1. Chapter 10
Nervous System

2. Divisions of the Nervous System
Central nervous system (CNS)
Consists of brain and spinal cord
Peripheral nervous system (PNS)
Consists of nerve network
The peripheral nervous system consists of everything outside of the brain and spinal cord.

3. Central and Peripheral Nervous Systems
Because the nervous system performs so many functions, it is helpful to further subdivide the peripheral nervous system, as shown here.

4. Neuroglia Supportive cells of the nervous system
Oligodendrocytes: Form myelin sheath in the CNS
Ependymal cells: Line spinal cord and cavities of the brain
Microglia: Perform phagocytosis
Astrocytes: Form blood–brain barrier
Schwann cells: Form myelin sheath in peripheral nervous system
Neuroglia—also called glial cells—are the supportive cells of the nervous system. They perform various functions that enhance the performance of the nervous system.
Other than Schwann cells (which are found in the peripheral nervous system), all neuroglia reside in the central nervous system.
Some ependymal cells secrete CSF, whereas others have cilia to propel circulation of CSF.

5. Blood–Brain Barrier Formed as astrocytes wrap around capillaries
Protects brain from foreign substances
Astrocytes—the most numerous of the glial cells—are pervasive throughout the brain.
A tiny “foot” exists at the end of each of the astrocyte’s star-like projections. Some of the feet latch on to a capillary; others connect with a neuron. This allows the astrocyte to funnel glucose from the bloodstream to the neuron.
The feet of the astrocytes join with endothelial cells lining the capillaries to create a membrane called the blood–brain barrier (BBB).
The BBB is a semipermeable membrane that exists throughout the brain. It allows small molecules (like oxygen, carbon dioxide, and water) to diffuse across to the brain but blocks larger molecules. This helps protect the brain from foreign substances. It also prevents most medications from reaching brain tissue.

6. Neurons Handle communication Three classes
Sensory (afferent) neurons: Detect stimuli
Interneurons: Connect pathways
Motor (efferent) neurons: Relay messages
Neurons handle the nervous system’s role of communication.
Sensory (afferent) neurons detect stimuli (such as touch, pressure, heat, cold, or chemicals) and transmit information about the stimuli to the central nervous system (CNS).
Interneurons (which are found only in the CNS) connect the incoming sensory pathways with the outgoing motor pathways.
Motor (efferent) neurons relay messages from the brain to the muscle or gland cells.

7. Neuron Structure Cell body Dendrites Axon Myelin sheath
Neurons assume a variety of shapes and sizes; even so, neurons have three basic parts: a cell body and two extensions called an axon and dendrite.
The cell body (also called the soma) is the control center and contains the nucleus.
Dendrites receive signals from other neurons and conduct the information to the cell body. Some neurons have only one dendrite; others have thousands.
The axon carries nerve signals away from the body. Nerve cells have only one axon; the length of the axon ranges from a few millimeters to a meter.
The axons of many (but not all) neurons are encased in a myelin sheath, which insulates the axon. The myelin sheath (which consists mostly of lipids) is formed by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system.
Gaps in the myelin sheath, called nodes of Ranvier, occur at evenly spaced intervals.
The end of the axon branches extensively, with each axon terminal ending in a synaptic knob. Within the synaptic knobs are vesicles containing a neurotransmitter.
Nodes of Ranvier
Synaptic knob

8. Peripheral nervous system Central nervous system
Myelin Sheath
Peripheral nervous system
Central nervous system
In the peripheral nervous system, Schwann cells wrap themselves around the axon, laying down multiple layers of cell membrane. The inside layers form the myelin sheath; the nucleus and most of the cytoplasm of the Schwann cell are located in the outermost layer. The outer layer, called the neurilemma, is essential for an injured nerve to regenerate.
In the central nervous system, projections from one oligodendrocyte wrap themselves around the axons of nearby nerves. Because the nucleus of the cell is located away from the myelin sheath, there is no neurilemma. This prevents injured CNS neurons from regenerating.
Myelin helps speed impulse conduction. (Typically, unmyelinated nerve fibers perform functions in which speed isn’t essential, such as stimulating the secretion of stomach acid. Nerve fibers stimulating skeletal muscles, where speed is more important, are myelinated.)
Myelination begins during the 14th week of fetal development but is not complete until late adolescence.

9. Impulse Conduction Caused by an electrical current
Membrane potential: When ions with opposite electrical charges are separated by a membrane
Polarization: When a membrane has an excess of positive ions on one side and an excess of negative ions on the other
Impulse transmission results from the flow of charged particles from one point to another.
In the body, whenever ions with opposite electrical charges are separated by a membrane, the potential exists for them to move toward one another. This is called membrane potential.
A membrane that exhibits membrane potential (an excess of positive ions on one side of the membrane and an excess of negative ions on the other side) is said to be polarized.

10. Impulse Conduction: Step 1 Resting Potential
When a neuron is not conducting an electrical signal, the interior has a negative electrical charge and the exterior has a positive charge.
The outside of the cell is rich with sodium ions (Na+), whereas the inside contains an abundance of potassium ions (K+).
The interior also contains other large, negatively-charged proteins and nucleic acids. These additional particles give the cell’s interior its overall negative charge.
Because of the membrane’s permeability, a certain amount of sodium and potassium ions leaks across the membrane. However, the sodium-potassium pump constantly works to restore the ions to the appropriate side.
This state of being inactive and polarized is called resting potential. The neuron is resting, but it has the potential to react if a stimulus comes along.

11. Impulse Conduction: Step 2 Depolarization
A stimulus (such as chemicals, heat, or mechanical pressure) causes channels on the neuron’s membrane to open and Na+ from outside the membrane rushes into the cell.
The addition of the positively charged ions changes the charge of a region of the cell’s interior from negative to positive. As the membrane becomes more positive, it is said to depolarize.

12. Impulse Conduction: Step 3 Action Potential
If the depolarization goes above the threshold level, adjacent channels also open, which allows more Na+ to enter. This creates an action potential, which means that the neuron has become active as it conducts an impulse along the axon. (Another term for action potential is nerve impulse.)
The action potential continues down the axon as one segment stimulates the segment next to it.

13. Impulse Conduction: Step 4 Repolarization
The sudden influx of Na+ triggers other channels to open; this allows K+ to flow out of the cell.
Soon after K+ begins to exit, the Na+ channels shut to prevent any more Na+ from flowing into the cell. This repolarizes the cell; however, Na+ and K+ are now flip-flopped, with the outside containing more K+ and the inside containing more Na+.

14. Impulse Conduction: Step 5 Refractory Period
As long as Na+ and K+ are on the wrong sides of the membrane, the neuron won’t respond to a new stimulus even though the membrane is polarized. This is called the refractory period.
The sodium-potassium pump returns Na+ to the outside and K+ to the inside. When this is completed, the nerve is again polarized and in resting potential until it receives another stimulus.
View animation on “nerve impulse conduction in unmyelinated fibers”

15. Impulse Conduction: Myelinated Fibers1.
Myelin blocks the free movement of ions across the cell membrane; the only place ion exchange can occur is at the nodes of Ranvier.
Electrical changes occur at the nodes of Ranvier, creating an action potential. The current flows under the myelin sheath to the next node, where it triggers another action potential.
Because the action potentials only occur at the nodes, the impulse seems to “leap” from node to node. This type of signal conduction is called saltatory conduction. 2.
View animation on “Impulse in myelinated fibers”

16. View animation on “Synapses”
As impulses move from one neuron to the next, they pass through a synapse:
When an action potential reaches a synaptic knob, the membrane depolarizes. Ion channels open and calcium ions enter the cell.
The infusion of calcium causes vesicles to bind to the cell wall and release their store of a neurotransmitter into the synapse.
The neurotransmitter binds to receptors on the postsynaptic membrane. Each neurotransmitter has a specific receptor. (For example, the neurotransmitter epinephrine can only bind to receptors specific to epinephrine.)
The specific neurotransmitter determines whether the impulse continues (called excitation) or whether it is stopped (called inhibition). If the impulse is inhibitory, K+ channels open and the impulse stops. If the neurotransmitter is excitatory—as shown here—Na+ channels open, the membrane becomes depolarized, and the impulse continues.
The receptor releases the neurotransmitter, after which it is reabsorbed by the synaptic knobs and recycled or destroyed by enzymes (as shown here).
View animation on “Synapses”