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The Nervous System
master controlling and communicating system of body
Cells communicate by electrical signals that are rapid and cause immediate responses
Sensory input
(Nervous system Function)
monitoring stimuli occurring inside and outside the body
Integration
(Nervous system Function)
interpretation of sensory input
Motor output
(Nervous system Function)
response to stimuli by activating effector organs
The two principal cell types of the nervous system are
Neuroglial – cells that surround and support neurons
Neurons – excitable cells that transmit electrical signals
Anatomy of Neurons
Cell body
Dendrites
Axon hillock
Axon
Myelin sheath
Nodes of Ranvier
Telodendrites (terminal branches)
Axon terminals
Cell body
contains nucleus and organelles
Dendrites
branching extensions
Receptive to neurotransmitters from pre-synaptic neurons and transmit graded potential towards cell body
Axon hillock
where cell body tapers into axon
site where action potential originates
Axon
single process extending from cell body
transmits action potential away from cell body
Myelin sheath
formed by schwann cells
wrapping around the axon
resulting in aligned layers of plasma membrane
Nodes of Ranvier
gaps in myelin sheath
Telodendrites (terminal branches)
distant branches of axon
Axon terminals
enlarged distal ends containing secretory vesicles filled with neurotransmitters
What is a synapses?
junctions between neurons
Function as control or decision point that can be excitatory or inhibitory
Occurs between axon terminals and a cell body, dendrite, axon hillock, muscle or gland
Structure of Chemical Synapses
Presynaptic neuron
Synaptic cleft
Postsynaptic neuron
Presynaptic neuron
transmits impulse towards the synapse, axon terminal with vesicles containing neurotransmitters
Synaptic cleft
fluid filled space between pre and post synaptic neuron
Postsynaptic neuron
transmits impulse away from synapse, contains receptors for neurotransmitters
Types of Ion Channels found in Neurons
Ligand-gated channels
Mechanically gated channels
Voltage-gated channels
Leaky channels
Ligand-gated channels
chemically gated
open when neurotransmitters bind
Found on dendrites, cell bodies, and axon hillocks
Mechanically gated channels
open in response to physical forces
Voltage-gated channels
open or close in response to changes in membrane potential
Found along axon
Leaky channels
always open
non-gated
found everywhere
Electricity
When opposite charges are separated, they contain potential energy and when they come together electrical energy is released
In cells, the separation of charges by the plasma membrane is “membrane potential”
Principles of Electricity
Voltage
Voltage
the measurement of potential energy created by charge separation
measured in millivolts
The voltage depends on the quantity of charge and the distance between the charges
Membrane Potentials
**Resting Membrane Potential-**potential difference across the membrane in a resting neuron
2 types of gradient within a resting membrane potential
**Chemical gradient-**higher concentration of Na+ in the extracellular fluid and a higher concentration of K+ in the intracellular fluid
**Electrical gradient-**The inside of the membrane is negatively charged and the outside is slightly positive
Factors contributing to the resting membrane potential
Membrane is 50 – 75X more permeable to K+ so K+ ions leak out faster than Na+ leak in
Intracellular proteins - fixed anions inside the cell
Sodium-Potassium pump maintains the chemical and electrical gradient – 3 Na+ out for every 2 K+ in
What happens in Membrane Potentials
Stimuli will trigger disruptions in RMP(resting membrane potential)
Triggers a graded potential – a localized change in membrane potential
Short lived and dissipates as it travels
Changes in Membrane Potential
(what happens if stimulus is excitatory?)
If the stimulus is excitatory it will cause depolarization of the membrane
Depolarization – the membrane potential becomes less negative
When neurons are stimulated Na+ channels open and Na+ rushes into the cell down its electrochemical gradient
Graded Potentials
(What does the magnitude of stimulus depend on?)
Magnitude of the stimulus depends on how many Na+ channels open
This determines the distance that the graded potential will travel
Amount of Na+ channels affected by the stimulus depends on
(graded potential)
Frequency of stimuli - summation
Amplitude of stimuli - strength
Strong graded potentials can initiate action potentials if the threshold potential is reached at the trigger zone (axon hillock
Threshold potential
=-55mV
The critical level of membrane potential must reach to open voltage-gated Na+ channels on the axon to produce an action potential
Action Potential
(What does stronger stimuli increase?)
brief reversal of the membrane potential
neuron sends information down an axon, away from the cell body
Wave of depolarization followed by repolarization
Stronger stimuli increases the frequency of axon potential
Repolarization
the membrane returns to its resting membrane potential
Voltage gated Na+ channels close
Voltage gated K+ channels fully open and K+ efflux restores the resting membrane potential
Membrane potential becomes more negative as K+ rushes out
Hyperpolarization
(What happens when the K+ permeability last longer?)
the inside of the membrane becomes more negative than the resting potential
Voltage gated K+ channels are sluggish to close
K+ permeability lasts longer and membrane potential dips below resting potential
Restoring the Resting Membrane Potential:
Repolarization restores the electrical gradient
Na/K pump restores resting ionic concentrations
Refractory Periods
time required for a neuron to generate another action potential
Absolute Refractory Period
when another AP cannot be generated
From the opening of the Na+ activation gates until the resetting of the activation gates
Ensures that each action potential is separate
Enforces one-way transmission of nerve impulses
Think of flushing the toilet
Relative Refractory Period
the interval of time during which a second action potential can be initiated, but initiation will require a greater stimulus than before due to a raise in threshold. Refractory periods are caused by the inactivation gate of the Na+ channel.
Na+ gates are reset
K+ gates are still open
Hyperpolarization is occurring
Factors Influencing Conduction Velocity
Myelination of axon
Saltatory conduction
Diameter of the axon
Alcohol, sedatives, and anesthetics
Insufficient blood flow to neurons
Myelination of axon
(Acts as….)
increases impulse rate
Acts as insulator preventing charge leakage
Saltatory conduction
voltage gated channels are concentrated nodes
electrical impulses jump from node to node instead of traveling down the entire axon
Diameter of the axon
the larger the diameter the quicker the impulse travels, less resistance to current flow so adjacent membranes depolarize quicker
Alcohol, sedatives, and anesthetics
(Is pain still present?)
slow or block nerve impulses by reducing permeability to Na+.
Pain is still present but the brain can’t detect it
Insufficient blood flow to neurons
(Why does foot tingle?)
slows impulses, caused by cold or pressure
Foot falls asleep, then tingles when neurons fire again
Note 
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