The Nervous System
31.1 The Nervous System Maintains Homeostasis
1. Nervous systems maintain homeostasis by enabling animals to respond to environmental changes.
2. A nervous system receives information, integrates it, and may respond to it, often with movement.
31.2 Neurons Are Functional Units of a Nervous System
3. A neuron has a cell body; dendrites, which receive impulses and transmit them toward the cell body; and an axon, which conducts impulses away from the cell body.
4. A sensory neuron carries information toward the brain and spinal cord (central nervous system, or CNS). A motor neuron carries information from the CNS and stimulates an effector (a muscle or gland). An interneuron conducts information between two neurons and coordinates responses.
5. A neural impulse, measured as an action potential, is an electrical change. In a neuron at rest, K+ concentration is 30 times greater inside the cell than outside, and the Na+ concentration is 10 times greater outside than inside. The resulting diffusional gradients combined with negatively charged proteins within the cell give the interior a negative charge. When an action potential begins, membrane channels open and allow Na+ in, creating a positive charge inside. The positively charged Na+ depolarizes the membrane. At the peak of depolarization, Na+ channels close. Repolarization occurs as K+ leaves the cell, restoring the negatively charged resting potential. The action potential spreads along the nerve fiber.
6. Myelination increases the speed of neural impulse transmission. A myelin sheath forms around some nerve fibers as fatty Schwann cells wrap around the fiber. The gaps between these insulating cells are nodes of Ranvier. In a myelinated fiber, the neural impulse “jumps” from one node to the next, an action called saltatory conduction.
31.3 Neurotransmitters Pass the Message from Cell to Cell
7. The several types of neurotransmitters, which are chemicals that transmit a neural impulse, include modified and unmodified amino acids. Neuromodulators alter a neuron’s response to a neurotransmitter. Neurotransmitters and neuromodulators interact in complex ways.
8. An action potential reaching the end of an axon causes vesicles in the presynaptic neuron to approach the cell membrane and release neurotransmitters into the synaptic cleft. These chemicals diffuse across the cleft and bind to receptors on the postsynaptic neuron.
9. Used neurotransmitter is enzymatically destroyed or reabsorbed into the presynaptic cell.
10. If the neurotransmitter is excitatory, it slightly depolarizes the postsynaptic membrane, making an action potential more probable. An inhibitory neurotransmitter hyperpolarizes the membrane, making an action potential less likely. Synaptic integration sums excitatory and inhibitory messages, finely controlling neuron activity.
31.4 Evolutionary Trends in Nervous Systems
11. Nervous systems are groups of interacting cells that help coordinate the activities of animals. The form of a nervous system is adapted to a species’ environment.
12. The simplest invertebrate nervous systems are nerve nets, which detect stimuli from any direction. Flatworms have simple brains, paired sense organs, and two nerve cord ladders that allow localized motor control. Annelids have a more elaborate brain and ladder organization, with acute senses. Arthropods have even more complex sensory organs and behaviors.
13. The vertebrate nervous system includes the CNS and the peripheral nervous system (PNS), which includes all neural tissue outside of the CNS.
31.5 The Human Central Nervous System
14. The human spinal cord is a tube of neural tissue encased in the vertebral canal. The white matter on the periphery of the cord conducts impulses to and from the brain. The spinal cord is a reflex center. A reflex is a quick, automatic, protective response that travels through a reflex arc. A reflex arc usually consists of a sensory receptor, a sensory neuron, a spinal interneuron, a motor neuron, and an effector, such as a muscle or gland.
15. The brain has three regions. The hindbrain includes the medulla oblongata, which controls many vital functions; the cerebellum, which coordinates unconscious movements; and the pons, which bridges the medulla and higher brain regions and connects the cerebellum to the cerebrum. The midbrain processes visual and auditory sensory information; the tectum integrates this information. The forebrain consists of the telencephalon, which includes the cerebrum and the diencephalon, which contains the thalamus, a relay station between lower and higher brain regions; the hypothalamus, which regulates vital physiological processes and regulates levels of some pituitary hormones; and the pineal and pituitary glands. The reticular formation filters sensory input and is important for arousal.
16. The cerebrum has an inner layer of white matter and an outer layer of convoluted gray matter, which comprises the cerebral cortex, where information is processed and integrated. The cerebrum’s two hemispheres receive sensory input from and direct motor responses to the opposite side of the body. The left hemisphere specializes in language and analytical reasoning, and the right hemisphere regulates spatial, intuitive, and creative abilities.
17. Short-term memory may depend on temporal electrical activity in neuronal circuits. Short-term memories may consolidate into long-term memories, which depend on permanent chemical or structural changes in neurons. Memories form in the hippocampus.
31.6 The Human Peripheral Nervous System
18. The PNS consists of the somatic (voluntary) nervous system and the autonomic (involuntary) nervous system. The somatic nervous system includes cranial and spinal nerves that transmit sensations from sensory receptors or stimulation to voluntary muscles. The autonomic nervous system receives sensory information and conveys impulses to smooth muscle, cardiac muscle, and glands. Within the autonomic nervous system, the sympathetic nervous system controls physical responses to stressful events, while the parasympathetic nervous system dominates during rest.
31.7 Protection of the Nervous System
19. Bones of the skull and vertebrae, cerebrospinal fluid, the blood-brain barrier, and meninges protect the CNS. PNS neurons can regenerate. The CNS has stem cells. Undamaged neurons can sometimes take over some functions of damaged neurons.