BioDigital Systems: Availble for free (individual) or for a fee (groups/businesses), this interactive system will first require you to sign in via your Facebook or Google account to gain access. Once inside the system, you can zoom and rotate your virtual skeleton. Eleven systems in total are able to view and examine. Note: this site allows you to repeatedly quiz yourself on all eleven systems that are covered within the site.
Cranial Nerves: This resource offers micrographs, MRIs, and 3d reconstruction views of nerves and various other areas of the brain. There are simple and detailed views of the 12 major nerves in the brain. Note: in addition to images of the nerves in the brain, you can also quiz yourself and/or watch a video about these nerves.
Human Anatomy Learning Modules: These learning modules include:
Back & Vetebral Column // Thorax // Abdomen // Pelvis & Perineum // Lower Extremity // Upper Extremity // Head & Neck
This site not only includes helpful skeletal images, but also practice questions for each of the covered regions.
Anatomy Drill & Practice: This site covers the human body, the chemical, cellular, and tissue levels of organization, the integumentary system, skeletal system, muscular system, nervous system, cardiovascular system, respiratory system, digestive system, excretory system, and reproductive system.
This site not only includes images of and information on the above listed systems, but it also includes interactive drills and practice questions for students. NOTE: Flash required for the quizzes/practice questions.
The AK Lectures are a series of lectures from a (external) educational platform designed to "promote collaboration between our users and help spread knowledge to every part of the world."
These lectures vary in length, and will open in a new window when you click on the provided link.
Introduction to Nervous System: The nervous system can be divided into the central nervous system, which includes the brain and the spinal cord, and the peripheral nervous system, which includes everything else. The peripheral nervous system is subdivided into the autonomic and the somatic nervous system. The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous system. The CNS only consists of interneurons, which are neurons that connect other neurons to one another. The peripheral nervous system consists of motor neurons (also called efferent neurons) and sensory neurons (also known as afferent neurons). Motor neurons originate in the central nervous system and travel to the effector organ or tissue. Sensory neurons accept the signal from receptors and move the electric signal to the central nervous system. The term nucleus refers to a collection of neuron cell bodies within the CNS while ganglia refers to a collection of cell bodies within the PNS. When discussing the autonomic nervous system, the two types of neurons within that system are the pre-ganglionic and post-ganglionic neurons. The somatic simple arc describes the movement of the electric signal from the receptor to the effector organ. Notice that all sensory neurons enter the spinal cord dorsally (from the back) while all motor neurons leave the spinal cord ventrally (from the front).
Neuron Structure and Function: The basic functional unit of the nervous system is the neuron, also known as the nerve cell. The neuron is a specialized type of cell that is capable of generating electric signals, propagating those electrical signals and passing them down to a different, adjacent cell. Neurons have lost their ability to divide via mitosis and are always in the G0 phase of the cell cycle. All neurons consist several important structures. They have dendrites, which are projections that receive electrical signals and pass them down to the cell body. The cell body, which contains the nucleus and the rest of the organelles transmits the signal onto the axon hillock, which is a region that is capable of generating the action potential. The axon hillock sends the action potential across a long extension of the neuron known as the axon. At the end of the axon is the axon terminal (also called the synaptic terminal or synaptic bouton). The axon terminal is a specialized region that is able to transmit signals to other adjacent cells.
Resting Membrane Potential of Neuron: When the neuron is not generating any action potential on its membrane, it is said to be at rest and the voltage difference between the two sides of the membrane is known as the resting membrane potential. For most neuron cells, the resting membrane potential is around -70 millivolts. This electric potential difference (also known as the voltage difference) is generated as a result of the movement of ions across the cell membrane. There is a higher concentration of sodium ions on the outside of the cell with respect to the inside. Conversely, there is a higher concentration of potassium ions inside the cell than the outside. The cell membrane contains integral proteins that passively allow the diffusion of these ions down their electrochemical gradient. The cell membrane is naturally much more permeable to potassium than to sodium. Due to this fact, the cell membrane will be more negative on the inside than on the outside.
Initiation of Action Potential: The neuron is a specialized cell that is capable of generating an action potential on the cell membrane of the axon hillock. It does this by using special voltage-gated ion channels that respond to changes in voltage across the membrane of the cell. There are two types of voltage-gated channels, one for sodium and one for potassium. At the resting membrane potential of about -70 millivolts, these two voltage-gated ion channels are closed. During stimulation, when the stimulus has reached or exceeded the threshold value of around -45 millivolts, this change in voltage will signal the voltage-gated sodium channels to open up. An influx of sodium ions down their electrochemical gradient will cause the inside of the cell to become much more positive than the outside. This will cause the cell membrane to reverse polarity and this period is known as depolarization. As the voltage increases, the opening of the sodium channels causes even more sodium channels to open up. The permeability of sodium channels is now greater than the permeability of potassium channels. Eventually the cell membrane will reach a voltage of + 45 mV. This will signal the cell to inactive the sodium channels and open the potassium channels. This stage is known as the depolarization period. As the potassium channels are open, the potassium will move down its electrochemical gradient and to the outside the cell. This will cause a decrease in the amount of positive charge on the inside and eventually this will shift the polarity of the membrane back to normal. Since the potassium at this point is slightly more permeable than normal, the voltage of the membrane will drop slightly below the normal resting membrane potential. This stage is known as hyperpolarization. To return the membrane to the normal potential, the cell must use the sodium-potassium ATPase pumps, which move 3 sodium ions to the outside and 2 potassium ions to the inside (against the electrochemical gradient). Action potentials are all-or-nothing, meaning that a certain stimulus is needed to achieve the action potential. It also means that increasing the stimulus will not change the amplitude (height) of the action potential. But increasing the stimulus will increase the frequency of the action potential. There are two types of refractory periods - absolute refractory and relative refractory. During absolute refractory, no amount of stimulus will be able to generate another action potential because the sodium voltage-gated channels are either open or inactivated. However, during the relative refractory period, some of the sodium voltage-gated channels are being recovered from the inactivated phase and therefore a higher-than-normal stimulus can generate another action potential.
Propagation of Action Potential: The action potential propagates along the axon of the cell in a rather simple manner. When the cell membrane of the axon hillock is stimulated enough, depolarization will take place. The influx of sodium ions into the cell will cause the inside of the cell to become positively charged, thereby reversing the polarity. Although the adjacent part of the cell will still be negative inside, it will begin to become more positive and this eventually will trigger the opening of sodium voltage-gated channels in that adjacent section of the cell membrane. As the sodium channels close at the location of the stimulus, the sodium channels will begin to open in the adjacent region. In this manner, the action potential will move along the axon, away from the cell body. The reason it does not move in reverse is because the region right behind the action potential is experiencing repolarization and will be in its absolute refractory period.
Mylenation and Saltatory Conduction: If we study the movement of the action potential along the axon of the neuron from a purely physics perspective, we see that the action potential is nothing more than an electric current while the axon is a type of a biological wire. So what will influence the speed of movement of this electric current? Recall the resistance can influence the speed; a higher resistance means a lower speed and vice versa. The resistance of the wire itself depends directly on the length of the wire and inversely on the cross-sectional area of the wire. That means thicker and shorter axons will produce less resistance and therefore will propagate the action potential at a much higher rate. Since there is a limit to how long and thick we can make our axons in the body, our body has another way of increasing the speed of propagation. Special cells called glial cells (Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system) attach onto and move around the axon and deposit an insulating material called myelin. This myelin sheath insulates the axon so that the action potential cannot pass through the membrane at those regions. Gaps called nodes of Ranvier appear at regular intervals between the myelin on the axon. These gaps do not contain myelin and contain a high concentration of sodium voltage-gated channels. When the axon hillock is stimulated, the electric current flows through the cytosol of the axon until it reaches the first node of Ranvier, at which point (if the stimulus is high enough) it will stimulate depolarization. This will cause the current to continue traveling through the cytosol and eventually reach second node. In this manner, the action potential seems to jump from one node to the next. This sort of movement of the action potential is termed saltatory conduction.
Synaptic Terminal (Neuromuscular Junction): Neurons are cells that are capable of accepting, generating and sending electrical signals (i.e. action potentials) to other cells. The region where this sort of transmission of the action potential takes place from one cell to the next is known as the synapse. The synapse is made up of the pre-synaptic cell and the post-synaptic cell. At the end of the pre-synaptic cell (the neuron) is the axon terminal (also called the synaptic terminal of synaptic bouton). The synaptic cleft is the region between the two cells. One common type of a synapse is the neuromuscular junction, which is the synapse between the neuron and a muscle fiber. When the action potential arrives to the synaptic terminal, it causes the opening of calcium channels and calcium ions rush into the cytosol of the pre-synaptic cell. This causes the synaptic vesicles carrying the neurotransmitter (acetylcholine for the case of the neuromuscular junction) to fuse with the membrane and release the neurotransmitter into the synaptic cleft. The acetylcholine then binds onto special receptors on the protein channels found on the post-synaptic cell membrane. This opens the channels and causes sodium ions to rush into the cell, causing depolarization, which can cause the muscle to contract. This is the method by which the cell passes down the action potential from the neuron to the muscle cell. As long as the acetylcholine is still found in the synaptic cleft, it will continue binding to the protein channel and continue generating the action potential. To stop this process, an enzyme called acetylcholinesterase hydrolyzes the acetylcholine into acetate and choline. These products are then shuttled back into the pre-synaptic cell and can be used to generate acetylcholine molecules.
Secondary Messenger Systems: Most signal-transduction pathways (pathways that involve the passing down of a signal from one cell to another) involve a set of molecules called primary messenger and secondary messenger molecules. These systems, which are usually controlled by G-protein complexes found on the membrane of the cell, are called secondary messenger systems. To demonstrate how these systems work, lets take a look at the secondary messenger system of the activation of protein kinase A (protein that catalyzes the phosphorylation of other proteins). The primary messenger (also known as first messenger) in this signal-transduction pathway is epinephrine and it attaches to a binding site on the extracellular side of a transmembrane protein called beta-adrenergic receptor. On the cytoplasmic side of the transmembrane are a group of proteins (alpha, beta and gamma subunits) which are bound together. The binding of the epinephrine induces conformational changes to the protein and causes the alpha subunit (a G protein) to dissociate from the complex and move onto another membrane protein called adenylate cyclase, which converts ATP into cyclic AMP. Cyclic AMP is the secondary messenger and it goes on to convert protein kinase A into its active form.
Neuroglia (Glial Cells): Aside from neurons, the nervous system also consists of support cells called neuroglia or glial cells. These cells are responsible for supporting neurons' activity and improving their functionality in various important ways. The central and peripheral nervous systems, the two divisions of the nervous system, consist of their own types of neuroglia. The central nervous system contain astrocytes (cells that bring nutrients from the blood, establish the proper ion concentration outside the neuron and support the neuron physically), ependymal cells (cells that line the spinal cord and parts of the brain and are involved in cerebrofluid production and movement), oligodendrocytes (synthesize the myelin sheath around cells) and microglia (engulf harmful debris around the neurons). The peripheral nervous system consists of satellite cells (provide physical support and create a protective layer around the cell and also give the cell the nutrients they need) and Schwann cells (synthesize myelin around the axon of the neuron).
Central Nervous System: The central nervous system is composed of the brain and the spinal cord. The brain is divided into three regions - the forebrain, the midbrain and the hindbrain. The forebrain itself consists of the telencephalon (cerebrum, hippocampus and basal ganglia) and the diencephalon (thalamus and hypothalamus), the midbrain consists of the mesencephalon and the hindbrain consists of the metencephalon (cerebellum and pons) and the myelencephalon (medulla oblongata). The spinal cord consists of four segments. From the top to bottom of the spinal cord, these segments are the cervical, thoracic, lumbar and sacral segments. The spinal cord is responsible for receiving electrical signals from the peripheral nervous system and sending it into the brain for integration and processing. It is also responsible for accepting signals from the brain and sending them to other parts of the body. The spinal cord is also capable of participating in the simple reflex arc. The spinal cord consists of white matter, which is found on the outside of the spinal cord and gray matter, which is found towards the center of the spinal cord.
Somatic Nervous System: The somatic nervous system is one of the two divisions of the peripheral nervous system (the other one being the autonomic nervous system). The somatic nervous system innervates and controls skeletal muscle and consists of the motor and sensory divisions. The motor division only contains motor neurons, which carry signals away from the central nervous system and to the effector skeletal muscle. These always use the acetylcholine neurotransmitter at the neuromuscular junction and exit the spinal cord from the front (ventral) side. The cell bodies of the motor neurons begin in the spinal cord and a single axon travels and ends up at the effector muscle. The sensory division consists of sensory neurons that accept electrical signals from environmental stimuli and send these signals to the central nervous system. They enter the spinal cord from the back side (dorsal side). The cell bodies of sensory neurons are located in the back of the spinal cord, in a region called the dorsal root ganglia. The somatic nervous system also controls the simple reflex arc. There are two types of simple reflec arcs - monosynaptic (only one synapse) and polysynaptic arcs (more than one synapse).
Autonomic Nervous System (Sympathetic and Parasympathetic): The autonomic nervous system innervates smooth muscle, cardiac muscle as well as the glands of the body. The signal pathway in the autonomic nervous system usually consist of a series of two neurons - the preganglionic neuron and the postganglionic neuron. The autonomic nervous system consists of the motor and sensory divisions. The motor division can be subdivided into two - the sympathetic and the parasympathetic nervous system. The sympathetic nervous system is responsible for the fight-or-flight responses. This includes increasing the size of the pupil (via the radial smooth muscle in the iris), increasing the heart rate and respiratory rate, increasing sweating, decreasing the rate of digestion and inhibiting peristalsis. The overall effect is to move more oxygenated blood to the skeletal muscle while decreasing the blood flow to the digestive system. In the sympathetic division, the preganglionic neuron always begins in the spinal cord and extends outward from the ventral side of the spine. It contains a relatively short axon and synapses with the postganglionic cell. At the synapse, acetylcholine is used as the neurotransmitter. The postganglionic synapse uses either epinephrine or norepinephrine. The electrical signals carried to the adrenal medulla by the sympathetic division only involve a single neuron (the preganglionic neuron) in the pathway. It also used acetylcholine. The parasympathetic nervous system is responsible for controlling the rest-and-digest responses. Its effect is to increase the blood flow to the digestive and excretory systems while decreasing the blood flow to the skeletal muscle. It basically reverses the effects of the sympathetic nervous system. The preganglionic neurons in the parasympathetic system can begin either in the spinal cord or the brain and have relatively long axons. Both types of synapses use acetylcholine as the neurotransmitter. The most important nerve of the parasympathetic nervous system is the vagus nerve (10th cranial nerve) because it innervates the majority of the organs in the thoracic and abdominal regions, including the heart, the lungs, the kidneys, the liver, the small intestine, etc.
Structure of the Human Eye: The human eye is a specialized organ that is capable of detecting light stimuli and transforming it into electrical signals that are used by the brain to form images of our environment. The eye consists of many important structures that each serve a specific purpose. The outside most portion of the eye is the sclera, which is the white layer you see when you look into the mirror. It is composed of collagen and elastic fibers that form a protective layer around the eye. The cornea is a transparent material that allows light into the eye. Due to the fact that the cornea contains an index of refraction that is much higher than that of air (1.4 for the cornea compared to 1.0 for air), most of the bending of light occurs at the cornea. The anterior cavity of the eye is filled with a special fluid called the aqueous humor. It maintains pressure within that region of the eye and is secreted by the ciliary process (found in the ciliary body) through a canal called the canal of Schlemm. The iris consists of the smooth radial muscle and smooth circular muscle that controls the opening of the eye called the pupil. The sympathetic nervous system innervates the radial muscle while the parasympathetic system innervates the circular muscle. The choroid is the vascular portion of the eye that contains connective tissue and supplies the eye with the nutrients and oxygen that it needs to function properly. The lens of the eye focuses the rays of light onto a region of the eye called the retina. The shape of the lens is controlled by the ciliary muscles. Changing the shape of the lens will change the focal length and this can be used to focus the imagine onto the retina. The retina contains specialized cells called rods and cones. These cells contain a photochemical called pigment that is capable of absorbing the energy in light and transforming it into electrical signals. The photochemical in rods is called rhodopsin. The retina contains a region called the fovea, which is a region that contains a high concentration of cones. Cones are capable of distinguishing between the different colors (while the rods cannot) and that means the image will appear the sharpest at the fovea. Once the electrical signal is produced, it is sent up to the brain via the optic nerve.
Structure of the Human Ear: The human ear is a specialized organ that is capable of capturing mechanical waves and transforming them into electrical signals that the brain can use to analyze our surroundings. The ear consists of three regions - the outer ear, the middle ear and the inner ear. When a disturbance in the air creates a mechanical wave, it begins to propagate and eventually hits the outer portion of the ear known as the pinna (also known as auricle). The pinna serves to capture a good portion of the energy stored in the mechanical wave. Due to the large size and surface area of the pinna, it is able to amplify the amount of energy that goes into the ear canal (auditory canal). Once inside the auditory canal, the mechanical wave travels to the ear drum (tympanic membrane) of the middle ear. Due to the small size of the ear drum compared to the size of the pinna, the force that the mechanical wave exerts is greatly amplified (called a mechanical advantage in physics). The vibration of the membrane exerts a force on three bones collectively called the ossicles (malleus, incus and stapes), which are connected to one another. These three bones act as a lever system and by decreasing the lever arm (displacement) as we go from bone to bone, they amplify the force even more. This amplification is required in order to pass the air-liquid boundary that exists in the inner ear. The inner ear consists of a fluid called the perilymph and in order to move the mechanical wave into this fluid, we must overcome a considerable amount of resistance. The stapes bone is connected to the oval window, which is the beginning of the inner ear. As the oval window (a membrane) vibrates, it creates a mechanical wave inside the fluid, which moves through the cochlea. This movement causes another membrane called the round window to vibrate, which causes even more pressure variation inside the fluid. The hair cells found in the organ of Corti inside the cochlea contain extensions called micovilli that depolarize when they feel the pressure variation and send that action potential to the cochlear nerve, which connects with the vestibular nerve and travels up to the brain. The ear also contains a set of three canals called the semicircular canals. These three canals are oriented along the three directions (x, y and x) and also contain hair cells that are capable of depolarizing when the pressure varies inside the fluid (called endolymph). These semicircular canals are responsible for helping us balance and allow us to feel acceleration and deceleration. They send their action potentials to the vestibular nerve.