The timing and frequency of action potentials in the presynaptic neuron play a crucial role in the effectiveness of synaptic transmission. Previous multiple stimulations of a synapse can lead to increased efficacy, as seen in facilitation, posttetanic potentiation, and long-term potentiation. On the other hand, long-term depression is an example of reduced efficacy resulting from previous activation of the synapse.

Neurotransmitters, of which there are hundreds, are utilized by the nervous system. These neurotransmitters can be categorized into small-molecule transmitters (such as acetylcholine, amino acids, biogenic amines, and purines), peptides, and gases (such as CO and NO). The action of a neurotransmitter is determined by its postsynaptic receptors. Most nongaseous transmitters have both ionotropic and metabotropic receptors.

Small-molecule transmitters primarily act locally across a single synapse, and their duration of action is limited by reuptake and enzymatic degradation. Peptides have the ability to diffuse from their presynaptic release site, potentially affecting all cells within a local region. Gaseous transmitters are free to diffuse from their release site.

Ionotropic receptors consist of an ion channel that is gated by the binding of neurotransmitter to the receptor. Metabotropic receptors, on the other hand, activate second messengers upon neurotransmitter binding.

Many synapses have the ability to release multiple types of transmitters, and the specific transmitters released depend on the activity pattern of the terminal. Co-released transmitters can function independently or act synergistically or antagonistically.

The completion of this chapter should enable the student to answer the following questions:

1. What are the major modalities of somatosensory information, and what are the corresponding pathways that convey each from the periphery to the primary somatosensory cortex?
2. What body regions and categories of information are associated with the exteroceptive, proprioceptive, and enteroceptive divisions of the somatosensory system?
3. What are the primary receptors responsible for fine/discriminatory touch sensations?
4. What types of somatosensory information does the cerebellum receive?
5. What are the primary receptors for pain and temperature touch sensations?
6. What is referred pain?
7. What proteins are involved in transducing different categories of somatosensory information?
8. How do descending pathways regulate the flow of activity in ascending somatosensory pathways?

The somatosensory system provides the central nervous system (CNS) with information regarding the body's state and its interaction with the surrounding environment. This is achieved through various sensory receptors that convert mechanical (pressure, stretch, and vibrations) and thermal energies into electrical signals. These electrical signals, known as generator or receptor potentials, occur in the distal ends of axons of first-order somatosensory neurons. They then trigger action potential trains that convey information about the stimulus characteristics. The cell bodies of these neurons are located in dorsal root (Fig. 7.1A Fig. 4.8) and cranial nerve ganglia.
The efficacy of synaptic transmission depends on the timing and frequency of action potentials in the presynaptic neuron. Facilitation, posttetanic potentiation, and long-term potentiation are examples of increased efficacy of synaptic transmission in response to previous multiple stimulations of a synapse. Long-term depression is an example of reduced efficacy resulting from previous activation of the synapse.
13.
The nervous system uses hundreds of neurotransmitters. Neurotransmitters can be subdivided into a few broad functional classes: small-molecule transmitters (acetylcholine, amino acids, biogenic amines, and purines), peptides, and gases (CO, NO). The action of a neurotransmitter depends on its postsynaptic receptors. Most nongaseous transmitters have both ionotropic and metabotropic receptors.
14.
Small-molecule transmitters act locally, mainly across a single synapse, and their duration of action is limited by reuptake and enzymatic degradation. Peptides can diffuse from their presynaptic release site and thus have the potential to affect all cells within a local region. Gaseous transmitters are free to diffuse from their release site.
15.
Ionotropic receptors contain an ion channel whose state (open versus closed) is gated by the binding of neurotransmitter to the receptor. Metabotropic receptors activate second messengers on binding neurotransmitter.
16.
Many synapses can release multiple types of transmitters, and which ones they release depends on the activity pattern of the terminal. Co-released transmitters may function independently or act synergistically or antagonistically.
Fain GL. Molecular and Cellular Physiology of Neurons. 2nd ed. Cambridge, MA: Harvard University Press; 2014.
Hille B. Ion Channels of Excitable Membranes. 3rd ed. Sunderland, MA: Sinauer Associates; 2001.
Upon completion of this chapter the student should be able to answer the following questions: 1.
What are the major modalities of somatosensory information, and what are the corresponding pathways that convey each from the periphery to the primary somatosensory cortex?
2.
What body regions and categories of information are the exteroceptive, proprioceptive, and enteroceptive divisions of the somatosensory system associated with?
3.
What are the main receptors for fine/discriminatory touch sensations?
4.
What types of somatosensory information does the cerebellum receive?
5.
What are the main receptors for pain and temperature touch sensations?
6.
What is the phenomenon of referred pain?
7.
What proteins are involved in transducing different categories of somatosensory information?
8.
How do descending pathways act to regulate the flow of activity in ascending somatosensory pathways?
he somatosensory system provides information to the central nervous system (CNS) about the state of the body and its contact with the world. It does so by using a variety of sensory receptors that transduce mechanical (pressure, stretch, and vibrations) and thermal energies into electrical signals. These electrical signals are called generator or receptor potentials and occur in the distal ends of axons of first-order somatosensory neurons, where they trigger action potential trains that reflect information about the characteristics of the stimulus. The cell bodies of these neurons are located in dorsal root (
Fig. 7.1A
Fig.
4.8 ) and cranial nerve ganglia.