BASIC ANATOMY AND PHYSIOLOGY REVIEW
Nervous systems
The nervous and endocrine systems work together to coordinate the actions of all other systems of the body to produce behavior and maintain homeostasis.
Theendocrine system produces chemical messengers that are transported through the circulatory system. It requires seconds, minutes or hours.
The nervous system is more rapid, requiring only thousandths of a second.
Embryonic development
The nervous system originates from ectodermal tissue during embryonic development.
Neurons
Neurons are cells that transfer stimuli to other cells.
Structure of Neurons
Cell Body- contains nucleus and organelles
Dendrites- receive input
Axon - conducts impulses away from the cell body
Synaptic terminal - Neurotransmitters are manufactured in the cell body but released from synaptic terminals. The neurotransmitters stimulate other neurons.
Synapse - A synapse is the junction between the synaptic terminal and another cell. The other cell is called a postsynaptic cell.
Nerves and Ganglia
Axons and dendrites are bundled with axons or dendrites from other neurons to form nerves. Clusters of neuron cell bodies are called ganglia.
Central and Peripheral Nervous Systems
The nervous system can be divided into the central nervous system (CNS) which includes the brain and spinal cord and the peripheral nervous system (PNS) which includes everything else.
Classes of Neurons
Sensory neurons (afferent neurons) conduct sensory information toward the CNS. Sensory neurons have a long dendrite and a short axon.
The brain and spinal cord contain interneurons. These receive information and if they are sufficiently stimulated, they stimulate other neurons.
Motor neurons (efferent neurons)send information from interneurons to muscle or gland cells (effectors).
Myelination
Some neuroglia function to provide insulation for axons or dentrites. They do so by wrapping around the long fibers.
The insulation properties come from myelin contained within the cells.
The layer of insulation is referred to as a myelin sheath.
If these insulating cells are located in the peripheral nervous system, they are called Schwann cells.
Autonomic Nervous System
This part of the nervous system sends signals to the heart, smooth muscle, glands, and all internal organs.
It is generally without conscious control.
The autonomic nervous system uses two or more motor neurons:
The cell body of one of the motor neurons is in the CNS. The cell body of the other one is in a ganglion.
Sympathetic Division
The sympathetic nervous system stimulates the body. For example, it helps prepare the body to deal with emergency situations. This is often called the "fight or flight" response.
Stimulation from sympathetic nerves dilates the pupils, accelerates the heartbeat, increases the breathing rate, and inhibits the digestive tract.
The neurotransmitter is norepinephrine.
Sympathetic nerves arise from the middle (thoracic-lumbar) portion of the spinal cord.
Parasympathetic Division
When there is little stress, the parasympathetic system tends to slow down the overall activity of the body.
It causes the pupils to contract, it promotes digestion, and it slows the rate of heartbeat.
The neurotransmitter is acetylcholine.
The actual rate of stimulus to each organ is determined by the sum of opposing signals from the sympathetic and parasympathetic systems.
Parasympathetic nerves arise from the brain and sacral (near the legs) portion of the cord.
Summary of Brain Structure
Brain Structure / FunctionMedulla oblongata / Vital functions such as breathing, heart rate, and blood pressure
Reflexes such as vomiting, coughing, sneezing, hiccupping, swallowing, and digestion
Neurons cross
Pons / Breathing, connects spinal cord, cerebellum and higher brain centers
Cerebellum / Motor coordination
Midbrain / Receives visual, auditory, and tactile information
In mammals, this information is sent to the thalamus and higher brain centers. In lower vertebrates, the information is further processed in the midbrain.
Thalamus / Relays sensory information to the cerebral cortex.
Contains part of the reticular formation (controls arousal).
Hypothalamus / Maintains homeostasis, regulates the endocrine system
Contains part of the Limbic system (controls emotion)
Cerebrum / Processes sensory information and produces signals that move the skeletal muscles.
Cerebral Cortex / This is the outer layer of the cerebrum.
Thinking, intelligence, and cognitive functions are located here.
Processing of sensory information and motor responses
MUSCULAR SYSTEM
Types of Muscle
Skeletal Muscle
Skeletal muscle is voluntary.
The cells are long (up to 30 cm or 12 in), striated, and multinucleate.
Cells are striated
Cardiac Muscle
Cardiac muscle is found in the heart.
The cells are short, branched, and striated.
Smooth Muscle
Smooth muscle is involuntary.
It lines the gut, blood vessels, and reproductive tract.
Structure of Skeletal Muscle
Skeletal muscles are composed of many cells. Muscle cells are also called muscle fibers.
Myofibrils are strands found within muscle cells that are composed of the proteins actin and myosin. They extend from one end of the cell to the other and are capable of contraction, causing the cell to shorten.
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Muscle Contraction
During muscle contraction, actin and myosin filaments slide over each other as diagrammed below.
SARCOMEREis the unit of muscle contraction
Cross-bridges (not shown) form between the heads of myosin molecules and binding sites on the actin filaments.
After attachment, the myosin heads bend, causing the two filaments to move with respect to each other.
The energy is derived from ATP. ATP binds the myosin heads, causing them to be released from the actin filaments. When ATP is hydrolyzed to ADP + Pi, the myosin heads bend to their high-energy configuration. After bending to this configuration, they myosin heads bind to binding sites on the actin filaments. When ADP and Pi are released, the myosin heads return to their original low-energy configuration, causing the actin and myosin filaments slide past each other. ATP binds to the myosin heads, causing them to be released from the actin filaments and the process can begin again.
At death, there is no ATP to cause the heads to detach, and the body enters rigor mortis (it becomes stiff) because the myosin heads remain attached to the actin filaments.
Control of Muscle Contraction
Calcium triggers the contraction.
Function of Ca++
The actin filaments are composed of two rows of actin subunits that are wound around with tropomyosin threads.
In a relaxed muscle, contraction does not occur because the myosin binding sites on the actin filament are covered by the tropomyosin threads.
Troponin occurs at intervals along the threads. When Ca++ combines with troponin, the tropomyosin threads shift, exposing the myosin binding sites.
When the binding sites are exposed, the myosin heads attach and the filament contracts as previously described.
CIRCULATORY SYSTEM
Open Circulatory System
In an open circulatory system, blood is pumped from the heart through blood vessels but then it leaves the blood vessels and enters body cavities, where the organs are bathed in blood, or sinuses (spaces) within the organs.
Blood flows slowly in an open circulatory system because there is no blood pressure after the blood leaves the blood vessels. The animal must move its muscles to move the blood within the spaces.
In a closed system, blood remains within blood vessels, pressure is high, and blood is therefore pumped faster.
Arthropods and most mollusks (except cephalopods: nautilus, squid, octopus) have an open circulatory system.
Insects
The coelom of insects has been reduced to a cavity that carries blood (hemolymph). It is called a hemocoel..
dorsal heart aorta hemocoel
Ostia (openings in the heart) close when heart contracts. When heart relaxes, the ostia open and blood is sucked into openings.
The blood of insects is colorless because it lacks respiratory pigments; it functions to carry nutrients, not gases.
Animals with open circulatory systems generally have limited activity due to limitations in the oxygen delivery capability of the system. Insects are able to be active because gas exchange is via a tracheal system.
Closed Circulatory System
In a closed circulatory system, blood is not free in a cavity; it is contained within blood vessels. Valves prevent the backflow of blood within the blood vessels.
This type of circulatory system is found in vertebrates and several invertebrates including annelids, squids and octopuses.
The blood of animals with a closed circulatory system usually contains cells and plasma (liquid). The blood cells of vertebrates contain hemoglobin.
Earthworms
Earthworms have a dorsal and ventral blood vessel that runs the length of the animal. Branches from these vessels are found in each segment.
There are five vessels that pump blood from the dorsal vessel to the ventral vessel.
Earthworms have red blood (due to the pigment hemoglobin) but they have no cells. Hemoglobin binds with oxygen to carry it to the tissues.
Evolution of Vertebrate Circulatory System
Chambers of the Heart
Vertebrate hearts contain muscular chambers called atria (sing. atrium) and ventricles. Contraction of the chamber forces blood out. Blood flows in one direction due to valves that prevent backflow.
The atrium functions to receive blood that is returning to the heart. When it contracts, blood is pumped into the ventricle.
The ventricle is the main pumping chamber of the heart. When it contracts, blood is pumped away from the heart to the body, lungs, or gills.
Circulatory System of Fish
In the diagrams that follow, arrows represent the direction of blood flow in blood vessels (arteries and veins). Blood pressure is represented by the thickness of the arrows. Thick arrows indicate high blood pressure. Blood that is rich in oxygen is represented by red arrows. Blue arrows represent blood that is low in oxygen after it has passed through the body tissues.
Fish have a two-chambered heart with one atrium (A) and one ventricle (V).
The gills contain many capillaries for gas exchange, so the blood pressure is low after going through the gills. Low-pressure blood from the gills then goes directly to the body, which also has a large number of capillaries. The activity level of fish is limited due to the low rate of blood flow to the body.
Circulatory System of Amphibians
Amphibians have a 3-chambered heart with two atria and one ventricle.
Blood from the lungs (pulmonary flow) goes to the left atrium. Blood from the body (systemic flow) goes to the right atrium.
Both atria empty into the ventricle where some mixing occurs.
The advantage of this system is that there is high pressure in vessels that lead to both the lungs and body.
Circulatory System of Some Reptiles
Inmost reptiles, the ventricle is partially divided. This reduces mixing of oxygenated and unoxygenated blood in the ventricle. The partial division of the ventricle is represented by a dashed line below.
Circulatory System of Crocodilians, Birds, and Mammals
Birds and mammals (also crocodilians) have a four-chambered heart which acts as two separate pumps. After passing through the body, blood is pumped under high pressure to the lungs. Upon returning from the lungs, it is pumped under high pressure to the body. The high rate of oxygen-rich blood flow through the body enables birds and mammals to maintain high activity levels.
Blood Vessels
heart arteries arterioles capillaries venules veins heart
Arteries
Arteries carry blood away from heart.
Arteries have a thick, elastic layer to allow stretching and absorb pressure. The wall stretches and recoils in response to pumping, thus peaks in pressure are absorbed.
The arteries maintain pressure in the circulatory system much like a balloon maintains pressure on the air within it. The arteries therefore act as pressure reservoirs by maintaining (storing) pressure.
The elastic layer is surrounded by circular muscle to control the diameter and thus the rate of blood flow. An outer layer of connective tissue provides strength.
Arterioles
Smooth muscle surrounding the arteries and arterioles controls the distribution of blood. For example, blood vessels dilate when O2 levels decrease or wastes accumulate. This allows more blood into an area to bring oxygen and nutrients or remove wastes.
Capillaries
The smallest blood vessels are capillaries. They are typically less than 1 mm long. The diameter is so small that red blood cells travel single file.
The total length of capillaries on one person is over 50,000 miles. This would go around the earth twice.
The total cross-sectional area of the capillaries is greater than that of the arteries or veins, so the rate of blood flow (velocity) is lowest in the capillaries. Blood pressure is highest in the arteries but is considerably reduced as it flows through the capillaries. It is lowest in the veins.
As blood flows through the capillary and fluid moves out, the blood that remains behind becomes more concentrated. The osmotic pressure in the capillary is therefore greater near the veinule end and results in an increase in the amount of fluid moving into the capillary near this end.
The arrows on the diagram above represent the movement of blood into and out of the capillary. Long and thick arrows are used to represent a large amount of fluid movement. The total amount of movement out of the capillary is approximately equal to the amount of movement into the capillary. Notice that more blood tends to leave the capillary near the arteriole end and more tends to enter it near the veinule end.
The lymphatic capillaries collect excess fluid in the tissues.
Venules
Capillaries merge to form venules and venules merge into veins.
Venules can constrict due to the contraction of smooth muscle. When they are constricted there is more fluid loss in the capillaries due to increased pressure.
Veins
The diameter of veins is greater than that of arteries.
The blood pressure in the veins is low so valves in veins help prevent backflow.
The contraction of skeletal muscle during normal body movements squeezes the veins and assists with moving blood back to the heart.
The vena cava returns blood to the right atrium of the heart from the body. In the right atrium, the blood pressure is close to 0.
Human Circulation
Chambers of the heart
The heart is actually two separate pumps. The left side pumps blood to the body (systemic circulation) and the right side pumps blood to the lungs (pulmonary circulation). Each side has an atrium and a ventricle. See the diagram below
The atria function to receive blood when they are relaxed and to fill the ventricles when they contract.
The ventricles function to pump blood to the body (left ventricle) or to the lungs (right ventricle).
IMMUNE SYSTEM
Active and Passive Immunity
Active immunity is produced in individuals by administering foreign antigens. These antigens may come from weakened or dead microorganisms. This process is called vaccination.
Genetically engineered bacteria are currently being used to produce some antigens. Examples: malaria, hepatitis B.
After exposure to antigens in a vaccine, the level of antibodies in the blood begins to increase after several days, levels off, then declines. After a secondary exposure (called a booster), the level increases rapidly.
Memory B cells and memory T cells allow the individual to be actively immune. If they are exposed to the disease, a rapid immune response will occur because they already have large numbers of the correct B and T cells.
Passive immunity occurs when an individual receives antibodies instead of making their own. Passive immunity is short-lived because the person’s B and T cells have not been stimulated to produce antibodies. The immunity lasts only as long as the antibodies they received remain in their bloodstream.
Examples of Passive Immunity
Newborn babies have antibodies they received from their mother.
Breast-fed babies receive antibodies from their mother’s milk.
ENDOCRINE SYSTEM
Hypothalamus
The hypothalamus is part of the brain. It maintains homeostasis (constant internal conditions) by regulating the internal environment (examples: heart rate, body temperature, water balance, and the secretions of the pituitary gland).
Pituitary Gland
The pituitary contains two lobes. Hormones released by the posterior lobe are synthesized by neurons in the hypothalamus. Unlike the posterior lobe, the anterior lobe produces the hormones that it releases.
Refer to the diagram below as you read about the hypothalamus, pituitary, and each of the glands they control.
Pancreas
The pancreas is a digestive gland that secretes digestive enzymes into the duodenum through the pancreatic duct.
The islets of Langerhans are groups of cells within the pancreas that secrete insulin and glucagon. The islets are endocrine glands because they are ductless; the circulatory system carries their hormones to target cells.