Sensory receptors, including exteroceptors, interoceptors, and proprioceptors, detect stimuli from the environment. These stimuli then lead to responses, which are the effects of the stimuli. Sensory transduction is the process of transforming the energy of the stimuli into electrical signals.
 
In terms of the type of energy they transduce or the source of the input, sensory receptors can be classified. Central pathways are usually named based on their origin and termination or the type of information they convey.
 
Chemical substances are distributed along axons through fast or slow axonal transport. Axonal transport can be anterograde or retrograde in direction.
 
Glial cells, such as astrocytes, oligodendroglia, Schwann cells, ependymal cells, and microglia, play different roles in the central nervous system (CNS). For example, astrocytes regulate the CNS microenvironment, oligodendroglia form CNS myelin, Schwann cells form peripheral nervous system (PNS) myelin, ependymal cells line the ventricles, and microglia act as CNS macrophages. Myelin sheaths enhance the conduction velocity of axons.
 
The choroid plexus is responsible for the formation of cerebrospinal fluid (CSF). CSF differs from blood in its lower concentration of K+, glucose, and protein, and higher concentration of Na+ and Cl-. Additionally, CSF normally lacks blood cells.
 
The composition of the extracellular fluid in the CNS is regulated by CSF, the blood-brain barrier, and astrocytes.
 
Damage to the axon of a neuron leads to an axonal reaction (chromatolysis) in the cell body and wallerian degeneration of the axon distal to the injury. Regeneration of PNS axons is more likely than regeneration of CNS axons.
 
After studying this chapter, the student should be able to answer the following questions:
 
1. How is the response of a nerve membrane to small-amplitude stimuli similar to a passive electric circuit comprising batteries, resistors, and capacitors?
2. What factors determine the time and length constants of a nerve membrane? How do these constants shape the electric responses of the nerve membrane?
3. How does an action potential differ from the subthreshold responses of a membrane (i.e., the passive and local responses)?
4. What is the sequence of conductances that underlies the action potential?
5. How do the responses of Na+ and K+ channels to membrane depolarization compare? How does the presence of the Na+ channel inactivation gate cause the responses to differ?
6. How do the gating properties of Na+ and K+ channels relate to the absolute and relative refractory periods of the action potential?
7. How is the action potential propagated without decrement? What factors determine its propagation velocity?
8. What are the structural properties of myelin that enable it to increase conduction velocity?
9. Considering the all-or-none nature of action potentials, how does the central nervous system distinguish characteristics of different stimuli?
Sensory receptors include exteroceptors, interoceptors, and proprioceptors. Stimuli are environmental events that excite sensory receptors, responses are the effects of stimuli, and sensory transduction is the process by which stimuli are detected by transforming their energy into electrical signals.
8.
Sensory receptors can be classified in terms of the type of energy they transduce or by the source of the input. Central pathways are usually named by their origin and termination or for the type of information conveyed.
9.
Chemical substances are distributed along the axons by fast or slow axonal transport. The direction of axonal transport may be anterograde or retrograde.
10.
Glial cells include astrocytes (regulate the CNS microenvironment), oligodendroglia (form CNS myelin), Schwann cells (form PNS myelin), ependymal cells (line the ventricles), and microglia (CNS macrophages). Myelin sheaths increase the conduction velocity of axons.
11.
The choroid plexus forms CSF. CSF differs from blood in having a lower concentration of K+ , glucose, and protein and a higher concentration of Na+ and Cl−; CSF normally lacks blood cells.
12.
The extracellular fluid composition of the CNS is regulated by CSF, the blood-brain barrier, and astrocytes.
13.
Damage to the axon of a neuron causes an axonal reaction (chromatolysis) in the cell body and wallerian degeneration of the axon distal to the injury. Regeneration of PNS axons is more likely than regeneration of CNS axons.
Squire L, Berg D. Fundamental Neuroscience. 4th ed. Waltham, MA:
Kiernan JA, Rajakumar N. Barr’s The Human Nervous System: An Academic Press; 2012. Anatomical Viewpoint. 10th ed. Philadelphia: Lippincott Williams Vanderah T, Gould D. Nolte’s The Human Brain: An Introduction to its & Wilkins; 2013. Functional Anatomy. 7th ed. St. Louis: Mosby; 2015.
Upon completion of this chapter, the student should be able to answer the following questions: 1.
How is a nerve membrane’s response to small-amplitude stimuli like a passive electric circuit comprising batteries, resistors, and capacitors?
2.
What factors determine the time and length constants of a nerve membrane? How do these constants shape the electric responses of the nerve membrane?
3.
How does an action potential differ from the subthreshold responses of a membrane (i.e., the passive and local responses)?
4.
What is the sequence of conductances that underlies the action potential?
5.
How are the responses of Na+ and K+ channels to membrane depolarization similar? How does the presence of the Na+ channel inactivation gate cause the responses to differ?
6.
How do the gating properties of Na+ and K+ channels relate to the absolute and relative refractory periods of the action potential?
7.
How is the action potential propagated without decrement? What are the factors that determine its propagation velocity?
8.
What are the structural properties of myelin that underlie its ability to increase conduction velocity?
9.
Given the all-or-none nature of action potentials, how are the characteristics of different stimuli distinguished by the central nervous system?