaptic cleft

V – Integration of Neural Information Transfer

            Divergence – one neuron branches to affect many
            Convergence –signal from many neurons combined to affect one

Spatial Summation – many different neurons affect one.  Effects summed -> action potential OR inhibition, if inhibitory message stronger than excitation

Temporal Summation – graded potential over-lapping in time

Presynaptic inhibition – an inhibitory neuron blocks neurotransmitter release from a collateral of the presynaptic axon terminal
Postsynaptic inhibition – – presynaptic neuron releases an inhibitory neurotransmitter onto a postsynaptic cell, -> no action potential

Nervous system development

Growth cone of embryonic nerve cells go towards growth factors, molecules in extracellular matrix and signals from target
After synapse formed, maintenance depends on use
Plasticity in nervous system allows it to change throughout life

         Cell body necessary for other parts of neuron to survive
                        If cell body dies, whole neuron dies
                        If axon severed, distal part dies