Sammelsurium vorklinischer Quiz-Bilder

(1) The femur is an example of a long bone.
(2) The metaphysis, periosteum, cortical, and trabecular bone: The cortical bone consists of osteons, inner and outer circumferential lamellae, and interstitial lamellae. Blood is supplied through vessels that run through the vertical Haversian canal and the horizontal Volkmann's canal.
(3) Transition from periosteum to cortical bone: The periosteum consists of a fibrous and an osteogenic layer that contains osteoblasts and mesenchymal stem cells. Sharpey's fibers from the fibrous layer also run through the osteogenic layer; these fibers enter the compact bone, attaching periosteum and muscle tendons together.
(4) Osteoblasts from the osteogenic layer produce and secrete osteoid (whitish layer). The osteoid is then mineralized (light brown). The osteoblasts embedded in the bone matrix are called osteocytes. Osteocytes are connected with each other, and osteoblasts, via gap junctions (black arrows). The endosteum lines the inner surfaces of the bone and consists of cells similar to those in the osteogenic layer.
(5-6) Compact bone consists of osteons with concentric layers of lamellae and a central Haversian canal containing a blood vessel. The direction of the collagen fibers alternates from each concentric layer to the next.

Lamellar bone

Structure of a long bone

A ball and socket joint enables multi-directional movement and rotation. The joint is surrounded by a joint capsule containing a joint cavity filled with synovial fluid. Movement and control of the joint are facilitated by activation of the agonistic muscle (e.g., quadriceps muscles during knee extension) and opposing force produced by the antagonistic muscle (e.g., hamstring muscles during knee extension).

Structure of ball and socket synovial joints

Synovial joints (blue) and their axes of movement (black arrows)
1. Ball-and-socket joints (e.g., the shoulder and hip joints): The ball-like end of one bone fits into a cup-like socket of the adjacent bone. These are multiaxial joints with three axes of motion.
2. Condyloid joints (e.g., radiocarpal joint): The oval or ellipsoid end of one bone fits into a shallow depression of the adjacent bone. These are biaxial joints with two axes of motion.
3. Saddle joints (e.g., carpometacarpal joint of the thumb): The articulating surface of each bone has a concavity and two convexities that fit into one another. These are biaxial joints with two axes of motion.
4. Hinge joints (e.g., PIP, DIP, elbow): a narrow, deeper version of the condyloid joint. These are uniaxial joints with only one axis of motion.
5. Pivot joints (e.g., atlantoaxial joint): The rounded end of one bone fits into a ring formed by the adjacent bone. These are uniaxial joints that allow for rotational movement.
6. Modified hinge joints (e.g., knee joint): a combination of the hinge and pivot joint. These are biaxial joints, with one axis that is rotational.
7. Plane joints (e.g., intercarpal and intertarsal joints): The articulating surfaces of both bones are flat or slightly curved. These are multiaxial joints that can glide in multiple axes. However, the range of movement is minimal.

Types of synovial joints

Articular cartilage is divided into four zones that correspond to the different sections of its arch-like collagen fibrils.
From the subchondral bone, collagen fibrils arise approx. perpendicular to the bone surface and pass through mineralized (calcified zone, 1) and nonmineralized (radial zone, 2) cartilage. The calcified and radial zones blend into each other at a thin border area referred to as the tide mark (which can be seen via electron microscopy). The fibrils start to bend in the transitional zone (3) and eventually reach their vertices to form the tangential zone (4). The fibrils then bend and return through the four layers to reinsert into the bone.

Structure of articular cartilage

Structure of skeletal muscle

Skeletal muscle cells are elongated, multinucleated cells filled with myofibrils. There are numerous mitochondria located between the cells, which ensure a supply of energy for contraction. Myofibrils are surrounded by the sarcoplasmic reticulum (SR), which include the longitudinal tubules (L-tubules) and terminal cisterna that form a cufflike structure around the myofibrils. T-tubules, which are invaginations of the sarcolemma, are located between the terminal cisternae and allow electrical impulses to travel to the SR. Two terminal cisternae and one T-tubule form a triad, which is located at the transition between the I-band and the A-band.

Structure of skeletal muscle fibers

Upper diagram: Myosin II consists of three sections – the tail, the neck, and the head. The shaft is composed of two heavy myosin chains that are intertwined. In the neck region, two light chains attach to the heavy chains, which serve to regulate the myosin head. The myosin head has an actin binding site, where it can bind to the actin filament. ATP can bind and be cleaved via the ATP binding site as well as the ATPase domain, which leads to straightening of the myosin head as part of the crossbridge cycle.
Lower diagram: Myosin filaments (thick filaments) are formed by the assembly of several hundred myosin II molecules, which are connected in the M lines of the muscle. The myosin heads are arranged in the myosin filament of the skeletal muscle in a bipolar structure, i.e., oriented in opposite directions. There are no myosin heads around the M lines (H zone of the sarcomere).

Structure of myosin filaments

A sarcomere is composed of the myofilaments myosin (red) and actin (blue). Based on the way in which the filaments intertwine, the sarcomere can be divided into different sections. The anchorage zones of the filaments are termed M lines (for myosin) and Z lines (for actin). The zone containing only actin filaments is termed the I band. The region containing only myosin filaments is termed the H zone. The A band contains the entire length of the myosin filament. A sarcomere is the region between two Z lines, which each sarcomere comprised of ½ I - A - ½ I. The sarcomere is stabilized by several structural proteins. Titin (green) stabilizes the myosin heads between actin filaments. α-Actinin binds the actin filaments of two sarcomeres in the Z lines. The sarcomere is the smallest functional unit of the myofilament in striated muscle. Several aligned sarcomeres are called a myofibril and form the building blocks and functional unit of muscle cells (muscle fiber).

Structure of a sarcomere

Sarcomeres are the smallest functional unit of a striated muscle fiber, defined as the area between two Z lines. 
Z lines separate one sarcomere from the other and act as an anchoring point for actin filaments. M lines are the center of the H zone, to which myosin filaments are attached on opposite sides. 
I (isotropic) bands are zones containing only thin actin filaments, while A (anisotropic) bands contain both actin and myosin filaments. A bands are composed of three segments: The H zone (the pale band of the central segment on either side of the M line), that contains only thick myosin filaments, and the dark bands of the outer segments (between H and I bands) where actin and myosin filaments are overlapping.

Sarcomere under electron microscopy

The skin is composed of the cutis (epidermis and dermis) and the subcutis.
- The epidermis has five layers: stratum corneum, lucidum, granulosum, spinosum, and basale. 
- The dermis has two layers: papillary and reticular dermis. 
- The subcutis contains fat and neurovascular structures.

Layers of the skin

Structure of basement membrane

Layers of the skin and skin appendages

Types of capillaries

The microcirculatory bed is composed of a network of arterioles, metarterioles, capillaries, and venules. Oxygenated arterial blood flows from the arterioles to the capillary bed (where gas and nutrient exchange with tissues occurs) and then into the venules. Metarterioles are thoroughfare channels that link arterioles directly to venules. The tunica media of the metarterioles form precapillary sphincters that regulate blood flow to the capillary bed. Arteriovenous (A-V) anastomoses are another mechanism by which arterial blood can bypass the capillary network to enter the venous system.

Anatomy of the microcirculatory bed

The parenchyma of the lymph node is divided into a B-cell zone (cortex), a T-cell zone (paracortex), and the marrow. The cortex contains primary and secondary lymph follicles with germinal centers, which allow affinity maturation of B cells. At the hilum (concave side), blood vessels enter and efferent lymphatic vessels leave the node. The entering artery and vein branch into a large capillary network. Special high endothelial venules in the paracortex allow for lymphocyte extravasation.
Afferent lymphatic vessels enter through the capsule at many points in the periphery and open into the subcapsular sinus. The cortical sinus runs from the cortex via the paracortex to the medullary sinus, which then drains into the efferent lymphatic vessel.

Anatomy of the lymph node

Lymph is ultimately drained into the systemic circulation via the subclavian veins. The right lymphatic duct only drains roughly the upper right area of the body (green), whereas the thoracic duct is responsible for the rest of the body. The deep lymph nodes are not palpable.

Deep lymph nodes

Note that lymph is ultimately drained into the systemic circulation via the subclavian veins. The right lymphatic duct only roughly drains the upper right area of the body (red), whereas the thoracic duct is responsible for the rest of the body.
Note: the testes (paraaortic) and scrotum (superficial inguinal) drain into different lymph nodes, which means that testicular tumors do not naturally disseminate to the scrotal lymph drainage.

Palpable lymph nodes

There are three different secretory mechanisms in the exocrine glands.
(1) Exocytosis: Several vesicles, which detach from the Golgi apparatus, are transported to the cell membrane, to which they then attach, allowing the release of intravesicular contents (e.g., proteins, neurotransmitter) into the extracellular space.
(2) In apocrine secretion, the secretion (e.g., milk lipid droplets) is detached with part of the apical cytoplasm and the cell membrane. As a result, the gland cell becomes smaller.
(3) Holocrine secretion only occurs in sebaceous glands. The secretion is released by apoptosis of the gland cell. During the course of maturation, the cell continuously produces sebum. Cell organelles and the nucleus are degraded and the cell subsequently disintegrates, with its entire content forming part of the secretion.

Secretory mechanisms of exocrine glands

The secretory lobules of the exocrine glands assume different structures depending on their function and the substances they secrete. While acinar and alveolar lobules tend to secrete thin, serous substances, tubular ones usually secrete thick, mucous substances. Aside from the three basic types (1–3), various special and intermediate types (4–7) exist.

Classification of exocrine glands (by structure)

Fundamentals of anatomy

The 4 lobes of each cerebral hemisphere are separated by 3 main sulci: 
–The central sulcus separates the frontal lobe from the parietal lobe. 
–The lateral sulcus separates the temporal lobe from the frontal lobe and the parietal lobe. 
–The parieto-occipital sulcus separates the parietal lobe from the occipital lobe.

Lateral view of the cerebral lobes

The primary motor cortex, in the frontal lobe, is the origin of the fibers of the pyramidal tract.
The primary and secondary auditory cortices, where auditory input is processed, are located in the temporal lobe. The primary visual cortex is located at the occipital pole and is surrounded by the secondary visual cortex.
Different functions converge at association areas. The most well-known examples are the motor (Broca) and sensory (Wernicke) areas of speech, which are commonly located in the dominant hemisphere.

Lateral view of important areas of the brain

Basal view of the brain

Ventral surface of the brain

Sagittal section of the brain

Important structures of the brain

The insula is a portion of the cerebral cortex that lies deep within the lateral sulcus, covered completely by the frontal, parietal, and temporal opercula. The central sulcus divides the insula into an anterior zone, composed of short gyri, and a posterior zone, composed of long gyri. The anterior insula is mainly involved in processing olfactory, gustatory, autonomic, and limbic information. The posterior insula is principally involved in processing somatic (mainly noxious stimuli), auditory, and motor information. The insula also plays an important role in cognition, emotion, and motivation.

Insula

Cerebellum and brain stem

Dorsocaudal view (without brainstem)

Cerebellum

Ventral view (the brainstem is not shown)

Hippocampus, corpus callosum, and fornix

Within the brain, there are several interconnected CSF-filled spaces (ventricles). The largest of these are the lateral ventricles (first and second ventricles), which are located within the cerebral hemispheres. The third ventricle lies in the diencephalon, between the two thalamic hemispheres, and has several recesses. The fourth ventricle is located in the rhomboid fossa (dorsal to the pons/medulla oblongata). The lateral ventricles are connected to the third ventricle via the interventricular foramina. The third ventricle is connected to the fourth ventricle by the cerebral aqueduct. The fourth ventricle also connects the ventricular system to the subarachnoid space (which is also filled with CSF) via several openings (e.g., the median aperture).

Ventricular system of the brain

Limbic system in coronal view

Basal ganglia (sagittal section)

Basal ganglia

Basal ganglia (coronal section)

Histological layers of the neocortex

The pituitary gland lies within the pituitary fossa and is completely surrounded by the cavernous sinus. The internal carotid artery and the abducens nerve pass through the middle of the cavernous sinus, while the oculomotor, trochlear, ophthalmic, and maxillary nerve are located towards the lateral wall of the sinus.

Pituitary gland and cavernous sinus

Ventral, dorsal, and lateral aspects of the brainstem (medulla oblongata, pons, midbrain) and the diencephalon. The points of entry/exit and course of various cranial nerves are shown as well.

Brainstem and diencephalon

Left: Transverse section at the level of the superior colliculi 
The mesencephalon contains the tectum (yellow), the tegmentum (green), and the crus cerebri (blue).
Right: Transverse section 
Important functional structures are depicted.

Mesencephalon (midbrain)

Dorsal view
The mesencephalic tectum and rhomboid fossa have distinct structures and functions. The rhomboid fossa is flanked by three pairs of cerebellar peduncles (superior, middle, and inferior) that connect the brainstem to the ipsilateral cerebellar cortex.

Brainstem and cerebellar peduncles

Brainstem posterior view

Dorsal view
The rhomboid fossa is a depression on the dorsal surfaces of the pons and medulla that forms the anterior/ventral border (floor) of the fourth ventricle.

Rhomboid fossa

Sagittal section of the brainstem reveals the three major components (from cranial to caudal: midbrain, pons, medulla oblongata) and their associated structures. The parts of the CSF space that traverse the brainstem are shown in blue.

Brainstem and ventricular system

Brainstem and origin of cranial nerves II-XII

Ventral view of the brain
Anterior cerebrum: CN I and II. CN I, the olfactory nerve, is not visible in this view. The labeled olfactory tract forms the continuation of the olfactory pathway from the olfactory bulb, where CN I synapses. 
Midbrain: CN III and IV 
Pons: CN V, VI, VII, and VIII 
Medulla: CN IX, X, XI, and XII

Origin of the cranial nerves

Cranial nerve nuclei

Transverse section
Typically, the efferent cranial nuclei are located medially while the afferent nuclei are located more laterally.

Cranial nerve nuclei of the medulla oblongata

Optic nerve: 
- Intraorbital part: Each optic nerve arises from the axons of the retinal ganglion cells and leaves the eyeball through the lamina cribrosa of the sclera (at the posterior aspect of the eyeball). 
- Intracranial part: Each optic nerve enters the middle cranial fossa through the optic canal and ends inferior of the pituitary gland at the optic chiasm, where the nasal fibers (carrying information from the temporal visual fields) cross to the opposite side.
Optic tracts: arise from the optic chiasm, wind around the midbrain, and end at the lateral geniculate body of the thalamus
Optic radiations: arise from the lateral geniculate body and terminate in the visual cortex of the occipital lobe

Optic nerve and visual pathway

1. Retina: The visual pathway begins at the retina. The temporal field of vision is projected onto the nasal portion of the retina and the nasal field of vision is projected onto the temporal portion of the retina. 
2. Optic nerves: The axons of the retinal ganglion cells merge to form the optic nerve, which carries the nasal and temporal fibers of the ipsilateral retina. 
3. Optic chiasm: The two optic nerves meet at the optic chiasm, where the nasal fibers of each retina cross over to the contralateral side. The temporal fibers remain uncrossed. 
4. Optic tracts: Each optic tract carries contralateral nasal fibers and ipsilateral temporal fibers (i.e., the right optic tract carries the nasal fibers of the left eye and the temporal fibers of the right eye). I.e., each optic tract carries the contralateral visual field of each eye. 
5. Lateral geniculate nucleus (LGN): The optic tracts synapse onto the ipsilateral LGN. 
6. Optic radiation: consists of neurons that carry visual information from the ipsilateral LGN to the ipsilateral primary visual cortex 
7. Primary visual cortex: The visual pathway terminates at the primary visual cortex, which is located at the posterior aspect of the occipital lobe. It is responsible for the perception of sight. The right visual cortex receives visual information from the right nasal field and the left temporal field of vision, i.e. the left visual field of each eye.

Visual pathway

The afferent pathway (blue tract) is initiated when light hits the retina: 
- Retinal photoreceptors → ipsilateral optic nerve → nuclei of bilateral pretectal areas (nasal fibers of the optic nerve cross to the contralateral side at the optic chiasma)
The efferent pathway (green tract) transmits neural impulses to the iris sphincter muscle: 
- Nuclei of pretectal area → bilateral Edinger Westphal nuclei (parasympathetic preganglionic nucleus of the oculomotor nerve) → bilateral oculomotor nerves (synapse at the ciliary ganglion) → bilateral iris sphincter muscle (and ciliary muscle) → bilateral pupillary constriction (and accomodation).
Shining a light into one eye causes constriction of the ipsilateral pupil (direct pupillary reflex) as well as that of the contralateral pupil (indirect or consensual pupillary reflex).

Pupillary light reflex

Pons (transverse plane)

The axons of the sensory cells in the olfactory epithelium (olfactory nerve filaments, 1) collect into the olfactory nerve (2) that enters the skull through the cribriform plate and passes into the second neuron in the olfactory bulb. Subsequently, as olfactory tract (4), it runs to the olfactory trigone where it splits into the medial olfactory stria (5) and the lateral olfactory stria (6). Both send their impulses into various regions of the brain for interpretation as smell.

Olfactory nerve I

The circle of Willis is depicted in light red with the main arteries of the brain in dark red.

Circle of Willis

Caudal view
The circle of Willis is formed around the pituitary stalk by the following paired arteries: 
- Anterior cerebral arteries (terminal branches of the internal carotids) 
- Internal carotid arteries (proximal to the origin of the middle cerebral arteries) 
- Posterior cerebral arteries (terminal branches of the vertebral arteries)
The anterior communicating artery (ventral to the optic chiasm) connects the anterior cerebral arteries.
The posterior communicating artery (a branch of internal carotid artery; lies on the lateral aspect of the pituitary stalk) connects the internal carotid artery and the posterior cerebral artery.
The basilar artery, formed by the fusion of the two vertebral arteries, lies in the midline groove of the dorsal aspect of the pons. Pontine and labyrinthine branches arise from the basilar artery.
The anterior spinal artery, formed by the fusion of the branches of the vertebral artery, lies in the anterior median fissure of the spinal cord.

Arterial supply of the brain

The cerebral hemispheres are drained by superficial and deep cerebral veins, which empty into the dural venous sinuses.
The superficial cerebral veins can be classified into the superior cerebral veins, middle cerebral veins, and inferior cerebral veins. All of them drain the blood within the white matter of the brain via bridging veins into the dural venous sinuses.
The deep cerebral veins drain the blood within the cerebral medulla into the straight sinus. The basal veins (veins of Rosenthal) and the internal cerebral veins unite into the great cerebral vein (vein of Galen), which then drains into the straight sinus.
Eventually, the blood collected by the sinuses drains into the internal jugular vein.

Venous drainage of the brain

The blood-brain barrier consists of three layers: 
Inner layer: capillary endothelial cells connected through tight junctions (TJ) 
Medial layer: basement membrane of the capillaries 
Outer layer: foot processes of the astrocytes, which communicate through gap junctions (GJ)

Blood-brain barrier

The transverse width of the spinal cord varies significantly depending on the level. It is largest at the cervical (C4/5) and lumbar (L4/5) enlargements, where neurons of the brachial and lumbosacral plexus are located. The white matter is a collection of nerve fibers and decreases in size as it descends, as descending fibers decrease from cranial to caudal, and ascending fibers increase from caudal to cranial.
Important structures of the gray and white matter are marked in the image.

Transverse sections of the spinal cord

Macroscopically, the gray matter is subdivided into the posterior and anterior horn. At the thoracic and lumbar levels, the spinal cord also includes a lateral horn, as seen here.
Histologically, the horns are subdivided (according to Rexed) into ten laminae (I-X).
Functionally, the gray matter can is subdivided into nuclei.
Certain laminae approximately correspond to certain nuclei: 
Lamina II ≈ substantia gelatinosa 
Lamina III-IV ≈ nucleus proprius 
Lamina V-VI ≈ posterior thoracic nucleus 
Lamina VII (thoracolumbar) ≈ intermediolateral nucleus 
Lamina VII (sacral) ≈ sacral parasympathetic nuclei (not depicted here)

Microstructure of the gray matter of the spinal cord

Somatic afferent fibers from the skeletal muscles, joints, and skin and visceral afferent fibers (e.g., from the meninges) become part of the spinal nerve and reach the dorsal root ganglion, where their somata are located. Because these are pseudounipolar neurons, the signals are conducted to the posterior horn of the spinal cord without synaptic transmission at this point.
From the anterior horn of the spinal cord, somatic efferent fibers emerge via the anterior root. These then combine with the afferent fibers to form the spinal nerve.
The lateral horns of the spinal cord segments C8 to L1–L3 contain somata of sympathetic neurons. Their visceral efferent fibers pass via the anterior root, the spinal nerve, and the white ramus communicans to the paravertebral ganglia of the sympathetic trunk, where some of them synapse. 
The fibers that synapse here then leave the paravertebral ganglion via the gray ramus communicans and run with peripheral nerves to provide sympathetic innervation to the blood vessels, sweat glands, and arrector pili muscles of the skin. 
The fibers that do not synapse in the paravertebral ganglion provide, e.g., sympathetic innervation to the intestinal smooth muscles and the adrenal medulla. These fibers either synapse in prevertebral ganglia in near proximity to the target organ (e.g., the smooth muscle of the intestine) or in ganglia that are located in the organ itself (e.g., in the adrenal gland).
The paravertebral ganglia also contain fibers that connect to other paravertebral ganglia.

Segment of the spinal cord with spinal roots, nerves, and fibers

Transverse section of the lumbar spine

Lumbar spinal cord segment
Longitudinal and segmental vessels supply the spinal cord with blood. The spinal branch splits into the anterior and posterior radicular arteries (segmental system). The posterior radicular artery anastomoses with the posterior spinal artery of each side, the anterior radicular artery with the anterior spinal artery. The anterior and posterior spinal arteries connect vertical segments (longitudinal system). Approx. at the level of T8 the anterior spinal artery and the artery of Adamkiewicz (arteria radicularis magna) fuse. The anterior and posterior spinal arteries give off small vessels for direct blood supply to the spinal cord: the sulcocommissural artery and vasocorona connecting the anterior and posterior spinal artery.

Arterial supply of a spinal segment

Arterial blood vessels of a thoracic spinal cord segment

Left: Transverse sections of the spinal cord at different levels reveal a significant difference in size between the cervical and thoracic segments. At the lumbar level, the spinal cord terminates and only spinal nerves descend the vertebral canal.
Left/middle: Spinal cord in relation to the spine, vertebral canal opened ventrally.
Right/middle (ventral view) and right (dorsal view): Segments and important structures like enlargements and fissures of the spinal cord shown without the surrounding spine.

Important structures of the spinal cord

Sympathetic nervous system:
The sympathetic trunk consists of 22 paired ganglia that are lateral to the spine. In the caudal area, the trunks of both sides of the ganglion connect unpaired in front of the tailbone.
The 22 ganglia can be grouped into cervical, thoracic, lumbar, and sacral ganglia. The 3 cervical ganglia contain fibers from C8 to Th1 and the switch to postganglionic neurons occurs in them. The 3rd cervical ganglion and 1st thoracic ganglion often merge to the cervicothoracic ganglion. The sympathetic fibers of the abdominal and pelvic organs pass through the sympathetic trunk without being interconnected. The thoracic ganglia receive their fibers from Th1 to Th12 and also form the greater and lesser splanchnic nerve. The lumbar ganglia receive fibers from L1 to L2 and form the lumbar splanchnic nerves. The sacral ganglia (fibers from S1–S4/S5) give rise to the sacral splanchnic nerves and the sympathetic innervation of all organs then occurs through different plexuses (or ganglia).

Sympathetic trunk

Parasympathetic nervous system with the most important ganglia and target organs:
The preganglionic neurons are located in the brainstem (cranial nerve nucleus) and the sacral spinal cord. Unlike the sympathetic ganglia, the parasympathetic ganglia are generally located near to or directly in the wall of their target organs.
In the cranial space, the preganglionic neurons are interconnected in the 4 head ganglia: ciliary ganglion (behind the eye), pterygopalatine ganglion (pterygopalatine fossa), submandibular ganglion (submandibular gland), or otic ganglion (base of skull). 
The fibers are led by the oculomotor nerve, facial nerve, or glossopharyngeal nerve. The fibers of the 4th parasympathetic cranial nerve (vagus nerve) are switched in several other ganglia (not shown here).
In the caudal space, the parasympathetic nerve fibers are led with the splanchnic nerves and switched in the pelvic ganglion (= inferior hypogastricus plexus).

Parasympathetic ganglia

Striking the patellar tendon results in two reflexes that occur simultaneously:
(a) Monosynaptic reflex: Stretching of the patellar tendon is detected by muscle spindles in the quadriceps femoris muscle. An impulse is passed to the spinal cord via somatic afferent (Ia) fibers of pseudounipolar neurons, the somata of which are located in the dorsal root ganglion. In the anterior horn, the afferent neurons synapse directly with α motor neurons that innervate the same muscle and thus cause contraction.
(b) Polysynaptic reflex: The afferent neurons synapse with inhibitory interneurons in a different segment of the spinal cord. These interneurons then inhibit the α motor neurons of the antagonistic muscle.

Stretch reflex

The extrapyramidal tracts are descending (motor) tracts involved in the regulation of involuntary muscular activity, e.g., reflexes, balance, facial expressions. They all synapse (directly or through interneurons) at the anterior horn cells of the spinal cord.
The rubrospinal tract originates in the red nucleus (midbrain) and crosses to the contralateral side at the ventral tegmental decussation.
The tectospinal tract originates in the superior colliculus (midbrain) and crosses to the contralateral side at the posterior tegmental decussation (not pictured).
The vestibulospinal tract originates in the vestibular nuclei (pons and medulla) and remains ipsilateral.
The reticulospinal tract originates in the reticular formation (pons and medulla) and remains ipsilateral.

Extrapyramidal tracts

Fibers of the pyramidal tract originate from the primary motor cortex and descend to the brain stem via the internal capsule. Bulbar corticonuclear fibers extend to each of the cranial nerve nuclei, which innervate the facial muscles. The remaining fibers descend from the brainstem as two divisions of the corticospinal tract. Approximately 80% of these fibers decussate at the level of the medulla (pyramidal decussation) to the contralateral side to form the lateral corticospinal tract. They then descend into the spinal cord to innervate the limbs and digits. The remaining approximately 20% of fibers descend from the brainstem ipsilaterally, forming the anterior corticospinal tract. These decussate at the spinal level of the trunk muscles they innervate.

Pyramidal tract

Left image: the dorsal column (gracile fasciculus and cuneate fasciculus) carries information of fine touch, vibration, pressure, and proprioception (gracile fasciculus: primarily from the lower half of the body; cuneate fasciculus: primarily from the upper half of the body).
Right side: the anterolateral column (anterior spinothalamic tract and lateral spinothalamic tract) carries information of crude touch, pressure, pain, and temperature. 
1) Anterior spinothalamic tract: The first neuron is located within the spinal ganglia. Axons from the first neuron split and ascend or descend several segments (a). Having reached another segment, they synapse to the second neuron in the posterior horn of the spinal cord, and the axons decussate to the contralateral side of the spinal cord within the anterior white commissure (b). Further ascend in the anterior spinothalamic tract (c, d). Synapse to the 3rd neuron in the thalamus (e). 
2) Lateral spinothalamic tract: The first neuron is located within the spinal ganglia. Synapse to the 2nd neuron in the gray matter on the same segment. The axon of the 2nd neuron decussates to the contralateral side of the spinal cord within the anterior white commissure (a). Ascend in the lateral spinothalamic tract (b, c, d). Synapse to the 3rd neuron in the thalamus (e).
Both pathways end in the somatosensory cortex and transmit information that enters consciousness.

Ascending conscious tracts of the spinal cord

A neural network formed by the anterior (ventral) divisions of the last two lumbar nerves and the first four sacral nerves (L4–S4).
The illustration depicts the nerve roots and branches of the sacral plexus that supply motor and sensory innervation to the lower limbs and pelvis.
Note that the nerve roots of the pudendal nerve are variably described in the literature as S2-S4 or S1-S4 (as depicted here).

Sacral plexus

Lateral view of the right cervical plexus
The hypoglossal and accessory nerve (gray) are not part of the cervical plexus and are provided here as anatomical landmarks only.
The branches of the cervical plexus (gray) make up the ansa cervicalis and innervate the infrahyoid muscles.

Cervical plexus

Brachial plexus

Each skeletal muscle is innervated by a group of alpha motor neurons. However, these motor neurons are not located in the same spinal cord segment, but organized in columns spanning multiple segments. The fibers of these segments form a plexus, from which the peripheral nerves emerge.
For example, the biceps brachii muscle is innervated by the musculocutaneous nerve that derives fibres from C5–C7. Similarly, the radial nerve derives fibres from C5–T1.

Motor neuron columns in the spinal cord and plexus formation

Peripheral nerves contain different sleeves of connective tissue: 
- The epineurium is the outermost layer that surrounds the nerve. 
- There are multiple nerve fiber bundles in each nerve, each of which is enclosed by a perineurium. 
- Individual nerve fibers are enclosed in an endoneurium.

Cross-section of a peripheral nerve

Neurons are classified based on the number of neuronal processes (neurites) that project from the cell body (soma).
– Unipolar: A single axon projects from the soma (dendrites may branch from the axon). 
– Pseudounipolar: A single neurite projects from the soma but then branches into one axon and one dendrite. 
– Bipolar: One axon and one dendrite project from the soma. 
– Multipolar: One axon and multiple dendrites project from the soma.

Types of neurons

Structure of a neuron

Illustration of different types of synapses, according to their insertion: axodendritic (between axon and dendrite); axosomatic (between axon and perikaryon); axoaxonic (between two axons) and axodendrosomatic (between axon, body, and dendrites).

Types of synapses

Schematic representation of the body's dermatomes (i.e., areas of skin that are innervated by a single sensory spinal root)
Note: Nearly all dermatome maps refer to publications by Head and Campbell (1900), Foerster (1933), or Keegan and Garrett (1948), which differ significantly in some areas. After careful review of the current literature, this illustration has been based on Keegan and Garrett's work, as it is most likely to be based on valid methodology and correlates with established clinical findings.

Dermatome map

Nervous system

Skull (frontal view)

Skull (lateral view)

Cranial bones (frontal view)

Bones of the skull (lateral view)

Cranial bones (posterior view)

Base of the skull (inferior view of external surface)

Base of the skull (superior view of internal surface)

Base of skull (superior view of internal surface)

Base of the skull (important vessels and nerves)

Cranial fossa (cranial view)

Canals and foramina of base of skull (inferior view)

Inside of calvaria

Calvaria (cranial view)

Skull of an infant (cranial view)
Anterior fontanelle: diamond-shaped structure located at the junction of the frontal, sagittal, and coronal sutures between the two frontal and two parietal bones. Normally closes between 12–18 months of age.
Posterior fontanelle: triangular-shaped structure located at the junction of the sagittal and lambdoid sutures between the occipital and two parietal bones. Normally closes between 2–3 months of age.

Cranial sutures and fontanelles

There are four paired paranasal sinuses that lie within the corresponding cranial bones. Unlike the other paranasal sinuses, the ethmoid sinuses are composed of groups of small air cells that drain separately into the nasal meatuses.

Paranasal sinuses

Medial wall of the orbit

Lateral wall of the orbit

Right orbit

Bony orbit (frontal view)

Frontal section of the orbita and neighboring structures

The joint capsule is reinforced and stabilized by the lateral ligament and the stylomandibular ligament. Not pictured is the sphenomandibular ligament, which lies medial to the mandible.

Left temporomandibular joint with ligaments

Left eye; transverse section; cranial view

Anatomy of the eye

Longitudinal section; lateral view
The eyeball can be divided into structures that refract light and structures that perceive light. The former focus incoming light to project a punctiform image onto the retina, while the latter convert the luminous stimuli into electric signals and transmit these to the brain via the optic nerve.

Functional anatomy of the eye

The lens is a transparent structure and an important part of the refractive system of the eye. It is maintained in position between the iris and the vitreous body by suspensory ligaments (zonular fibers) attached to the ciliary body. Contraction or relaxation of the ciliary muscle changes the tension of these fibers and so adjusts the convexity of the lens, allowing the eye to focus on objects at varying distances (accommodation).
The lens lies directly behind the iris with its central part directly behind the pupil. It is surrounded by the elastic lens capsule, which provides structural support and is important in returning the lens to a more convex shape once tension on the zonular fibers is reduced. On the anterior surface of the lens until the equator, between the lens capsule and the lens fibers, lies the single-layer epithelium. In the equatorial region it continuously produces new lens fibers, forming layers around the old fibers. This creates a structure of newer outer fibers and older inner fibers, with the nucleus formed of the oldest fibers.

Intraocular lens

Aqueous humor is produced in the non-pigmented ciliary epithelium. From the posterior chamber it flows through the pupil into the anterior chamber. About 85% of aqueous humor is then drained through the canal of Schlemm into the episcleral veins. About 15% is drained through the ciliary muscle into the vortex veins and choridal vessels (uveoscleral pathway).

Flow of aqueous humor

The anterior chamber angle is formed by the iris, cornea, and sclera, and drains the aqueous humor. A trabecular meshwork with spaces of Fontana connects the anterior chamber and the Schlemm's canal, which is located in the sclera. The aqueous humor drains via the Schlemm's canal first into the sclera, then into the episcleral veins.

Anterior chamber angle

The retina consists of the retinal pigment epithelium and the nerve layer. The nerve layer (also referred to as neural retina) is further subdivided into nine layers, each containing specific cells. 
Most prominently, the nerve layer contains the 1st (photoreceptors), 2nd (bipolar cells), and 3rd (ganglion cells) neurons of the visual pathway.

Layers of the retina

Anatomy of the eye and surrounding structures

Main image: ventral view of lacrimal apparatus; inset: sagittal section of cornea and tear film
Not shown: meibomian glands (produce lipid layer) and conjunctival goblet cells (produce mucous layer)

Lacrimal apparatus and tear film layers

The oculomotor nerve innervates all extraocular muscles (1–5) apart from the superior oblique muscle (6; innervated by the trochlear nerve) and the lateral rectus muscle (7; innervated by the abducens nerve).

Innervation of the extraocular muscles

The insertions of the extraocular muscles to the eyeball are shown in the right orbit. In the left orbit, the eyeball has been removed revealing the tendinous ring (the common origin of the extraocular muscles – except for the inferior oblique muscle, which arises from the medial orbital surface).

Ventral view of extraocular muscles

Right eye; lateral view
The orbit contains 6 muscles that are attached to the eyeball itself and move it, as well as the levator palpebrae superioris muscle, which inserts into the upper eyelid and elevates it. The medial rectus muscle is only partially visible here as it is mostly obscured by the lateral rectus. The levator palpebrae superioris muscle is shown resected near its origin to reveal the superior oblique muscle.

Anatomy of the extraocular muscles

Multiple nerves pass through the bony orbit. The most important nerves are the optic nerve (carrying special somatic afferent information, i.e. all the visual information) and the ophthalmic nerve (carrying general somatic afferent information).

Nerves of the eye and the orbital cavity

The ear is subdivided into the outer, middle, and inner ear. The outer ear consists of the auricle and the external acoustic porus. The tympanic membrane separates the outer ear from the middle ear. The middle ear comprises the tympanic cavity, mastoid process, and eustachian tube. The inner ear contains the organ of hearing, which is connected to the brain via the cochlear nerve, and the organs of balance, which are connected to the brain via the vestibular nerve.

Outer, middle, and inner ear

Arterial supply of the ear

Frontal section of the enlarged tympanic cavity with its main structures

Tympanic cavity

The ossicular chain is formed by the sequential articulation of the malleus, incus, and stapes. 
The malleus is attached to the inner aspect of the tympanic membrane. 
The base of the stapes is attached to the oval window that covers the scala vestibuli of the inner ear (not shown here).
The ossicular chain transmits and amplifies sound waves from the tympanic membrane to the inner ear.

Ossicles (middle ear)

The boundaries of the tympanic cavity are shown here by color in a schematic cube (upper part of the picture) and as an enlarged frontal section (lower part of the picture).
For a better overview, the paries caroticus has been removed in the lower part of the picture.

Borders of the tympanic cavity

Cranial view of the right petrous temporal bone
The external, middle, and inner ear, as well as important surrounding structures, are shown.

Location of the inner ear in the cranium

The inner ear consists of a bony labyrinth with a membranous labyrinth inside it, separated by a space containing perilymph fluid. It contains the cochlea and vestibular system. The cochlea (on the right) is responsible for the conversion of sound waves into neuronal signals, which are transmitted via the cochlear nerve (depicted by a cartoon DJ). The vestibular system (on the left) consists of three semicircular canals, the saccule, and the utricle. Semicircular canals detect angular acceleration in three planes: the superior or anterior semicircular canal in the sagittal plane, the posterior semicircular canal in the coronal plane, and the lateral or horizontal semicircular canal in the transverse plane. The saccule and utricle contain cells that detect linear motion in the vertical and horizontal planes, respectively. The neuronal signals from the semicircular canals are transmitted via the vestibular nerve (depicted by a dancing cartoon).

Anatomy of the inner ear

Lateral view of the left nasal cavity; turbinates removed

Nasal meatuses and recesses

Left: lateral nasal wall; right: nasal septum
Branches of the maxillary nerve innervate the following regions: 
– Posterior nasal cavity (posterior superior lateral nasal and posterior inferior nasal branches of the pterygopalatine ganglion) 
– Soft palate (lesser palatine nerves) 
– Hard palate (greater palatine and nasopalatine nerves)
The anterior ethmoidal branches of the ophthalmic nerve innervate the anterior nasal cavity.

Sensory innervation of the nasal cavity and the palate

The olfactory epithelium covers the superior concha, the roof of the nose, and the upper parts of the nasal septum. This epithelium contains the olfactory receptor neurons, whose axons unite into small bundles (olfactory nerve filaments), pass through the cribriform plate, and enter the olfactory bulb, where they synapse on second-order neurons. The axons of these second-order neurons form the olfactory tract, which divides into the medial and lateral olfactory striae.

Anatomy illustration quizzes

Fundamentals of anatomy

Bones

Muscles

Lymphoreticular system

Exocrine

Nervous system

Brain

Cerebrum and cerebellum

Brainstem and cranial nerves

Neurovasculature

Spinal cord and tracts

Nerve plexuses

Miscellaneous

Head and neck

Extraocular structures

Nose

Throat and mouth

Neck and thyroid gland

Thorax and cardiopulmonary system

Trachea and lungs

Thoracic vasculature

Thoracic cavity and mediastinum

Thoracic wall and spine

Gastrointestinal system

Esophagus and stomach

Hepatobiliary system and pancreas

Small intestine, colon, and anus

Vasculature and nervous innervation

Peritoneal cavity and the abdominal wall

Abdominal regions

Urology and renal system

Breast and reproductive system

Upper limb

Shoulder, clavicle, and axilla

Hand and wrist

Vasculature and nervous innervation

Lower limb

Pelvis and hip

Leg and knee

Foot and ankle

Vasculature and nervous innervation

Embryogenesis

Development of specific structures

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