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The Nose and its Adjacent Structures
Several bones that help form the walls of the nasal cavity have air-containing spaces called the paranasal sinuses, which serve to warm and humidify incoming air. Sinuses are lined with a mucosa. Each paranasal sinus is named for its associated bone:…
Comparison of Somatic and Visceral Reflexes
The afferent inputs to somatic and visceral reflexes are essentially the same, whereas the efferent branches are different. Somatic reflexes, for instance, involve a direct connection from the ventral horn of the spinal cord to the skeletal muscle.…
Autonomic Varicosities
The connection between autonomic fibers and target effectors is not the same as the typical synapse, such as the neuromuscular junction. Instead of a synaptic end bulb, a neurotransmitter is released from swellings along the length of a fiber that…
Connections of Parasympathetic Division of the Autonomic Nervous System
Neurons from brain-stem nuclei, or from the lateral horn of the sacral spinal cord, project to terminal ganglia near or within the various organs of the body. Axons from these ganglionic neurons then project the short distance to those target…
Corticospinal Tract
The major descending tract that controls skeletal muscle movements is the corticospinal tract. It is composed of two neurons, the upper motor neuron and the lower motor neuron. The upper motor neuron has its cell body in the primary motor cortex of…
Ventral and Dorsal Visual Streams
From the primary visual cortex in the occipital lobe, visual processing continues in two streams—one into the temporal lobe and one into the parietal lobe.
Retinal Disparity
Because of the interocular distance, which results in objects of different distances falling on different spots of the two retinae, the brain can extract depth perception from the two-dimensional information of the visual field.
Topographic Mapping of the Retina onto the Visual Cortex
The visual field projects onto the retina through the lenses and falls on the retinae as an inverted, reversed image. The topography of this image is maintained as the visual information travels through the visual pathway to the cortex.
The Sensory Homunculus
A cartoon representation of the sensory homunculus arranged adjacent to the cortical region in which the processing takes place.
Segregation of Visual Field Information at the Optic Chiasm
Contralateral visual field information from the lateral retina projects to the ipsilateral brain, whereas ipsilateral visual field information has to decussate at the optic chiasm to reach the opposite side of the brain.
Vestibulo-ocular Reflex
Connections between the vestibular system and the cranial nerves controlling eye movement keep the eyes centered on a visual stimulus, even though the head is moving. During head movement, the eye muscles move the eyes in the opposite direction as…
Auditory Brain Stem Mechanisms of Sound Localization
Localizing sound in the horizontal plane is achieved by processing in the medullary nuclei of the auditory system. Connections between neurons on either side are able to compare very slight differences in sound stimuli that arrive at either ear and…
Ascending Sensory Pathways of the Spinal Cord
The dorsal column system and spinothalamic tract are the major ascending pathways that connect the periphery with the brain.
Comparison of Color Sensitivity of Photopigments
Comparing the peak sensitivity and absorbance spectra of the four photopigments suggests that they are most sensitive to particular wavelengths.
Retinal Isomers
The retinal molecule has two isomers, (a) one before a photon interacts with it and (b) one that is altered through photoisomerization.
Photoreceptor
(a) All photoreceptors have inner segments containing the nucleus and other important organelles and outer segments with membrane arrays containing the photosensitive opsin molecules. Rod outer segments are long columnar shapes with stacks of…
Structure of the Eye
The sphere of the eye can be divided into anterior and posterior chambers. The wall of the eye is composed of three layers: the fibrous tunic, vascular tunic, and neural tunic. Within the neural tunic is the retina, with three layers of cells and two…
Extraocular Muscles
The extraocular muscles move the eye within the orbit.
The Eye in the Orbit
The eye is located within the orbit and surrounded by soft tissues that protect and support its function. The orbit is surrounded by cranial bones of the skull.
Rotational Coding by Semicircular Canals
Rotational movement of the head is encoded by the hair cells in the base of the semicircular canals. As one of the canals moves in an arc with the head, the internal fluid moves in the opposite direction, causing the cupula and stereocilia to bend.…
Linear Acceleration Coding by Maculae
The maculae are specialized for sensing linear acceleration, such as when gravity acts on the tilting head, or if the head starts moving in a straight line. The difference in inertia between the hair cell stereocilia and the otolithic membrane in…
Frequency Coding in the Cochlea
The standing sound wave generated in the cochlea by the movement of the oval window deflects the basilar membrane on the basis of the frequency of sound. Therefore, hair cells at the base of the cochlea are activated only by high frequencies, whereas…
Cochlea and Organ of Corti
a given region of the basilar membrane will only move if the incoming sound is at a specific frequency. Because the tectorial membrane only moves where the basilar membrane moves, the hair cells in this region will also only respond to sounds of this…
Hair Cell
The hair cell is a mechanoreceptor with an array of stereocilia emerging from its apical surface. The stereocilia are tethered together by proteins that open ion channels when the array is bent toward the tallest member of their array, and closed…
Cross Section of the Cochlea
The three major spaces within the cochlea are highlighted. The scala tympani and scala vestibuli lie on either side of the cochlear duct. The organ of Corti, containing the mechanoreceptor hair cells, is adjacent to the scala tympani, where it sits…
Transmission of Sound Waves to Cochlea
A sound wave causes the tympanic membrane to vibrate. This vibration is amplified as it moves across the malleus, incus, and stapes. The amplified vibration is picked up by the oval window causing pressure waves in the fluid of the scala vestibuli…
Structures of the Ear
The external ear contains the auricle, ear canal, and tympanic membrane. The middle ear contains the ossicles and is connected to the pharynx by the Eustachian tube. The inner ear contains the cochlea and vestibule, which are responsible for audition…
The Olfactory System
(a) The olfactory system begins in the peripheral structures of the nasal cavity. (b) The olfactory receptor neurons are within the olfactory epithelium. (c) Axons of the olfactory receptor neurons project through the cribriform plate of the ethmoid…
The Tongue
The tongue is covered with small bumps, called papillae, which contain taste buds that are sensitive to chemicals in ingested food or drink. Different types of papillae are found in different regions of the tongue. The taste buds contain specialized…
Receptor Classification by Cell Type
Receptor cell types can be classified on the basis of their structure. Sensory neurons can have either (a) free nerve endings or (b) encapsulated endings. Photoreceptors in the eyes, such as rod cells, are examples of (c) specialized receptor cells.…
Nerve Plexuses of the Body
There are four main nerve plexuses in the human body. The cervical plexus supplies nerves to the posterior head and neck, as well as to the diaphragm. The brachial plexus supplies nerves to the arm. The lumbar plexus supplies nerves to the anterior…
The Cranial Nerves
The anatomical arrangement of the roots of the cranial nerves observed from an inferior view of the brain.
Close-Up of Nerve Trunk
Zoom in on this slide of a nerve trunk to examine the endoneurium, perineurium, and epineurium in greater detail (tissue source: simian).
Nerve Structure
The structure of a nerve is organized by the layers of connective tissue on the outside, around each fascicle, and surrounding the individual nerve fibers (tissue source: simian).
Spinal Cord and Root Ganglion
The slide includes both a cross-section of the lumbar spinal cord and a section of the dorsal root ganglion (see also [link]) (tissue source: canine).
Dorsal Root Ganglion
The cell bodies of sensory neurons, which are unipolar neurons by shape, are seen in this photomicrograph. Also, the fibrous region is composed of the axons of these neurons that are passing through the ganglion to be part of the dorsal nerve root…
Cerebrospinal Fluid Circulation
The choroid plexus in the four ventricles produce CSF, which is circulated through the ventricular system and then enters the subarachnoid space through the median and lateral apertures. The CSF is then reabsorbed into the blood at the arachnoid…
Dural Sinuses and Veins
Blood drains from the brain through a series of sinuses that connect to the jugular veins.
Circle of Willis
The blood supply to the brain enters through the internal carotid arteries and the vertebral arteries, eventually giving rise to the circle of Willis.
Cross-section of Spinal Cord
The cross-section of a thoracic spinal cord segment shows the posterior, anterior, and lateral horns of gray matter, as well as the posterior, anterior, and lateral columns of white matter
The Cerebellum
The cerebellum is situated on the posterior surface of the brain stem. Descending input from the cerebellum enters through the large white matter structure of the pons. Ascending input from the periphery and spinal cord enters through the fibers of…
The Brain Stem
The brain stem comprises three regions: the midbrain, the pons, and the medulla.
The Diencephalon
The diencephalon is composed primarily of the thalamus and hypothalamus, which together define the walls of the third ventricle. The thalami are two elongated, ovoid structures on either side of the midline that make contact in the middle. The…
Frontal Section of Cerebral Cortex and Basal Nuclei
The major components of the basal nuclei, shown in a frontal section of the brain, are the caudate (just lateral to the lateral ventricle), the putamen (inferior to the caudate and separated by the large white-matter structure called the internal…
Brodmann's Areas of the Cerebral Cortex
Brodmann mapping of functionally distinct regions of the cortex was based on its cytoarchitecture at a microscopic level.
Lobes of the Cerebral Cortex
The cerebral cortex is divided into four lobes. Extensive folding increases the surface area available for cerebral functions.
The Cerebrum
The cerebrum is a large component of the CNS in humans, and the most obvious aspect of it is the folded surface called the cerebral cortex.
Spinal Bifida
(a) Spina bifida is a birth defect of the spinal cord caused when the neural tube does not completely close, but the rest of development continues. The result is the emergence of meninges and neural tissue through the vertebral column. (b) Fetal…
Primary and Secondary Vesicle Stages of Development
The embryonic brain develops complexity through enlargements of the neural tube called vesicles; (a) The primary vesicle stage has three regions, and (b) the secondary vesicle stage has five regions.
Early Embryonic Development of Nervous System
The neuroectoderm begins to fold inward to form the neural groove. As the two sides of the neural groove converge, they form the neural tube, which lies beneath the ectoderm. The anterior end of the neural tube will develop into the brain, and the…
Receptor Types
(a) An ionotropic receptor is a channel that opens when the neurotransmitter binds to it. (b) A metabotropic receptor is a complex that causes metabolic changes in the cell when the neurotransmitter binds to it (1). After binding, the G protein…
The Synapse
The synapse is a connection between a neuron and its target cell (which is not necessarily a neuron). The presynaptic element is the synaptic end bulb of the axon where Ca2+ enters the bulb to cause vesicle fusion and neurotransmitter release. The…
Graded Potentials
Graded potentials are temporary changes in the membrane voltage, the characteristics of which depend on the size of the stimulus. Some types of stimuli cause depolarization of the membrane, whereas others cause hyperpolarization. It depends on the…
Leakage Channels
In certain situations, ions need to move across the membrane randomly. The particular electrical properties of certain cells are modified by the presence of this type of channel.
Voltage-Gated Channels
Voltage-gated channels open when the transmembrane voltage changes around them. Amino acids in the structure of the protein are sensitive to charge and cause the pore to open to the selected ion.
Ligand-Gated Channels
When the ligand, in this case the neurotransmitter acetylcholine, binds to a specific location on the extracellular surface of the channel protein, the pore opens to allow select ions through. The ions, in this case, are cations of sodium, calcium,…
Cell Membrane and Transmembrane Proteins
The cell membrane is composed of a phospholipid bilayer and has many transmembrane proteins, including different types of channel proteins that serve as ion channels.
The Motor Response
On the basis of the sensory input and the integration in the CNS, a motor response is formulated and executed.
The Sensory Input
Receptors in the skin sense the temperature of the water.
Testing the Water
(1) The sensory neuron has endings in the skin that sense a stimulus such as water temperature. The strength of the signal that starts here is dependent on the strength of the stimulus. (2) The graded potential from the sensory endings, if strong…
The Process of Myelination
Myelinating glia wrap several layers of cell membrane around the cell membrane of an axon segment. A single Schwann cell insulates a segment of a peripheral nerve, whereas in the CNS, an oligodendrocyte may provide insulation for a few separate axon…
Glial Cells of the PNS
The PNS has satellite cells and Schwann cells.
Glial Cells of the CNS
The CNS has astrocytes, oligodendrocytes, microglia, and ependymal cells that support the neurons of the CNS in several ways.
Other Neuron Classifications
Three examples of neurons that are classified on the basis of other criteria. (a) The pyramidal cell is a multipolar cell with a cell body that is shaped something like a pyramid. (b) The Purkinje cell in the cerebellum was named after the scientist…
Parts of a Neuron
The major parts of the neuron are labeled on a multipolar neuron from the CNS.
Somatic, Autonomic, and Enteric Structures of the Nervous System
Somatic structures include the spinal nerves, both motor and sensory fibers, as well as the sensory ganglia (posterior root ganglia and cranial nerve ganglia). Autonomic structures are found in the nerves also, but include the sympathetic and…
Optic Nerve Versus Optic Tract
This drawing of the connections of the eye to the brain shows the optic nerve extending from the eye to the chiasm, where the structure continues as the optic tract. The same axons extend from the eye to the brain through these two bundles of fibers,…
What Is a Nucleus
(a) The nucleus of an atom contains its protons and neutrons. (b) The nucleus of a cell is the organelle that contains DNA. (c) A nucleus in the CNS is a localized center of function with the cell bodies of several neurons, shown here circled in red
Central and Peripheral Nervous System
The structures of the PNS are referred to as ganglia and nerves, which can be seen as distinct structures. The equivalent structures in the CNS are not obvious from this overall perspective and are best examined in prepared tissue under the…
Muscles of the Lower Leg
The muscles of the anterior compartment of the lower leg are generally responsible for dorsiflexion, and the muscles of the posterior compartment of the lower leg are generally responsible for plantar flexion. The lateral and medial muscles in both…
Intrinsic Muscles of the Foot
The muscles along the dorsal side of the foot (a) generally extend the toes while the muscles of the plantar side of the foot (b, c, d) generally flex the toes. The plantar muscles exist in three layers, providing the foot the strength to…
Hip and Thigh Muscles
The large and powerful muscles of the hip that move the femur generally originate on the pelvic girdle and insert into the femur. The muscles that move the lower leg typically originate on the femur and insert into the bones of the knee joint. The…
Intrinsic Muscles of the Hand
The intrinsic muscles of the hand both originate and insert within the hand. These muscles provide the fine motor control of the fingers by flexing, extending, abducting, and adducting the more distal finger and thumb segments.
Muscles That Move the Forearm
The muscles originating in the upper arm flex, extend, pronate, and supinate the forearm. The muscles originating in the forearm move the wrists, hands, and fingers.
Muscles That Move the Humerus
(a, c) The muscles that move the humerus anteriorly are generally located on the anterior side of the body and originate from the sternum (e.g., pectoralis major) or the anterior side of the scapula (e.g., subscapularis). (b) The muscles that move…
Muscles That Position the Pectoral Girdle
The muscles that stabilize the pectoral girdle make it a steady base on which other muscles can move the arm. Note that the pectoralis major and deltoid, which move the humerus, are cut here to show the deeper positioning muscles.
Muscles of the Pelvic Floor
The pelvic floor muscles support the pelvic organs, resist intra-abdominal pressure, and work as sphincters for the urethra, rectum, and vagina.
Intercostal Muscles
The external intercostals are located laterally on the sides of the body. The internal intercostals are located medially near the sternum. The innermost intercostals are located deep to both the internal and external intercostals.
Muscles of the Diaphragm
The diaphragm separates the thoracic and abdominal cavities.
Muscles of the Abdomen
(a) The anterior abdominal muscles include the medially located rectus abdominis, which is covered by a sheet of connective tissue called the rectus sheath. On the flanks of the body, medial to the rectus abdominis, the abdominal wall is composed of…
Muscles of the Neck and Back
The large, complex muscles of the neck and back move the head, shoulders, and vertebral column.
Posterior and Lateral Views of the Neck
The superficial and deep muscles of the neck are responsible for moving the head, cervical vertebrae, and scapulas.
Muscles of the Anterior Neck
The anterior muscles of the neck facilitate swallowing and speech. The suprahyoid muscles originate from above the hyoid bone in the chin region. The infrahyoid muscles originate below the hyoid bone in the lower neck.
Muscles That Move the Tongue
Tongue muscles can be extrinsic or intrinsic. Extrinsic tongue muscles insert into the tongue from outside origins, and the intrinsic tongue muscles insert into the tongue from origins within it. The extrinsic muscles move the whole tongue in…
Muscles That Move the Lower Jaw
The muscles that move the lower jaw are typically located within the cheek and originate from processes in the skull. This provides the jaw muscles with the large amount of leverage needed for chewing.
Muscles of the Eyes
(a) The extrinsic eye muscles originate outside of the eye on the skull. (b) Each muscle inserts onto the eyeball.
Muscles of Facial Expression
Many of the muscles of facial expression insert into the skin surrounding the eyelids, nose and mouth, producing facial expressions by moving the skin rather than bones.
Overview of the Muscular System
On the anterior and posterior views of the muscular system above, superficial muscles (those at the surface) are shown on the right side of the body while deep muscles (those underneath the superficial muscles) are shown on the left half of the body.…
Muscle Shapes and Fiber Alignment
The skeletal muscles of the body typically come in seven different general shapes.
Prime Movers and Synergists
The biceps brachii flex the lower arm. The brachoradialis, in the forearm, and brachialis, located deep to the biceps in the upper arm, are both synergists that aid in this motion.
Motor Units
A series of axon-like swelling, called varicosities or “boutons,” from autonomic neurons form motor units through the smooth muscle.
Muscle Contraction
The dense bodies and intermediate filaments are networked through the sarcoplasm, which cause the muscle fiber to contract.
Smooth Muscle Tissue
Smooth muscle tissue is found around organs in the digestive, respiratory, reproductive tracts and the iris of the eye.
Cardiac Muscle
Intercalated discs are part of the cardiac muscle sarcolemma and they contain gap junctions and desmosomes.
Atrophy
Muscle mass is reduced as muscles atrophy with disuse.
Types of Muscle Contractions
During isotonic contractions, muscle length changes to move a load. During isometric contractions, muscle length does not change because the load exceeds the tension the muscle can generate.
Muscle Metabolism
(a) Some ATP is stored in a resting muscle. As contraction starts, it is used up in seconds. More ATP is generated from creatine phosphate for about 15 seconds. (b) Each glucose molecule produces two ATP and two molecules of pyruvic acid, which can…
Skeletal Muscle Contraction
(a) The active site on actin is exposed as calcium binds to troponin. (b) The myosin head is attracted to actin, and myosin binds actin at its actin-binding site, forming the cross-bridge. (c) During the power stroke, the phosphate generated in the…
The Sliding Filament Model of Muscle Contraction
When a sarcomere contracts, the Z lines move closer together, and the I band becomes smaller. The A band stays the same width. At full contraction, the thin and thick filaments overlap completely.
Relaxation of a Muscle Fiber
Ca++ ions are pumped back into the SR, which causes the tropomyosin to reshield the binding sites on the actin strands. A muscle may also stop contracting when it runs out of ATP and becomes fatigued.
Contraction of a Muscle Fiber
A cross-bridge forms between actin and the myosin heads triggering contraction. As long as Ca++ ions remain in the sarcoplasm to bind to troponin, and as long as ATP is available, the muscle fiber will continue to shorten.
The T-tubule
Narrow T-tubules permit the conduction of electrical impulses. The SR functions to regulate intracellular levels of calcium. Two terminal cisternae (where enlarged SR connects to the T-tubule) and one T-tubule comprise a triad—a “threesome” of…
Behaviour, Development and Evolution
"The role of parents in shaping the characters of their children, the causes of violence and crime, and the roots of personal unhappiness are central to humanity. Like so many fundamental questions about human existence, these issues all relate to…
General Biology
The word biology means, "the science of life", from the Greek bios, life, and logos, word or knowledge. Therefore, Biology is the science of Living Things. That is why Biology is sometimes known as Life Science.
Radiation In The Environment
In the post-nuclear era the word ‘radiation’, in the popular lexicon, has taken on a specialized and negative meaning. Largely because of media misinterpretation the word now suggests to the non-scientist the malignant effects associated with the…
Biostratigraphic And Geological Significance Of Planktonic Foraminifera
Foraminifera are marine, free-living, amoeboid protozoa
(in Greek, proto = first and zoa = animals). They are
single-celled eukaryotes (organisms the cytoplasm of
which is organized into a complex structure with internal
membranes and contains a…
(in Greek, proto = first and zoa = animals). They are
single-celled eukaryotes (organisms the cytoplasm of
which is organized into a complex structure with internal
membranes and contains a…
Microbiology
Microbiology is designed to cover the scope and sequence requirements for the single-semester Microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications of microbiology in healthcare…
Environmental Biology
Environmental Biology enables students to develop a nuanced understanding of today’s most pressing environmental issues. This text helps students grasp the scientific foundation of these issues so that they can better understand the world around them…
Organic Chemistry With a Biological Emphasis: Volumes II
A free, open-access organic chemistry textbook (volumes I and II) in which the main focus is on relevance to biology and medicine. This is a PDF version of a wiki project called Chemwiki at the University of California, Davis. There are also…
Organic Chemistry With a Biological Emphasis: Volumes I
Open-access organic chemistry textbook (volumes I and II) in which the main focus is on relevance to biology and medicine. This is a PDF version of a wiki project called Chemwiki at the University of California, Davis. There are also supplementary…
Microbiology
This Microbiology textbook, adapted from the Boundless version, is intended to provide health care professionals with a working knowledge of infectious disease. Much of the book is devoted to the structure/function relationships in pathogenic…
All About Cells
Life. Every single living thing has something in common—do you know what that is? The clue is in the title! In this learning package you will learn all about cells—the parts and the functions of those parts. By learning about the basic building…
Clinical Biochemistry
The pre-analytical phase is defined as the period from the physician’s indication of the test up to the laboratory
analysis of the biological material. In other words, this phase involves an individual’s preparation for collection of the biological…
analysis of the biological material. In other words, this phase involves an individual’s preparation for collection of the biological…
Concepts of Biology-1st Canadian Edition
In this survey text, directed at those not majoring in biology, we dispel the assumption that a little learning is a dangerous thing. We hope that by skimming the surface of a very deep subject, biology, we may inspire you to drink more deeply and…
What is DNA
DNA (deoxyribose nucleic acid) is extremely important. All living things have DNA, or in the case of some viruses they have RNA instead.
In eukaryotes, it is found within the nucleus and is arranged into chromosomes.
DNA determines the…
In eukaryotes, it is found within the nucleus and is arranged into chromosomes.
DNA determines the…