Fundamentals of the Nervous System and Nervous Tissue
• master controlling & communicating system of body
• Functions:
Sensory input:
Integration:
Motor output:
Organization of the Nervous System
• Central nervous system (CNS)
• Brain and spinal cord
• Integration and command center
• Peripheral nervous system (PNS)
• Paired spinal and cranial nerves
• Carries messages to and from the spinal cord and brain
Neurons and support cells
Histology of Nerve Tissue
• Two principal cell types of nervous system :
• _____________________– excitable cells that transmit electrical signals
• __________________________________ – cells that surround and wrap neurons
Supporting Cells: (neuroglia or glia):
• ____________________________________________ scaffolding for neurons
• _____________________and__________________________ neurons
• ________________________ young neurons to the proper connections
• Promote __________________and________________________
Astrocytes
• Most abundant, versatile, and highly branched glial cells
• cling to neurons ; cover capillaries
• Function:
• Support and brace neurons
• Anchor neurons to their nutrient supplies
• Guide migration of young neurons
• Control the chemical environment (blood-brain barrier)
Microglia
• small, ovoid cells with spiny processes
• Phagocytes- monitor health of neurons
Ependymal Cells
• squamous- to columnar-shaped cells
• line central cavities of brain and spinal column
Oligodendrocytes: branched cells that wrap CNS nerve fibers
Schwann Cells: (neurolemmocytes) – surround fibers of the PNS
Satellite Cells: surround neuron cell bodies with ganglia
Neurons (Nerve Cells)
Neurons (Nerve Cells)
• Structural units of nervous system
• Composed of: _________________________, ______________________ +
____________________
• Long-lived
• Amitotic (
)
• high metabolic rate
• plasma membrane functions in:
• Electrical signaling
• Cell-to-cell signaling during development
Neuron Classification
Structural Classification of Neurons
 Multipolar neurons – many extensions from the cell body
 Bipolar neurons – one axon and one dendrite
 Unipolar neurons – have a short single process leaving the cell body

Nerve Cell Body (Soma)
• Contains: _________________________________________________
• Major biosynthetic center
• Focal point for the outgrowth of neuronal processes
• NO centrioles (does NOT divide)
• Well developed Nissl bodies (rough ER)
• Axon hillock –
Processes
• Armlike extensions from the soma
• Called _____________________ in CNS and _______________________ in PNS
• Two types: ___________________________ + ________________________________
Dendrites of Motor Neurons
• Short, tapering, diffusely branched processes
• receptive, or input, regions of the neuron
• Electrical signals conveyed as graded potentials (not action potentials)
Axons: Structure
• Slender processes of uniform diameter arising from the hillock
• Long axons = nerve fibers
• Usually only one unbranched axon per neuron
• Axonal terminal – branched terminus(end) of an axon
Axons: Function
• Generate and transmit action potentials
• Secrete neurotransmitters from the axonal terminals
Myelin Sheath
• Whitish, fatty (lipoprotein), segmented sheath around most long axons
• functions :
• ________________________________ of the axon
• Electrically __________________________ fibers from one another
• ___________________________________________ of nerve impulse transmission
• Formed by Schwann cells in the PNS
• Schwann cell:
• Envelopes an axon in a trough
• Encloses the axon with its plasma membrane
• Concentric layers of membrane make up the myelin sheath
• Neurilemma – remaining nucleus and cytoplasm of a Schwann cell
Nodes of Ranvier (Neurofibral Nodes
• Gaps in myelin sheath between adjacent Schwann cells
• sites where collaterals can emerge
Unmyelinated Axons
• Schwann cell surrounds nerve fibers but coiling does not take place
• Schwann cells partially enclose 15 or more axons
Axons of the CNS
• myelinated and unmyelinated fibers present
• Myelin sheaths formed by oligodendrocytes
• Nodes of Ranvier widely spaced
• no neurilemma
Regions of the Brain and Spinal Cord
• _________________ matter – dense collections of myelinated fibers
• _________________ matter – mostly soma and unmyelinated fibers
Fundamentals of the Nervous System and Nervous Tissue
Role of Ion Channels
• Types:
• Passive, or leakage, channels –
• Chemically gated channels • Voltage-gated channels –
Operation of a Chemically Gated Channel
Operation of a Voltage-Gated Channel
When gated channels are open:
• Ions move quickly across the membrane
• Movement is along their electrochemical gradients
• An electrical current is created
• Voltage changes across the membrane
Electrochemical Gradient
• Ions flow along chemical gradient when they move from area of high concentration to area
of low concentration
• Ions flow along electrical gradient when they move toward area of opposite charge
• Electrochemical gradient – the electrical and chemical gradients taken together
•
Resting Membrane Potential (Vr)
potential difference (–70 mV) across the membrane of a resting neuron
generated by different concentrations of Na+, K+, Cl, and protein anions (Ax)
Ionic differences are the consequence of:
• Differential permeability of the neurilemma to Na+ and K+
• Operation of the sodium-potassium pump
Membrane Potentials: Signals
• Used to integrate, send, and receive information
• Membrane potential changes are produced by:
• Changes in membrane permeability to ions
• Alterations of ion concentrations across the membrane
• Types of signals:
– graded potentials
– action potentials
Changes in Membrane Potential
• Caused by three events:
• Depolarization – the inside of the membrane becomes less negative
• Repolarization – the membrane returns to its resting membrane potential
• Hyperpolarization –the inside of the membrane becomes more negative than the
resting potential
Graded Potentials
• Graded potentials:
• short-lived, local changes in membrane potential
• Decrease in intensity with distance
• Can only travel over short distances
• magnitude varies directly with the strength of the stimulus
• Sufficiently strong graded potentials can initiate action potentials
Action Potentials
• brief reversal of membrane potential with a total amplitude of 100 mV
• Action potentials only generated by ___________________________________ and
___________________
• do not decrease in strength over distance
• principal means of neural communication
• action potential in axon of neuron: nerve impulse
•
•
•
Action Potential: Resting State
Action Potential: Depolarization Phase
Action Potential: Repolarization Phase
Action Potential: Undershoot
Phases of the Action Potential
•
1 – resting state
•
2 – depolarization phase
•
3 – repolarization phase
•
4 – hyperpolarization
Conduction Velocities of Axons
• Conduction velocities vary widely among neurons
• Rate of impulse propagation determined by:
• Axon diameter –larger diameter, faster impulse
• Presence of myelin sheath – myelination dramatically increases impulse speed
Saltatory Conduction (sauter = “to jump (Fr.)”
Current passes through a myelinated axon only at nodes of Ranvier
Voltage regulated Na+ channels concentrated at nodes
Action potentials triggered only at nodes; jump from one node to the next
Much faster than conduction along unmyelinated axons
Synapses
• Junction that mediates information transfer from one neuron:
• To another neuron
• To an effector cell
• Presynaptic neuron – conducts impulses toward the synapse
• Postsynaptic neuron – transmits impulses away from the synapse
Electrical synapses:
• far less common than chemical synapses
• Correspond to gap junctions found in other cell types
• Contain intercellular protein channels
• Permit ion flow from one neuron to the next
• BI-directional !!!
• found in brain; abundant in embryonic tissue
Chemical Synapses
-Specialized for the release and reception of neurotransmitters
-Typically composed of two parts:
• Axonal terminal of the presynaptic neuron; contains synaptic vesicles
• Receptor region on the dendrite(s) or soma of the postsynaptic neuron
Synaptic Cleft
• Fluid-filled space separating the presynaptic and postsynaptic neurons
• Prevent nerve impulses from directly passing from one neuron to next as in an electrical synapse
• Transmission across the synaptic cleft:
• chemical event (as opposed to an electrical one)
• Ensures unidirectional communication between neurons
Synaptic Cleft: Information Transfer
• Nerve impulse reaches axonal terminal of the presynaptic neuron
• Neurotransmitter released into the synaptic cleft
• Neurotransmitter crosses synaptic cleft; binds to receptors on postsynaptic neuron
• Postsynaptic membrane permeability to ions changes,
excitatory or inhibitory effect
Termination of Neurotransmitter Effects
• Neurotransmitter bound to a postsynaptic neuron:
• Produces a continuous postsynaptic effect
• Blocks reception of additional “messages”
• Must eventually be removed from receptor
• Removal of neurotransmitters occurs when they:
• Are degraded by enzymes
• Are reabsorbed by astrocytes or the presynaptic terminals
• Diffuse from the synaptic cleft
Synaptic Delay
• Neurotransmitter must be released, diffuse across the synapse, and bind to receptor
• Synaptic delay – time needed to do this (0.3-5.0 ms)
• Synaptic delay is the rate-limiting step of neural transmission
Neurotransmitters
• Chemicals for neuronal communication with body and brain
• 50 different neurotransmitter identified
• Classified chemically and functionally
Functional Classification of Neurotransmitters
-Two classifications: excitatory and inhibitory
*Excitatory neurotransmitters cause depolarizations (glutamate)
*Inhibitory neurotransmitters cause hyperpolarizations (GABA and glycine)
-Some neurotransmitters have both excitatory and inhibitory effects
Determined by the receptor type of the postsynaptic neuron
Example: aceytylcholine
Excitatory at neuromuscular junctions
Inhibitory with cardiac muscle