Nervous Tissue Midterm 2

Nervous System

Central nervous system and Peripheral nervous system

Central Nervous System (CNS)

Brain and spinal cord

Peripheral Nervous System (PNS)

All nervous system found outside of the brain and spinal cord, includes cranial and spinal nerves, ganglia, enteric plexus and sensory receptors

Ganglia

group of neuronal cell bodies outside of CNS

Nucleus

group of neuronal cell bodies within the CNS

Enteric plexus

Interconnections of neurons along the digestive tract, includes myenteric plexus and submucosal plexus

Somatic nervous system

Consciously aware of sensory input, consciously aware of motor output

Autonomic nervous system

Not consciously aware of sensory input, not consciously aware of motor output, works “automatically”, it’s subdivided into sympathetic and parasympathetic, and enteric nervous system

Sympathetic and parasympathetic division

“fight or flight” response, “rest and digest” response

Enteric nervous system

“Gut brain”, interweaving of nerve fibers we find along digestive tract

Sensory receptors

Small structures associated with or located at receiving end of sensory neuron, when stimulated it converts into electrical signal that travels along sensory neuron toward its opposite end, triggering release of neurotransmitters

Somatic sensory receptors

Associated with somatic sensory neurons, detect stimuli that we become consciously aware of (touching a hot surface)

Autonomic sensory receptors

Associated with autonomic sensory neurons, detect stimuli we aren’t consciously aware of (blood oxygen level)

Sensory neurons (afferent neurons) “A” for Arrives at CNS

Transmit sensory information from environment or internal organs to CNS, pseudounipolar shape, sensations arrive from dorsal aspect of spinal cord

Somatic: transmits conscious sensations to CNS (touch, temperature, pain sensations)

Autonomic: transmits unconscious sensations to CNS, viscera sensations (blood pressure, level of oxygen in blood)

Integrative function

CNS interprets sensory information and determines appropriate outcome

Motor neurons (efferent neurons) “E” for Exits CNS

Transmit motor information from CNS to appropriate effectors (muscles and glands), multipolar shape

Somatic: control skeletal muscles consciously (contracting biceps brachii, sucking your belly in for a picture)

Autonomic: control smooth & cardiac muscles and glands unconsciously (sweat gland secretion, heart rate, breathing rate)

Sensory path to CNS (I)

Sensory information is detected in periphery (PNS) and transformed into electrical signal where it travels through sensory neuron (responsible for carrying out info) to CNS, where it’s then processed and interpreted

CNS information interpretation (II)

Sensory info is interpreted and we’re aware of it (someone presses our finger), sensation arrived at CNS using somatic sensory neuron.

If we aren’t aware of it (blood flow in vessels), sensation arrived at CNS using autonomic sensory neuron (it worked automatically)

After CNS interpretation (III)

Specific outcome is decided, outcome of contracting muscle like moving our finger, or outcome of relaxing muscle in blood vessels’ wall, decision Exits CNS using motor neuron (efferent)

Once Exited of CNS (IV)

When we’re consciously moving a body part, sensation exited CNS using somatic motor neuron. When we’re unconscious, sensation exited CNS using autonomic motor neuron (works automatically)

Neurons = Nerve cells

Highly specialized cells capable of communication and forming networks, involved in sensations, thinking and muscle activities. They’re surrounded by plasma membrane, contain nucleus and cytoplasmic organelles

*Approx. 85 billion neurons in brain, each communicate 1000-10000 other neurons

Dendrites

Branched extensions of cell body, receive incoming signals from neurons or sensory receptors, take info towards neuronal cell body

Cell body (perikaryon/soma)

Contains nucleus

Axon hillock (point of no return)

Cone-shaped region extending from neuronal cell body that becomes an axon, electrical signal propagates down the axon when it’s received until it reaches terminal button, triggering release of neurotransmitters into synaptic cleft

Terminal button (synaptic bulb)

End of axon, within button there’s vesicles filled with neurotransmitters. Vesicles fuse with plasma membrane when electrical signal arrives, neurotransmitters are released into synaptic gap via exocytosis

Nerve fibers=Neuronal fibers

Very long process(es) that extend from neuronal cell body, all neurons are nerve cells which have at least one nerve fiber. (fiber usually means axon, but not always). Whole bundle of fibers is a nerve

Independent of structure of neuron:

Dendrites will always receive stimulus, convert into electrical signal and move it towards neuronal cell body. Axon carries signal away from neuronal cell body

Structure: multipolar neuron

Multitude (several) processes leave neuronal cell body, two or more dendrites and one axon. Cell body covered in dendrites (like little hairs). Most common neurons in nervous system

Structure: bipolar neuron

two processes leave neuronal cell body, one process is dendrite and other is axon, each process extends in opposite directions

Structure: unipolar neuron

unique (single) branch leaves neuronal cell body, very rare in humans (mostly found in invertebrates)

Structure: pseudounipolar neuron

Unique (single) short process leaves neuronal cell body and splits in two processes that extend in opposite directions, cell bodies are at dorsal root ganglion

Peripheral process: extends to periphery, functions like dendrite, conducts electrical signal toward neuronal cell body

Central process: extends to CNS, functions like axon, conducts electrical signal away from neuronal cell body

Special sensory neurons

Specific type of sensory neuron, carries info from special senses (smell, taste, vision, hearing, balance), bipolar shape

Interneurons

Transfers info from one neuron to another, found “between” neurons multipolar shape

Special sense of gustation

Taste receptors are modified epithelial cells. *similar but NOT bipolar neurons

Neurons in charge of carrying taste information are pseudounipolar (bc cell bodies are located in cranial nerve sensory ganglia)

Purkinje cells (specialized neurons)

Exclusive to CNS, they’re large interneurons so they’re multipolar, found in cerebellum and are involved in coordination and fine-tuning of motor activity

Pyramidal cells (specialized neurons)

Exclusive to CNS, they’re motor neurons so they’re multipolar, pyramidal-shaped, found in motor cortex and is involved with voluntary motor control

Communication in the body

Electrical signals: change in voltage caused by ions moving across plasma membrane, they travel through the neuron

Chemical signals: neurotransmitters, chemical compounds that transmit information between neurons

Nerve Impulse=Action Potential=Electrical Signals

All the same when referencing transmission of information along neurons.

Action potential is initiated by depolarization of cell membrane leading to generation of electrical impulse.

Involves opening and closing of gated ion channels to regulate ion flow across membrane

Polarized cells

Resting membrane potential is when cells are at “rest”, cells are more negative inside than outside environment.

Negativity inside is caused by negative charged proteins stuck within the cell

Sodium/Potassium Active Pump

3 Na⁺ pumped outside of cell, 2 K⁺ pumped inside of cell, uses ATP for energy

Depolarized cell

Cell is not “at rest”, it’s excited! Cells’ more positive inside than outside environment. Change in polarity is an electrical signal.

Ions (Na⁺ and K⁺) due to small size and different concentrations inside and outside of cell, are responsible for creating electrical signals when membrane gate opens, allowing flow of ions

Excitable cells

Capable of changing polarity, muscle cells and neurons capable to undergo depolarization and repolarization, fundamental process in generation of action potential

Voltage-gated ion channel

Gated ion channel opens in response to change in voltage (bc of electrical signal)

gate opens for passage of Na⁺ ions: Voltage-gated Na⁺ channel

gate opens for passage of Ca⁺² ions: Voltage-gated Ca⁺² channel

Ligand-gated ion channel

Gated ion channel opens in response to a ligand (neurotransmitter) binding a receptor on the channel

gate opens to neurotransmitter allowing passage of Na⁺ ions: Ligand-gated Na⁺ channel

Synapse

Communication between neurons

Synaptic transmission: electrical conduction along neuron (I)

Neuron at resting membrane potential. Stimulus at dendrite of sensory neuron opens voltage-gated Na⁺.

Positive ions (Na⁺) flow into neuron and depolarize it, creating electrical signal.

Signal travels to cell body, passes axon hillock entering the axon which carries signal towards terminal button.

Synaptic transmission: events at terminal button (II)

Electrical signal reaches terminal buttons opening Ca²⁺ channels.

Calcium ions flow into terminal buttons, triggers vesicles filled with neurotransmitters to fuse with plasma membrane.

Fusion releases stored neurotransmitters into synaptic gap (cleft)

Synaptic transmission: crossing synapse & postsynaptic activation (III)

Neurotransmitters flow through synaptic gap and bind to receptors associated with ligand-gated ion channels on the following neuron, causing the gate to open, allowing ions to cross plasma membrane to transmit info from presynaptic neuron to postsynaptic neuron.

Signals our body uses to communicate

Electrical and chemical

Synaptic communication

Requires a lot of energy, mitochondria are present in synaptic end bulb of pre-synaptic neuron

Neurotransmitters

Two excitable cells never touch, they’re chemicals used to transfer information from one excitable cell to another, facilitates transmission of info from one neuron to another

Excitatory neurotrasmitters

Bind receptors associated with with ligand-gated Na⁺ channels. Opening of channels allow Na⁺ to enter postsynaptic cell, depolarization occurs and excitability of postsynaptic cell

Inhibitory neurotransmitters

Bind receptors associated with ligand-gated Cl^- channels. Opening of channel allows CI^- ions to enter postsynaptic cell and K⁺ ions to exit, results in hyperpolarization (making inside of the cell more negative) and inhibition of postsynaptic cell

Neurotransmitter: Acetylcholine

Excitatory in skeletal muscle, inhibitory in cardiac muscle

Neurotransmitter: Adrenaline (epinephrine)

Excitatory in cardiac muscle (increases heart rate), inhibitory on respiratory smooth muscle (relaxes muscle)

Neurotransmitter: Endorphins

Inhibitory effects, causes relaxation and sleepiness

*Narcotic drugs mimic effects of endorphins (endogenous morphine)

Removal of neurotransmitters

Must be removed from synaptic cleft to terminate signal transmission

Reuptake

Neurotransmitters are taken back up into presynaptic neuron for recycling

Enzymatic degradation

Enzymes in synaptic cleft break down neurotransmitters into inactive metabolites

Diffusion

Neurotransmitters diffuse into nearby tissue fluid

Electrical signal flow

Travels through neuron in wave-like motion, nerve fibers can be myelinated or unmyelinated

Unmyelinated

Nerve fiber is bare, nothing wrapping around it, has lower conduction velocity (0.5m/s)

Myelinated

Nerve fiber is wrapped in segments by entire cells, has higher conduction velocity (130m/s)

Myelination: function of phospholipid bilayer in nerve cell membranes

Wrapping insulates segments of nerve fiber, prevents ions from flowing into or out of that specific segment

Myelinated neurons: Myelin sheath

Part of nerve fiber that has another cell wrapped around it

Myelinated neurons: Node of Ranvier

Part of nerve fiber that does NOT have another cell wrapped around it

Saltatory conduction

Jumping motion of conducting electrical signals from one Node of Ranvier to the next to reach terminal button.

*Done since ions can’t get in or out of nerve fiber that’s wrapped by another cell

Myelinated neurons conduction

higher conduction velocity since electrical signal jumps, skipping parts of nerve fiber allowing signal to reach terminal button faster (than if it traveled along entire length of fiber)

Nerve Impulse

Remains constant over time and distance, no info is lost regardless of distance needed to travel

*If it lost impulse strength hitting your toe wouldn’t hurt much

Glial cells (neuroglia)

Include all nervous tissue cells that aren’t neurons, outnumber neurons in nervous system, maintain homeostasis and support neuronal function. Unlike neurons, they proliferate throughout our whole life

”Glued” to neurons

Oligodendrocytes

Create myelin sheath in CNS neurons (facilitate rapid conduction) and inhibit regeneration (with astrocytes) of neurons. They’re cells with few tree-like branches that can myelinate more than one segment of nerve fiber at a time

Astrocytes

Star-shaped cells, provide physical and nutritional support to neurons and form blood-brain barrier (BBB) by surrounding blood vessels within CNS. Forms scar tissue acting as physical barrier, hindering nerve fiber regeneration

“big stars” (astro), “most popular” glial cells bc they’re most numerous!

Microglial Cells

Small cells responsible for phagocytosing damaged nervous tissue and infecting particles within CNS

“small eaters”

Ependymal cells

Produces cerebrospinal fluid (CSF). They’re ciliated cells that surround specific blood vessels in the CNS. They filter blood plasma to produce CSF and use cilia to give direction the CSF needs to flow

Satellite cells (PNS)

Found within ganglion, surrounds neuronal cell bodies. Regulate exchange of materials between neuron and interstitial fluid.

*They’re like “satellites” surrounding neuronal cell body

Schwann cells

Creates myelin sheath in PNS neurons (facilitate rapid conduction), aids regeneration of neurons. Cells form myelin sheath that stay in place, serving as a pathway for fiber to regrow when a PNS neuronal fiber is damaged

Regeneration

Regenerate means generate again, bring back to life, and exclusively occurs in PNS. Schwann cells aid in axon regeneration by forming pathway for nerve regrowth

Neurogenesis

Generating (genesis) neurons (neuro). New neurons are generated from stem cells, VERY RARE! Mainly occurs in specific regions of the brain (hippocampus, area involved with memory)

Plasticity

Brain’s ability to reorganize itself by forming new neural connections, it happens throughout life. (think of plastic bag)

Synaptic plasticity involves strengthening/weakening of synaptic connections based on experience.

Structural plasticity involves changes in brain’s physical structure, like formation of new synapses between neurons

Gray matter

Dark areas in spinal cord and brain, found in inner area of spinal cord and outer area of brain (cortex). Processes and integrates sensory and motor information. Consists of glial cells, neuronal cell bodies, dendrites, and nerve fibers.

White matter

Lighter areas in spinal cord and brain, found in outer area of spinal cord and inner area of brain. Forms tracts/pathways to facilitate communication among different regions of brain and spinal cord. Consists of glial cells, dendrites, and nerve fibers. Presence of myelin (fat) contributes to white appearance.