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Chapter The Nervous System The CNS is like the power plant of the nervous system. It creates the signals that control the functions of the body. The PNS is like the wires that go to individual houses. The PNS can be broken down into the autonomic nervous system, which controls bodily functions without conscious control, and the sensory-somatic nervous system, which transmits sensory information from the skin, muscles, and sensory organs to the CNS and sends motor commands from the CNS to the muscles. Autonomic Nervous System Figure The postganglionic neuron, in turn, acts on a target organ. Autonomic responses are mediated by the sympathetic and the parasympathetic systems, which are antagonistic to one another. Which of the following statements is false? The parasympathetic pathway is responsible for resting the body, while the sympathetic pathway is responsible for preparing for an emergency. Most preganglionic neurons in the sympathetic pathway originate in the spinal cord.
Slowing of the heartbeat is a parasympathetic response. Parasympathetic neurons are responsible for releasing norepinephrine on the target organ, while sympathetic neurons are responsible for releasing acetylcholine. The autonomic nervous system serves as the relay between the CNS and the internal organs. It controls the lungs, the heart, smooth muscle, and exocrine and endocrine glands. The autonomic nervous system controls these organs largely without conscious control; it can continuously monitor the conditions of these different systems and implement changes as needed.
Signaling to the target tissue usually involves two synapses: a preganglionic neuron originating in the CNS synapses to a neuron in a ganglion that, in turn, synapses on the target organ, as illustrated in Figure There are two divisions of the autonomic nervous system that often have opposing effects: the sympathetic nervous system and the parasympathetic nervous system. Examples of functions controlled by the sympathetic nervous system include an accelerated heart rate and inhibited digestion. Figure The sympathetic and parasympathetic nervous systems often have opposing effects on target organs. Most preganglionic neurons in the sympathetic nervous system originate in the spinal cord, as illustrated in Figure The axons of these neurons release acetylcholine on postganglionic neurons within sympathetic ganglia the sympathetic ganglia form a chain that extends alongside the spinal cord. The acetylcholine activates the postganglionic neurons.
Postganglionic neurons then release norepinephrine onto target organs. As anyone who has ever felt a rush before a big test, speech, or athletic event can attest, the effects of the sympathetic nervous system are quite pervasive. This is both because one preganglionic neuron synapses on multiple postganglionic neurons, amplifying the effect of the original synapse, and because the adrenal gland also releases norepinephrine and the closely related hormone epinephrine into the blood stream.
The physiological effects of this norepinephrine release include dilating the trachea and bronchi making it easier for the animal to breathe , increasing heart rate, and moving blood from the skin to the heart, muscles, and brain so the animal can think and run. The strength and speed of the sympathetic response helps an organism avoid danger, and scientists have found evidence that it may also increase LTP—allowing the animal to remember the dangerous situation and avoid it in the future. Parasympathetic preganglionic neurons have cell bodies located in the brainstem and in the sacral toward the bottom spinal cord, as shown in Figure The axons of the preganglionic neurons release acetylcholine on the postganglionic neurons, which are generally located very near the target organs. Most postganglionic neurons release acetylcholine onto target organs, although some release nitric oxide. Effects of acetylcholine release on target organs include slowing of heart rate, lowered blood pressure, and stimulation of digestion.
Sensory-Somatic Nervous System The sensory-somatic nervous system is made up of cranial and spinal nerves and contains both sensory and motor neurons. Sensory neurons transmit sensory information from the skin, skeletal muscle, and sensory organs to the CNS. Motor neurons transmit messages about desired movement from the CNS to the muscles to make them contract. Without its sensory-somatic nervous system, an animal would be unable to process any information about its environment what it sees, feels, hears, and so on and could not control motor movements. Unlike the autonomic nervous system, which has two synapses between the CNS and the target organ, sensory and motor neurons have only one synapse—one ending of the neuron is at the organ and the other directly contacts a CNS neuron.
Acetylcholine is the main neurotransmitter released at these synapses. Humans have 12 cranial nerves, nerves that emerge from or enter the skull cranium , as opposed to the spinal nerves, which emerge from the vertebral column. Each cranial nerve is accorded a name, which are detailed in Figure Some cranial nerves transmit only sensory information. For example, the olfactory nerve transmits information about smells from the nose to the brainstem. Other cranial nerves transmit almost solely motor information.
For example, the oculomotor nerve controls the opening and closing of the eyelid and some eye movements. Other cranial nerves contain a mix of sensory and motor fibers. For example, the glossopharyngeal nerve has a role in both taste sensory and swallowing motor. The human brain contains 12 cranial nerves that receive sensory input and control motor output for the head and neck. Spinal nerves transmit sensory and motor information between the spinal cord and the rest of the body.
Each of the 31 spinal nerves in humans contains both sensory and motor axons. The sensory neuron cell bodies are grouped in structures called dorsal root ganglia and are shown in Figure Each sensory neuron has one projection—with a sensory receptor ending in skin, muscle, or sensory organs—and another that synapses with a neuron in the dorsal spinal cord. Motor neurons have cell bodies in the ventral gray matter of the spinal cord that project to muscle through the ventral root. These neurons are usually stimulated by interneurons within the spinal cord but are sometimes directly stimulated by sensory neurons. Spinal nerves contain both sensory and motor axons.
The somas of sensory neurons are located in dorsal root ganglia. The somas of motor neurons are found in the ventral portion of the gray matter of the spinal cord. Summary The peripheral nervous system contains both the autonomic and sensory-somatic nervous systems. The autonomic nervous system provides unconscious control over visceral functions and has two divisions: the sympathetic and parasympathetic nervous systems. The parasympathetic nervous system is active during restful periods. The sensory-somatic nervous system is made of cranial and spinal nerves that transmit sensory information from skin and muscle to the CNS and motor commands from the CNS to the muscles.
Exercises Which of the following statements is false? The parasympathetic pathway is responsible for relaxing the body, while the sympathetic pathway is responsible for preparing for an emergency. Activation of the sympathetic nervous system causes: increased blood flow into the skin a decreased heart rate.
Reproduction permitted for individual classroom use. Label the three major parts of the nervous system. Toxic effects of lead on the central nervous system are observed more in children. In adults, the effects of lead toxicity occur in the peripheral nervous system. Use a reflex hammer to test the knee jerk response OR use the back of your hand and a swift, firm hit under the patella. Have students note the response. Macular Degeneration 1. Introduction — The macula is an area in the center of the retina at the back of the eye. In your plan, include the following items First, read the KidsHealth. Related KidsHealth Links. True or false: The skin is the largest organ in the body. Nominations must focus on a s ecific ESD ro'ect or ro ramme. Check out www.
The publisher's final edited version of this article is available at Cell See other articles in PMC that cite the published article. How the nervous system has changed in the human lineage and how it differs from that of closely related primates is not well understood. Here, we consider recent comparative analyses of extant species that are uncovering new evidence for evolutionary changes in the size and the number of neurons in the human nervous system, as well as the cellular and molecular reorganization of its neural circuits. We also discuss the developmental mechanisms and underlying genetic and molecular changes that generate these structural and functional differences. As relevant new information and tools materialize at an unprecedented pace, the field is now ripe for systematic and functionally relevant studies of the development and evolution of human nervous system specializations.
The question of what makes us human has fascinated humankind throughout modern history. Today, we view the brain as the core component of human identity, and an understanding of this organ is consequently essential for answering why we as a species are what we are. The remarkable abilities of the human brain are among the most obvious features that set us apart from our taxonomically closest living relatives, the other great apes of Africa. However, our understanding of how the nervous system—in particular, the brain—has changed in the human lineage remains incomplete Bae et al. Although humans share most of their genetic, molecular, and cellular features with other non-human primates NHPs , there are compelling differences in cognitive and behavioral capacities between humans and NHPs. Syntactical-grammatical language, symbolic thought, self-reflection, long-term planning ability, autobiographical memory, the theory of mind, and the capacity to create art are among the most distinctively human aspects of cognition and behavior Gazzaniga, ; Lieberman, ; Passingham, ; Penn et al.
Whether these differences represent incremental, quantitative advances in cognitive ability rather than a novel or qualitative leap is an open question. Historically, some cognitive faculties present in humans were thought to represent such a leap Lashley, ; Penn et al. For example, great apes have a more sophisticated natural behavioral repertoire than previously thought, and some behavioral skills are transmitted culturally, similar to how human infants learn from observing adults Horner et al. This implies that human mental abilities have gradually evolved from ancestral forms and functions, with culture in addition to genomic information playing a critical role in the emergence and transmission of complex behavioral skills. These observations also suggest that a greater understanding of the evolutionary differences separating humans and NHPs will yield new insights into human neurodevelopment, function, and disease.
Importantly, as more and better-designed studies are conducted, it is likely that further behavioral and cognitive homologies and differences between humans and NHPs will also be identified and provide insights into how the underlying structure and function of neural circuits have changed in the human lineage. Comparative studies of the human and NHP brain have historically been difficult to accomplish in systematic and functionally relevant manners. Most of the procedures used in experimental biology are not, due to ethical and legal limitations, applicable to human and NHP brains.
Therefore, it is not surprising that much of our understanding of the molecular and cellular mechanisms governing the development and function of the primate brain is derived from experimental studies of a handful of relatively distantly related model organisms. While the use of these model systems has revolutionized the biomedical sciences, extrapolating from them can be problematic; the prolonged development of the human brain, its remarkable cellular complexity and connectivity, and the reliance of the brain on experience for the shaping and refinement of synaptic connections all conspire to make translating data across experimental contexts difficult.
Furthermore, the type of studies that can be accomplished with the human or NHP brain is also hampered by experimental limitations, particularly those typical of in vitro assays, imaging modalities, and tissue collected postmortem, often only available to researchers after substantial delays. As a result, characterizing the genome-to-structure-to-function relationships in the nervous system and comparing these relationships between humans and NHPs is especially complicated Enard, ; Franchini and Pollard, ; Reilly and Noonan, ; Silver, Fortunately, recent advances in methods and tools are easing many of these limitations. Increased tissue banking, multi-institutional and multi-disciplinary consortia, and the development of improved protocols for tissue processing and preservation allow greater access to high-quality developmental and adult postmortem human and NHP tissues with reduced postmortem intervals.
The development of genomic methods and in vitro systems, including induced pluripotent stem cells iPSCs and primary human neural cells, have also expanded the range of studies possible. Non-invasive technologies, such as diffusion tensor imaging, that can be applied to living human and NHP subjects are similarly allowing new avenues of research. Together, these advances have enabled comparative studies of cognition, behavior, neuroanatomy, neurophysiology, proteome, transcriptome, and genome that have provided the necessary foundation to begin a comprehensive exploration of the mechanisms underlying human cognitive capacities. In this Review, we summarize current advances in our understanding of potentially distinguishing features of the function, organization, and development of the human central nervous system CNS.
Emphasis is given to studies on evolutionary changes in the size and number of neurons of the human brain and the organization of neural circuits—in particular, the long-distance projection systems associated with language and digital dexterity. We also highlight the importance of conducting comparative studies throughout development, as some of the most distinctively human aspects of cognition and behavior are apparent as early as infancy and toddlerhood. Next, we consider the search for the changes in genomic content and molecular processes that make these features possible. We also provide a perspective on the emerging experimental investigations linking the development of human evolutionary specializations to genetic changes and related molecular and cellular mechanisms.
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