1. 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)

2. 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”
Sensory communicate information about the external and internal environment to the CNS
Approx 12 billion neurons in human body, most in brain and spinal cord
Sensory – receive stimuli from external enviro
Motor – transmit impulses from brain and spinal cord to the muscle or gland that will produce the stimulus
Interneurons – connect sensory and motor neurons and carry stimuli in brain and spinal cord; more than 90% of neurons in the body are association neurons

3. 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

7. 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!
Motor and associaton neurons possess a profusion of highly branched dendrites which enables them to receive information from many different sources simultanously
Only one axon per neuron

8. Neuron

9. Nervous System: Neuroglia
Neurons are supported structurally and functionally by supporting cells called neuroglia
1/10th 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
Each Schwann cell tightly wraps around the axon numerous times to form a multi-layered insulation
In the CNS, myelinated axons form the white matter and unmyelinated dendrites form the gray matter; in the PNS, myelinated axons are bundled together like wires in a cable to form nerves

11. 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)
Ion transfer occurs here, action pot jumps from one node to another rather than propogating smoothly

12. 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
The nerve impulse is an electrochemical event that occurs in the neuron
The minus sign indicates that the inside of the cell is negative with respect to the outside

13. 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
Negative charge maintained by the active transport of Na ions out of the cytoplasm
Sodium, potassium pump – helps establish and maintain conc gradients resulting in high K and low Na inside the cell, and high Na and low K outside of the cell
Ion leakage channels – in the cell membrane are more numerous for K than for N; form pores through the membrane; more numerous for K than for Na allowing more K to diffuse out of cell than for Na to diffuse into cell

14. 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

15. 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)

16. Remember: Cells contain relatively high [K+] inside the cell, but low [Na+]
Na/K pump – transfers 3 Na outside of the cell and simultaneously transports 2 Ks inside the cell; requires ATP
maintains cell membrane potential

17. 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

18. 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”

19. 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
allow cell to change its membrane potential in response to stimuli

21. Graded Potentials
Depends on stimulus, 1 weak, 2 stronger, 3 inhibitory (produces hyperpolarization); resulting change will be the sum of all 3 (if they occur together)

22. 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
These are graded potentials b/c their size depends on either the strength of the stimulus or the amount of ligand available to bind with their receptors

23. 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 result when depolarization reaches a threshold
Also in muscle cells

24. 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
only open at certain membrane potentials (theres a threshold, above it – open, below it – closed)

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

26. 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

27. 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

28. 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)
Na+ is the primary ion responsible for changing cell membrane potential

29. 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

30. 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

31. 3. Top curve 2. Rising Phase 1. Resting Phase 4. Falling Phase
Membrane potential (mV)
2. Rising Phase
Stimulus causes above threshold voltage
+50 1 2 3
–70
Time (ms)
1. Resting Phase
Equilibrium between diffusion of K+ out
of cell and voltage pulling K+ into cell
Voltage-gated
potassium channel
Potassium
channel
sodium channel
Potassium channel
gate closes
Sodium channel
activation gate closes.
Inactivation gate opens.
activation gate opens
3. Top curve
Maximum voltage reached
gate opens
4. Falling Phase
gate open
Undershoot occurs as excess potassium
diffuses out before potassium channel closes
Equilibrium
restored
Na+ channel
inactivation gate
closes K+
Na+
closed
At rest, voltage-gated ion channels are closed, but there is some leakage of K. In response to stimulus, the cell begins to depolarize, and once a threshold is met, an action potential is produced; rapid depolarization occurs (bringing in Na+ into cell) because voltage-gated Na+ channels open, allowing Na+ to diffuse into the axon. At the top of the spike, Na channels close, and K+ channe;s (that were previously closed) open. Repolarization occurs because of the diffusion of K outside of the cell. And undershoot occurs before the membrane returns to its resting potential

32. 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

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

34. 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
Squid have a “giant” axon, allows for very rapid escape response
Saltare means to jump

35. Saltatory Conduction

36. Saltatory Conduction