In the hierarchical organization of animal bodies, groups of cells with a common structure and function constitute tissues. The endocrine system and the nervous system are the two means of communication between different parts of the body. Circadian rhythm is daily fluctuations in metabolism and behavior aligned with the light and dark cycles in the environment.
Food in the digestive tract causes nervous and hormonal responses that control the secretion of digestive juices and promote the movement of ingested material through the tract. The many dissolved substances include inorganic salts in the form of dissolved ions, also called electrolytes. In the remainder of this chapter we will focus on the process of gas exchange.
The humoral immune response involves the activation and clonal selection of B cells, resulting in the production of secreted antibodies. In animals, hormones are secreted into the extracellular fluid, circulate in the hemolymph or blood, and communicate regulatory messages throughout the body. In the case of the caterpillar, a hormone called ecdysteroid stimulates the growth of adult cells, the programmed death of larval cells, and even the behavior that produces the motionless pupal stage.
In the case of birds and other reptiles, as well as monotremes (egg-laying mammals), the zygotes consist of eggs with calcium- and protein-containing shells and several internal eggs. Embryos of marsupials, such as kangaroos and opossums, spend only a short period in the womb; the embryos then crawl out and complete the development of the fetus, attached to a mammary gland in the mother's pouch. However, embryos of eutherian (placental) mammals, such as humans, remain in the uterus throughout fetal development.
NAWAL KAMILAH ISMAIL
Neuron, Synapses, and Signaling
- Neuron Organization and Structure Reflect Function in Information Transfer
- Ion Pumps and Ion Channels Establish The Resting Potential of A Neuron
- Action Potentials Are The Signals Conducted by Axons
- Neurons Communicate With Other Cells at Synapses
The pump uses ATP to actively transport Na out of the cell and K into the cell. Although there is a significant concentration gradient of sodium across the membrane, very little net diffusion of Na occurs because there are very few open sodium channels. Because the membrane is only weakly permeable to chloride and other anions, this efflux of K results in a net negative charge within the cell.
In resting neurons, the plasma membrane has many open potassium channels but few open sodium channels. Neurons have gated ion channels that open or close in response to stimuli, leading to changes in membrane potential. Changes in membrane potential that vary continuously with stimulus strength are known as graded potentials.
When a graded depolarization brings the membrane potential up to threshold, many voltage-gated ion channels open, causing an influx of Na that rapidly brings the membrane potential to a positive value. A negative membrane potential is restored by the inactivation of sodium channels and by the opening of many voltage-gated potassium channels, which increases K efflux. A nerve impulse travels from the axon hillock to the synaptic terminal by propagation of a series of action potentials along the axon.
The speed of conduction increases with the diameter of the axon and, in many vertebrate axons, with myelination. In response to the binding of neurotransmitter, ligand-gated ion channels in the postsynaptic membrane open (as shown here) or, less commonly, close. Synaptic transmission ends when the neurotransmitter diffuses out of the synaptic cleft, is taken up by the synaptic terminal or by another cell, or is broken down by an enzyme.
At many synapses, the neurotransmitter binds to ligand-gated ion channels in the postsynaptic membrane, producing an excitatory or inhibitory postsynaptic potential (EPSP or IPSP). The neurotransmitter then diffuses out of the cleft, is taken up by surrounding cells, or degraded by enzymes. Temporal and spatial summation at the axon terminal determines whether a neuron generates an action potential.
Nervous Systems
- Nervous Systems Consist of Circuits of Neurons and Supporting Cells
- The Vertebrate Brain is Regionally Specialized
- The Cerebral Cortex Controls Voluntary Movement and Cognitive Functions
- Changes in Synaptic Connections Underlie Memory and Learning
- Many Nervous System Disorders Can Be Explained in Molecular Terms
The sympathetic and parasympathetic divisions of the autonomic nervous system have antagonistic effects on a diverse set of target organs, while the enteric division controls the activity of many digestive organs. Each side of the cerebral cortex is divided into four lobes, and each lobe has specialized functions, some of which are listed here. Some areas on the left side of the brain (shown here) have different functions than those on the right side (not shown).
Parts of the frontal and temporal lobes, including Broca's area and Wernicke's area, are essential for generating and understanding language. These functions are concentrated in the left hemisphere of the brain, just like mathematical and logical operations. In the somatosensory cortex and the motor cortex, neurons are divided according to the part of the body that generates sensory input or receives motor commands.
Primates and cetaceans, which are capable of higher cognition, have an extensive complex neocortex, the outermost part of the cerebral cortex. The programmed death of neurons and elimination of synapses in embryos establishes the basic structure of the nervous system. In the adult, remodeling of the nervous system may involve loss or addition of synapses or strengthening or weakening of signaling at synapses.
In long-term memory, these temporary connections are replaced by connections within the cerebral cortex. Long-term potentiation (LTP) is a persistent increase in the strength of synaptic transmission and appears to be an important process in memory storage and learning. Drugs that increase the activity of biogenic amines in the brain can be used to treat bipolar disorder and depressive disorders.
The compulsive drug use that characterizes addiction reflects altered activity of the brain's reward system, which normally provides motivation for actions that enhance survival or reproduction. Alzheimer's disease is a dementia in which neurofibrillary tangles and amyloid plaques form in the brain. Alzheimer's disease leads to the death of neurons in many areas of the brain, including the hippocampus and cerebral cortex.
ANIMAL FORM AND FUNCTION Sensory and Motor Mechanisms
Pressure waves in the fluid vibrate the basilar membrane, depolarizing the hair cells and causing action potentials that travel through the auditory nerve to the brain. Each region of the basilar membrane vibrates more strongly at a certain frequency and leads to the excitation of a specific auditory area of the cerebral cortex. In the vertebrate eye, a single lens is used to focus light onto photoreceptors in the retina.
In humans, sensory cells within the taste buds express a single type of receptor that is specific for one of the five taste sensations—sweet, sour, salty, bitter, and umami (elicited by glutamate). Olfactory receptor cells line the upper part of the nasal cavity and extend axons to the olfactory bulb of the brain. Muscle cells (fibers) of vertebrate skeletal muscles contain myofibrils, consisting of thin filaments (mostly) actin and thick filaments of myosin.
Myosin heads, activated by the hydrolysis of ATP, bind to the thin filaments, forming cross-bridges and then are released when ATP is re-bound. The cardiac muscle, found only in the heart, consists of striated cells electrically connected by intermediate discs and can generate action potentials without input from neurons. In smooth muscle, contractions are slow and can be initiated by the muscles themselves or by stimulation by neurons in the autonomic nervous system.
For any of the three main forms of locomotion, larger animals are more efficient than smaller ones. Variation in the mating system and variation in the method of fertilization affect paternity certainty, which in turn has a significant impact on mating behavior and parental care. Game theory provides a way to think about evolution in situations where the fitness of a particular behavioral phenotype is influenced by other behavioral phenotypes in the population.
Research with two species of voles has revealed that variation in a single gene can determine differences in complex behaviors involved in both mating and parenting. When behavioral variation within a species matches variation in environmental conditions, it can be evidence of past evolution. Field and laboratory studies have documented the genetic basis for a change in migratory behavior of certain birds and have revealed behavioral differences in snakes that correlate with geographic variation in prey availability.
