Nervous System
• All animals must respond to environmental stimuli
– Sensory receptors – detect stimulus
– Motor effectors – respond to stimulus
• The nervous system links the two
• Consists of neurons (nerve cells) and supporting cells (neuroglia)

Nervous System
• Vertebrates have three types of neurons
(nerve cell)
• Sensory – carry impulses from sensory receptors to the central nervous system (CNS)
• Motor – carry impulses from the CNS to effectors
(muscles and glands)
• Interneurons (association neurons) – located in the brain and spinal cord; provide more complex reflexes and higher associative functions such as learning and memory; “integrators”

Nervous System
• The central nervous system (CNS) consists of the brain and the spinal cord
• The peripheral nervous system (PNS) consists of sensory and motor neurons; a network of nerves extending into different parts of the body; carries sensory input to the CNS and motor output away from the CNS

Nervous System: Neurons
• The basic structure of the neuron (consists of):
– Cell body – an enlarged region containing nucleus
– Dendrite – short cytoplasmic extensions extending from the cell body; receive stimuli
– Axon – single, long extension that conducts impulses away from the cell body
• The axons controlling the muscles in a person’s feet can be more than 1 meter (3 ft) long!; axons from the skull to the pelvis in a giraffe are 3 meters (9 ft) long!

Neuron

Nervous System: Neuroglia
• Neurons are supported structurally and functionally by supporting cells called neuroglia
– 1/10 th size of a neuron, but 10x more abundant
– Schwann cells and Oligodendrocytes – produce the myelin sheaths in the PNS and the CNS, respectively
• Myelin sheaths surround and insulate the axon of many types of neurons; “myelinated” axons

Nervous System: Neuroglia
• Other neuroglia provide neurons with nutrients, remove wastes
• Small gaps known as nodes of Ranvier interrupt the myelin sheath at intervals of 1-
2μm; uninsulated, capable of generating electrical activity (sites of action potential)

Conduction of the nerve impulse
• Upon stimulation of a nerve cell, electrical changes spread or propagate from one part of the cell to another
• Neuron function depends on a changeable permeability to ions; an electrical difference exists across the plasma membrane
• Membrane potential – voltage measured across a membrane due to differences in electrical charge; inside of cell is negative relative to outside

Conduction of the nerve impulse
• When a neuron is not being stimulated, it maintains a resting potential; -70mV (average; ranges from -40 to -90; “polarized”)
• The inside of the cell is negatively charged relative to the outside
– Polarization is established by maintaining an excess of Na+ ions on the outside, and an excess of K+ ions on the inside
• Most animal cells have a low concentration of Na+ and a high K+ relative to their surroundings

Conduction of the nerve impulse
• A certain amount of Na+ and K+ ions are always leaking across the membrane through
leakage channels, but Na+/K+ pumps actively restore the ions to their appropriate sides
– The Na+/K+ pump: brings in 2 K+ for every 3 Na+ pumped out
– Ion leakage channels: allow more K+ to diffuse out than Na+ to diffuse in

Conduction of the nerve impulse
• Other ions, such as large, negatively-charged proteins and amino acids, reside within the cell
• It is these large, negatively-charged ions that contribute to the overall negative charge on the inside of the cell membrane relative to the outside
• Negative pole: Cytoplasm (inside cell)
• Positive pole: Extracellular (outside cell)

Remember: Cells contain relatively high [K+] inside the cell, but low [Na+]

Conduction of the nerve impulse
• A nerve impulse is generated when the difference in electrical charge disappears
– Occurs when a stimulus contacts the tip of a dendrite and increases the permeability of the cell membrane to Na+ ions
– Na+ ions rush into the cystoplasm, and the difference in electrical charge across the membrane disappears (depolarized)
– Remember, the concentration of Na+ inside the cell is low relative to its surroundings

Conduction of the nerve impulse
• Some stimuli open K+ channels
– As a result, K+ leaves the cell (remember: high
[K+] inside the cell)
– Membrane potential becomes more negative
(more negative inside the cell)
– “hyperpolarization”
• Some stimuli open Na+ channels
– Causes Na+ to enter the cell
– Membrane potential becomes less positive
– “depolarization”

Conduction of the nerve impulse
• When the strength of stimuli determines how many ion channels will open; graded response
• Caused by the acvtivation of a gated ion channels which behave like a door that can open or close, unlike ion leakage channels that are always open
• Each gated channel is selective, opening only to allow diffusion of one type of ion
• Normally closed in a resting cell

Graded Potentials

Action Potentials
• Permeability changes are measurable as depolarizations or hyperpolarizations of the membrane potential
• Depolarization – makes membrane potential less negative (more positive)
• Hyperpolarization – makes membrane potential more negative
• Ex: -70mV  -65mV = Depolarization
-70mV  -75mV = Hyperpolarization

Action Potentials
• Action potentials are rapid, reversals in voltage across the plasma membrane of axons
• Once a threshold of depolarization is reached
(-50 to -55 mV), an action potential will occur
• An ‘all or nothing’ response, not graded
• Magnitude of the action potential is independent of strength of depolarizing stimuli
• Action potentials are the signals by which neurons communicate and spread messages

Action Potentials
• An action potential is caused by a different class of ion channels, voltage-gated ion channels
• These channels open and close in response to changes in membrane potential; only open at certain membrane potentials; flow of ions controlled by these channels creates the action potential

Action Potentials
• Voltage-gated channels are very specific; each ion has its own channel
– Voltage-gated Na+ channels
– Voltage-gated K+ channels

Action Potentials
• When the threshold voltage is reached, Na+ channels open rapidly
• Influx of Na+ causes the membrane to depolarize
• K+ channels open slowly, eventually repolarizing the membrane
• Action potential consists of three phases:
• Rising, falling, and undershoot

Action Potentials – the Spoiler
• At threshold, membrane is depolarized enough that Na+ voltage-gated channels open; Na+ moves into interior of cell, becoming less negative; rapid depolarization
(  45mV), then stops
– Stops because channels will close after a specific amount of time has elapsed

Action Potentials II – the Spoiler
• K+ voltage-gated channels will also open, before membrane potential reaches zero; K+ moves out of cell, making cell become more negative, returns cell to resting
• Na+/K+ pump is also activated, moving 3 Na+ out for every 2 K+ in, makes cell more negative
• Returns cell to rest (~-70mV)

Action Potentials III – the Spoiler
• Excess K+ diffuse out before K+ channel closes, or over-activity of the Na+/K+ pump; results in undershoot (hyperpolarization)
• Entire process occurs very rapidly: 2-4ms from start to finish

Action Potentials
• Each action potential, in its rising, reflects a reversal in membrane polarity
• Positive charges due to Na+ influx can depolarize adjacent region to threshold, causing the next region to produce an action potential of its own
• The previous region then repolarizes back to its resting membrane potential

2. Rising Phase
Stimulus causes above threshold voltage
Sodium channel activation gate opens
1. Resting Phase
Equilibrium between diffusion of K + of cell and voltage pulling K + out into cell
Voltage-gated potassium channel
Potassium channel
Voltage-gated sodium channel
3. Top curve
Maximum voltage reached
Potassium gate opens
K +
Na +
Na + channel inactivation gate closes
+50
0
–70
1 2
Time (ms)
3
4. Falling Phase
Undershoot occurs as excess potassium diffuses out before potassium channel closes
Potassium gate open
Potassium channel gate closes
Equilibrium restored
Sodium channel activation gate closes.
Inactivation gate opens.
Na + channel inactivation gate closed

Action Potentials
• Action potentials are localized events
• They DO NOT travel down the membrane
• They are generated anew in a sequence along the neuron as they propogate along axon
• During undershoot, the membrane is less likely to depolarize
• This keeps the action potential moving in one direction

resting repolarized depolarized
Cytoplasm
Cell membrane
+ + + + + + + + +
– – – – – – – – –
– – – – – – – – –
+ + + + + + + + +
– – + + + + + + +
+
+ + – – – – – – –
Na +
+ – – – – – –
–– + + + + + + +
–
K +
+ + – – + + + + +
–
– – + + – – – – –
– +
Na
+
+
– – – –
+ +
– –
+ + + + +
–
K +
K +
+ + + + – – – + +
–
– – – – + + + – –
– – –
+ +
Na +
+
–
+ + + + – – – + +
–
+
K +
K +
+ + + + + + + – –
–
– – – – – – –
+ +
– – – – – – Na
+
+
+
+ + + + + +
– –
K +

Saltatory Conduction
• Two ways to increase velocity of conduction:
– Increase diameter of axon; reduces resistance to current flow; found primarily in invertebrates
– Axon is myelinated; impulse jumps from node to
node (Nodes of Ranvier – the only site of action potentials) = saltatory conduction
– one action potential still serves as stimulus for the next one, but the impulse (depolarization at one end) spreads quickly beneath the insulating myelin to trigger the opening of voltage-gated ion channels at the next node

Saltatory Conduction http://www.flickr.com/photos/photoklick/2829645922/

Saltatory Conduction