1. The Nervous System

2. The Nervous System Overall Function COMMUNICATION
Works with the endocrine system in regulating body functioning, but the nervous system is specialized for SPEED

3. Neurons A neuron is the functional unit of the nervous system
Neurons are specialized for transmitting signals from one location in the body to another
Neurons consist of a large cell body (contain a nucleus and other organelles), and neuronal processes
Axons
Conduct messages AWAY from cell body
Dendrites
Conducts messages TOWARD cell body

4. Neuron Structure

5. PARTS OF THE NEURON
Cell body: this is where most of the neuron’s organelles (including the nucleus) are located
Dendrites: highly branched extensions from the cell body that RECEIVE signals from other neurons
Axon: a large extension from the cell body that TRANSMITS signals to other neurons or “effector” cells
Axon hillock: where the axon joins the cell body
Myelin sheath: a fatty layer of cells that “insulates” the axon (not present in most invertebrates)
Synaptic terminal: the branching ends of the axon that release a “neurotransmitter” to send a message
Synapse: the space between the synaptic terminal and the effector cell

6. Supporting cells of the nervous system
Glia is the term given to the many cells that support the neurons in the nervous system
Astrocytes: provide structural support for neurons in the CNS. They also regulate extracellular ion concentrations (important when we talk about membrane potentials)
Oligodendrocytes (in the CNS) and Schwann cells (in the PNS): responsible for creating the myelin sheath on the axon

7. Organization of the nervous system
Organisms have different types of nervous systems based on their complexities
The simplest organisms will have a web-like arrangement of nerves throughout the body the act as a nerve net
These organisms are able to react to stimuli, but do not show any higher activity
Example: Hydra
A little more complicated organism also have bundled fiber-like extensions of neurons called nerves, along with nerve nets
This allows nerve nets to control more complex movements
Example: Sea star

8. Organization of the nervous system: More complicated organisms
Central Nervous System (CNS)
Consists of brain and spinal chord
In more primitive organisms, this could include a cluster of neurons (called ganglia) along a ventral nerve and a brain
Peripheral Nervous System (PNS)
Consists of all of the peripheral nerves that connect with the CNS

9. Central and Peripheral Nervous Systems
The central nervous system consists of the brain and spinal cord
This is where integration occurs
Made of interneurons
The peripheral nervous system consists of the nerve cells that communicate signals between the CNS and the rest of the body
Sensory neurons
Carry info from the sensory receptors to the brain
Motor neurons
Carry info from the brain to effector cells (to do whatever the brain said!)
Central and Peripheral Nervous Systems

10. Other divisions of the nervous system
Autonomic Nervous System
Regulates internal environment (digestion, cardiovascular, excretion and hormone release
Called the “involuntary” nervous system
Three parts:
Sympathetic
Parasympathetic
Enteric
Somatic Nervous System
Carries signals to and from the skeletal muscles
Responds to external stimuli
Called the “voluntary” nervous system

11. Autonomic nervous system
Sympathetic: corresponds to increased arousal or energy output (fight or flight response)
Increased heart rate
Dilate blood vessels and respiratory passages
Convert glycogen to glucose
Release epinephrine (adrenaline)
Inhibits digestion
Parasympathetic: corresponds to self-maintenance and relaxation (“rest” and “digestion”)
Opposite of sympathetic nervous system
Enteric: network of neurons responsible for digestion (digestive tract, pancreas, and gallbladder)

12. Information processing
Regardless of the complexity of the nervous system, there are 3 general stages to information processing:
Sensory input
Integration
Motor output/effect

13. Communication Lines Stimulus (input) Receptors (sensory neurons)
Integrators
(interneurons)
motor neurons
Effectors
(muscles, glands)
Response
(output)

14. Major Nervous System Processes
Input
The conduction of signals from sensory neurons to integration centers in the nervous system
Detect external stimuli (light, sound, heat, smell, touch, taste)
Detect internal conditions (blood pressure, blood CO2 levels, muscle tension)
Integration
The process by which the information from the environmental stimulation of the sensory receptors is sent and interpreted by interneurons in the CNS
The complexity of the CNS has to do with the amount of connections between interneurons

15. Major Nervous System Processes
Motor Output
The conduction of signals from the processing center of the CNS to the motor neurons which communicate with muscle cells or gland cells (effector cells) that actually carry out the body’s responses to stimuli

16. Action Potentials: how the nerves conduct signals
In order to actually TRANSMIT a signal, the voltage (charge) across the membrane (membrane potential) has to change
A signal will cause the ion channels to open, letting some of the ions (Na+, K+) through, trying to achieve EQUILIBRIUM
This depolarizes the membrane
This causes the signal to be passed along the neuron, which is known as an ACTION POTENTIAL (like a wave of electricity)

17. Resting potential: not transmitting a signal
Resting Potential: charge difference across the plasma membrane of a neuron when not transmitting signals
Fluid just outside cell is more positively charged than fluid inside because of large negatively charged proteins in the cytoplasm
Potassium (K+): Higher inside than outside
Sodium (Na+): Higher outside than inside
Potential is measured in millivolts
Resting potential is usually about -60mV to -80mV (inside of the membrane is “-” and outside is “+”)

18. Resting potential
The resting potential of a neuron creates an ionic gradient
Remember the concentration gradient in the H+ pump to make ATP
There are many open potassium ion channels in the plasma membrane and few sodium ion channels (ungated)
This causes a net flow of Na+ and K+ across the membrane
This is what creates the voltage (flow of ions)
To maintain the levels of Na+ and K+, the cells utilize the sodium-potassium pump (remember active transport)

19. Gated ion channels
Neurons also have 3 gated ion channels (controls the flow of ions)
Stretch-gated ion channels: sense stretching of the cell and cause the gates to open
Ligand-gated ion channels: open or close when a specific chemical binds to the channel
Voltage-gated ion channels: open or close when the membrane potential changes

20. Action Potentials: transmitting a signal
Depending on external stimuli, gated ion channels can open or close
Some stimuli can cause a hyperpolarization which makes the membrane potential of the cell greater than resting potential
Example: opening K+ gated channels allows the movement of K+ out of the cell (remember: at rest K+ is more concentrated inside the cell)
Increases membrane potential to -92 mV (losing “+” out of cell)
Some stimuli can cause a depolarization which makes the membrane potential of the cell less than resting potential
Example: opening Na+ gated channels allows the movement of Na+ into the cell (remember: at rest Na+ is more concentrated outside the cell
Decreases membrane potential to +62 mV (gaining “+” in cell)

21. Action Potentials: transmitting a signal
A change in membrane potential is called a graded potential
Action potentials are either ALL or NOTHING
Either there is enough change in the voltage to pass the message along, or there isn’t
The neuron either “fires” or it doesn’t fire
In order to “fire”, the membrane potential must hit a threshold (the membrane voltage that sets the reaction)
If the threshold is reached, then the neuron undergoes an action potential (these are what carries a signal along the axon)

22. All or Nothing All action potentials are the same size
If stimulation is below threshold level, no action potential occurs
If it is above threshold level, cell is always depolarized to the same level
Action potential is initiated at the axon hillock and travels down the axon to the axon terminal

23. Structure of a Neuron dendrites INPUT ZONE cell body axon OUPUT ZONE
TRIGGER ZONE
CONDUCTING ZONE
axon endings

24. Action potential
Step 1: Neuron is in the resting potential, the gated-ion channels are closed
Step 2: A stimulus causes some Na+ ion channels to open allowing Na+ to diffuse through the membrane. This causes the membrane to be depolarized. The depolarization causes even more Na+ ion channels to open (positive feedback) until a threshold is reached in the membrane potential
Step 3: Once the threshold is reached, positive feedback progresses at a rapid rate to create an action potential (the voltage that allows the membrane to conduct the signal)

25. Action potential
Step 4: After the action potential is reached, the Na+ gates close, preventing the influx of any more Na+ ion. At the same time, the K+ ion channels open. This allows the K+ ions to diffuse out of the membrane (high concentration of K+ inside the membrane compared to outside). This release of K+ ions rapidly lowers the membrane potential.
Step 5: As the membrane potential lowers, it falls a little below the resting potential, undershoot The K+ ion channels close and the membrane eventually returns to its resting potential

26. Steps in the Action Potential
An action potential is very quick (each one only takes 1-2 milliseconds
After an action potential, it takes a little bit of time to return all of the Na+ and K+ concentrations to their original levels
Na+ / K+ pumps the Na+ and K+ back to original positions
During this time, a second action potential cannot by initiated (refractory period)

27. Recording of Action Potential
+20
-20
Membrane potential (millivolts)
threshold
-40
resting membrane potential
-70 1 2 3 4 5
Figure 34.6b Page 583
Time (milliseconds)

28. Transmitting signal along axon
Transmitting the signal
In order to propagate the signal, the membrane potential must be depolarized along the length of the axon
To make this occur, when the Na+ is being let into the cell (depolarization) in one part of the axon, it creates an electric current that causes depolarization in an adjacent area
Behind the zone of depolarization is where the membrane is returning to resting potential (repolarization)
The refractory period prevent the action potential from being sent “backwards” along the neuron

29. Action Potential 1 2 3 4 Figure 34.5d Page 583 Na+ Na+ Na+ K+ K+ K+ K+

30. Speed of conduction
In general, the speed of a signal along an axon is dependent on a few things
The smaller the axon diameter, the slower the speed of signal conduction
Simple invertebrates (worms) may have conduction speeds of centimeters/second
Larger axon diameters allow increased speed of signal conduction
Complex invertebrates (squid or octopi) have conduction speeds of about 100 meters/second
In the vertebrate axon, there is a myelin sheath which increases speed due to insulation
There are gaps in the myelin sheath (Nodes of Ranvier), where the depolarization can “jump” to. This greatly increases conduction rate (about 120 meters/second)

31. Communication between neurons

32. NEURON TO NEURON COMMUNICATION
As the action potential travels along the axon it stops at the axon terminal (synaptic terminal)
Action potentials do not travel between different neurons
Yet, it is still necessary to send the “signal” from one neuron to the next
To do this, there has to be a way to send a signal across the space that exists between one neuron and another (synaptic cleft or gap junction)

33. Chemical Synapse
Gap between axon terminal of one neuron and dendrite of adjacent neuron
Action potential in axon ending of presynaptic cell causes voltage-gated calcium channels to open
Flow of calcium into presynaptic cell causes release of neurotransmitter into synaptic cleft
plasma membrane of axon ending of presynapic cell
plasma membrane of postsynapic cell
synaptic vesicle
synaptic cleft
membrane receptor
Figure 34.7a Page 584

34. Acetylcholine (bridges gaps between motor neurons & muscle cells),
Neurotransmitters
Neurotransmitters are substances that carry the “message” across the synapse
Important neurotransmitters:
Acetylcholine (bridges gaps between motor neurons & muscle cells),
norepinephrine, dopamine, serotonin work in CNS

35. Synaptic Transmission
Neurotransmitter diffuses across cleft and binds to receptors on membrane of postsynaptic cell
Binding of neurotransmitter to receptors opens ion channels in the membrane of postsynaptic cell

36. Ion Gates Open neurotransmitter ions receptor for neurotransmitter
gated channel protein

37. Synaptic Transmission
Enzymes in synaptic cleft will degrade neurotransmitters after action potential is initiated on the post-synaptic cell. The neurotransmitters are recycled after they are broken down.
Example: Acetylcholine is broken down by the enzyme acetylcholine esterase

38. Indirect synaptic transmission
The neurotransmitter does not bind directly to an ion channel gate.
Instead, it activates a signal transduction pathway (Remember cell signaling again)
Utilizes a second messenger (AMP to cAMP again)
These signals take longer to activate, but last for a longer period of time

39. Nerve A bundle of axons enclosed within a connective tissue sheath
myelin sheath
nerve
fascicle
Nerve
A bundle of axons enclosed within a connective tissue sheath

40. Reflexes Automatic movements made in response to stimuli
In the simplest reflex arcs, sensory neurons synapse directly on motor neurons; interneurons in CNS aren’t involved.
Most reflexes involve an interneuron

41. Stretch Reflex STIMULUS Biceps stretches. sensory neuron motor neuron
Response
Biceps contracts.

42. Structure of the Spinal Cord
ganglion
nerve
meninges
(protective
coverings)
vertebra
Figure 34.19a Page 593

43. Divisions of Brain Division Main Parts Forebrain Cerebrum
Olfactory lobes
Thalamus
Hypothalamus
Limbic system
Pituitary gland
Pineal gland
Midbrain
Tectum
Hindbrain
Pons
Cerebellum
Medulla oblongata
anterior end of the
spiral cord
Figure Page 594

44. Cerebrospinal Fluid Surrounds the spinal cord
Fills ventricles within the brain
Blood-brain barrier controls which solutes enter the cerebrospinal fluid

45. Anatomy of the Cerebrum
Largest and most complex part of human brain (Responsible for thinking & higher level functions)
Outer layer (cerebral cortex) is highly folded
A longitudinal fissure divides cerebrum into left and right hemispheres
Corpus collosum connects the two hemispheres

46. Lobes of the Cerebrum Parietal Frontal Occipital Temporal
Primary somatosensory cortex
Primary motor cortex
Parietal
Frontal
Occipital
Temporal

47. Limbic System Controls emotions and has role in memory
(olfactory tract)
cingulate gyrus
thalamus
amygdala
hypothalamus
hippocampus

48. Other Parts of the Brain
Cerebellum - Controls muscle coordination and posture
Medulla oblongata- Controls heart rate & breathing rate

49. Variations in Nervous Systems Among Animals

50. Example: problem with nervous system
Multiple Sclerosis:
A condition in which nerve fibers lose their myelin
Slows conduction
Symptoms include visual problems, numbness, muscle weakness, and fatigue