Anatomy Chapter 14 Nervous Tissue

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Nervous Tissue

Nervous system is composed of all tissue types, but primarily of nervous tissue- neurons and glial cells.

Central nervous system (CNS)

Composed of the brain and spinal cord.
The brain is protected and enclosed within the skull.
The spinal cord is housed and protected within the vertebral column.
Structural Organization

Peripheral nervous system (PNS)

Composed of the cranial nerves (nerves that extend from the brain), spinal nerves (nerves that extend from the spinal cord), and ganglia (clusters of neuron cell bodies located outside the CNS).
Structural Organization

Functional Organizaion: Sensory and Motor Nervous System

The CNS and PNS perform three general functions:
-Collection information. Receptors detect changes in the internal or external environment and pass them on to the CNS as sensory input.
-Processing and evaluating information. After processing sensory input, the CNS determines what, if any, response is required.
-Responding to information. After seeking an appropriate response, the CNS initiates specific nerve impulses called motor output. This travels through structures of the PNS to effectors (cells that receive impulses from motor neurons: muscles or glands).

Sensory Nervous System

(Afferent = inflowing)
-Responsible for receiving information from receptors and transmitting this information to the CNS.
-It is responsible for input.
-Contains both PNS and CNS components: nerves of the PNS transmit the sensory information, and certain parts of the brain and spinal cord in the CNS interpret this information.
-Has two components: Somatic Sensory and Visceral Sensory.

Somatic Sensory

Component of Sensory Nervous System.
General somatic senses: touch, pain, pressure, vibration, temperature, and proprioception (sensing the position or movement of joints and limbs)- and the special senses: taste, vision, hearing, balance, and smell.
Voluntary control.

Visceral Sensory

Component of Sensory Nervous System.
Transmit nerve impulses from blood vessels and viscera to the CNS. The visceral senses primarily include temperature and stretch (of muscles of the organ wall). Involuntary control.

Motor Nervous System

(Efferent = conducting outward)
Responsible for transmitting motor impulses from the CNS to muscles or glands.
The motor nervous system is responsible for output.
Contains both CNS and PNS components: Parts of the brain and spinal cord (CNS) initiate nerve impulses, which travel through motor nerves that in turn transmit these impulses to effector organs.

Somatic Motor

Component of Motor Nervous System.
Conducts nerve impulses from the CNS to the skeletal muscles, causing them to contract.
Voluntary.

Autonomic Motor

Component of Motor Nervous System.
Innervates internal organs and regulates smooth muscle, cardiac muscle, and glands without our control.
Also known as visceral motor system or involuntary motor system.

Neurons

Basic structural unit of the nervous system. Conduct nerve impulses from one part of the body to another.
Characteristics:
-High metabolic rate (need a lot of O2 and Glucose)
-Extreme longevity (can last a life time)
-Nonmitotic (during development, mitotic activity is lost)

Cell Body

Neuron.
Also called soma.
The neuron's control center and is responsible for receiving, integrating, and sending nerve impulses.
Enclosed by a plasma membrane and contains cytoplasm surrounding a nucleus. The nucleus contains a prominent nucleolus, reflecting the high metabolic activity of neurons. Many mitochondria are present. Also large amounts of free ribosomes.

Chromatophillic

Neuron.
Both free and bound ribosomes go by two names: chromatophilic substance, because they stain darkly with dyes and Nissl bodies (who discovered them).
They think it accounts for the grey color of the gray matter.

Dendrites

Neuron.
Tend to be shorter, smaller processes that branch off the cell body. Some neurons only have one dendrite, while others have many. They conduct nerve impulses toward the cell body for precessing. The more dendrites a neuron has, the more nerve impulses that neuron can receive from other cells.

Axon

Neuron.
Typically longer nerve cell process emanating from the cell body.
Sometimes called a nerve fiber.
Neurons either have one axon or no axon at all, those that do not have one is called anaxonic.
The axon transmits a nerve impulse away from the cell body toward another cell. The axon transmits output information to other cells.

Anaxonic

Neuron.
Neuron that has no axon.
Small neurons that provide no clues to distingush axon from dendrite; they are only found in CNS; they are uncommon and their function is unknown.

Axon Hillock

Neuron.
The axon connects to the cell body at a triangular region called the axon hillock. The axon hillock is devoid of chromatophilic substance, and so it lacks those dark staining regions when viewed under the microscope.

Axon Collateral

Neuron.
Axon may give rise to a few side branches called axon collaterals. Most axons and their collaterals branch extensively at their distal end into an array of fine terminal extensions called telodendria, or axon terminals.

Telodendria

Neuron.
Axon terminals.

Synaptic knobs

The extreme tips of the fine extensions of telodendria are slightly expanded regions called synaptic knobs.

Perikaryon

Neuron.
The cytoplasm within the cell body is called perikaryon.

Neurofilaments

Neuron.
Intermediate filaments that aggregate to form bundles called neurofibrils. Neurofibrils extend as a complex network into both dendrites and axons, where their tensile strength provides support for these processes.

Structural Classification of Neurons

Unipolar
Bipolar
Multipolar

Unipolar Neurons

Have a single, short neuron process that emerges from the cell body and branches like a T.
The combined peripheral process (from dendrites o the cell body) and central process (from the cell body into the CNS) together denote the axon, because these processes generate and conduct impulses and are often myelinated.
Most sensory neurons of the PNS are unipolar.

Bipolar Neurons

Have two neuron processes that extend from the cell body-one axon and one dendrite.
Relatively uncommon in humans and primarily limited to special senses.
Ex. Located in the olfactory epithelium of the nose and in the retina of the eye.

Multipolar Neurons

Most common type of neuron. Multiple neuron processes- many dendrites and a single axon extend from the cell body.
Ex. Include motor neurons that innervate muscle and glands.

Sensory Neurons

Also known as afferent neurons.
Transmit nerve impluses from sensory receptors to the CNS.
Specialized to detect changes in their environment called stimuli. This can be in the form of touch, pressure, heat, light, or chemicals.
Most are unipolar, the rest are bipolar (olfactory/retina).
The cell bodies are located outside the CNS and housed within structures called posterior (dorsal) root ganglia.

Motor Neurons

Also known as efferent neurons.
Transmit nerve impulses from the CNS to muscles or glands.
Most of them extend to muscle cells, and the nerve impulses they transmit cause these cells to contract.
The muscle and gland cells that receive nerve impulses from motor neurons are called effectors, because their stimulation produces a response or effect.
The cell bodies of most motor neurons lie in the spinal cord, whereas the axons primarily travel in cranial or spinal nerves to muscles and glands.
All are multipolar.

Interneurons

Known as association neurons.
Lies entirely within the CNS.
Multipolar
Receive nerve impulses from many other neurons and carry out integrative function of the nervous system. They retrieve, process, and store information and "decide" how the body responds to stimuli.
They facilitate communication between sensory and motor neurons.
Interneurons outnumber all other neurons in both their total number and different types; it is estimated that 99% of our neurons are interneurons.

Glial Cells

Also known as neuroglia.
Occur within both CNS and PNS.
Differ from neurons in that they are smaller and capable of mitosis.
Do not transmit nerve impulses.
They protect and nourish neurons.
They provide an organized, supporting framework for all nervous tissue.
Outnumber neurons.
Astrocytes, ependymal cells, microglial cells, and oligodendrocytes.

Astrocyte

Located in CNS
Large cell with numerous cell processes
In contact with neurons and capillaries
Most common type of glial cell
Functions: Helps for blood-brain barrier
Regulates tissue fluid composition
Provides structural support and organization to CNS
Replaces damaged neurons
Assists with neuronal development

Ependymal Cell

Located in CNS
Simple cuboidal epithelial cell lining cavities in brain and spinal cord.
Cilia on apical surface
Functions:
Lines ventricles of brain and central canal of spinal cord
Assists in production and circulation of CSF
They work together with nearby capillaries to form a network called choroid plexus, which produces CSF.

Microglial Cell

Located in CNS
Small cell with slender branches from cell body
Least common type of glial cell
Functions:
Wanders through CNS and replicate in response to an infection
Defends against pathogens
Removes Debris
Phagocytizes waste

Oligodendrocyte

Located in CNS
Rounded, bulbous cell with slender cytoplasmic extensions
Extensions wrap around CNS axons
Functions:
Myelinates and insulates CNS axons (myelin sheath)
Allows faster nerve impulse conduction through the axon

Satellite Cell

Located in PNS
Flattened cell clustered around neuronal cell bodies in a ganglion (a collection of neuron cell bodies located outside of the CNS).
Functions:
Protects and regulates nutrients for cell bodies in ganglia.

Neurolemmocyte (Schwann Cell)

Located in PNS
Flattened cell wrapped around a portion of an axon in the PNS
Functions:
Myelinates and insulates PNS axons
Allows for faster nerve impulse conduction through the axon

Gliomas

Glial cell tumor
can be benign and slow-growing or malignant

Nerve Impulse

The main activity of axons is nerve impulse conduction.
it is the rapid movement of an electrical charge along a neuron's plasma membrane.
A nerve impulse is also called an action potential.
It is caused by an actual voltage (potential) change that moves along the plasma membrane of the axon.
Its ability to propagate is affected by myelin.

Myelination of PNS Axons

1. Neurolemmocyte starts to wrap around a portion of an axon.
2. Neurolemmocyte cytoplasm and plasma membrane begin to form consecutive layers around axon.
3. The overlapping inner layers of the neurolemmocyte plasma membrane form the myelin sheath.
4. Eventually, the neurolemmocyte cytoplasm and nucleus are pushed to the periphery of the cell as the myelin sheath is formed.

Myelin Sheath

The insulating covering around the axon consisting of concentric layers of myelin.
Mainly consists of the plasma membrane of glial cells (oligodendrocytes and neurolemmocyte) and contains large proportion of fats and a lesser amount of proteins.
The high lipid content gives the axon a distinct, glossy-white appearance.

Difference between myelin sheaths in CNS and PNS

CNS: Oligodendrocytes can myelinate many axons, not just one. Cytoplasmic extensions wrap successively around a portion of each axon, and successive plasma membrane layers form the myelin sheath.
PNS: Neurolemmocytes can myelinate a portion of a single axon only. Multiple neurolemmocytes on one axon.

Unmyelinated Axons

Not all axons are myelinated.
Can be found in PNS and CNS
In PNS: Associated with a neurolemmocyte, but no myelin sheath covers them. Axon rests in a portion of the neurolemmocyte rather than being wrapped by successive layers of plasma membrane.
In CNS: unmyelinated axons are not associated with oligodendrocytes.

Neurofibril Nodes

Also known as nodes of Ranvier
Gaps in between myelin sheaths
At these nodes, change in voltage occur across the plasma membrane and result in movement of a nerve impulse.
Saltatory Conductions: Nerve impulse "jumps" from node to node.
Continuous Conductions: In unmyelinated axon, nerve impulse must travel entire length of axon. Requires more energy.
Myelinated axons produce faster nerve impulses and requires less energy.

PNS axon regeneration depends upon three factors:

1. The amt of damage
2. Neurolemmocyte secretion of nerve growth factors to stimulate outgrowth of severed axons
3. The distance between the site of the damaged axon and the effector organ (as the distance to the effector increases, the possibility of repair decreases)

Wallerian Degeneration

Regeneration via neurolemmocytes of damaged axons.
1. Axon is severed by trauma
2. End of the proximal portion of severed end seals off by membrane fusion and swells. Severed distal portion of the axon and its myelin sheath degenerate; macrophages remove debris by phagocytosis.
3. Neurolemmocytes form a regeneration tube in conjunction with the remaining endoneurium of severed axon.
4. Axon regenerates, and remyelination occurs. Regeneration tubes guides the axon sprout as it begins to grow rapidly through the regeneration tube.
5. Innervation is restored as the axon reestablishes contact with its original effector.

Nerve

A cablelike bundle of parallel axons.
Has three successive CT wrappings: endoneurium, Perineurium, and Epineurium.

Endoneurium

An individual Axon in a myelinated neuron is surrounded by neurolemmocytes and then wrapped in the endoneurium, a delicate layer of areolar CT that separates and electrically isolates each axon. Also within this layer are capillaries that supply each axon.

Fascicles

Groups of axons are wrapped into separate bundles called fascicles.

Perineurium

Groups of axons are wrapped into separate bundles called fascicles by a cellular dense irregular CT layer called perineurium. This layer supports blood vessels supplying the capillaries within the endoneurium.

Epineurium

All of the fascicles are bundles together by a superficial CT covering called epineurium. This thick layer of dense irregular CT encloses the entire nerve, providing both support and protection to the fascicles within the layer.

Synapses

Axons terminate as they contact other neurons, muscle cells, or gland cells at specialized junctions called synapses where the nerve impulse is transmitted to the other cell.
As the axon approaches the cell onto which it will terminate, it generally branches repeatedly into several telodendria, and each telodendrion loses its myelin covering.
The endings usually form swellings called synaptic knobs at the ends of the axon branches.

CNS Synapse

Consists of the close association of a presynaptic neuron and a postsynaptic neuron at a region where their plasma membranes are separated by a very narrow space called the synaptic cleft.
Presynaptic neurons transmit nerve impulses through their axons toward a synapse; post-synaptic neurons conduct nerve impulses through their dendrites and cell bodies away from the synapse.

Axodendritic Synapse

Most common type
Occurs between the synaptic knobs of a presynaptic neuron and the dendrites of the postsynaptic neuron.
Occur either on the expanded tips of narrow dendritic spines or on the shaft of the dendrite.

Axosomatic Synapse

Occurs between synaptic knobs and the cell body of the postsynaptic neuron.

Axoaxonic Synapse

Least common synapse
Far less understood
Occurs between the synaptic knob of a presynaptic neuron and the synaptic knob of a postsynaptic neuron. The action of this synapse appears to influence the activity of the synaptic knob.

Electrical Synapses

The plasma membranes of the presynaptic and postsynaptic cells are bound together. Electrical synapses are fast and secure, and they permit two-way signaling.
Gap junctions formed by connexons between both plasma membranes, between the cells.
Primarily occurs between smooth muscle cells and cardiac muscle at the intercalated discs.

Chemical Synapses

Most numerous type of synapse
Facilitates most of the interactions between neurons and all communications between neurons and effectors.
The presynaptic membrane releases a signaling molecule called a neurotransmitter.
Acetylcholine (ACh) is most common type of neurotransmitter.

Conduction of a nerve impulse from the presynaptic neuron to the postsynaptic neuron

1. Nerve impulse travels through the axon and reaches its synaptic knob.
2. This causes an increase in Ca2+ movement into the synaptic knob through voltage-gated calcium ion channels in the membrane.
3. This causes synaptic vesicles to move to and bind to the inside surface of the membrane; neurotransmitter molecules within the synaptic vesicles are released into the synaptic cleft by exocytosis.
4. Neurotransmitter molecules diffuse across the synaptic cleft to the plasma membrane of the postsynaptic cell.
5. NT mole. attach to specific protein receptors in the plasma mem. of the postsynaptic cell, causing ion gates to opne.
6. Influx of NA+ moves into the postsynaptic cell through the open gate, affecting the charge across the mem.
7. Change in the postsynaptic cell voltage causes a nerve impulse to begin in the postsynaptic cell.
8. Enzyme AChE resides in the synaptic cleft and rapidly breaks down molecules of ACh that are released into the synaptic cleft. Thus, AChE is needed to that ACh wil not continuously stimulate the postsynaptic cell.

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