46.1. Nervous Tissue (p. 818)
A. Neurons In Motor Neurons
1. Neurons vary in size and shape, but they all have three parts. (Fig. 46.1) [transp. 253]
a. Dendrites receive information and conduct impulses toward the cell body.
b. The cell body contains the nucleus and other organelles, and manufactures neurotransmitters.
c. The single axon conducts impulses away from the cell body; stimulates or inhibits a neuron, muscle, or gland.
2. Neurotransmitters are usually stored at the ends of axons in vesicles.
B. Myelination
1. A long axon is called a nerve fiber.
2. Long-axons are covered by a myelin sheath.
3. A myelin sheath is formed by the membranes of neuroglial cells.
4. In the PNS, neuroglial cells called neurolemmocytes perform this function, leaving gaps called neurofibril nodes.
C. Types of Neurons
1. Motor neurons have many dendrites and a single axon; conduct impulses from CNS to muscle fibers or glands.
2. Sensory neurons are unipolar---the process that extends from the cell body divides into two processes, both of which are the axon; conduct impulses from the periphery toward the CNA.
3. Interneurons are multipolar---have highly brauched dendrites within the CNS and convey messages between various parts of the CNS.
D. Transmitting the Nerve Impulse
1. Italian Luigi Galvani discovered in 1786 that a nerve is stimulated by an electric current.
2. The impulse is too slow to be due to movement of electrons as in electrical current.
3. Julius Bernstein suggested an impulse is movement of unequally distributed ions on either side of membrane.
4. A Nobel Prize in 1963 went to British researchers who confirmed this idea.
a. They and other researchers inserted a tiny electrode into the giant axon of a squid.
b. The electrode was attached to a voltmeter and oscilloscope to trace a change in voltage over time.
c. The voltage measured the difference in electrical potential between the inside and outside of the membrane.
d. The oscilloscope indicates the polarity changes.
E. Resting Potential Is Baseline
1. When an axon is not conducting an impulse, the oscilloscope records a membrane potential equal to -65 mV, indicating that the inside of the neuron is more negative than the outside. (Fig. 46.2) [transp. 254]
2. This is the resting membrane potential because the axon is not conducting an impulse.
3. This polarity is due to a difference in electrical charge on either side of the plasma membrane of the axon.
a. The inside of the plasma membrane is more negatively charged than the outside.
b. There is a higher concentration of K+ ions inside the axon, and there is a higher concentration of Na+ ions outside the axon. (Fig. 46.2) [transp. 254]
c. A sodium-potassium pump maintains this unequal distribution of Na+ and K+ ions.
4. The sodium-potassium (Na+ -K+ ) pump is an active transport system that moves Na+ ions out and K+ ions into the axon.
5. The pump is always working because membrane is permeable to these ions and they tend to diffuse toward the lesser concentration.
6. Since the membrane is more permeable to potassium ions than to sodium ions, there are always more positive ions outside the membrane; this accounts for some of the polarity.
7. Large negatively charged proteins in the cytoplasm of the axon also contribute to the resting potential.
F. Action Potential Brings Changes
1. When an axon is stimulated to conduct a nerve impulse, the rapid change in polarity is called the action potential.
2. The action potential is due to special protein-lined channels in the membrane that can open to allow either sodium or potassium ions to pass; these are sodium and potassium gates.
3. A stimulus causes some sodium gates to open, allowing Na+ ions to rush into the axon in depolarization.
4. First the trace on an oscilloscope goes from -65 mV to +40 mV; this is a depolarization phase indicating the cytoplasm is now more positive than the tissue fluid. (Fig. 46.2) [transp. 254]
5. The trace returns to -65 mV again; this is repolarization phase indicating inside of the axon is negative again.
6. More and more gates open when membrane potential decreases in an adjacent area; this generates an impulse.
7. At completion of the action potential, there are more potassium ions outside and more sodium ions inside.
8. Nerve impulses travel down thick nerve fibers faster than down thin nerve fibers.
9. In vertebrates, the primary way in which nerve impulse conduction is spread is not by increased size but because most long fibers have a myelin sheath.
a. Myelinated sheath has neurofibril nodes, gaps where one neurolemmocyte ends and the next begins.
b. The action potential "leaps" from one neurofibril node to another during saltatory conduction.
c. Saltatory conduction may reach rates of 200 meters/second.
G. Transmitting at Synapses
1. Charles Sherrington noticed in 1897 that nerve impulses move in one direction, and there is a very short delay.
2. He called the minute space between neurons the synapse.
3. A synapse consists of a presynaptic membrane, the synaptic cleft, and a postsynaptic membrane. (Fig. 46.3) [transp. 255]
a. When an action potential arrives at the presynaptic axon bulb, it increases the permeability of the plasma membrane to calcium ions (Ca2+).
b. Ca2+ ions enter and interact with actin filaments, causing the actin filaments to pull synaptic vesicles to the presynaptic membrane. (Fig. 46.3b) [transp. 255]
c. When the vesicles merge with the membrane, neurotransmitters are discharged into the synaptic cleft.
d. The neurotransmitter molecules diffuse across the synaptic cleft to the postsynaptic membrane, where they bind with a receptor in a lock-and-key manner. (Fig. 46.3c) [transp. 255]
e. This binding creates a depolarization (less negative) or a hyperpolarization (more negative).
f. The degree of change can vary and therefore postsynaptic potentials are termed graded potentials.
H. Graded Potentials Sum Up
1. A neuron compares and integrates information, then passes it to another neuron.
2. Integration is accomplished by summation---adding up the graded potentials received.
a. If negative inputs prevent threshold, postsynaptic neuron does not fire.
b. If positive inputs result in a threshold value, postsynaptic neuron fires.
I. Neurotransmitters Act Quickly
1. Acetylcholine (Ach) and norepinephrine (NE) are well know neurotransmitters active in both the peripheral nervous system and the central nervous system.
2. They are excitatory or inhibitory, according to the type of receptor at the postsynaptic membrane.
3. Once the neurotransmitter has been released into a synaptic cleft, it has only a short time to act.
4. In some synapses, the synaptic cleft contains enzymes that rapidly degrade the neurotransmitter acetylcholine.
5. In other synapses, the synaptic ending rapidly absorbs the neurotransmitter, possibly for repackaging in synaptic vesicles or for chemical breakdown.
6. The short existence of neurotransmitters in the synapse prevents continuous stimulation (or inhibition) of postsynaptic membranes.
46.2. How the Nervous System Evolved (p. 823)
A. Ability to Resond
1. In complex animals, the ability to respond to stimuli depends on functions of the nervous, endocrine, sensory, and musculoskeletal systems.
2. Working together, the nervous and endocrine systems coordinate the actions of all the other systems of the body to produce effective behavior and to keep the internal environment within safe limits for life.
3. Since hormones may take from seconds to hours to produce their effects, the nervous system is used for rapid communication.
4. The nervous system receives and processes information before sending out signals to the muscles and glands for an appropriate response; it integrates and controls the other systems of animals.
B. Invertebrate Nervous Organization
1. Comparative study of invertebrates indicates the possible evolutionary steps that may have led to the centralized nervous system found in vertebrates.
2. The most primitive sponges, with only a cellular level of organization, respond by closing the osculum.
3. Hydra are cnidaria and possess two cell layers separated by mesoglea.
a. Their simple nervous system extends like a net throughout the body within the mesoglea. (Fig. 46.4)
b. The nerve net is composed of neurons that are in contact with one another and with muscle fibers.
c. Hydra can contract, extend, move tentacles to capture prey, and turn somersaults.
d. More complex cnidaria (sea anemones and jellyfish) may have two nerve nets.
1) A fast-acting nerve net enables major responses, particularly in times of danger.
2) Another nerve net coordinates slower and more delicate movements.
4. The planarian nervous system is bilaterally symmetrical; it is composed of two lateral nerve cords, on either side of the longitudinal axis, that allow a rapid transfer of information from anterior to posterior.
a. The nervous system of planaria exhibits cephalization; at their anterior end, planaria have a simple brain composed of a cluster of neurons or ganglia.
b. Cerebral ganglia receive sensor information from photoreceptors in the eyespots and sensory cells in the auricles. (Fig. 46.4)
c. Transverse nerve fibers between nerve cords keep movement of two sides of a planarian body coordinated.
5. Annelids, arthropods, and molluscs are complex with true nervous systems. (Fig. 46.4c, d, e)
a. The central nervous system (CNS) consists of a brain and a ventral solid nerve cord.
1) The nerve cord has a ganglion in each segment of the body that controls muscles of that segment.
2) The brain still receives sensory information and controls the ganglia so entire animal is coordinated.
b. The presence of a brain and other ganglia indicate an increase in the number of neurons among invertebrates.
C. Vertebrate Nervous Organization
1. Vertebrate nervous systems exhibit cephalization and bilateral symmetry. (Fig. 46.4d)
a. The vertebrate nervous system is composed of central and peripheral nervous systems.
1) Central nervous system consists of brain and spinal cord; both develop from dorsal hollow nerve cord.
2) The peripheral nervous system consists of cranial and spinal nerves.
b. Paired eyes, ears, and olfactory structures gather information from the environment.
c. A vast increase in the number of neurons accompanied the evolution of the vertebrate nervous system; an insect may have one million neurons while a vertebrate may contain a thousand to a billion times more.
2. The Vertebrate Brain
a. The vertebrate brain develops at the anterior end of the dorsal hollow nerve cord.
b. The vertebrate brain is divided into the hindbrain, midbrain, and forebrain.
1) Vertebrates have a well-developed hindbrain that regulates organs below a level of consciousness; in humans its regulates lung and heart function even when we sleep, and coordinates motor activity.
2) Optic lobes are part of a midbrain; originally a center for coordinating reflex responses to visual input.
3) Forebrain receives sensory input from other two sections and regulates their output; cerebrum is highly developed in mammals and is associated with conscious control of the body, and the outer layer called the cerebral cortex is large and complex.
D. The Human Nervous System (Fig. 46.5) [transp. 256]
1. Three specific functions of the nervous system are:
a. to receive sensory input
b. to perform integration
c. to stimulate motor output
2. The central nervous system (CNS) is located in the midline of the body and integrates sensory information and controls the body.
3. The peripheral nervous system (PNS) lies outside the CNS and contains the cranial and spinal nerves.
4. The peripheral nervous system is divided into the somatic and autonomic systems.
5. The CNS and PNS of the human nervous system work together to perform the functions of the nervous system.
46.3. The Peripheral Nervous System Contains Nerves (p. 825)
A. Introduction to the PNS (Fig. 46.5) [transp. 252]
1. The peripheral nervous system lies outside the CNS and is composed of cranial nerves and spinal nerves.
a. Cranial nerves connect to the brain.
b. Spinal nerves lie on either side of the spinal cord.
2. Somatic system controls skeletal muscles.
a. Autonomic system controls smooth and cardiac muscles, and glands.
b. Autonomic system has two parts:
1) Sympathetic system
2) Parasympathetic system
B. Humans have 31 pairs of spinal nerves emerging from the spinal cord. (Fig. 46.6) [transp. 257]
1. Each spinal nerve leaves the spinal cord by two short branches (spinal roots), which lie within the vertebral column. (Fig. 46.6-46.7) [transp. 257 and 258]
a. The dorsal or sensory root contains fibers of sensory neurons that conduct nerve impulses to the cord.
b. The ventral root contains the axons of motor neurons that conduct nerve impulses away from the cord.
c. Since the two roots join before the nerve leaves the vertebral column, all spinal nerves are mixed nerves that conduct impulses to and from the spinal cord.
d. Pairs of spinal nerves exit a vertebral column between vertebrae; this is evidence of segmentation.
2. The spinal cord is a thick, whitish nerve cord that extends down the back, and is protected by the vertebrae.
3. The cord contains a tiny central canal filled with cerebrospinal fluid, gray matter consisting of cell bodies and short fibers, and white matter made of myelinated fibers.
C. Somatic System Serves Skin and Muscles
1. The somatic system includes all nerves that serve the musculoskeletal system and the exterior sense organs.
2. Exterior sense organs are receptors that receive environmental stimuli and initiate nerve impulses.
D. Reflexes Are Automatic
1. Reflexes are automatic, involuntary responses to changes occurring inside or outside the body.
2. In the somatic system, outside stimuli often initiate a reflex action, some of which require the brain.
3. A spinal reflex or reflex arc involves the following pathway. (Fig. 46.7) [transp. 258]
a. A receptor at end of a dendrite generates an impulse in a sensory neuron that moves toward the cell body in the central nervous system.
b. A cell body of a sensory neuron is located in the dorsal-root ganglion, just outside the cord.
c. Impulses travel along the axon and pass to many interneurons, one of which connects with a motor neuron.
d. The short dendrites and the cell body of the motor neuron lead to the axon which leaves the cord by the ventral root of the spinal nerve.
e. Nerve impulses travel along the axon to muscle fibers that contract.
4. The reflex response occurs because the sensory neuron stimulates several interneurons.
5. Impulses extend to all parts of the CNS including the cerebrum, which makes a person conscious of the stimulus and reaction.
E. Autonomic System Serves Internal Organs (Fig. 46.8) [transp. 259]
1. Autonomic system is a part of the PNS; its neurons control the internal organs automatically.
2. Sensory neurons from internal organs allow us to feel internal pain; their cell bodies are in dorsal-root ganglia.
3. There are two divisions: sympathetic and parasympathetic.
a. Both function automatically and usually subconsciously.
b. Both innervate internal organs.
c. Both utilize two motor neurons and one ganglion for each impulse to an effector organ.
1) The first neuron has a cell body within the CNS and a preganglionic fiber.
2) The second neuron has a cell body within the ganglion and a postganglionic fiber.
F. Sympathetic System: Fight or Flight
1. Most preganglionic fibers of the sympathetic system arise from the middle (thoracic-lumbar) portion of the spinal cord and almost immediately terminate in ganglia that lie near the cord (the thoracolumbar portion).
2. The preganglionic fiber is short, but the postganglionic fiber that contacts an organ is long.
3. The sympathetic system is especially important during emergency situations (the "fight or flight" response).
4. To defend or flee, muscles need a supply of glucose and oxygen; the sympathetic system accelerates heartbeat dilates bronchi, and increases breathing rate.
5. To divert energy from less necessary digestive function, the sympathetic system inhibits the digestion.
6. The neurotransmitter released by the postganglionic axon is mainly norepinephrine, similar to epinephrine (adrenaline) used as a heart stimulant.
G. Parasympathetic System: Relaxed State
1. The parasympathetic system consists of a few cranial nerves, including the vagus nerve, and fibers that arise from the bottom (sacral) portion of the spinal cord (therefore, the craniosacral portion).
2. The preganglionic fibers are long and the postganglionic fibers are short.
3. System is sometimes called "housekeeper system"; it promotes internal responses resulting in a relaxed state.
4. The parasympathetic system causes the pupil of the eye to constrict, promotes digestion of food, and retards heartbeat.
5. The neurotransmitter released is acetylcholine.
46.4. Central Nervous System: Brain and Spinal Cord (p. 830)
A. Introduction to the CNS
1. The central nervous system, which consists of a spinal cord and the brain, is where nerve impulses are interpreted.
2. Both are wrapped in three connective tissue coverings called meninges.
3. The spaces between the meninges are filled with cerebrospinal fluid that cushions and protects the CNS.
4. The cerebrospinal fluid is contained in the central canal of the spinal cord and within the ventricles of the brain, which are interconnecting spaces that produce and serve as reservoirs for the cerebrospinal fluid.
B. Spinal Cord Communicates
1. The spinal cord has two main functions: it is the center for many reflex actions and it provides the means of communication between the brain and the spinal nerves.
2. The spinal cord is composed of white and gray matter. (Fig. 46.7) [transp. 258]
a. Gray Matter
1) Unmyelinated cell bodies and short fibers give the gray matter its color.
2) In cross section, the gray area looks like a butterfly or the letter H.
3) It contains portions of sensory neurons and motor neurons and short interneurons that connect them.
b. White Matter
1) Myelinated long fibers of interneurons run together in bundles called tracts and give white matter its color.
2) Tracts conduct impulses between brain and spinal nerves; ascending tracts are dorsal and descending tracts are ventral.
3) The tracts cross over near the brain so that the left side of the brain controls the right side of the body.
C. The Brain Commands
1. Human brain is divided into medulla oblongata, cerebellum, pons, midbrain, hypothalamus, thalamus, and cerebrum. (Fig. 46.10) [transp. 260]
2. The brain has four cavities called ventricles: two lateral ventricles and a third and fourth ventricle.
D. Brain Stem
1. The medulla oblongata, the pons, and the midbrain all form the brain stem.
2. The medulla oblongata lies between spinal cord and pons, anterior to cerebellum. (Fig. 46.10)
a. It contains a number of vital centers for regulating heartbeat, breathing, and vasoconstriction.
b. It contains the reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing.
c. It contains nerve tracts that ascend or descend between the spinal cord and the brain's higher centers.
3. The pons contains bundles of axons traveling between the cerebellum and the rest of the CNS.
a. The pons functions with the medulla to regulate the breathing rate and has reflex centers concerned with head movements in response to visual or auditory stimuli.
4. Besides acting as a relay station for tracts passing between the cerebrum and the spinal cord or cerebellum, the midbrain has reflex centers for visual, auditory, and tactile responses.
E. Diencephalon
1. The hypothalamus and the thalamus are in a portion of the brain known as the diencephalon, where the third ventricle is located.
2. The hypothalamus forms the floor of the third ventricle.
3. The hypothalamus maintains homeostasis by exerting control over virtually all internal organs.
a. It contains centers for regulating hunger, sleep, thirst, body temperature, water balance, and blood pressure.
b. It also controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.
4. The thalamus is in the roof of the third ventricle.
a. It is the last portion of the brain for sensory input before the cerebrum.
b. It serves as central relay station for sensory impulses traveling upward from body and from brain to cerebrum.
c. It receives all sensory impulses and channels them to appropriate regions of cerebrum for interpretation.
F. Cerebellum
1. Cerebellum lies under posterior portion of cerebrum; it is separated from brain stem by the fourth ventricle.
2. The cerebellum integrates impulses from higher centers to coordinate muscle actions, maintains equilibrium and muscle tone, and sustains normal posture. (Fig. 46.9b)
3. Receiving information from inner ear indicating body position, it sends impulses to muscles maintaining balance.
G. Cerebrum
1. The cerebrum is the largest and foremost part of the brain.
2. It consists of two large cerebral hemispheres connected by a bridge of nerve fibers, the corpus callosum.
3. The outer portion is the cerebral cortex which is highly convoluted and consists of gray matter.
4. Cerebral cortex in each hemisphere contains four surface lobes; different functions are associated with each lobe. (Fig. 46.11)
a. The frontal lobe controls motor functions and permits us to control muscles consciously.
b. The parietal lobe is responsible for the sensations of temperature, touch, pressure, and pain from skin.
c. The occipital lobe interprets visual input.
d. The temporal lobe has sensory areas for hearing and smelling.
5. Comparison of vertebrates shows a continual evolutionary increase in the relative size of the cerebrum.
a. The cerebral cortex is most convoluted in humans.
b. In fishes and amphibians, the cerebrum is largely olfactory; in reptiles, birds and mammals, it receives information from other parts of the brain and coordinates sensory data and motor functions.
c. Only the cerebrum is responsible for consciousness, intelligence and reason.
d. The cerebrum overrides the functioning of the brain stem and diencephalon.
e. Researchers map regions that receive sensory information and control various parts of the body.
H. Limbic System
1. The limbic system involves portions of both conscious and unconscious brain.
2. It lies just beneath the cortex and contains neural pathways that connect portions of the frontal lobes, the temporal lobes, the thalamus, and the hypothalamus. (Fig. 46.12)
3. It includes several masses of gray matter deep within each hemisphere, called the basal nuclei.
4. Stimulation of different regions of the limbic system cause rage, pain, pleasure or sorrow; these are likely to increase our chance of survival.
5. Learning and memory
a. Limbic system is also involved in learning and memory; what permits memory development is not known.
b. Studies of the simpler systems of slugs and snails reveals learning is accompanied by an increase in number of synapses; forgetting involves a decrease in number of synapses.
c. Within individual neurons, learning involves changes in gene regulation and nerve protein synthesis.
d. Work with monkeys shows the limbic system is absolutely essential to short-term and long-term memory.
e. Involvement of the limbic system explains why emotionally charged events result in most vivid memory.
f. Since limbic system communicates with sensory areas, specific stimuli can awaken complex memories.