Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 31

Nervous tissue and systems maintain homeostasis by enabling animals to respond to environmental changes. Nervous tissue includes neurons and glial. Neurons transmit electrochemical changes that stimulate or inhibit cell surface receptors. The complexity of a species' nervous tissue or system reflects adaptations to the environment.
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. A sensory neuron carries information towards 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.
A neuron's message, or nerve impulse, is measured as an action potential that occurs as an electrical change. In a neuron at rest, K{pos} concentration is 30 times greater inside the cell than outside, and the Na{pos} 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{pos} in, creating a positive charge inside. The positively charged Na{pos} depolarizes the membrane. At the peak of depolarization, Na{pos} channels close. Repolarization occurs as K{pos} leaves the cell, restoring the negatively charged resting potential. The action potential spreads along the nerve fiber.
Myelination increases the speed of nerve 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 nerve impulse "jumps" from one node to the next, increasing speed of conduction. Transmission of an action potential along a myelinated fiber is called saltatory conduction.
When an action potential reaches the end of an axon, it causes vesicles in the terminal portion of the presynaptic cell to move toward the membrane and release neurotransmitters into the synaptic cleft. These chemicals diffuse across the cleft and bind receptors on the postsynaptic cell. 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. This neural integration, the summing of excitatory and inhibitory messages, finely controls neuron activity. After it functions, a neurotransmitter is enzymatically destroyed or reabsorbed into the presynaptic cell.
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.
In the vertebrate nervous system, the CNS begins developing when the embryonic notochord stimulates ectoderm to develop into the neural tube. The neural tube closes at both ends, and then the anterior end swells, forming a brain. Neurons extend from the brain ventricles outward to form the outer brain layers. Other ectoderm forms the peripheral nervous system (PNS), which includes all neural tissue outside of the CNS.
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.
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 many vital physiological processes and regulates levels of some pituitary hormones; and the pineal and pituitary glands. The reticular activating system filters sensory input and is important for arousal.
The cerebrum has an inner layer of white matter and an outer layer of convoluted gray matter. It has two hemispheres, which 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.
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.
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.
Nervous system disorders may have molecular and/or environmental causes.
Bones of the skull and vertebrae, cerebrospinal fluid, the blood-brain barrier, and meninges protect the CNS. PNS neurons can regenerate, but those in the CNS cannot. Undamaged neurons can sometimes take over some functions of damaged neurons.