06/26/08 Bscc2094 Anatomy And Physiology Ii Lecture Notes Nerve Cells

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CENTRAL NERVOUS SYSTEM (CNS)

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Brain and Spinal Cord

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Foramen Magnum

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Largest foramen in the skull, located at the base. Constitutes a dividing line.

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Spinal Cord

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connects and carries messages to and from the brain via the foramen magnum

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PERIPHERAL NERVOUS SYSTYEM (PNS)

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Cranial and spinal nerves outside the CNS.

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PERIPHERAL NERVOUS SYSTYEM (PNS)

Cranial Nerves

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Emerge from the base of the brain and travel through the holes in the skull. There are 12 of them, named with roman numerals I-XII

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PERIPHERAL NERVOUS SYSTYEM (PNS)

Spinal Nerves

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Emerge from the spinal cord out of the vertebra via the IVF. Numbered 1 - 31.

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Nerve

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A bundle containing many axons which follows a defined pathway of transmission and serves a specific area of the body.

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Motor

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Eferrent. Motor neurons transmits signals from CNS to any effector (muscle or gland cell). Its axon is an efferent nerve fiber.

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Sensory

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Afferent - An axon that conducts information from a receptor to the CNS. An afferent nerve fiber. (Think "assigned" to be "aware" of sensations and "advise" the CNS about them.

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Nuclei (Nucleus)

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CNS nerve cell bodies

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Ganglia

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PNS Nerve cell bodies (some CNS ganglia) - knotlike swelling in a nerve where the cell bodies of neurons are concentrated (i.e. dorsal ganglia = shingles)

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Plexus

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A network of neurons/nerves which usually overlap an area or region and coordinate together. (i.e. enteric plexus, brachial plexus)

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PNS Divisions

Somatic Nervous System (SNS)

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- Sensory (afferent) from the periphery to the CNS
- motor (efferent) from the CNS to the periphery (muscles) - voluntary

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PNS Divisions

Autonomic Nervous System (ANS)

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Involuntary, divided into opposing sympathetic and parasympathetic
-sensory from viscera
-motor to the smooth muscles, cardiac muscles, gland secretion (mostly motor)
Sympathetic (SANS)- fight/flight
Parasympathetic (PANS) rest/digest
HR and GI movement can go up/down w/either

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PNS Divisions

Enteric Nervous System (ENS)

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Independent. stimulated by changes w/in gut (i.e. chemical, stretch)

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Neuron (nerve cell) Characteristics

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electrical excitability, cell body, axon, dendrite (others include Nissl Bodies RER, axon hillock, axoplasm=cytoplasm, axon terminals, synaptic end bulbs, synaptic vesicles)

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Multipolar

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Many dendrites + 1 axon with a cell body between them

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Bipolar

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1 dendrite + 1 axon with a cell body between them

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Unipolar

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1 dendrite + 1 axon without a cell body between them

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Associate / Interneuron / Internuncial Neuron

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90% of neurons. Links impulses from sensory to motor neuron located in the CNS (known as: Purkinje cells; Renshaw cells; pyramidal cells).

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Dendrites

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Extending from the soma, multiple short, branched processes which receive signals from other cells and transmit messages to the soma (cell body)

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Nissl Bodies

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Dark-staining regions of compartmentalized Rough ER unique to neurons, and a helpful clue to identifying them in tissue sections with mixed cell types.

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Cell Body / Soma

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The control center of the neuron, with a single centrally located nucleus with a large nucleolus.

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Nucleus

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At center of cell body

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Schwann Cell

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Cells that envelop nerve fibers of the PNS. Winds repeatedly around nerve fiber, produces Myelin sheath (similar to oligodendrocyte in CNS). It's right in the sheath. Assists in regeneration of damaged fibers.

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Axon

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nerve fiber, extends from cell body but still part of the cell. Some axons are more than a meter long and extend from the brainstem to the foot.

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Myelin

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Insulating layer around a nerve fiber (think rubber coating on wire). Formed by oligodendrocytes in CNS and Schwann Cells in PNS.

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Nerve Impulse

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Proceeds in one direction, from soma down the axon, to pre synaptic area, through synaptic cleft, to post synaptic area.

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Nodes of Ranvier

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Gaps between the segments of the myelin sheath.

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Synaptic Bulb/Knob

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terminal button at the end of the extensive complex of fine branches at the distal end of an axon. Forms a junction (synapse) with next cell. Contains synaptic vessels.

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Neuroglia aka Glial Cells

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CNS support and protect. Do not transmit action potential. Brain tumor origin. Multiply in area of brain injury/trauma)

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Neuroglia of CNS

Microglial Cells (Microglia)

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Tiny phagocytic cells that engulf foreign substances, microorganisms, dead tissue. Become concentrated in damaged areas, so Drs look for them as clues to sites of injury.

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Neuroglia of CNS

Astrocyte

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forms Blood Brain Barrier - cover brain surface and nonsynaptic regions of neurons. supportive framework. nourish neurons. Most abundant glial cells of CNS

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Ependymal Cells

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CSF (cerebrospinal Fluid) production! Resembles cuboidal epithelium but without basement membrane and with rootlike processes into underlying nervous tissue. Line the ventricles (cavities) of brain and spinal cord at the central canal.

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Neoroglia of PNS

Schwann Cells

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Myelinating cells of PNS. Neurolemmocyte. Neurolemma regeneration possible. Nodes of Ranvier.

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Neuroglia of CNS

Oligodendrocyte

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Myelinating cells of CNS. No neurolemma, no regeneration. Few create myelin for many nerve cells. Attaches and branches between many nerve cells like taping up wires.

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Neuruglia of PNS

Sattelite Cells

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Support cells in PNS ganglia. Not sure of function. Surround neuron cell bodies in PNS.

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Myelin

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insulates/increases conduction of the nerve impulse

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CNS Features

White matter

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myelanted (i.e. columns outside horns of spinal cord) On the outside in the spinal cord, on the inside in the brain

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CNS Features

Gray matter

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unmyelinated (i.e. cortex of the brain. Inside in the spine, outside of the brain.

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CNS Features

Horns

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Gray matter in the spinal cord which forms an H (anterior, posterior, and lateral)

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CNS Features

Tracts

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bundles of axons ascending and descending, carrying impulses up (sensory) and down (motor) and in and out CNS (spinal cord)

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Membrane Potential (MP)

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An electrical difference across the plasma membrane by ions

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Resting Membrane Potential

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potential of excitable cell membrands to have a current (AP)

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Sodium-Potassium Pump (NA+ K+ Pump)

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Active Transport that pumps 3 intercellular sodium molecules out of the cell, and 2 extracellular potassium molecules in.

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Depolarization

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Influx of Na+, changing the normally negatively charged inside of a cell to a positive charge. Change in polarity to +

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Repolarization

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Efflux of K+. Potassium rushes out of the cell to try and reverse the depolarization

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Hyperpolarization

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Due to a large K+ efflux, the membrane potential becomes extra negative inside. This is fixed through Active Transport (NA+/K+ Pump)

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Action Potential (AP)

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Events of decrease, eventually reverse of membrane potential and restoration

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All Or Nothing

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when a “sufficient – threshold” stimulus causes a depolarization to reach a “certain level” opening of voltage gated channels leading to an Action Potential (AP). At the NMJ, all or nothing will determine muscle fiber contraction. There is no in-between, no partial contraction of the “muscle fiber” (not muscle!).

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Threshold

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The point where depolarization is critical and further depolarization leads to an AP (at the AP it becomes all-or-nothing).

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Properties of Action Potentials

Absolute Refractory Period

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That period during which, no matter how strong the stimulus, it cannot induce a second action potential.

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Properties of Action Potentials

Relative Refractory Period

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That period during which a greater than normal stimulus is required to induce a second action potential.

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Synapse

Pre-Synaptic

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Area before the synaptic gap

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Synapse

Post-Synaptic

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Area after the synaptic gap

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Synaptic cleft

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gap of 60-100 nm between synaptic knob and motor end plate

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Synaptic delay

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An interval which constitutes the time from the arrival of a signal at the axon terminal of a presynaptic cell to the beginning of an action potential in the postsynaptic cell.

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Synaptic fatigue

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no neurotransmitter remaining in presynaptic knobs

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Neurotransmitters

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chemical that crosses the synaptic cleft and stimulates the next cell.

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Neurotransmitters

Acetylcholine

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NMJ, most synapses of autonomic nervous system, retina, many parts of brain. Excites skeletal muscles, inhibits cardiac muscle, has excitatory or inhibitory effects on smooth muscle/glands depending on location.

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Neurotransmitters

Norepinephrine

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A Catecholamine. Sympathetic nervous system, cerebral cortex, hypothalamus, brain stem, cerebellum, spinal cord. involved with dreaming, waking, mood, excites cardiac muscle, excite or inhibit smooth muscle and glands depending on location.

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Neurotransmitters

Serotonin

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A monoamine. Hypothalamus, limbic system, cerebellum, retina, spinal cord. also secreted by blood platelets and intestinal cells. Sleep induction, alertness, sensory perception thermoregulation, mood. 5-hydroxtryptamine (5_HT) @ raphe nucleus

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Neurotransmitters

Glutamine / Glutamate

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Gls. CNS. An excitatory Amino Acid, glutamic acid, glutamate. Cerebral cortex and brainstem, retina. 75% of all excitatory synaptic transmission in brain. Inv. with learning, memory.

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Neurotransmitters

Dopamine

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A catecholamine. Hypothalamus, limbic system, cerebral cortex, retina. highly concentrated in substantia nigra of midbrain. Inv. with elevation of mood, control of skeletal muscles.

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Neurotransmitters

GABA

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An Amino Acid. Gamma Aminobutyric Acid)Thalamus, hypothalamus, cerebellum, occipital lobes of cerebrum, retina. Most common inhibitory neurotransmitter in brain. Inhibitory via opening C1 channels.

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Neuromuscular Junction (NMJ)
or
Myoneural Junction

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A synapse between a nerve fiber and a muscle fiber. Functional connection between distal end of nerve fiber, middle of muscle fiber. synaptic knob and motor end plate

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Synaptic vesicle

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secretory vesicle in synaptic knob; contains acetylcholine

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Acetylcholine (ACh)

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The neurotransmitter released by a somatic motor fiber that stimulates a skeletal muscle fiber (also used elsewhere in the nervous system)

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Acetylcholinesterase (AChE)

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An enzyme in the sarcolemma and basal lamina of muscle fiber in synaptic region, responsible for degrading ACh and stopping the stimulation of muscle fiber. And recycling of ACh.

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ACh receptor

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a transmembrane protein in the sarcolemma of the motor end plate that binds to ACh

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Choline acetyltransferase

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Acetyl CoA + Choline --> acetylcholine

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What happens at the NMJ?

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Pre-synaptic neuron releases Ach, which has been stored as vesicles (champagne bubbles).
Ach is released into the synaptic cleft.

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Ion Channels

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these channels when open allow the passage of ions (cations and anions)

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Ion Channels

Leakage Channels

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Always open

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Ion Channels

Gated Channels

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These open and close in response to a stimulus. Neuron & muscle fiber plasma membrane.

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Ion Channels

Voltage-Gated Channels

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These open in response to change in membrane potential. Generation and conduction of an action potential.

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Ion Channels

Ligand Gated Channels

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These open and close in response to a specific chemical stimulus. Examples:
1. Acetylcholine → open channel
2. Acetylcholine → G protein → → open channel.

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Ion Channels

Mechanical Gated Channels

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These open in response to mechanical stimulus (pressure, touch, vibration.)

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Conduction

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Propagation. Connonly called nerve conduction. Na+ ion inflow -->depolarization (increase) --> adjacent Na+ voltage gates open

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Continuous conduction

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Step-like unmyelinated depolarization of adjacent membrane (like rock going through water slowly)

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Saltatory conduction

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myelinated conduction involving use of Nodes of Ranier (i.e. local anesthetics/lidocaine -->block because prevents Na+ voltage gates from opening, so no pain to CNS. Think rock skipping on water.

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Nerve Fibers

Fastest

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Myelinated. Skin sensory (touch, pressure, temp). Muscle motor (contraction).

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Nerve Fibers

Fast

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Myelinated. Viscera sensory. Viscara motor ANS (ganglia) -->organs

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Nerve Fibers

Slowest

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Skin (temp, touch) and viscera sensory (pain). Visera motor ANS --> organs

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Excitatory Post Synaptic Potentials (EPSP)

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A depolarizing post-synaptic potential. A single EPSP normally doesn't initiate an impulse.

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Inhibitory Post-Synaptic Potentials (IPSP)

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A hyperpolarizing post-synapitc potential. Inhibitory

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Spatial Summation

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multiple pre-synaptic ends (knobs) simultaneously releasing the same neurotransmitter

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Temporal Summation

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A single pre-synaptic end releasing neurotransmitter in rapid succession (numerous firings)

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Neutotransmitters

Agonist

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enhances neurotransmission or mimics a neurotransmitter

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Neurotransmitters

Antagonist

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blocks or inhibits the action of a neurotransmitter.

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Neurotransmitters - Cholinergic

Acetylcholine (Ach) PNS/CNS

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1. excitatory. Somatic path. → Ach → skeletal muscle → contraction
2. inhibitory: ANS: parasym → Ach → G protein → heart muscle → slow HR

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Neurotransmitters - Catecholamine

Noripinephrine

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sympathetic release

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Neurotransmitters - Catecholamine

Epinephrine

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brain neurotransmitter
(Norepi/Epi: adrenal medulla release)

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Neurotransmitters - Catecholamine

Dopamine

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CNS: emotional (psycosis); skeletal muscle tone & coordination (Parkinson’s); prolactin; CNS: Basal Ganglia: substantia nigra; PNS: GI receptors: nausea, vomiting.

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Neurotransmitters - Amino Acids

Aspartate (Asp.)

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Spinal Cord. Affects similar to those of glutamate. Excitatory. CNS.

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Neurotransmiters

Glycine

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Inhibitory via opening C1 Channels

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Neurotransmitters

ATP/ADP/AMP

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excitatory neurotransmitters.