1. Copyright © 2016 by Elsevier Inc. All rights reserved.
Nervous System
Chapter 9
Copyright © 2016 by Elsevier Inc. All rights reserved.

2. Learning Objectives Lesson 9.1: Nervous System
List the organs and divisions of the nervous system and describe the generalized functions of the system as a whole.
Identify the major types of cells in the nervous system and discuss the functions of each.
Identify the anatomical components of a reflex arc and explain its function.
Explain the mechanisms of transmission of a nerve impulse along a nerve fiber and across a synapse.
Copyright © 2016 by Elsevier Inc. All rights reserved.

3. Learning Objectives Lesson 9.1: Nervous System (Cont.)
Identify the major anatomical components of the brain and spinal cord and briefly comment on the functions of each.
Compare and contrast cranial and spinal nerves.
Discuss the anatomical and functional characteristics of the two divisions of the autonomic nervous system.
Copyright © 2016 by Elsevier Inc. All rights reserved.

4. Organization of the Nervous System
Central nervous system (CNS)
Brain
Spinal cord
Peripheral nervous system (PNS)
All nerves
Autonomic nervous system (ANS)
The nervous system is divided into two principal divisions: central nervous system (CNS) and peripheral nervous system (PNS).
The autonomic nervous system (ANS) is a subdivision of the PNS.
The ANS consists of structures that regulate the body’s automatic or involuntary functions.
Copyright © 2016 by Elsevier Inc. All rights reserved.

5. Divisions of the Nervous System
Figure 9-1 shows the divisions of the nervous system and what is included in each, as well as how they work together.
Copyright © 2016 by Elsevier Inc. All rights reserved.

6. Cells of the Nervous System
Neurons
Neuron structure: Each neuron consists of three main parts (Figure 9-2):
Cell body of neuron
Dendrites: Branching projections that conduct impulses to cell body of neuron
Axon: Elongated projection that conducts impulses away from cell body of neuron
Types of neurons: Classified according to function
Sensory (afferent) neurons: Conduct impulses to the spinal cord and brain
Motor (efferent) neurons: Conduct impulses away from brain and spinal cord to muscles and glands
Interneurons: Conduct impulses from sensory neurons to motor neurons or among a network of interneurons; also known as central or connecting neurons
Axon is surrounded by a segmented wrapping of a material called myelin.
Nodes of Ranvier are gaps between adjacent Schwann cells.
Copyright © 2016 by Elsevier Inc. All rights reserved.

7. Neuron
Figure 9-2, A, is a diagram o f a typical neuron, showing dendrites, a cell body, and an axon.
Figure 9-2, B, is a segment of a myelinated axon cut to show detail of the concentric layers of the Schwann cell filled with myelin.
Courtesy Dennis Streete.
Copyright © 2016 by Elsevier Inc. All rights reserved.

8. Copyright © 2016 by Elsevier Inc. All rights reserved.
Nerves and Tracts
Nerve: Bundle of peripheral axons
Tract: Bundle of central axons
White matter: Tissue composed primarily of myelinated axons (nerves or tracts)
Gray matter: Tissue composed primarily of cell bodies and unmyelinated fibers
Nerve coverings: Fibrous connective tissue
Endoneurium: Surrounds individual fibers within a nerve
Perineurium: Surrounds a group (fascicle) of nerve fibers
Epineurium: Surrounds the entire nerve
Peripheral nerve fibers usually have a myelin sheath.
Peripheral nerves often look white.
Copyright © 2016 by Elsevier Inc. All rights reserved.

9. Copyright © 2016 by Elsevier Inc. All rights reserved.
The Nerve
Figure 9-4: Each nerve contains axons bundled into fascicles.
A connective tissue called epineurium wraps the entire nerve.
Perineurium surrounds each fascicle.
Courtesy Dr. Richard Kessel, Professor of Biological Sciences, University of Iowa, Iowa City.
Copyright © 2016 by Elsevier Inc. All rights reserved.

10. Copyright © 2016 by Elsevier Inc. All rights reserved.
Knee-Jerk Reflex
Figure 9-5: The neural pathway involved in the knee-jerk (patellar) reflex.
A three-neuron reflex response that causes contraction of muscles to pull the leg away from the irritant is called the withdrawal reflex.
Copyright © 2016 by Elsevier Inc. All rights reserved.

11. Central Nervous System
Divisions of the brain: Brainstem
Consists of three parts of brain, named in ascending order
Medulla oblongata
Pons
Midbrain
Structure
White matter with bits of gray matter scattered through it
Functions
All three parts of brainstem conduct impulses to the higher parts of the brain
Sensory tracts in the brainstem conduct impulses to the higher parts of the brain
Motor tracts conduct from the higher parts of the brain to the spinal cord
The CNS is centrally located.
The brain is protected in the cranial cavity of the skull.
The brain and the spinal cord are protected by three membranes called meninges.
Small bits of gray matter mix closely and intricately with white matter to form reticular formation.
Copyright © 2016 by Elsevier Inc. All rights reserved.

12. Central Nervous System
Major Regions of the
Central Nervous System
Figure 9-10: A, Sagittal sections of the brain and spinal cord. B, Section of preserved brain.
From Vidic B, Suarez FR: Photographic atlas of the human body, St Louis, 1984, Mosby.
Copyright © 2016 by Elsevier Inc. All rights reserved.

13. Functions of Major Division of the Brain
The functions of major divisions of the brain are shown in Table 9-1.
Copyright © 2016 by Elsevier Inc. All rights reserved.

14. Central Nervous System (Cont.)
Divisions of the brain: Cerebellum
Structure
Second largest part of the human brain
Gray matter outer layer is thin but highly folded, forming a large surface area for processing information
Arbor vitae: Internal, treelike network of white matter tracts
Functions
Helps control muscle contractions to produce coordinated movements for maintaining balance, moving smoothly, and sustaining normal postures
Variety of additional coordinating effects, assisting the cerebrum and other regions of the brain
Cerebellum lies under the occipital lobe of the cerebrum.
Recent studies show that the cerebellum may have far more functions than earlier observed.
Copyright © 2016 by Elsevier Inc. All rights reserved.

15. Central Nervous System (Cont.)
Divisions of the brain: Diencephalon (hypothalamus, thalamus, and pineal gland)
Hypothalamus
Consists mainly of the posterior pituitary gland, pituitary stalk, and gray matter
Acts as the major center for controlling the ANS; therefore, it helps control the functioning of most internal organs
Controls hormone secretion by anterior and posterior pituitary glands; therefore, it indirectly helps control hormone secretion by most other endocrine glands
Contains centers for controlling body temperature, appetite, wakefulness, and pleasure
The diencephalon is small but important.
Hypothalamus is located below the thalamus.
Copyright © 2016 by Elsevier Inc. All rights reserved.

16. Central Nervous System (Cont.)
Divisions of the brain: Diencephalon (hypothalamus, thalamus, and pineal gland)
Thalamus
Dumbbell-shaped mass of gray matter extending toward each cerebral hemisphere
Relays sensory impulses to cerebral cortex sensory areas
In some way produces the emotions of pleasantness or unpleasantness associated with sensations
Pineal gland (pineal body)
Small body resembling a pine nut behind the thalamus
Adjusts output of “time-keeping hormone” melatonin in response to changing levels of external light (sunlight and moonlight)
Thalamus produces sensations, associates sensations with emotions, and plays a part in the arousal mechanism.
The pineal gland receives sensory information about the strength of light and adjusts its output of melatonin.
Copyright © 2016 by Elsevier Inc. All rights reserved.

17. Central Nervous System (Cont.)
Divisions of the brain: Cerebrum (Figure 9-11)
Largest part of the human brain
Outer layers of gray matter are the cerebral cortex; made up of lobes; composed mainly of dendrites and cell bodies of neurons
Interior of the cerebrum composed mainly of white matter
Tracts: Nerve fibers arranged in bundles
Basal nuclei: Islands of gray matter regulate automatic movements and posture
Functions of the cerebrum
Mental processes of all types, including sensations, consciousness, memory, and voluntary control of movements
Cerebrum is largest and uppermost part of the brain.
Cerebrum has ridges (gyri) and grooves (sulci).
Copyright © 2016 by Elsevier Inc. All rights reserved.

18. Copyright © 2016 by Elsevier Inc. All rights reserved.
The Cerebrum
Figure 9-11, A, shows the lobes of the cerebrum.
From Vidic B, Suarez FR: Photographic atlas of the human body, St Louis, 1984, Mosby.
Copyright © 2016 by Elsevier Inc. All rights reserved.

19. Copyright © 2016 by Elsevier Inc. All rights reserved.
The Cerebrum (Cont.)
Figure 9-11, B, shows the functional regions of the cerebral cortex.
From Vidic B, Suarez FR: Photographic atlas of the human body, St Louis, 1984, Mosby.
Copyright © 2016 by Elsevier Inc. All rights reserved.

20. Central Nervous System (Cont.)
Spinal cord (Figure 9-12)
Columns of white matter, composed of bundles of myelinated nerve fibers, form the outer portion of the H-shaped core of the spinal cord; bundles of axons called tracts
Interior composed of gray matter made up mainly of neuron dendrites and cell bodies
Spinal cord tracts provide two-way conduction paths: Ascending and descending
Spinal cord functions as the primary center for all spinal cord reflexes; sensory tracts conduct impulses to the brain, and motor tracts conduct impulses from the brain
If you are of average height, your spinal cord is about 17 to 18 inches long.
If an injury cuts the spinal cord all the way across, impulses can no longer travel to the brain from any part of the body located below the injury.
Copyright © 2016 by Elsevier Inc. All rights reserved.

21. Spinal Cord and Spinal Nerves
The spinal cord and spinal nerves are shown in Figure 9-12.
Inset is a dissection of the cervical region of the spinal cord; the spinal cord is viewed from behind (posterior aspect).
From Vidic B, Suarez FR: Photographic atlas of the human body, St Louis, 1984, Mosby.
Copyright © 2016 by Elsevier Inc. All rights reserved.

22. Central Nervous System (Cont.)
Coverings (Figure 9-14)
Cranial bones and vertebrae
Cerebral and spinal meninges
Dura mater
Pia mater
Arachnoid mater
Fluid spaces
Subarachnoid spaces of meninges
Central canal inside cord
Ventricles in brain
The meninges form the protective covering around the spinal cord.
The meninges are surrounded by hard bone of the skull and vertebrae, which protects from injury.
Copyright © 2016 by Elsevier Inc. All rights reserved.

23. Spinal Cord and Its Coverings
Figure 9-14: The meninges, spinal nerves, and sympathetic trunk are visible in this illustration.
Copyright © 2016 by Elsevier Inc. All rights reserved.

24. Peripheral Nervous System
Cranial nerves (Figure 9-17 and Table 9-2)
Twelve pairs: Attached to undersurface of the brain
Connect brain with the neck and structures in the thorax and abdomen
Spinal nerves
Thirty-one pairs: Contain dendrites of sensory neurons and axons of motor neurons
Conduct impulses necessary for sensations and voluntary movements
Dermatome: Skin surface area supplied by a single cranial or spinal nerve
The nerves connecting the brain and spinal cord to other parts of the body constitute the PNS.
Other structures in the ANS are considered part of the PNS and connect the brain and spinal cord to various glands of the body and to the cardiac and smooth muscles in the thorax and abdomen.
Copyright © 2016 by Elsevier Inc. All rights reserved.

25. Copyright © 2016 by Elsevier Inc. All rights reserved.
Cranial Nerves
Figure 9-17: View of the undersurface of the brain shows attachments of the cranial nerves.
Copyright © 2016 by Elsevier Inc. All rights reserved.

26. Copyright © 2016 by Elsevier Inc. All rights reserved.
Cranial Nerves (Cont.)
The names of each cranial nerve and a brief description of their functions are listed here.
Copyright © 2016 by Elsevier Inc. All rights reserved.

27. Autonomic Nervous System
ANS consists of motor neurons that conduct impulses from the CNS to cardiac muscle, smooth muscle, and glandular epithelial tissue; regulates the body’s automatic or involuntary functions (Figure 9-19)
Functional anatomy:
Preganglionic autonomic neurons conduct impulses from spinal cord or brainstem to an autonomic ganglion
Postganglionic neurons conduct from autonomic ganglia to cardiac muscle, smooth muscle, and glandular epithelial tissue
Copyright © 2016 by Elsevier Inc. All rights reserved.

28. Autonomic Nervous System (Cont.)
Functional anatomy: Autonomic or visceral effectors are the tissues to which autonomic neurons conduct impulses (i.e., cardiac and smooth muscle and glandular epithelial tissue)
Composed of two divisions
Sympathetic system
Parasympathetic system
Motor nerves that control the voluntary actions of skeletal muscles are sometimes called the somatic nervous system.
Copyright © 2016 by Elsevier Inc. All rights reserved.

29. Innervation of Major Target Organs by the Autonomic Nervous System
Figure 9-19: The sympathetic pathways are highlighted in orange, and the parasympathetic pathways are highlighted in green.
Copyright © 2016 by Elsevier Inc. All rights reserved.

30. Autonomic Nervous System (Cont.)
Autonomic conduction paths
Consist of two-neuron relays (that is, preganglionic neurons from the CNS to autonomic ganglia, synapses, postganglionic neurons from ganglia to visceral effectors)
In contrast, somatic motor neurons conduct all the way from the CNS to somatic effectors with no intervening synapses
Conduction paths to visceral and somatic effectors from the CNS differ somewhat.
Impulses travel over preganglionic neurons from the spinal cord (brainstem) to autonomic ganglia.
Copyright © 2016 by Elsevier Inc. All rights reserved.

31. Autonomic Nervous System (Cont.)
Sympathetic division: Structure
Dendrites and cell bodies of sympathetic preganglionic neurons are located in the gray matter of the thoracic and upper lumbar segments of the spinal cord
Axons leave the spinal cord in the anterior roots of spinal nerves, extend to sympathetic or collateral ganglia, and synapse with several postganglionic neurons whose axons extend to spinal or autonomic nerves to terminate in visceral effectors
A chain of sympathetic ganglia is in front of and at each side of the spinal column
Branches of the preganglionic axon may ascend or descend to terminate in ganglia above and below their point of origin.
Copyright © 2016 by Elsevier Inc. All rights reserved.

32. Autonomic Nervous System (Cont.)
Sympathetic division: Functions
Serves as the emergency or stress system, controlling visceral effectors during strenuous exercise and when strong emotions (anger, fear, hate, or anxiety) are triggered
Group of changes induced by sympathetic control is called the fight-or-flight response
Together, all these varied sympathetic responses make us ready for strenuous muscular work. We need such physiological preparation when facing a threat.
Copyright © 2016 by Elsevier Inc. All rights reserved.

33. Autonomic Nervous System (Cont.)
Parasympathetic division: Structure
Parasympathetic preganglionic neurons
Have dendrites and cell bodies in the gray matter of the brainstem and the sacral segments of the spinal cord
Terminate in parasympathetic ganglia located in the head and the thoracic and abdominal cavities close to visceral effectors
Each parasympathetic preganglionic neuron synapses with postganglionic neurons to only one effector
Parasympathetic division: Functions
Dominates control of many visceral effectors under normal, everyday conditions
Counterbalances sympathetic function
We can think of parasympathetic function as counterbalancing sympathetic function.
Copyright © 2016 by Elsevier Inc. All rights reserved.

34. Autonomic Nervous System (Cont.)
Autonomic neurotransmitters
Cholinergic fibers
Preganglionic axons of parasympathetic and sympathetic systems and parasympathetic postganglionic axons release acetylcholine
Adrenergic fibers
Axons of sympathetic postganglionic neurons release norepinephrine (noradrenaline)
The term autonomic nervous system implies that this part of the nervous system is independent from other parts but that is not true.
Copyright © 2016 by Elsevier Inc. All rights reserved.

35. Autonomic Nervous System (Cont.)
Autonomic nervous system as a whole
Regulates the body’s automatic functions in ways that maintain or quickly restore homeostasis
Many visceral effectors are doubly innervated (that is, they receive fibers from parasympathetic and sympathetic divisions and are influenced in opposite ways by the two divisions)
The limbic system is also known as the emotional brain.
According to some physiologists, the slightly altered state of consciousness known as meditation leads to decreased sympathetic activity and a group of changes opposite to those of the flight-or-flight response.
Copyright © 2016 by Elsevier Inc. All rights reserved.