Fundamentals of the Nervous System and Nervous Tissue PDF

**Fundamentals of the Nervous System and Nervous Tissue** - The master **controlling and communicating** system of the body - Functions: - **Sensory input** - monitoring stimuli occurring inside and outside of the body - **Integration** - interpretation of sensory input...

**Fundamentals of the Nervous System and Nervous Tissue** - The master **controlling and communicating** system of the body - Functions: - **Sensory input** - monitoring stimuli occurring inside and outside of the body - **Integration** - interpretation of sensory input - **Motor output** - respond to stimuli by activating effector organs A diagram of a motor output Description automatically generated **[Anatomy of the Neuron]** - The two **principal cell types** of the nervous system are **neurons** and **supporting cells**: - **Neurons** - excitable cells that transmit electrical signals, composed of a *body, axon and dendrites* - Characteristics of a neuron - They have *extreme longevity*. Neurons can live and function for a lifetime - They *do not divide*. As the fetal neurons assume their roles as communication links in the nervous system, they lose their ability to undergo mitosis. They cannot replace themselves if destroyed. While most neurons are amitotic, some areas of the brain, like the olfactory bulb, can produce new neurons through a process called neurogenesis. - *amitotic* - an unusual form of cell division in which the nucleus and cytoplasm divide by constriction without the formation of chromosomes. They lack the necessary organelles for cell division, like centrioles. - They have an exceptionally *high metabolic rate*, requiring continuous and abundant supplies of oxygen and glucose. Neurons can't survive for more than a few minutes without oxygen - Their **plasma membrane** functions in: - Electrical signaling - Cell-to-cell signaling during development [**Structures of the Neuron**:]  - **Nerve Cell Body (Soma or Perikaryon)** - Contains the nucleus and a nucleolus - It's the ***major biosynthetic center*** - It's the focal point for the outgrowth of neuronal processes - Contains an axon hillock - cone-shaped area from which axons arise - Contains a distinctive organelle - **Chromatophilic (Nissl) bodies (Rough ER)** - These bodies, whose names means "color loving" or "easily stained", are large clusters of rough ER and free ribosomes that stain darkly with basic dyes. These cellular organelles continually renew the membranes of the cell and the protein part of the cytosol. - **Neurofibrils** are bundles of intermediate filaments (neurofilaments) that run in a network between the chromatophilic bodies. Like all other intermediate filaments, neurofilaments keep the cell from being pulled apart when it is subject to tensile forces - The cell body is the focal point for the outgrowth of the neuron processes during embryonic development. In most neurons, the plasma membrane of the cell body acts as a receptive surface that receives signals from other neurons - **Processes** - These are arm-like extensions extending from the soma - Called **tracts** **in CNS** and **nerves in PNS** - There are two types: **axons** and **dendrites** - **Dendrites** - Short, tapering and diffusely branching processes - They are the receptive, or **input**, regions of the neuron, providing an enlarged surface area for receiving signals from other neurons - Most neurons have numerous dendrites - By definition, (*dendro* = tree) dendrites conduct electrical signals *towards* the soma - **Axons** - Slender processes of uniform diameter arising from the axon hillock ("little hill") - Long axons are called **nerve fibers** - Usually there is only one unbranching axon per neuron - By definition, ("axis, axle"), axons are impulse generators and conductors that transmit nerve impulses *away* from the soma - Functions: - Generate and transmit **action potentials**, or **nerve impulses**; signals that are transmitted along the plasma membrane - **Secrete neurotransmitters** from the axonal terminals - Axons and the axon hillock lack ribosomes and all organelles involved in protein synthesis, so they must receive their proteins from the cell body. - Neurofilaments, actin filaments and microtubules are especially present in axons, where they provide structural strength. - These cytoskeletal elements also aid in the transport of substances to and from the soma since the axonal cytoplasm is constantly being renewed and recycled. This movement of substances is called **axonal transport**. - Some neurons have short axons, but in others it can be extremely long. For example, the axons of the motor neurons that control muscles in the foot extend from the lumbar region of the spine to the sole of the foot, a distance of 3-4 feet. Any long axon is called a **nerve fiber**. - Although axons branch far less frequently than dendrites, they do occasionally occur along their length. These branches, called **axon collaterals**, extend from axons at right angles, more or less. - Whether an axon remains undivided or has collaterals, it usually branches profusely at its terminus (end): Ten thousand of these **terminal branches** per neuron is not unusual. They end in knobs called **axon terminals**. - **Axonal Terminals** - - A nerve impulse is typically generated at the axon's initial segment and is conducted along the axon to the axon terminals, where it causes the release of chemicals called *neurotransmitters* into the extracellular space. The neurotransmitters excite or inhibit the neurons or target organ with which the axon is in close contact. A diagram of a cell cycle Description automatically generated - **Myelin Sheath** - Formed by **Oligodendrocytes** in the CNS - Formed by **Schwann cells** in the PNS - Whitish, fatty (lipoprotein), segmented sheath surrounding **thick** axons - In contrast, thin, slowly conducting axons lack a myelin sheath and are called **unmyelinated axons.** (see photos) - 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 unmyelinated axons, each of which occupies a separate tubular recess in the surface of the Schwann cell. Unmyelinated axons are found in portions of the autonomic nervous system as well as in some sensory fibers. - Both myelinated and unmyelinated axons are present in the CNS and PNS - Each segment of myelin consists of the plasma membrane of a supporting cell rolled in concentric layers around the axon - The outermost layer of the myelin sheath is called the **neurilemma**, which is the remaining nucleus and cytoplasm of the Schwann cell (see photos) - Because the adjacent Schwann cells along a myelinated axon do not touch one another, there are gaps in the myelin sheath, called **nodes of Ranvier**, which are spaced at regular intervals of about 1 mm apart. - In the myelin sheaths of the CNS, which are formed by **oligodendrocytes**, the **nodes of Ranvier** are spaced more widely. - Functions: - Protect the axon - Increases the speed of nerve impulse transmission, making impulse propagation more energy efficient - Insulation layer that prevents leakage of electrical current from the axon A diagram of a nervous system Description automatically generated A diagram of a nerve cell Description automatically generated - **Supporting cells (Neuroglia** or **Glial cells)** - cells that surround and wrap neurons. - Provide a supportive scaffolding for neurons - Segregate and insulate neurons - Promote health and growth of nervous tissue - Neuroglia of the CNS include: - **Astrocytes** - star-shaped cells, highly branched glial cells with bulbous ends (see photos) - Most abundant, versatile - Some cling to neurons and their synaptic endings (axonal terminals), and others cling to capillaries - Function: - Support and brace neurons - Anchor neurons to their nutrient supplies - Guide migration of young neurons - Involved with synapse formation in developing neural tissue, producing molecules necessary for neural growth and propagating calcium signals that may be involved with memory - Control the chemical environment - Take up and release ions, controlling the ionic environment around neurons. This is important because the concentrations of various ions outside the axons must be kept within narrow limits in order for nerve impulses to be generated and conducted - **Microglia** - small, elongated cells with spiny processes, like a thorny bush. - Phagocytes (type of WBC that engulf and destroy pathogens) that monitor the health of neurons - The smallest, least abundant neuroglia of the CNS - Unlike other neuroglial cells, they don't originate in nervous tissue. Instead, like other macrophages (a type of phagocyte that are specialized in tissue cleanup), they are derived from blood cells called monocytes. Monocytes that become microglia migrate to the CNS during the embryonic and fetal periods. - **Ependymal cells** - range in shape, from squamous to columnar - The CNS originates in the embryo as a hollow neural tube and retains a central cavity throughout life. **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. - Ependymal cells are ciliated, helping circulate the cerebrospinal fluid - **Oligodendrocytes** - branched cells that wrap CNS nerve fibers - These cells have fewer branches than astrocytes - They line up in small groups and wrap their cell processes around thicker axons in the CNS, producing insulating coverings called myelin sheaths - Neuroglia of the PNS include: - **Schwann cells** - surround nerve fibers of the PNS - Form the myelin sheath of nerve fibers in the PNS - Develop during the fetal period and the first year or so of postnatal life - To form the sheath, the Schwann cells first indent to receive the axon and then wrap themselves around the axon repeatedly like a jelly-roll. - **Satellite cells** - surround neuron cell bodies of ganglia (a cluster of neurons in the PNS) (see photo) - Resemblance to the moons, or satellites, around a planet  **[Gross Anatomy of the Nervous System:]** The brain and spinal cord have distinct regions of gray and white matter that reflect the arrangement of their neurons: - **Gray Matter** - mostly *soma and unmyelinated fibers* - Gray-colored zone that surrounds the hollow central cavity of the CNS - In the spinal cord, it's an H-shaped region in which the dorsal half contains cell bodies of interneurons and the ventral half contains cell bodies of motor neurons - Thus, gray matter is the site where neuron cell bodies are *clustered*. - More specifically, the gray matter of the CNS is a mixture of neuron cell bodies; dendrites; short, unmyelinated axons; and neuroglia. - **White Matter** - dense collections of *myelinated fibers* - External to the gray matter is the white matter, which contains no soma but millions of axons - The 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. - White matter consists of axons running between different parts of the CNS. - Within the white matter, axons traveling to similar destinations form axon bundles called **tracts.** - In two regions of the brain (cerebrum and cerebellum), there's an additional layer of gray matter located superficially called the **cortex**. A diagram of a brain Description automatically generated **[Neuron Classification:]** - **Structural** - grouped according to the number of processes that extend from the cell body - **Multipolar** - three or more processes - Usually have numerous dendrites and a single axon. However, some small multipolar neurons have no axon and rely strictly on their dendrites for conducting signals. - Well over 99% of neurons in the body belong to the multipolar class, including motor neurons and most interneurons. - **Bipolar** - two processes (axon and dendrite) - 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** - single, short process - A short, single process that emerges from the divides like an inverted T into two long branches. - Most unipolar 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*. - Unipolar neurons make up the typical sensory neurons. - **Functional** - grouped according to the direction the nerve impulse travels relative to the CNS - **Sensory (afferent)** - transmit impulses *toward* the CNS - Makes up the sensory division of the PNS - Virtually all sensory neurons are pseudounipolar - the short, single process near the neuron cell body divides into two longer branches. - One of these branches runs centrally into the CNS and is called the **central process** - Whereas the other branch extends peripherally to the receptors and is called the **peripheral process**. - They function as one, carrying impulses directly from the peripheral receptors to the CNS - Their cell bodies are in ganglia outside of the CNS - **Motor (efferent)** - carry impulses *away* from the CNS to effector organs (muscles and glands) - Make up the motor division of the PNS - Motor neurons are multipolar and their cell bodies are located in the CNS (except for some neurons of the autonomic nervous system) - Motor neurons form junctions with effector cells, stimulating muscles to contract or glands to secrete - **Interneurons (association neurons)** - shuttle signals through CNS pathways - Lie between motor and sensory neurons and are confined entirely to the CNS - Interneurons link together in chains that form complex neuronal pathways. - The fact that interneurons make up 99.98% of the neurons of the body reflects the vast amount of information process in the human CNS. - Almost all interneurons are multipolar, but they show great diversity in size and in the branching patterns of their processes. A diagram of the human body Description automatically generated  A diagram of a nervous system Description automatically generated **[Structure of a Nerve:]** A **nerve** is a cablelike organ in the PNS. Each nerve consists of many axons (nerve fibers) arranged in parallel bundles and enclosed by successive wrappings of connective tissue. - Almost all nerves contain both myelinated and unmyelinated sensory and motor fibers - Within a nerve, each axon is surrounded by Schwann cells. Covering the Schwann cells is a delicate layer of loose connective tissue called **endoneurium**. - Groups of axons are bound into bundles called **nerve fascicles** by a wrapping of connective tissue called the **perineurium**. - Finally, the whole nerve is surrounded by a tough fibrous sheath called the **epineurium**. - These 3 layers of connective tissue in nerves correspond exactly to those in skeletal muscles: endomysium, perimysium and epimysium. - The connective tissue in nerves also contains the blood vessels that nourish the axons and schwann cells. - The terms *neuron, nerve fiber and nerve* are easy to confuse. Keep in mind: - A **neuron** is a nerve cell. - A **nerve fiber** is a long axon. - A **nerve** is a collection of nerve fibers in the PNS.