Chapter 11 Fundamentals of Nervous Tissue and Cells Flashcards
Nervous System:
- Master controlling and communicating system of the body.
- Communicate via rapid and specific chemical signals.
3 Overlapping Functions of the Nervous System:
o Sensory Input: Monitor changes both inside and outside of the body.
o Integration: Processes and interprets sensory input and decides what should be done based on the stimulus.
o Motor Output: Activation of effector organs to cause a response.
Central Nervous System:
Consists of the brain and spinal cord of the dorsal body cavity.
Peripheral Nervous System:
Definition and 2 Types
Part of the nervous system outside of the CNS. Consists of nerves that extend from the brain and spinal cord.
o Spinal Nerves: Carry impulses to and from the spinal cord.
o Cranial Nerves: Carry impulses to and from the brain.
Sensory (Afferent) Fibers:
Definition and 2 Types
Convey impulses towards the central nervous system from sensory receptors located throughout the body
o Somatic Sensory Fibers: Convey impulses from skin, skeletal muscles, and joints.
o Visceral Sensory Fibers: Transmit impulses from visceral organs.
Motor (Efferent) Fibers:
Definition and 2 Types
Convey impulses away from the Central nervous system to the effector organs. Impulses activate muscles to contract and glands to secrete.
o Somatic Nervous System: Voluntary nervous system. Conscious control of our skeletal muscles. Composed of somatic motor nerve fibers that conduct impulses from the CNS to skeletal muscles.
o Autonomic Nervous System: Involuntary nervous system. We can’t consciously control the parts of this system. Consists of visceral motor nerve fibers that regulate the activity of smooth muscles, cardiac muscles, and glands.
The 2 Functional Subdivisions of the Autonomic Nervous System:
o Sympathetic Division: Works in opposition to Parasympathetic division.
o Parasympathetic Division Works in opposition to Sympathetic division.
What is the Nervous System Composed Mostly of?
Highly cellular nervous tissue.
The 2 Principal Types of Cells that Make Up Nervous Tissue:
o Neuroglia: Small supporting cells that surround and wrap the more delicate neurons.
o Neurons: Nerve cells that are excitable (Responsive to stimuli) and transmit electrical signals.
Neuroglia:
There are six types; Four in the CNS and two in the PNS.
The 4 Neuroglia of the CNS:
1) Astrocytes
SUPPORT!
- Most abundant and versatile glial cells.
- Radiating processes cling to neurons and their synaptic endings and cover nearby capillaries.
- Make exchanges between capillaries and neurons (controlling Capillary permeability).
- Guide migration of young neurons and formation of synapses between neurons.
- Control chemical environment around neurons, “mopping up” leaked potassium ions and recycling Neurotransmitters.
- They take in calcium, creating slow paced calcium pulses, and release extracellular chemical messengers for communication.
The 4 Neuroglia of the CNS:
2) Microglial Cells
INSULATION/ MYLEINATION!
- Small and ovoid with relatively long “thorny” processes.
- The processes touch nearby neurons, monitoring their health, and sense whether they’re injured or in danger.
- When invading microorganisms or dead cells are present, the cells transform into a special type of macrophage that phagocytizes the microorganisms or neuronal debris.
- Very important because immune system cells have limited access to the CNS.
The 4 Neuroglia of the CNS:
3) Ependymal Cells
CUSHIONING/PROTECTION!
- Squamous to columnar cells, many are ciliated.
- Line central cavities of the brain and spinal cord, where they form a permeable barrier between CSF that fills the cavities and tissue fluid bathing the cells of the CNS.
- Beating of their cilia helps circulate CSF and cushion the brain and spinal cord.
The 4 Neuroglia of the CNS:
4) Oligodendrocytes
PROTECTION!
- Branch out but have fewer processes than astrocytes.
- Line up along the thicker nerve fibers in the CNS and wrap their processes tightly around fibers, producing an insulating covering called a Myelin Sheath.
The 2 Neuroglia of the PNS:
1) Satellite Cells
Surround neuron cell bodies located in PNS and are thought to have many of the same functions in the PNS as astrocytes in the CNS.
The 2 Neuroglia of the PNS:
2) Schwann Cells
- Surround nerve fibers in the PNS and form myelin sheaths around the thicker nerve fibers.
- Functionally similar to oligodendrocyes.
- Vital to regeneration of damaged peripheral nerves.
Neurons
- Nerve cells, structural units of the nervous system.
- Typically large, highly specialized cells that conduct messages in the form of nerve impulses from one part of the body to another.
The 4 Characteristics of Neurons:
o Ability to conduct nerve impulses.
o Have extreme longevity (can function for a long time)
o Are amitotic (lose their ability to divide)
o Have exceptionally high metabolic rate and require abundant, continuous oxygen and glucose.
Neuron Cell Body Details:
o Consists of nucleus with a nucleolus surrounded by cytoplasm.
o Also called the Perikaryon or Soma.
o Biosynthetic center of a neuron; contains organelles needed to synthesize proteins and other chemicals.
o It’s protein/membrane-making machinery is most active and best developed in the body.
o Chromatophilic substance: the cell body’s rough ER, it stains darkly with basic dyes.
o Mitochondria, microtubules, and neurofibrals important in maintaining cell shape and integrity. Form a network throughout the body.
o Cell bodies sometimes contain pigment inclusions such as black melanin, red iron-containing pigment, or golden-brown pigment called lipofuscin (harmless by-product of lysosomal activity).
o Focal point for outgrowth of neuron processes during embryonic development.
o Part of the receptive region; receives information from other neurons.
o Most are in the CNS where they’re called nuclei but some in PNS where they’re called ganglia.
Neuron Processes:
- Arm-like processes that extend from the cell body of all neurons.
- In Bundles of neuron processes in the CNS: called Tracts and in the PNS: called nerves.
The 2 Types of Neuron Processes:
- Dendrites
- Axons
Dendrites:
o Short, tapering, diffusely branching extensions.
o Motor neurons have hundreds clustering close to cell body.
o All organelles present in cell body also occur in dendrites.
o Main receptive or input regions; provide large surface area for receiving signals from other neurons.
o In brain, finer dendrites specialized for collecting information.
o Dendritic spines: Thorny appendages with spiky ends that represent points of close contact with other neurons.
o Convey incoming messages toward cell body. Usually short-distance graded potentials rather than APs.
Axon Structure:
o Axon Hillock: The initial region of the axon arises from the cone-shaped area of the cell body, and then narrows to uniform diameter for the rest of it’s length.
o Nerve Fiber: Any long axon
o Axon collaterals: branches along the length of an axon; they extend from the axon at right angles.
o Terminal Branches: branches at the end of the axon, the endings called axon terminals or terminal boutons.
Functional Characteristics of the Axon:
o Axon is conducting region of the neuron; it generates nerve impulses and transmits them.
o Axolemma: Plasma membrane where the nerve impulses transmit along.
o Secretory region of the neuron: axon terminals
o Neurotransmitters: Stimulated when impulse reaches axon terminals. These signal chemicals to be released into the extracellular space where they excite or inhibit neurons with which the axon is in close contact.
o Contains organelles found in dendrites and cell body with exception of rough endoplasmic reticulum and golgi apparatus; structures involved in protein synthesis and packaging.
o Axon depends on 1) Cell body to renew necessary proteins and membrane components and 2) Efficient transport mechanisms to distribute them.
Transport Along the Axon:
o Substances travel continuously along the axon both toward and away from the cell body with help from motor proteins and cytoskeletal elements.
o Anterograde movement: Movement away from the cell body. Substances include mitochondria, cytoskeletal elements, membrane components used to renew the axon plasma membrane, and enzymes needed to synthesize certain NTs.
o Retrograde movement: Movement towards the cell body. Substances include organelles returning to the cell body to be degraded or recycled. This specific movement is an important means of intracellular communication to “advise” the cell body of conditions at axon terminals, and deliver cell body vesicles containing signal molecules.
o There is one basic bidirectional transport mechanism that is responsible for axonal transport. It uses ATP-dependent “motor proteins”, depending on direction of transport. The components are then propelled along the microtubules.
o Certain bacteria and diseases are brought about through retrograde axonal transport.
Myelin Sheath:
- A whitish, fatty protein-lipoid segmented structure that covers axons for protection and insulation.
- Composed of myelinated fibers which conduct nerve impulses more rapidly.
Nonmyelinated Fibers:
Conduct nerve impulses much more slowly.
Myelination in the PNS:
o The Myelin Sheaths in the PNS are formed by schwann cells, which indent the axon and wrap themselves around it. This wrapping is loose but the schwann cell cytoplasm is squeezed from between the membrane layers.
o Wrapping process completion, many concentric layers of schwann cell plasma membrane enclose the axon. The thickness of this myelin sheath depends on the number of spirals.
o Plasma membranes of myelinating cells contain much less protein than plasma membranes of most body cells.
o Channel and carrier proteins are relatively absent. Making myelin sheaths good electrical insulators.
o Nodes or Ranvier: adjacent schwann cells along an axon don’t touch one another making these gaps. They occur at regular intervals along a myelinated axon.
o Schwann cells sometimes surround peripheral nerve fibers but the coiling process doesn’t occur. A single schwann cell can partially enclose 15+ axons. Each occupies a separate recess in schwann cell surface. Nerve fibers associated with schwann cells in this manner are said to be nonmyelinated and typically thin fibers.
Myelination in the CNS:
o CNS contains both myelinated and nonmyelinated axons.
o In CNS oligodendrocytes form the myelin sheaths. Oligodendrocytes have multiple flat processes that can coil around as many as 60 axons at the same time.
o CNS myelin sheath gaps separate adjacent sections of an axon’s myelin sheath. But these myelin sheaths lack outer collar of perinuclear cytoplasm because cell extensions do the coiling and squeezed-out cytoplasm is forced back in towards the nucleus.
o Smaller diameter axons are nonmyelinated, and covered by long extensions of adjacent glial cells.
o White Matter: Regions of the brain and spinal cord containing dense collections of myelinated fibers. Primarily fiber tracts.
o Gray matter: Contains mostly nerve cell bodies and nonmyelinated fibers.
The 3 Steps of Myelination of a Nerve Fiber:
o 1) Schwann cell envelops an axon.
o 2) Schwann cell rotates around axon, wrapping its plasma membrane loosely around it in successive layers.
o 3) Schwann cell cytoplasm forced between the membranes. The tight membrane wrappings surrounding the axon form a myelin sheath.
Classification of Neurons:
Structural Classification
o Multipolar Neurons: Have three or more processes- One axon and the rest dendrites. Most common neuron type in humans, 99% belong to this class. Major neuron type of the CNS.
o Bipolar Neurons: Two processes- axon and a dendrite. Extend from opposite sides of the cell body. Rare neurons found in some special sense organs (retina of eye, and olfactory mucosa).
o Unipolar Neurons: Single short process that emerges from the cell body and divides T-like into proximal and distal branches. More accurately called Pseudounipolar neurons. Found chiefly in ganglia in PNS, where they function as sensory neurons.
o Peripheral Process: Often associated with a sensory receptor.
o Central Process: enters the CNS.
Classification of Neurons:
Functional Classification
o Sensory or Afferent Neurons: Transmit impulses from sensory receptors in the skin or internal organs toward or into the CNS. Virtually all unipolar, and their bodies are located in sensory ganglia outside the CNS. Peripheral processes often very long.
o Motor or Efferent Neurons: Carry impulses away from the CNS to the effector organs. Multipolar. Except for some neurons of ANS their cell bodies are located in the CNS.
o Interneurons: Association neurons. Lie between motor and sensory neurons in neural pathways and shuttle signals through CNS pathways where integration occurs. Most confined within CNS. Make up 99% of neurons of the body, including most in CNS. Almost all Multipolar, but there is a diversity in size and branching patterns.
Action Potential / Nerve Impulse:
- When a neuron is adequately stimulated, an electrical impulse is generated and conducted along the length of its axon.
- Always the same regardless of the source or type of stimulus and underlies virtually all functional activities of the nervous system.
Voltage:
- The measure of potential energy generated by separated charge.
- Either measured in volts or millivolts.
- Always measured between two points and is called potential between the points.
Current:
-The flow of electrical charge from one point to another.
Resistance:
- Hindrance to charge flow provided by substances through which the current must pass.
- High Resistance: Insulators
- Low Resistance: Conductors.
Ohm’s Law
Current = Voltage/Resistance.
o Current is proportional to voltage (greater voltage=greater current).
o There’s no net current flow between points that have the same potential.
o Current is inversely related to resistance. (Greater resistance=smaller current)
Membrane Ion Channels:
o Leakage/Non-Gated Channels: Channels that are always open.
o Chemically Gated/Ligand-Gated Channels: Open when appropriate chemical binds.
o Voltage-Gated Channels: Open and Close in response to changes in membrane potential.
o Mechanically Gated Channels: Open in response to physical deformation of the receptor.
o Electrochemical Gradient: Electrical and concentration gradients constitute this. Ions that flow along these gradients underlie all electrical events in neurons.
