Physical Therapists must know the anatomy of the body, as well as how it functions. The following is a general overview of systems anatomy - what students should know before they begin the professional phase of physical therapy education.
Systems Overview Edit
The NERVOUS SYSTEM is divided, into two parts based on location: Central Nervous System (CNS)
The CNS consists of the brain and spinal cord.
Neurons that lie completely within the CNS and connect via synapses only with other neurons are called interneurons. Peripheral Nervous System (PNS) The PNS is made up of nerves and ganglia This includes:
- twelve pairs of cranial nerves
- thirty-one pairs of spinal nerves:
- 8 cervical
- 12 thoracic
- 5 lumbar
- 5 sacral
- 1 coccygeal
- all sensory ganglia
- dorsal root ganglia
- cranial sensory ganglia
- all autonomic ganglia
Neurons and glia are the cells of the nervous system.
- Neurons are cells that have a cell body, or soma, and several processes or extensions from the cell body.
- Glia are cells which are not conductive, but provide structural as well as nutritive functions. In the PNS, the most common glial cell is the Schwann cell. In the CNS, the oligodendrocytes are common, but there are also other types of glial cells.
- Dendrites are the receptive processes of nerves: they receive incoming signals.
- Axons are usually longer processes that convey signals from the cell body along the axon to synapses, which convey the signal to another neuron, muscle, or gland.
- Synapses are the electrochemical junctions between neurons at which one neuron can excite or inhibit action potentials in another neuron.
- Nerve Fibers are axons (single processes of a single neuron) with its myelin sheath (if it has one).
Nerves are bundles of parallel nerve fibers (primarily axons) in the peripheral nervous system (outside the brain and spinal cord).
- Each individual axon arises from (belongs to) a single neuron. Axons may branch in the CNS, but not usually in ther periphery.
- There are no synapses within a nerve.
- The impulses are conducted along each axon separately, with no "cross talk" between axons.
- Myelin: Some axons are tightly surrounded by layer upon layer of cell membrane of Schwann cells that forms an insulating covering called myelin. Myelin increases the speed of conduction of action potentials within the nerve.
- Non-myelinated axons lie separately, usually surrounded loosely by a Schwann cell, but without the multilayered myelin.
- Connective Tissue Sheaths of Nerves: Within a nerve, several axons are together surrounded by a fine endoneurium; Several of these bundles (a fascicle) are ensheathed by a more dense perineurium, and the entire nerve is surrounded by an epineurium: all of these layers are connective tissue. They provide support and protection, and contain tiny blood vessels (nervi vasorum) and nerves (nervi nervorum) that supply the nerves.
- Action Potentials are the electrochemical signals conveyed within a single neuron from its dendrite to the cell body and then out all of its' axons.
- Afferents are axons that convey action potentials toward the CNS (brain or spinal cord). These are sensory axons.
- Efferents are axons that convey action potentials away from the CNS - toward the periphery to end in on muscle cells or glands. These are also called motor axons. A single nerve usually contains both afferents and efferents.
- Sensory roots are the fibers(axons) of sensory neurons as they enter the spinal cord at each spinal cord segment. They enter the dorso-lateral part of the spinal cord, and so are called dorsal roots. So all the axons in the dorsal roots are sensory.
- Motor roots are the fibers (axons) of motor neurons as they leave the spinal cord at each spinal cord segment.They leave from the ventro-lateral part of the spinal cord, and so are called ventral roots. So all the axons in the ventral roots are motor. (Sympathetic fibers also leave the spinal cord via the motor roots.)
- Spinal nerves: While still within the vertebral column, the dorsal and ventral roots on each side, and at each segmental level, join to form spinal nerves (one on each side of the body, since we are bilaterally symmetrical.)
- Ganglia are clusters of neuron cell bodies in the PNS (Peripheral Nervous System).
- There are 2 types of ganglia:
- Sensory ganglia (or dorsal root ganglia) contain ONLY the cell bodies of sensory neurons, and glial cells. There are no synapses in sensory ganglia. There are many sensory functions represented in these ganglia, including: touch, temperature, pain, itch, vibration. In the head, there are sensory ganglia for some of our special senses: sight, hearing, olfaction, and taste, as well as the general senses.
- Autonomic ganglia, which can further be divided into 2 categories:
- Sympathetic ganglia - which contain ONLY the cell bodies of post-ganglionic sympathetic neurons, and glia. These ganglia DO contain synapses between the pre- and post-ganglionic sympathetic neurons.
- Parasympathetic ganglia - which contain ONLY the cell bodies of post-ganglionic parasympathetic neurons, and glia. These ganglia DO contain synapses between the pre- and post-ganglionic parasympathetic neurons.
- Ganglia are "pure" in the sense that they contain only one class of neurons (only sensory, sympathetic, or parasympathetic.
- There are 2 types of ganglia:
Functional Categories of Neurons Edit
Every individual neuron has only one job. It serves only one of the following functions:
SENSORY (afferent) - these neurons convey impulses (sensation or "feeling") from the periphery (for example, a fingertip) to the Central Nervous System. or
MOTOR (efferent) - these neurons convey impulses (commands) away from the Central Nervous System, toward the periphery, to contract a muscle, for example. or
INTERNEURONS (connecting) - these neurons, located exclusively in the Central Nervous System, communicate with sensory and motor neurons, and other interneurons, and allow for higher processing.
Every individual peripheral neuron innervates only one class of target structures. Each sensory and motor neuron can serve only one type of structure:
SOMATIC - refers to the musculoskeletal system of the body, and includes bones, muscle, and skin.
VISCERAL - refers to the organs, including the gut and associated organs, glands, and BLOOD VESSELS.
Each neuron of the peripheral nervous system has only a single function: it is either sensory or motor AND it serves either somatic or visceral structures. So all peripheral neurons fall into ONE of the following categories:
- GSA: somatic sensory (general somatic afferent)
- GVA: visceral sensory (general visceral afferent)
- GSE: somatic motor (general somatic efferent)
- GVE: visceral motor (general visceral efferent)
Nerve Fibers are axons (single processes of a single neuron) with its myelin sheath (if it has one) and neurolemmal sheath (both made of Schwann cells). Each nerve fiber, like each neuron, can only have a single function (e.g., sensory or motor).
Nerves are bundles of parallel nerve fibers (primarily axons) in the peripheral nervous system (outside the brain and spinal cord).
This bundle may contain individual nerve fibers that have different functions: e.g., a dorsal primary ramus has nerve fibers that serve all four primary functions: sensory (both GSA and GVA) as well as motor (both GVE and GSE) fibers.
Individual nerve fibers are covered by a delicate layer of endoneurium, which is connective tissue.
Groups of nerve fibers surrounded by a perineurium, which is a more dense and organized layer of connective tissue that forms a barrier similar to the blood brain barrier.
These bundles of fibers within a nerve are called fascicles or fasciculi.
The entire nerve is covered with a thin protective layer of tissue called the epineurium.
Ganglia are clusters of neuron cell bodies in the PNS (Peripheral Nervous System).
A single ganglion can contain only one of the following types of neurons: sensory, sympathetic, or parasympathetic. So any ganglion can be classified as 1) sensory, or 2) autonomic. If autonomic, it must be either sympathetic or parasympathetic, but not both. Sympathetic and parasympathetic neurons are not found in the same ganglia. Similarly, sensory ganglia do not contain any autonomic neurons.
Sensory ganglia contain the cell bodies of sensory neurons.
These cell bodies have a central process that projects into the spinal cord (or brain stem) and a peripheral process that extends into the peripheral tissue (e.g., the skin).
There are no synapses in sensory ganglia.
There are paired sensory ganglia for each spinal cord segment; these are called spinal ganglia or dorsal root ganglia. They lie within the intervertebral foramina.
The spinal ganglia contain neurons that convey all types of general sensation: touch, temperature, pressure, tickle, itch, pain; although any single neuron can only convey one type of information.
There are sensory ganglia in the head associated with some of the cranial nerves. In addition to the general sensations, some of these neurons convey taste, sight, hearing, and smell, although some of the neurons are not found in typical ganglia.
Neurons in these ganglia convey sensory information from the body to the CNS. There are 2 categories of sensation that are based on the source and quality of sensation. The 2 categories also differ in the size of neuron and nerve fiber and the type of sensory receptor at the distal end of the peripheral process.
somatic sensation arises from the skin, fascia, skeletal muscles, tendons, joints, and bone.
visceral sensation arises from smooth muscle, (including smooth muscle in blood vessels), cardiac muscle, organs and glands.
A DERMATOME is the area of skin innervated by a single spinal cord segment. For example, the C6 spinal cord segment supplies a strip of skin over the lateral aspect of the arm, forearm, and thumb. This area of skin is the C6 DERMATOME. see Netter plate 150.
Autonomic ganglia can be either: a) sympathetic or, b) parasympathetic, but not both. In either case, the ganglia contain the cell bodies of the postganglionic neuron and synapses between the pre- and postganglionic neurons.
Postganglionic Sympathetic neurons that supply the vasculature and glands of the musculoskeletal system are found in ganglia along the paired sympathetic trunks (or chains). These are longitudinal series of ganglia connected by axons that lie on either side of the vertebral column from cervical to sacral levels.
Postganglionic Parasympathetic neurons that supply the organs of the body are found within the organs they supply, e.g., the stomach (Myenteric plexus), and thus do not form discreet ganglia. Those that supply glands of the head are discreet ganglia within the head.
SPINAL NERVES Edit
The Spinal Nerve is formed by the union of fibers from the sensory or dorsal root and the motor or ventral root. It also contains postganglionic sympathetic fibers. Therefore, if a spinal nerve is lesioned, both sensory and motor functions, somatic and visceral, are damaged. Notice that the spinal nerve is formed distal to the dorsal root ganglion. The spinal nerve then divides into two major branches: The smaller of the two is the dorsal primary ramus: This one innervates the deep or intrinsic muscles of the back and the skin overlying them. The larger branch is the ventral primary ramus: It innervates the limbs and body wall, except the deep back muscles and skin.
Sensations arise in the tissues of the body. The general senses which we have in our limbs and trunk actually arise from several different tissues and are of a variety of types. We can sense course touch, pressure, tickle, itch, pain, heat, cold and stretch. These senses arise in specific tissues and must be converted to a neural signal to be carried through the peripheral nervous system to the central nervous system - that is, so it can then ascend in the spinal cord to the brain, where, after at least one relay station, it reaches cortex and so can become conscious.
The conversion of the initial process in the tissue to a neural signal is generally done by a receptor. There are many types of sensory receptors, some specialized to tissue types - e.g., muscle spindles and golgi tendon organs, and others that are found broadly throughout many connective tissues, such as Ruffini corpuscles. Nociceptors are sensory receptors, usually free endings, that detect painful stimuli. Nociceptors are of three classes - those sensitive to 1) extremes of temperature, 2) mechanical damage, and chemical indicators of injury. If the stimulus is very strong, however, it will activate any of the types of nociceptor.
Two types of axons—Type A and Type C fibers—carry painful sensations. Myelinated Type A fibers carry sensations of fast pain, or prickling pain. They conduct impulses quickly because they are myelinated - but the diameter of these axons is much smaller than that of non-nociceptive sensations like touch. An injection or a deep cut produces this type of pain. These sensations very quickly reach the CNS, where they often trigger somatic reflexes. They are also relayed to the primary sensory cortex and so receive conscious attention. In most cases, the arriving information permits the stimulus to be localized to an area several inches in diameter.
Slower, Type C fibers carry sensations of slow pain, or burning and aching pain. These are the smallest axons, and they are not myelinated - two reasons that they carry impulses slowly. These sensations cause a generalized activation of the reticular formation and thalamus. The individual becomes aware of the pain but has only a general idea of the area affected.
Pain receptors are tonic receptors. Significant peripheral adaptation does not occur, and the receptors continue to respond as long as the painful stimulus remains. Painful sensations cease only after tissue damage has ended. However, central adaptation may reduce the perception of the pain while the pain receptors are still stimulated. This effect involves the inhibition of centers in the thalamus, reticular formation, lower brain stem, and spinal cord.
Sensory Receptors (Martini) Nociceptors (Martini) THE AUTONOMIC NERVOUS SYSTEM (ANS)
The autonomic nervous system (or visceral motor system) is the motor outflow to visceral structures like the smooth muscle of the gut, piloarrector muscles and blood vessels; cardiac muscle; and other organs and glands.
Like the somatic motor system, impulses travel from the CNS to the periphery.
Unlike the somatic motor system, the visceral motor system has a two neuron chain between the CNS and the periphery.
The first type of neuron, called preganglionic neurons have their cell bodies in the central nervous system.
The cell bodies of the second type of neuron, called postganglionic neurons have their cell bodies in the peripheral nervous system, either in ganglia or dispersed in the walls of organs. (Remember a ganglion is a cluster of a single type of neuronal cell bodies in the peripheral nervous system.)
TWO DIVISIONS OF THE AUTONOMIC NERVOUS SYSTEM Edit
There are two divisions of the autonomic nervous system: Sympathetic and Parasympathetic. The two divisions differ in the location of pre- and postganglionic neurons and in function.
PARASYMPATHETIC Division of the Autonomic Nervous System: Edit
The Parasympathetic System functions mainly in maintaining normal body function: e.g., peristalsis, GI secretions, constricting the pupil to avoid damage to the eye from excessive light.
The preganglionic parasympathetic neurons are located in the brainstem and in the sacral spinal cord. Because of this origin, the parasympathetic division is often call the Cranio-Sacral part of the ANS.
The axons of these neurons form synapses with postganglionic neurons located in ganglia in or near their target organ (e.g., the stomach).
Parasympathetic ganglia are spread diffusely in the walls of or near the organs they supply in the thorax, abdomen and pelvis, but are present in discreet ganglia in the head and neck.
There are NO parasympathetic ganglia or fibers in the back or extremities: They are found exclusively in the body cavities and in the head and neck. There is no parasympathetic innervation of blood vessels.
SYMPATHETIC Division of the ANS: Edit
The Sympathetic System functions in emergency, fight or flight circumstances, preparing the body to face danger or run, increasing heart rate and contractility, increasing blood flow to skeletal muscle and away from the digestive tract, opening the pupils to let light in, and increasing respiration rate.
The preganglionic sympathetic neurons are located in the intermediolateral cell column of the spinal cord from T (thoracic) 1 to L (lumbar) 2 segmental levels. It is referred to as the Thoraco-Lumbar part of the ANS.
Axons of these fibers emerge from the spinal cord and join with the ventral root to reach and synapse with neurons in the appropriate ganglia, which contain the postganglionic sympathetic neurons.
At each spinal level, there are a pair of paravertebral sympathetic ganglia, connected by a longitudinal sympathetic trunk (or chain) consisting of axons. Postganglionic fibers from these ganglia join with each spinal nerve to supply the vasculature of the trunk and limbs and the arrector pili muscles of the skin. Other fibers emerge from the sympathetic trunk to supply the heart and lungs via cardiac and pulmonary plexuses.
There are also prevertebral sympathetic ganglia, which lie along the major abdominal blood vessels. These postganglionic neurons supply the viscera of the abdomen and pelvis.
Preganglionic axons may travel up or down through the sympathetic trunk before synapsing in a paravertebral (chain) sympathetic ganglion, or emerge from the trunk without synapsing to proceed to prevertebral ganglia which supply the abdominal and pelvic organs.
SOME SYNONYMS Edit
- Afferent = Sensory, impulses travel toward the CNS, e.g., carrying touch, temperature, position, or pain signals.
- Dorsal=Posterior , e.g., the dorsal root is sometimes referred to as the posterior root.
- Dorsal Root Ganglion = Spinal Ganglion = Sensory Ganglion
- Efferent = Motor, impulses travel toward the periphery, e.g., to a muscle or gland.
- Nerve fiber = axon of a single neuron
- Sympathetic Chain = Sympathetic Trunk, contains preganglionic and postganglionic axons traveling to higher or lower sympathetic ganglia.
- Ventral=Anterior, e.g., the ventral root is sometimes referred to as the anterior root.
- Visceral Motor System = Autonomic Nervous System