Information Flow and the Neuron

Animals sense and respond to the environment.  The ability to perceive such danger, for example of an approaching tornado, and take evasive action is dependent on the nervous system.  However to understand how our nervous system works, you should have to start with how it’s neuron functions and its structure.

 

I.                     Cells of the Nervous System

A.     The neuron is the basic unit of communication in the nervous system.

1.      Three Classes

a)      Sensory Neurons are receptors for any specific stimulus to spinal cord and brain.

b)      Interneurons integrate input and output signals in the brain and spinal cord.

c)      Motor Neuron relay information away from the motor brain and spinal cord to the muscle or glands (effectors)

B.     Neuroglia – variety of specialized cells that protect, structurally support, and functionally assist the neuron

 

II.                   Neuron – The Communication Specialists

A.     Functional Zones of a Neuron

1.      The numerous, usually short extension that receives information (input zones) are dendrites.

2.      The usually single, often long extension (conducting zones) that transmits impulses to other cells at its branched endings (output zones) are axons.’

3.      Signals arise in trigger zones


 

 

B.     A Neuron at Rest, Then Moved to Action

1.      Neuron maintains a voltage difference across plasma membrane if not bothered.

a)      The inside, more negatively charged than the outside

b)      The resting membrane potential

2.      (Action Potential) when neuron receives signals, an abrupt, temporary reversal in the voltage difference, the inside becomes more positive.

3.      Stimulation of a neuron disturbs the distribution of electric charge across its plasma membrane.

C.     Restoring and Maintaining Readiness

1.      Three factors resulting membrane potential

a)      The concentration of K+, Na+, and other charged molecules are not the same on the two sides of the plasma membrane.

b)      Channel proteins spanning the membrane actively influence the diffusion of specific types of ions.

c)      Transport proteins spanning the membrane actively pump Na+ and K+ ions.

2.      There are more K+ ions inside and more Na+ ions outside the resting neuron membrane.

a)      Na+ ions have a tendency to leak out through the channel proteins.

b)      Most of the Na+ channels are gated and is close most of the time, keeping the outside high.

c)      Still small amount of sodium do leak in and must be pumped out (K+ pumped in) by the sodium-potassium pump.


 

 

III.                 Action Potentials

A.     Approaching Threshold

1.      Graded – the signals at the input zones vary in magnitude depending on the intensity and duration of the stimulus.

2.      Local – the signal does not usually spread beyond the input zone

a)      If stimulation is strong enough, an adjacent trigger zone may respond.

3.      When minimum threshold level is reached by a stimulus, gated channels open and Na+ rushes in.

B.      All or Nothing Spike

1.      Action potential = all or nothing events

2.      Sodium gates closes, potassium gates open at the membrane site if charged reversal.

3.      Sodium potassium membrane pumps are operational for restoring the resting potential.

C.     Propagation of Action Potentials

1.      Actions potential = self-propagating, don’t diminish in magnitude

2.      In a period after disturbance, membrane remain insensitive to stimulation; gate is shut, potassium gates open.


 


 

 

IV.                 Node to Node Hopping along Sheathed Axons

A.     Axons are wrapped in a myelin sheath, which consists of the plasma membrane of neuroglial cell called schwann cells.

B.     Action potentials move quickly along myelinated axons for they jump from node to node.

C.     With Multiple Sclerosis, myelin sheaths in the spinal cord’s nerve tract deteriorate.

 

V.                   Chemical Synapses

A.     A. Chemical Synapse – is a junction between a neuron and an adjacent cell, separated by a synaptic cleft into which a neurotransmitter substance is released.

1.      Presynaptic cell – neuron that releases the neurotransmitter molecules into the cleft.

a)      Gated proteins open to allow calcium ions enter the neuron.

b)      Calcium causes synaptic vesicles to fuse with the plasma membrane, releasing neurotransmitters into the cleft.

2.      The neurotransmitter binds to receptors on the membrane of the postsynaptic cell.

a)      Have excitatory effects – drive cell’s membrane to the threshold of an action potential.

b)       Have inhibitory effects – drive the membrane away from threshold.

3.      Acethylcholine (Ach) – has both excitatory and inhibitory effects


 

 

VI.                 Synaptic Integration

A.     Smorgasbord of Signals

1.      Serotin

2.      Norepinephrine

3.      Dopamine

4.      GABA

5.      Neuromodulators are substances that enhance or reduce the effects of a neurotransmitter on target neurons.

B.     Synaptic Integration

1.      Excitatory and Inhibitory signals compete at the input zone

a)      EPSP – summation of signals that brings the membrane closer to threshold (depolarizating effect)

b)      IPSP – drives the membrane away from the threshold by hyperpolarizing effect.

C.     Removing Neurotransmitter from the Synaptic Cleft

1.      Some molecules diffuse out; others are actively pumped back into the preynaptic cells by the membrane transport proteins; acetylcholinesterase degrades many.

 

VII.               Paths of Information Flow

A.     Blocks and Cables of Neurons

1.      Neuron circuits or pathways determine the direction a signal will travel.

2.      In the brain, neurons are organized into regional blocks that receive, integrate, and then send out signals.

B.     Signals between brain or spinal cord and regions travel by nerves.

1.      Axons of sensory neurons, motor neurons, or both are bundled together in a nerve.

2.      Nerve pathways, or “tracts”, bundles within the brain and spinal cord

C.     Reflex Arcs

1.      Reflexes are simple, stereotyped movements made in response to sensory stimuli.

In stretch reflex, receptors of sensory neurons (muscle spindles) transmit impulses to the spinal cord where direct synapses with motor neurons occur.

In the withdrawal reflex, interneurons in the spinal cord can activate or suppress motor neurons as necessary for a coordinated response.