2 major parts--The CNS and PNS
CNS consists of brain and spinal cord, PNS of cranial and spinal nerves
PNS has 2 functional subdivisions:
Highly cellular; less than 20% extracellular space
2 main cell types: neurons and supporting cells
Supporting cells:
Account for firmness of nervous tissue; assist, segregate and insulate neurons, have additional special functions
6 types:
Neuroglial cells found in CNS, consist of 4 types:
Nonneuroglial cells are found in PNS.
Conduct messages (nerve impulses)
Other characteristics:
Cell body
large nucleus (perikaryon or soma) and nucleolus.
granular cytoplasm
biosynthetic center of cell. Lots of free ribosomes and rough ER (Nissl bodies or chromatophilic substance). Elaborate Golgi apparatus. Lots of mitochondria. Bundles of microtubules and neurofilaments for intracellular transport and structure.
Most located in CNS. Clusters in CNS are nuclei, clusters in PNS are ganglia.
Neuron processes
make up most of PNS. Bundles of processes are tracts in the CNS, nerves in the PNS.
2 types of processes: Dendrites and axons.
Use motor neurons as model.
Dendrites are short, heavily branched. Form receptive or input regions. Much surface area. Have dendritic spines in brain for contact with other neurons. Conduct electrical signals (graded potentials, not nerve impulses) toward cell body.
Single axon on each neuron.
Arises from axon hillock.
Uniform thickness through most of length.
Ranges from absent to most of neuron's length.
A long axon is called a nerve fiber.
Rare axon collaterals.
10K + telodendria, each with axonal terminal (aka synaptic knobs, boutons).
Axons the conducting component of neurons. Generate nerve impulses and carry them away from cell body.
Axonal terminal store neurotransmitters and are the secretory component of the neuron.
Axons have organelles except Nissl bodies. lots of conducting structures to move synthates from body to axon and terminals.
Plasma membrane of axon is the axolemma.
Larger nerve fibers have fat/protein-based myelin sheaths on axons. These conduct impulses 150X faster than unmyelinated fibers.
In PNS, myelin sheath formed of Schwann cells, oligodendrocytes in CNS.
Schwann cells good insulators, few transport channels/proteins. Neurilemma or sheath of Schwann is outermost layer of rolled cell.
Nodes of Ranvier between Schwann cells. Schwann cells associated with axons but not wrapped--unmyelinated fibers.
Oligodendrocytes not wrapped, have central body, processes that may wrap 60+ axons. No neurilemma.
White matter in CNS--myelinated fibers. Grey matter unmyelinated.
Structural
Based on number of processes off cell body.
Multipolar have 3+ processes. Most common type.
Bipolar have 1 axon, 1 dendrite from opposite sides. Rare.
Unipolar have one process that divides into two processes. common.
Functional
Grouped by direction of travel of nerve impulse relative to CNS.
Sensory or afferent neurons carry impulses from receptors in skin/organs to CNS. Most unipolar, bodies in ganglia.
Motor or efferent neruons carry impulses from CNS to effector organs. Multipolar.
Association neruons or interneruons are between motor and sensory. Multipolar. Most within CNS. Most common neurons (99%)
Voltage a measure of the potential energy between 2 opposite charges.
Potential measured in volts or millivolts (mV)
Current is movement of charge. In biological systems, this is via a flow of ions rather than electrons. Resistance is hindrance to flow.
Membranes of neurons and many muscle cells may develop a potential across the membranes.
Ion channels are associated with proteins in membranes. Passive or leakage channels are always open, active or gated channels can open in response to neurotransmitters (chemically-gated) or changes in membrane potential (voltage-gated).
Ions move in response to both electrical and chemical gradients = electrochemical gradient.
Resting membrane potential ca. -70mV (inside -). Membrane is polarized.
Less Na+ and more K+ inside cell than outside.
This condition due to
Changes in the potentials of membranes can serve as signals
2 types of these signals: graded potentials (short distances) and action potentials (long distances)
Depolarization is when membrane potential is reduced (inside becomes less negative)
Hyperpolarization is when membrane potential becomes more negative than resting potential
Graded potentials are short-term, local changes in potential.
Triggered by environmental stimulus (receptor potential) or neurotransmitters (post-synaptic potential) released across a synapse
Depolarization creates local current flow on both sides of membrane as charges adjust. Effect decreases with distance and with movement of ions through membrane (not part of flow).
Action potentials are the main way neurons communicate. Neurons and muscle cells have excitable membranes, allowing them to generate action potentials. In a neuron, a transmitted action potential is a nerve impulse. Only axons generate action potentials. Graded potentials activate action potential at (usually) the axon hillock.
Generation of action potential a 3-step process: (see fig 11.12)
Na/K pump will be required to restore original ionic distributions.
Propagation of action potential occurs when local ion flows on either side of membrane (not through) associated with developing action potential prompt depolarization of adjacent area, in much the same way that a graded potential stimulates action potential. Series of action potentials one after another travel down axolemma.
Action potential an all-or-none phenomenon. Dependent on amount of current crossing membrane to reach threshold stimulus.
Absolute and relative refractory periods following action potential.
Conduction velocity dependent on diameter of axon (larger=faster) and myelin sheath. Sheath is insulator, so current can only cross axolemma at Nodes of Ranvier, and there is no dissipation of local depolarizing ion flow due to leakage between nodes. Saltatory conduction.
Synapses
Most between neurons: axodendritic or axosomatic connections.
Also occur between axons, between dendrites, and between dendrites and cell bodies.
Neuron that conducts signal toward synapse is presynaptic, away form is postsynaptic.
Synapes can be chemical or electrical in nature.
Electrical synapses have protein channels connecting cytoplasm of adjacent neurons. Ion flow. Rapid transmission of signals. Common type of synapse in embryonic tissue, uncommon in adults, except in cardiac/smooth muscles where integration of action of cells necessary
Chemical synapses have 2 parts:
These are separated by synaptic cleft.
Unidirectional communication
Steps of signal transfer across synapse:
Greater # vesicles = greater effort at effector
Mechanisms for termination of neurotransmitter effects:
Probably more than 100 neurotransmitters.
Chemical classification:
Functional classification:
Strength of stimulus governed by frequency of impulse
