Exam 4 Review - University of Texas at Arlington - Human Anatomy

Integration

The process through the central nervous system processes and interprets sensory input and makes decisions about what should be done at each moment.

Motor Output

The dictation of a response by the central nervous system by activating the effector organs, like our muscles or glands

The Central Nervous System (CNS)

Consists of the brain and the spinal cord, which occupy the cranium and the vertebral canal, respectively. Known as the integrating and command center of the nervous system: It receives incoming sensory signals, interprets these signals, and dictates motor responses based on past experiences, reflexes, and current conditions.

The Peripheral Nervous System (PNS)

Consists mainly of the nerves that extend from the brain and spinal cord. Contains both cranial nerves and spinal nerves which serve different functions. These system nerves serve as communication lines that link all regions of the body to the central nervous system. Ganglia are also included in this system.

Cranial Nerves

Part of the PNS, they carry signals to and from the brain.

Spinal Nerves

Part of the PNS, carry signals to and from the spinal cord. 

Ganglia

Areas where the cell bodies of neurons are clustered.

Sensory (Afferent) Division

Part of the nervous system where signals are picked up by sensory receptors located throughout the body and carried by nerve fibers of the PNS into the CNS (afferent means “carrying toward”).

Motor (Efferent) Division

Part of the nervous system where signals are carried away from the CNS by nerve fibers of the PNS to innervate the muscles and glands, causing these organs either to contract or to secrete (efferent means “carrying away”).

The Somatic Body Region

Consists of the structures external to the ventral body cavity—in other words, the structures of the outer tube (skin, skeletal musculature, bones).

The Visceral Body Region

Mostly contains the viscera within the ventral body cavity—which means the structures of the body’s inner tube (digestive tube, lungs, heart, bladder, and so on).

Somatic Sensory System

The sensory innervation of the outer tube: skin, body wall, and limbs.

Visceral Sensory System

The sensory innervation of the viscera

Somatic (Voluntary) Motor System

The motor innervation of the outer tube, specifically skeletal muscles

Autonomic Nervous System (ANS)

Also known as the visceral motor system or involuntary motor system. The involuntary motor innervation of the inner tube, specifically smooth muscle, cardiac muscle, and glands, as well as some outer tube structures: arrector pili muscles, smooth muscle in the vessels, and sweat glands.

General Somatic Senses

The senses whose receptors are spread widely throughout the outer tube of the body. These include the many senses experienced on the skin and in the body wall, such as touch, pain, pressure, vibration, and temperature.

Proprioception

A sense that detects the amount of stretch in muscles, tendons, and joint capsules. Informs you of the position and movement of your body in space, giving you a “body sense.”

Special Somatic Senses

Senses whose receptors are confined to relatively small areas rather than spread widely throughout the body. Most of these senses are confined to the head, including hearing and balance, or equilibrium (with receptors in the inner ear), and vision (with receptors in the eye).

General Visceral Senses

Include stretch, pain, and temperature, which can be felt widely in the digestive and urinary tracts, reproductive organs, and other viscera. Sensations such as hunger and nausea are also included.

Special Visceral Senses

These senses, referred to also as the chemical senses, have their sensory receptors localized to the tongue (taste) and nasal cavity (smell), respectively.

Nervous Tissue

Tissue which makes up the majority of the nervous system whose cells are densely packed and tightly intertwined. Made up of just two main types of cells: neurons and neuroglia.

Neurons

The excitable nerve cells that transmit electrical signals from one part of the to another, developed from the neural tube and crest. These signals are transmitted along the plasma membrane, or neurilemma, in the form of nerve impulses, or action potentials. They have extreme longevity. They can live and function for a lifetime, over 100 years. In the fetal stage after they lose their ability to undergo mitosis. They cannot replaces themselves if destroyed. They also have a very high metabolic rate meaning they require a continuous and abundant supply of oxygen and glucose. They will begin to die after just a few minutes with oxygen.

Neuroglia

Nonexcitable supporting cells that surround and wrap the neurons, developed from the neural tube and crest. They provide a supportive scaffolding for neurons and cover all nonsynaptic parts of the neurons, thus insulating the neurons and keeping activity from other neurons from interfering with each other. Smaller than neurons with darker-staining nuclei, outnumber neurons 10 to 1 and make up half the mass of the brain. They can divide. 

Cell Body of A Neuron

Sometimes also called a soma. Range in size from 5-140 micrometers. All consist of a single nucleus surrounded by cytoplasm. The plasma membrane of this part of the neuron acts as a receptive surface that receives signals from other neurons. Serves as the focal point for the outgrowth of the neuron processes during embryonic development.

Chromatophilic Substance

Large clusters of rough endoplasmic reticulum and free ribosomes that stain darkly with basic dyes. These cellular organelles continually renew the membranes of the cell and the protein components of the cytosol.

Neurofibrils

Bundles of intermediate filaments (neurofilaments) that run in a network between the chromatophilic substance. Like all other intermediate filaments, these filaments keep the cell from being pulled apart when it is subjected to tensile forces.

Neuron Processes 

Armlike protrusions that extend from the cell body. There are two types: axons and dendrites. 

Dendrites

Processes that branch from the cell body like the limbs on a tree. Virtually all organelles that occur in the cell body also occur in these processes, and chromatophilic substance extends into the basal part of each process. These processes function as receptive sites, providing an enlarged surface area for receiving signals from other neurons. By definition, they conduct electrical signals toward the cell body.

Axon

Thin processes of uniform diameter throughout their length. By definition, they are impulse generators and conductors that transmit nerve impulses away from their cell body. Chromatophilic substance and the Golgi apparatus are absent from it. They lack ribosomes and all organelles involved in protein synthesis, so they must receive their protein components from the cell body. Neurofilaments, actin microfilaments, and microtubules are especially evident in them, where they provide structural strength. Those with larger diameters conduct impulses faster than those with smaller diameters because of a basic law of physics: The resistance to the passage of an electrical current decreases as the diameter of any “cable” increases.

Axon Hillock

A cone-shaped region of the cell body from which the axon arises. Chromatophilic substance and the Golgi apparatus are absent from it.

Axonal Transport

The transport of substances to and from the cell body and axon, which is aided by the cytoskeletal elements in the axon as the axonal cytoplasm is continuously recycled and renewed

Nerve Fiber

Any long axon of a neuron.

Axon Collaterals

Branches along an axon’s length that extend from it at more or less right angles

Terminal Arborization

The end of the axon where it begins to branch profusely . For there to be ten thousand of these branches per neuron is not unusual.

Terminal Boutons

Also known as axon terminals. Knobs at the ends of the branches in the terminal arborization. Where neurotransmitters are released to the post-synaptic neuron. Mitochondria are abundant here because the secretion of neurotransmitters requires a great deal of energy.

Synapse

Specialized cell junction between two neurons, at which the neurons communicate. Most transmit info through chemical messages. Some in the CNS transmit signals electrically. Determine the direction of informational flow through the nervous system.

Presynaptic Neuron

The neuron that conducts signals toward the synapse

Postsynaptic Neuron

The neuron that transmits signals away from the synapse

Axodendritic Synapses

Synapses that occur between the terminal boutons of one neuron and the dendrites of another neuron

Axosomatic Synapses

Synapses that occur between axons and neuron cell bodies

Synaptic Vesicles

Membrane-bound sacs filled with neurotransmitters, the molecules that transmit signals across the synapse. Located on the presynaptic side in the terminal bouton

Multipolar Neurons

Have more than two processes. Usually, these neurons have numerous dendrites and a single axon. However, some small ones have no axon and rely strictly on their dendrites for conducting signals. Well over 99% of neurons in the body belong to this class.

Bipolar Neurons

Have two processes that extend from opposite sides of the cell body. These very rare neurons occur in some of the special sensory organs (inner ear, olfactory epithelium of the nose, retina of the eye), where they mostly serve as sensory neurons.

Unipolar Neurons

Have a short, single process that emerges from the cell body and divides like an inverted T into two long branches. Most of these neurons start out as bipolar neurons whose two processes fuse together near the cell body during development. Therefore, they are more properly called pseudounipolar neurons. These neurons are found in sensory ganglia in the PNS, where they function as sensory neurons. The short, single process near the neuron cell body divides into two longer branches. Do not formally contain dendrites since the peripheral process, while serving a similar function to a dendrite, is more similar to an axon in fine structure.

Central Process

The branch of a unipolar neuron that runs centrally into the CNS

Peripheral Process

The branch of a unipolar neuron that extends peripherally to the receptors

Sensory Neurons

Also known as afferent neurons. Make up the sensory division of the PNS, transferring impulses toward the CNS from sensory receptors in the PNS. Most are psuedounipolar, and their cell bodies are in ganglia outside the CNS. Some are bipolar in structure, these are restricted to some of the special sense organs.

Motor Neurons

Also known as efferent neurons. Make up the motor division of the PNS. These neurons carry impulses away from the CNS to effector organs such as muscles and glands. These neurons are multipolar and their cell bodies are located in the CNS. They form junctions with effector cells, stimulating muscles to contract or glands to secrete.

Interneurons

Lie between motor and sensory neurons. They link together into chains that form complex neuronal pathways. The fact that these neurons make up 99.98% of the neurons of the body reflects the vast amount of information processed in the human CNS. These multipolar neurons show great diversity in size and in the branching patterns of their processes

Astrocytes

Star-shaped glial cells that are the most abundant glial cells in the CNS. They have many radiating processes with bulbous ends. Some of these bulbs cling to neurons (including the axon terminals), whereas others cling to capillaries. Known functions include (1) regulating neurotransmitter levels by increasing the rate of neurotransmitter uptake in regions of high neuronal activity; (2) signaling increased blood flow through capillaries in active regions of the brain; and (3) controlling the ionic environment around neurons. They also help synapses form in developing neural tissue, produce molecules necessary for neural growth (brain-derived trophic factor, BDTF), and propagate calcium signals that may be involved with memory. No longer are these cells considered passive supportive cells for neurons; rather, they appear to have an active role in neural activity.

Microglial Cells

The smallest and least abundant neuroglia of the CNS. They have elongated cell bodies and cell processes with many pointed projections, like a thorny bush. They are phagocytes, the macrophages of the CNS. They migrate to, and then engulf, invading microorganisms and injured or dead neurons. Unlike other neuroglial cells, these cells do not originate in nervous tissue; like the other macrophages of the body, they are derived from blood cells called monocytes. The monocytes that become these cells migrate to the CNS during the embryonic and fetal periods.

Ependymal Cells

Form a simple epithelium that lines the central cavity of the spinal cord and brain. These cells provide a fairly permeable layer between the cerebrospinal fluid that fills this cavity and the tissue fluid that bathes the cells of the CNS. These cells bear cilia that help circulate the cerebrospinal fluid.

Oligodendrocytes

Have fewer branches than astrocytes, as their name implies. They line up in small groups and wrap their cell processes around the thicker axons in the CNS, producing insulating coverings called myelin sheaths.

Gliomas

Tumors formed by uncontrolled proliferation of glial cells. Two percent of all cancers are these, and their incidence is increasing. These are difficult cancers to treat, and the one-year survival rate is under 50%.

Satellite Cells

Glial cells in the PNS that surround neuron cell bodies within ganglia

Schwann Cells 

Glial cells that surround all axons in the PNS and form myelin sheaths around many of these axons.

Myelin Sheath

Develop during the fetal period and the first year or so of postnatal life. Produced by oligodendrocytes in the CNS and Schwann cells in the PNS. These sheaths are segmented structures that are composed of the lipoprotein myelin and surround the thicker axons of the body. Each segment consists of the plasma membrane of a glial cell rolled in concentric layers around the axon. Form an insulating layer that prevents the leakage of electrical current from the axon, increases the speed of impulse conduction along the axon, and makes impulse propagation more energy-efficient. Only thick, rapidly conducting axons are sheathed with myelin.

Formation of Myelin Sheath

Schwann cells first indent to receive the axon and then wrap themselves around the axon repeatedly. Initially the wrapping is loose, but the cytoplasm of the Schwann cell is gradually squeezed outward from between the membrane layers. When the wrapping process is finished, many concentric layers of Schwann cell plasma membrane ensheathe the axon in a tightly packed coil of membranes that is the true myelin sheath. The nucleus and most of the cytoplasm of the Schwann cell end up just external to the myeli​n layers.

Nodes of Ranvier

Also known as myelin sheath gaps. Occur at regular intervals about 1 mm apart. In myelinated axons, nerve impulses do not travel along the myelin-covered regions of the axonal membrane but instead jump from the membrane of one myelin sheath gap to the next in a way that greatly speeds impulse conduction.

Nonmyelinated Axons

Schwann cells surround such axons but do not wrap around them in concentric layers of membrane. A single Schwann cell can partly enclose 15 or more nonmyelinated axons, each of which occupies a separate tubular recess in the surface of the Schwann cell. They are found in portions of the autonomic nervous system and in some sensory fibers. Those in the CNS are covered by the many long processes of glial cells, such as astrocytes, that are so abundant in the CNS.

Tic Douloureux

Also known as trigeminal neuralgia. Affects the main sensory nerve of the face, the trigeminal nerve, which is compressed by an adjacent vessel and causes degeneration and loss of the myelin sheath that surrounds the sensory nerve fibers. Due to this loss of myelin, pain fibers are now also stimulated by touch (a process known as cross-talk).

Gray Matter

A gray-colored zone that surrounds the hollow central cavity of the CNS. In the spinal cord it is a butterfly-shaped region in which the dorsal half contains cell bodies of interneurons and the ventral half contains cell bodies of motor neurons. It is the site where neuron cell bodies are clustered. More specifically, this part of the CNS is a mixture of neuron cell bodies; dendrites; short, nonmyelinated neurons; and neuroglia. Synapses occur here.

White Matter

Contains no neuron cell bodies but millions of axons and neuroglia. Its white color comes from the myelin sheaths around many of the axons. Most of these axons either ascend from the spinal cord to the brain or descend from the brain to the spinal cord, allowing these two regions of the CNS to communicate with each other. Thus, white matter consists of axons running between different parts of the CNS.

Nerve

A cablelike organ in the peripheral nervous system. Each one consists of many axons (nerve fibers) arranged in parallel bundles and enclosed by successive wrappings of connective tissue. Almost all of them contain both myelinated and nonmyelinated sensory and motor axons.

Endoneurium

A delicate layer of loose connective tissue covering the myelinated axons in a nerve.

Nerve Fasicles

Groups of axons in a nerve covered by the perineurium

Perineurium

Connective tissue which surrounds each nerve fasicle 

Epineurium

A tough fibrous sheet which encloses the entire nerve

Reflex Arcs

Simple chains of neurons that cause our simplest, reflexive behaviors and reflect the basic structural plan of the nervous system. They account for reflexes. Has five essential components:

1. The Receptor

2. The Sensory Neuron

3. The Integration Center

4. The Motor Neuron

5. The Effector

Reflexes

Defined as rapid, automatic motor responses to stimuli

Somatic Reflexes

Reflexes resulting in the contraction of skeletal muscles

Visceral Reflexes

Reflexes that activate smooth muscle, cardiac muscle, or glands

Reflex Arc: Receptor

The site where the stimulus acts. They are located at the terminal end of the peripheral process of a sensory neuron.

Reflex Arc: Sensory Neuron

Transmits the afferent impulses to the CNS

Reflex Arc: Integration Center

Consists of one or more synapses in the gray matter of the CNS. In the simplest reflex arcs, its is a single synapse between a sensory neuron and a motor neuron. In more complex reflexes, it can involve multiple synapses that send signals through long chains of interneurons to other portions of the CNS, for instance, to portions of the brain.

Reflex Arc: Motor Neuron

Conducts efferent impulses from the integration center to an effector.

Reflex Arc: Effector

The muscle or gland cell that responds to the efferent impulses by contracting or secreting.

Monosynaptic Reflex

There is no interneuron between the sensory neuron and the motor neuron; thus, as its name implies, there is only one synapse in this reflex arc.

Knee-Jerk Reflex

The impact of a hammer on the patellar ligament stretches the quadriceps muscles of the thigh. This stretching initiates an impulse in a sensory neuron that directly activates a motor neuron in the spinal cord, which then signals the quadriceps muscle to contract. This contraction counteracts the original stretching caused by the hammer. It’s a monosynaptic reflex.

Stretch Reflexes

Monosynaptic reflexes that help maintain equilibrium and upright posture. In these reflexes, the sensory neurons sense the stretching of muscles that occurs when the body starts to sway, and then the motor neurons activate muscles that adjust the body’s position to prevent a fall. Because they contain just one synapse, these reflexes are the fastest of all reflexes—and speed is essential to keep one from falling to the ground.

Polysynaptic Reflex

One or more interneurons are part of the reflex pathway between the sensory and motor neurons. The presence of even one interneuron means that there have to be at least two synapses in this type of reflex arc, thus the name polysynaptic. Most of the simple reflex arcs in the body contain a single interneuron and therefore have a total of three neurons.

Withdrawal Reflexes

The reflexes by which we pull away from danger, are three-neuron polysynaptic reflexes. Pricking a finger with a tack initiates an impulse in the sensory neuron, which activates the interneuron in the CNS. The interneuron then signals the motor neuron to contract the muscle that withdraws the hand.

Diverging Circuit

A circuit in which one neuron is synapsed with several other postsynaptic neurons, which then either communicate directly with a motor neuron or cascade to send information to different areas of the CNS. One neuron creates a domino effect essentially.

Converging Circuit

When many neurons synapse on a single postsynaptic neuron. Results in convergence, which is when a single motor neuron receives both excitatory and inhibitory impulses from many other neurons. These impulses are integrated by the target motor neuron and influence whether it will initiate a nerve impulse.

Reverberating Circuit

One neuron in the circuit receives feedback from another neuron in the same circuit. A branch off the axon of one neuron circles back and synapses with a previous neuron in the circuit. In this pathway, the signal continues to be sent until either synaptic fatigue or inhibition by another signal interrupts the circuit. These circuits are involved in the control of many rhythmic activities, such as breathing and swinging the arms when walking.

Serial Processing

Occurs in neurons that synapse one-on-one in a series. Neurons linked this way pass their signal along a single pathway from one neuron to the next, like links in a chain. 

Parallel Processing

Information is sometimes sent from a single neuron along two or more parallel pathways, linking them “in parallel”. Occurs when a single sensory stimulus results in multiple perceptions. For example, as you watch a friend walking toward you, multiple pathways process visual stimuli from your retina in parallel, evaluating the color, shape, spatial location, and movement of your friend. The stimuli are also processed by the parts of your brain associated with recognition and memory from past experiences, enabling you to recognize your friend, recall his name, and remember when you last saw him. Occurs almost instantaneously, allows the brain to evaluate stimuli with incredible speed, and enables information to be integrated along numerous pathways.

Multiple Sclerosis

A progressive disease that destroys patches of myelin in the brain and spinal cord, disrupting neuronal signals in the CNS and leading to sensory disorders and weakened musculature. Characterized by periods of relapse (disability) and remission (recovery). Common symptoms include numbness or pain on the skin of some part of the body, disturbances of vision, muscle weakness or paralysis, difficulty in maintaining balance, slurred speech, bladder incontinence, fatigue, and depression. Known to be an autoimmune disease in which the immune system attacks the myelin around axons in the CNS, thereby interfering with the conduction of signals along the axons. The immune system cells called lymphocytes break down the myelin, and then macrophages consume the remains through phagocytosis. The damage is also accompanied by inflammation, which can destroy the axons themselves.

Neuronal Regeneration

If the neuronal body survives an injury, the axon may regenerate. In the PNS, if a nerve is severed, usually the immune system will attack the axon. Axon filaments can grow peripherally from the injured site at an approximate rate of 1.5 mm a day within regeneration tubes formed by the surviving Schwann cells that surrounded the original axons. Eventual reinnervation of the target organ, with partial recovery of function, is sometimes possible in the PNS. In the CNS, the neuroglia never form bands to guide the regrowing axons and even hinder such axons by secreting growth-inhibiting chemicals. Therefore, there is no effective regeneration after injury to the spinal cord or brain.

Medulla Oblongata

Contains the pyramids, the olive, inferior cerebral peduncles, and the reticular formation

Pyramidal Tracts

Large fiber tracts that originate from pyramid-shaped neurons in the cerebrum and descend through the brain stem and spinal cord carrying voluntary motor output to the spinal cord.

Decussation of The Pyramids

The point through which 70-90% of pyramidal fibers cross to the opposite side of the brain in the caudal medulla oblongata.

Inferior Olivary Nucleus

A large wavy fold of gray matter viewable in cross section. This brain nucleus is a relay station for sensory information traveling to the cerebellum, especially for proprioceptive information ascending from the spinal cord.

Reticular Formation

A loose cluster of brain nuclei running through the brain stem. Includes the raphe nuclei, medial nuclear group, and lateral nuclear group.

Pontine Nuclei

Relay brain nuclei in a path that connects a portion of the cerebral cortex with the cerebellum located in the pyramidal tracts in the Pons. Involved with the coordination of voluntary movements.

Substantia Nigra

Bandlike in appearance, neuronal cell bodies contain dark melanin pigment, is deep to the pyramidal tracts in the cerebral peduncle. This brain nucleus is functionally linked to the deep gray matter of the cerebrum, the basal nuclei (ganglia), and is involved in controlling voluntary movement. Degeneration of the neurons in the this is the cause of Parkinson’s disease.

Red Nucleus

Lies deep to the substantia nigra. Its reddish hue is due to a rich blood supply and to the presence of iron pigment in the cell bodies of its neurons. It has a minor motor function: helping to bring about flexion movements of the limbs. Closely associated with the cerebellum.

Periaqueductal Gray Matter

First, it is involved in the “fight-or-flight” (sympathetic) reaction, constituting the midbrain link between the part of the cerebrum that perceives fear and the autonomic pathway that produces the physiological reactions associated with fear: increase in heart rate, skyrocketing blood pressure, wild fleeing or defensive freezing, and the suppression of pain when the person is injured. Second, it seems to mediate the response to visceral pain (for instance, nausea), during which it decreases heart rate and blood pressure, produces a cold sweat, and discourages movement.

Superior Colliculi

Part of the midbrain, act in visual reflexes, for example, when the eyes track and follow moving objects even if the person is not consciously looking at the objects

Inferior Colliculi

Part of the midbrain, belong to the auditory system. Among other functions, they act in reflexive responses to sounds. For instance, when you are startled by a loud noise, your head and eyes reflexively turn toward the sound.