Biomechanics is the study of the structure and motion of the mechanical aspects of biological systems, at any level from whole organisms to organs and cell organelles, using the methods of mechanics. The word "biomechanics" and the related "biomechanical" come from the Ancient Greek βίος bios "life" and μηχανική, mēchanikē "mechanics", to refer to the study of the mechanical principles of living organisms their movement and structure. Biological fluid mechanics, or biofluid mechanics, is the study of both gas and liquid fluid flows in or around biological organisms. An studied liquid biofluids problem is that of blood flow in the human cardiovascular system. Under certain mathematical circumstances, blood flow can be modelled by the Navier–Stokes equations. In vivo whole blood is assumed to be an incompressible Newtonian fluid. However, this assumption fails. At the microscopic scale, the effects of individual red blood cells become significant, whole blood can no longer be modelled as a continuum.
When the diameter of the blood vessel is just larger than the diameter of the red blood cell the Fahraeus–Lindquist effect occurs and there is a decrease in wall shear stress. However, as the diameter of the blood vessel decreases further, the red blood cells have to squeeze through the vessel and can only pass in single file. In this case, the inverse Fahraeus -- the wall shear stress increases. An example of a gaseous biofluids problem is that of human respiration. Respiratory systems in insects have been studied for bioinspiration for designing improved microfluidic devices; the main aspects of Contact mechanics and tribology are related to friction and lubrication. When the two surfaces come in contact during motion i.e. rub against each other, friction and lubrication effects are important to analyze in order to determine the performance of the material. Biotribology is a study of friction and lubrication of biological systems human joints such as hips and knees. For example and tibial components of knee implant rub against each other during daily activity such as walking or stair climbing.
If the performance of tibial component needs to be analyzed, the principles of biotribology are used to determine the wear performance of the implant and lubrication effects of synovial fluid. In addition, the theory of contact mechanics becomes important for wear analysis. Additional aspects of biotribology can include analysis of subsurface damage resulting from two surfaces coming in contact during motion, i.e. rubbing against each other, such as in the evaluation of tissue engineered cartilage. Comparative biomechanics is the application of biomechanics to non-human organisms, whether used to gain greater insights into humans or into the functions and adaptations of the organisms themselves. Common areas of investigation are Animal locomotion and feeding, as these have strong connections to the organism's fitness and impose high mechanical demands. Animal locomotion, has many manifestations, including running and flying. Locomotion requires energy to overcome friction, drag and gravity, though which factor predominates varies with environment.
Comparative biomechanics overlaps with many other fields, including ecology, developmental biology and paleontology, to the extent of publishing papers in the journals of these other fields. Comparative biomechanics is applied in medicine as well as in biomimetics, which looks to nature for solutions to engineering problems. Computational biomechanics is the application of engineering computational tools, such as the Finite element method to study the mechanics of biological systems. Computational models and simulations are used to predict the relationship between parameters that are otherwise challenging to test experimentally, or used to design more relevant experiments reducing the time and costs of experiments. Mechanical modeling using finite element analysis has been used to interpret the experimental observation of plant cell growth to understand how they differentiate, for instance. In medicine, over the past decade, the Finite element method has become an established alternative to in vivo surgical assessment.
One of the main advantages of computational biomechanics lies in its ability to determine the endo-anatomical response of an anatomy, without being subject to ethical restrictions. This has led FE modeling to the point of becoming ubiquitous in several fields of Biomechanics while several projects have adopted an open source philosophy; the mechanical analysis of biomaterials and biofluids is carried forth with the concepts of continuum mechanics. This assumption breaks down when the length scales of interest approach the order of the micro structural details of the material. One of the most remarkable characteristic of biomaterials is their hierarchical structure. In other words, the mechanical characteristics of these materials rely on physical phenomena occurring in multiple levels, from the molecular all the way up to the tissue and organ levels. Biomaterials are classified in two groups and soft tissues. Mechanical deformation of hard tissues may be analysed with the theory of linear elasticity.
On the other hand, soft tissues undergo large deformations and thus their analysis rely on the finite strain theory and computer simulations. The interest in continuum biomechanics is spurred by the need for realism in the development of medical simulation
A nerve is an enclosed, cable-like bundle of nerve fibres called axons, in the peripheral nervous system. A nerve provides a common pathway for the electrochemical nerve impulses called action potentials that are transmitted along each of the axons to peripheral organs or, in the case of sensory nerves, from the periphery back to the central nervous system; each axon within the nerve is an extension of an individual neuron, along with other supportive cells such as Schwann cells that coat the axons in myelin. Within a nerve, each axon is surrounded by a layer of connective tissue called the endoneurium; the axons are bundled together into groups called fascicles, each fascicle is wrapped in a layer of connective tissue called the perineurium. The entire nerve is wrapped in a layer of connective tissue called the epineurium. In the central nervous system, the analogous structures are known as tracts; each nerve is covered on the outside by a dense sheath of the epineurium. Beneath this is a layer of flat cells, the perineurium, which forms a complete sleeve around a bundle of axons.
Perineurial septae subdivide it into several bundles of fibres. Surrounding each such fibre is the endoneurium; this forms an unbroken tube from the surface of the spinal cord to the level where the axon synapses with its muscle fibres, or ends in sensory receptors. The endoneurium consists of an inner sleeve of material called the glycocalyx and an outer, meshwork of collagen fibres. Nerves are bundled and travel along with blood vessels, since the neurons of a nerve have high energy requirements. Within the endoneurium, the individual nerve fibres are surrounded by a low-protein liquid called endoneurial fluid; this acts in a similar way to the cerebrospinal fluid in the central nervous system and constitutes a blood-nerve barrier similar to the blood-brain barrier. Molecules are thereby prevented from crossing the blood into the endoneurial fluid. During the development of nerve edema from nerve irritation, the amount of endoneurial fluid may increase at the site of irritation; this increase in fluid can be visualized using magnetic resonance neurography, thus MR neurography can identify nerve irritation and/or injury.
Nerves are categorized into three groups based on the direction that signals are conducted: Afferent nerves conduct signals from sensory neurons to the central nervous system, for example from the mechanoreceptors in skin. Efferent nerves conduct signals from the central nervous system along motor neurons to their target muscles and glands. Mixed nerves contain both afferent and efferent axons, thus conduct both incoming sensory information and outgoing muscle commands in the same bundle. Nerves can be categorized into two groups based on where they connect to the central nervous system: Spinal nerves innervate much of the body, connect through the vertebral column to the spinal cord and thus to the central nervous system, they are given letter-number designations according to the vertebra through which they connect to the spinal column. Cranial nerves innervate parts of the head, connect directly to the brain, they are assigned Roman numerals from 1 to 12, although cranial nerve zero is sometimes included.
In addition, cranial nerves have descriptive names. Specific terms are used to describe their actions. A nerve that supplies information to the brain from an area of the body, or controls an action of the body is said to "innervate" that section of the body or organ. Other terms relate to whether the nerve affects the same side or opposite side of the body, to the part of the brain that supplies it. Nerve growth ends in adolescence, but can be re-stimulated with a molecular mechanism known as "Notch signaling". If the axons of a neuron are damaged, as long as the cell body of the neuron is not damaged, the axons would regenerate and remake the synaptic connections with neurons with the help of guidepost cells; this is referred to as neuroregeneration. The nerve begins the process by destroying the nerve distal to the site of injury allowing Schwann cells, basal lamina, the neurilemma near the injury to begin producing a regeneration tube. Nerve growth factors are produced causing many nerve sprouts to bud.
When one of the growth processes finds the regeneration tube, it begins to grow towards its original destination guided the entire time by the regeneration tube. Nerve regeneration is slow and can take up to several months to complete. While this process does repair some nerves, there will still be some functional deficit as the repairs are not perfect. A nerve conveys information in the form of electrochemical impulses carried by the individual neurons that make up the nerve; these impulses are fast, with some myelinated neurons conducting at speeds up to 120 m/s. The impulses travel from one neuron to another by crossing a synapse, the message is converted from electrical to chemical and back to electrical. Nerves can be categorized into two groups based on function: An afferent nerve fiber conducts sensory information from a sensory neuron to the central nervous system, where the information is processed. Bundles of fibres or axons, in the peripheral nervous system are called nerves, bundles of afferent fibers are known as sensory nerves.
An efferent nerve fiber conducts signals from a motor neuron in the central nervous system to muscles. Bundles of these fibres are known as efferent nerves; the nervous system is the part of an animal that coordinates its actions by transmitting signals to and from different parts of its body. In vertebrates it consists of two main par
Anatomical terms of motion
Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints and specific sections of the body; the terminology used describes this motion according to its direction relative to the anatomical position of the joints. Anatomists use a unified set of terms to describe most of the movements, although other, more specialized terms are necessary for describing the uniqueness of the movements such as those of the hands and eyes. In general, motion is classified according to the anatomical plane. Flexion and extension are examples of angular motions, in which two axes of a joint are brought closer together or moved further apart. Rotational motion may occur at other joints, for example the shoulder, are described as internal or external. Other terms, such as elevation and depression, describe movement above or below the horizontal plane. Many anatomical terms derive from Latin terms with the same meaning. Motions are classified after the anatomical planes they occur in, although movement is more than not a combination of different motions occurring in several planes.
Motions can be split into categories relating to the nature of the joints involved: Gliding motions occur between flat surfaces, such as in the intervertebral discs or between the carpal and metacarpal bones of the hand. Angular motions occur over synovial joints and causes them to either increase or decrease angles between bones. Rotational motions move a structure in a rotational motion along a longitudinal axis, such as turning the head to look to either side. Apart from this motions can be divided into: Linear motions, which move in a line between two points. Rectilinear motion is motion in a straight line between two points, whereas curvilinear motion is motion following a curved path. Angular motions occur when an object is around another object decreasing the angle; the different parts of the object do not move the same distance. Examples include a movement of the knee, where the lower leg changes angle compared to the femur, or movements of the ankle; the study of movement is known as kinesiology.
A categoric list of movements of the human body and the muscles involved can be found at list of movements of the human body. The prefix hyper- is sometimes added to describe movement beyond the normal limits, such as in hypermobility, hyperflexion or hyperextension; the range of motion describes the total range of motion. For example, if a part of the body such as a joint is overstretched or "bent backwards" because of exaggerated extension motion it can be described as hyperextended. Hyperextension increases the stress on the ligaments of a joint, is not always because of a voluntary movement, it may be other causes of trauma. It may be used in surgery, such as in temporarily dislocating joints for surgical procedures; these are general terms. Most terms have a clear opposite, so are treated in pairs. Flexion and extension describe movements; these terms come from the Latin words with the same meaning. Flexion describes a bending movement that decreases the angle between a segment and its proximal segment.
For example, bending the elbow, or clenching a hand into a fist, are examples of flexion. When sitting down, the knees are flexed; when a joint can move forward and backward, such as the neck and trunk, flexion refers to movement in the anterior direction. When the chin is against the chest, the head is flexed, the trunk is flexed when a person leans forward. Flexion of the shoulder or hip refers to movement of the leg forward. Extension is the opposite of flexion, describing a straightening movement that increases the angle between body parts. For example, when standing up, the knees are extended; when a joint can move forward and backward, such as the neck and trunk, extension refers to movement in the posterior direction. Extension of the hip or shoulder moves the leg backward. Abduction is the motion of a structure away from the midline while adduction refer to motion towards the center of the body; the centre of the body is defined as the midsagittal plane. These terms come from Latin words with similar meanings, ab- being the Latin prefix indicating "away," ad- indicating "toward," and ducere meaning "to draw or pull".
Abduction refers to a motion that pulls a part away from the midline of the body. In the case of fingers and toes, it refers to spreading the digits apart, away from the centerline of the hand or foot. Abduction of the wrist is called radial deviation. For example, raising the arms up, such as when tightrope-walking, is an example of abduction at the shoulder; when the legs are splayed at the hip, such as when doing a star jump or doing a split, the legs are abducted at the hip. Adduction refers to a motion that pulls a structure or part toward the midline of the body, or towards the midline of a limb. In the case of fingers and toes, it refers to bringing the digits together, towards the centerline of the hand or foot. Adduction of the wrist is called ulnar deviation. Dropping the arms to the sides, bringing the knees together, are examples of adduction. Ulnar deviation is the hand moving towards the ulnar styloid. Radial deviation is the hand moving towards the radial styloid; the terms elevation and depression refer to movement below the horizontal.
They derive from the Latin terms with similar meaningsElevation refers to movement in a superior direction. For example
Anterior compartment of thigh
The anterior compartment of thigh contains muscles which extend the knee and flex the hip. The anterior compartment is one of the fascial compartments of the thigh that contains groups of muscles together with their nerves and blood supply; the anterior compartment contains the sartorius muscle and the quadriceps femoris group, which consists of the rectus femoris muscle and the three vasti muscles – the vastus lateralis, vastus intermedius, the vastus medialis. The iliopsoas is sometimes considered a member of the anterior compartment muscles, as is the articularis genus muscle; the anterior compartment is separated from the posterior compartment by the lateral intermuscular septum and from the medial compartment by the medial intermuscular septum. The nerve of the anterior compartment of thigh is the femoral nerve. Innervation for the quadriceps muscles come from the posterior division of the femoral nerve, while the anterior division gives a lateral and a medial branch, the second being responsible for the innervation of the sartorius muscle.
The iliacus and the psoas major and psoas minor muscles, sometimes considered part of the anterior compartment, do not share the same innervation. Whereas the iliacus is innervated by the femoral nerve, the psoas is innervated by ventral rami of L1-L3; when the external iliac artery crosses the inguinal ligament, it becomes the femoral artery, which supplies blood to the anterior compartment and is the largest blood vessel of the inferior member. The anterior compartment of thigh contains muscles which are extensors of the knee and flexors of the hip joints; the anterior compartment may be affected as part of a compartment syndrome. Antthigh at The Anatomy Lesson by Wesley Norman knee/muscles/thigh1 at the Dartmouth Medical School's Department of Anatomy Overview at stanford.edu
The public domain consists of all the creative works to which no exclusive intellectual property rights apply. Those rights may have been forfeited, expressly waived, or may be inapplicable; the works of William Shakespeare and Beethoven, most early silent films, are in the public domain either by virtue of their having been created before copyright existed, or by their copyright term having expired. Some works are not covered by copyright, are therefore in the public domain—among them the formulae of Newtonian physics, cooking recipes, all computer software created prior to 1974. Other works are dedicated by their authors to the public domain; the term public domain is not applied to situations where the creator of a work retains residual rights, in which case use of the work is referred to as "under license" or "with permission". As rights vary by country and jurisdiction, a work may be subject to rights in one country and be in the public domain in another; some rights depend on registrations on a country-by-country basis, the absence of registration in a particular country, if required, gives rise to public-domain status for a work in that country.
The term public domain may be interchangeably used with other imprecise or undefined terms such as the "public sphere" or "commons", including concepts such as the "commons of the mind", the "intellectual commons", the "information commons". Although the term "domain" did not come into use until the mid-18th century, the concept "can be traced back to the ancient Roman Law, as a preset system included in the property right system." The Romans had a large proprietary rights system where they defined "many things that cannot be owned" as res nullius, res communes, res publicae and res universitatis. The term res nullius was defined as things not yet appropriated; the term res communes was defined as "things that could be enjoyed by mankind, such as air and ocean." The term res publicae referred to things that were shared by all citizens, the term res universitatis meant things that were owned by the municipalities of Rome. When looking at it from a historical perspective, one could say the construction of the idea of "public domain" sprouted from the concepts of res communes, res publicae, res universitatis in early Roman law.
When the first early copyright law was first established in Britain with the Statute of Anne in 1710, public domain did not appear. However, similar concepts were developed by French jurists in the 18th century. Instead of "public domain", they used terms such as publici juris or propriété publique to describe works that were not covered by copyright law; the phrase "fall in the public domain" can be traced to mid-19th century France to describe the end of copyright term. The French poet Alfred de Vigny equated the expiration of copyright with a work falling "into the sink hole of public domain" and if the public domain receives any attention from intellectual property lawyers it is still treated as little more than that, left when intellectual property rights, such as copyright and trademarks, expire or are abandoned. In this historical context Paul Torremans describes copyright as a, "little coral reef of private right jutting up from the ocean of the public domain." Copyright law differs by country, the American legal scholar Pamela Samuelson has described the public domain as being "different sizes at different times in different countries".
Definitions of the boundaries of the public domain in relation to copyright, or intellectual property more regard the public domain as a negative space. According to James Boyle this definition underlines common usage of the term public domain and equates the public domain to public property and works in copyright to private property. However, the usage of the term public domain can be more granular, including for example uses of works in copyright permitted by copyright exceptions; such a definition regards work in copyright as private property subject to fair-use rights and limitation on ownership. A conceptual definition comes from Lange, who focused on what the public domain should be: "it should be a place of sanctuary for individual creative expression, a sanctuary conferring affirmative protection against the forces of private appropriation that threatened such expression". Patterson and Lindberg described the public domain not as a "territory", but rather as a concept: "here are certain materials – the air we breathe, rain, life, thoughts, ideas, numbers – not subject to private ownership.
The materials that compose our cultural heritage must be free for all living to use no less than matter necessary for biological survival." The term public domain may be interchangeably used with other imprecise or undefined terms such as the "public sphere" or "commons", including concepts such as the "commons of the mind", the "intellectual commons", the "information commons". A public-domain book is a book with no copyright, a book, created without a license, or a book where its copyrights expired or have been forfeited. In most countries the term of protection of copyright lasts until January first, 70 years after the death of the latest living author; the longest copyright term is in Mexico, which has life plus 100 years for all deaths since July 1928. A notable exception is the United States, where every book and tale published prior to 1924 is in the public domain.
Anterior cruciate ligament
The anterior cruciate ligament is one of a pair of cruciate ligaments in the human knee. The two ligaments are called cruciform ligaments, as they are arranged in a crossed formation. In the quadruped stifle joint, based on its anatomical position, it is referred to as the cranial cruciate ligament; the term cruciate translates to cross. This name is fitting because the ACL crosses the posterior cruciate ligament to form an “X”, it assists in controlling excessive motion. This is done by limiting mobility of the joint; the anterior cruciate ligament is one of the four main ligaments of the knee, providing 85% of the restraining force to anterior tibial displacement at 30 degrees and 90 degrees of knee flexion. The ACL is the most injured ligament of the four located in the knee; the ACL originates from deep within the notch of the distal femur. Its proximal fibers fan out along the medial wall of the lateral femoral condyle. There are two bundles of the ACL: the anteromedial and the posterolateral, named according to where the bundles insert into the tibial plateau.
The tibia plateau is a critical weight-bearing region on the upper extremity of the tibia. The ACL attaches in front of the intercondyloid eminence of the tibia, where it blends with the anterior horn of the lateral meniscus; the purpose of the ACL is to resist the motions of anterior tibial translation and internal tibial rotation. This function prevents anterior tibial subluxation of the lateral and medial tibiofemoral joints, important for the pivot-shift phenomena; the ACL has been proven to have mechanoreceptors that detect changes in direction of movement, position of the knee joint, changes in acceleration and tension. A key factor in instability after ACL injuries is having altered neuromuscular function secondary to diminished somatosensory information. For athletes who participate in sports involving cutting and rapid deceleration it is important for the knee to be stable in terminal extension, the screw-home mechanism. An ACL tear is one of the most common knee injuries, with over 100,000 tears occurring annually in the US.
Most ACL tears are a result of a non-contact mechanism such as a sudden change in a direction causing the knee to rotate inward. As the knee rotates inward additional strain is placed on the ACL, since the femur and tibia, which are the two bones that articulate together forming the knee joint, move in opposite directions causing the ACL to tear. Most athletes will require reconstructive surgery on the ACL, in which the torn or ruptured ACL is removed and replaced with a piece of tendon or ligament tissue from the patient or from a donor. Conservative treatment has poor outcomes in ACL injury since the ACL is unable to form a fibrous clot as it receives most of its nutrients from the synovial fluid which washes away the reparative cells making it difficult for new fibrous tissue to form; the two most common sources for tissue are the hamstrings tendon. The patellar ligament is used, since bone plugs on each end of the graft are extracted which helps integrate the graft into the bone tunnels, during reconstruction.
The surgery is arthroscopic, meaning. The camera sends video to a large monitor. In the event of an autograft, the surgeon will make a larger cut to get the needed tissue. In the event of an allograft, in which material is donated, this is not necessary since no tissue is taken directly from the patient's own body; the surgeon will drill a hole forming the tibial bone tunnel and femoral bone tunnel, allowing for the patient's new ACL graft to be guided through. Once the graft is pulled through the bone tunnels, two screws are placed into the tibial and femoral bone tunnel. Recovery time ranges between one and two years or longer, depending if the patient chose an autograft or allograft. A week or so after the occurrence of the injury, the athlete is deceived by the fact that he/she is walking and not feeling much pain; this is dangerous as some athletes start resuming some of their activities such as jogging which, with a wrong move or twist, could damage the bones as the graft has not become integrated into the bone tunnels.
It is important for the injured athlete to understand the significance of each step of an ACL injury to avoid complications and ensure a proper recovery. ACL reconstruction is the most common treatment for an ACL tear, however it is not the only treatment available for individuals; some individuals may find it more beneficial to complete a non-operative rehab program. Both individuals who are going to continue with physical activity that involves cutting and pivoting, individuals who are no longer participating in those specific activities are candidates for the non-operative route. A study was completed comparing operative and non-operative approaches to ACL tears and there were few differences noted by both surgical and nonsurgical groups. However, there was no significant differences in regard to knee function or muscle strength reported by the patient; the main goals to achieve during rehabilitation of an ACL tear is to regain sufficient functional stability, maximize full muscle strength, decrease risk of re-injury.
There are three phases involved in non-operative treatment. These phases include the Acute Phase, the Neuromuscular Training Phase, the Return to Sport Phase. During the acute phase, the rehab is focusing on the acute symptoms that occur right after the injury and is causing an impairment; the use
The motor cortex is the region of the cerebral cortex involved in the planning and execution of voluntary movements. Classically the motor cortex is an area of the frontal lobe located in the posterior precentral gyrus anterior to the central sulcus; the motor cortex can be divided into three areas: 1. The primary motor cortex is the main contributor to generating neural impulses that pass down to the spinal cord and control the execution of movement. However, some of the other motor areas in the brain play a role in this function, it is located on the anterior paracentral lobule on the medial surface. 2. The premotor cortex is responsible for some aspects of motor control including the preparation for movement, the sensory guidance of movement, the spatial guidance of reaching, or the direct control of some movements with an emphasis on control of proximal and trunk muscles of the body. Located anterior to the primary motor cortex. 3. The supplementary motor area, has many proposed functions including the internally generated planning of movement, the planning of sequences of movement, the coordination of the two sides of the body such as in bi-manual coordination.
Located on the midline surface of the hemisphere anterior to the primary motor cortex. The posterior parietal cortex is sometimes considered to be part of the group of motor cortical areas, it is thought to be responsible for transforming multisensory information into motor commands, to be responsible for some aspects of motor planning, in addition to many other functions that may not be motor related. The primary somatosensory cortex the part called area 3a, which lies directly against the motor cortex, is sometimes considered to be functionally part of the motor control circuitry. Other brain regions outside the cerebral cortex are of great importance to motor function, most notably the cerebellum, the basal ganglia, pedunculopontine nucleus and the red nucleus, as well as other subcortical motor nuclei. In the earliest work on the motor cortex, researchers recognized only one cortical field involved in motor control. Alfred Walter Campbell was the first to suggest that there might be two fields, a "primary" motor cortex and an "intermediate precentral" motor cortex.
His reasons were based on cytoarchitectonics, or the study of the appearance of the cortex under a microscope. The primary motor cortex contains cells with giant cell bodies known as "Betz cells"; these cells were mistakenly thought to be the main outputs from the cortex, sending fibers to the spinal cord. It has since been found that Betz cells account for about 2-3% of the projections from the cortex to the spinal cord, or about 10% of the projections from the primary motor cortex to the spinal cord; the specific function of the Betz cells that distinguishes them from other output cells of the motor cortex remains unknown, but they continue to be used as a marker for the primary motor cortex. Other researchers, such as Vogt and Vogt and Otfrid Foerster suggested that motor cortex was divided into a primary motor cortex and a higher-order motor cortex. Wilder Penfield notably disagreed and suggested that there was no functional distinction between area 4 and area 6. In his view both were part of the same map, though area 6 tended to emphasize the muscles of the back and neck.
Woolsey who studied the motor map in monkeys believed there was no distinction between primary motor and premotor. M1 was the name for the proposed single map that encompassed both the primary motor cortex and the premotor cortex. Although sometimes "M1" and "primary motor cortex" are used interchangeably speaking, they derive from different conceptions of motor cortex organization. Despite the views of Penfield and Woolsey, a consensus emerged that area 4 and area 6 had sufficiently different functions that they could be considered different cortical fields. Fulton helped to solidify this distinction between a primary motor cortex in area 4 and a premotor cortex in area 6; as Fulton pointed out, as all subsequent research has confirmed, both primary motor and premotor cortex project directly to the spinal cord and are capable of some direct control of movement. Fulton showed that when the primary motor cortex is damaged in an experimental animal, movement soon recovers; the premotor cortex is now divided into four sections.
First it is divided into a lower premotor cortex. Each of these is further divided into a region more toward the front of the brain and a region more toward the back. A set of acronyms are used: PMDr, PMDc, PMVr, PMVc; some researchers use a different terminology. Field 7 or F7 denotes PMDr. PMDc is studied with respect to its role in guiding reaching. Neurons in PMDc are active during reaching; when monkeys are trained to reach from a central location to a set of target locations, neurons in PMDc are active during the preparation for the reach and during the reach itself. They are broadly tuned, responding best to one direction of reach and less well to different directions. Electrical stimulation of the PMDc on a behavioral time scale was reported to evoke a complex movement of the shoulder and hand that resembles reaching with the hand opened in preparation to grasp. PMDr may participate in learning to associate arbitrary sensory stimuli with specific movements or learning ar