The master controlling and communicating system of the body
Functions Sensory input – monitor
internal and external stimuli
Integration – interpretation of sensory input
Motor output – response to stimuli by activating effector organs
Central nervous system (CNS) Consists of Brain and spinal cordControls entire organism Integrates incoming information and responses
Peripheral nervous system (PNS)Link between CNS, body and environment Consists of Spinal and cranial nervesCarries messages to and from the spinal cord and brain
Sensory (afferent) divisionSensory afferent fibers –
carry impulses from skin, skeletal muscles, and joints (sensory receptors) to the brain
Visceral afferent fibers – transmit impulses from visceral organs to the brain
Motor (efferent) division Transmits impulses from the
CNS to effector organs (muscles, glands)
Two Divisions:
Somatic nervous system (Voluntary)Conscious control of skeletal
musclesConducts impulses from the
CNS to skeletal muscles
Autonomic nervous system (ANS)Regulates smooth muscle,
cardiac muscle, and glandsSubconscious or involuntary
control
Sympathetic Nervous System “Flight or fright system” Most active during physical activity
Parasympathetic nervous systemRegulates resting or vegetative functions such
as digesting food or emptying of the urinary bladder
The two principal cell types of the nervous system are:Neurons – excitable cells that transmit electrical signals
Supporting cells – cells that surround and wrap neurons
Structural units of the nervous system
Receive stimuli and transmit action potentials
Long-life
Amitotic
Have a high metabolic rate
Each neuron consists of:BodyAxondendrites
Cell Body (soma or perikaryon)
Contains the nucleus and a nucleolus & usual organelles
Has no centrioles (amitotic nature)
Has well-developed Nissl bodies (rough ER)
Nissl bodies -primary site of protein synthesis
Contains an axon hillock – cone-shaped area from which axons arise
Short, branched cytoplasmic extensions
They are the receptive, or input, regions of the neuron
Electrical signals are conveyed toward the cell body
Slender processes of uniform diameter arising from the axon hillock
Initial segment: beginning of axon
Axoplasm : Cytoplasm of axon
Axolemma : Plasma membrane of axon
Long axons are called nerve fibers
Usually there is only one unbranched axon per neuron
Rare branches, if present, are called axon collaterals
Presynaptic (Axon) terminal – branched terminus of an axon
Trigger zone: site where action potentials are generated; axon hillock and part of axon nearest to cell body
AP are conducted along the axons to axonal terminals and release neurotransmitters
AP conduction away from cell body
Neurons can be classified by structure: Multipolar Most common in both CNS & PNS Single axon, many dendrites (motor
neurons and interneurons of CNS) Bipolar two processes (one axon and one
dendrite) Are sensory neurons found in the
retina, olfactory nerve Unipolar single short process extending from
cell body Divides into two branches and
functions as both dendrite and axon (sensory neurons , dorsal root ganglia)
Neurons can be classified by function:Sensory (afferent) — transmit impulses from
receptors toward the CNS
Motor (efferent) — carry impulses from CNS to muscles and glands
Interneurons (association neurons) Link sensory and motor neurons within CNS Make up 99% of neurons in body
Neuroglia or glial cells: Supporting cells Surround neurons Mitotic Nonconducting 6 types
2 in PNS4 in CNS
1) Astrocytes In CNS only
Anchor neurons to capillaries
Regulate what substances reach the CNS from the blood (blood-brain barrier)
Regulate extracellular brain fluid composition
Pick up excess K+
Recapture released (recycle) neurotransmitters
2) Ependymal CellsCNS only
Line the cavities of the brain and spinal cord
Ciliated
Circulate the cerebrospinal fluid (CSF)
3) MicrogliaCNS only
Migrate toward injured neurons
Specialized macrophages
Phagocytize necrotic tissue, microorganisms, and foreign substances that invade the CNS
4) OligodendrocytesCNS only
Wrap extensions around neuron fibers (axons)
Form myelin sheath
1) Schwann Cells or NeurolemmocytesPNS only
Wrap around axons of neurons in the PNS
Forms myelin sheath
2) Satellite Cells PNS only
Surround neuron cell bodies
Provide support and nutrients to neuronal cell bodies
Protect neurons from heavy metal poisons (lead, mercury) by absorbing them
Myelinated Axons: Whitish, fatty (protein-lipid), segmented sheath around most long axons
Functions: Protect the axon Electrically insulate fibers from one another Increase the speed of nerve impulse
transmission
Formed by Schwann cells in the PNS
In CNS formed by oligodendrocytes
Nodes of Ranvier : Gaps in the myelin sheath between adjacent Schwann cells
Unmyelinated Axons : Schwann cell surrounds nerve fibers but coiling does not take place
White matter – dense collections of myelinated fibers
Gray matter – mostly nerve cell bodies and unmyelinated fibers
In brain: gray is outer cortex as well as inner nuclei; white is deeper
In spinal cord: white is outer, gray is deeper
Synapse Junction between one neuron and
another
Where two cells communicate with each other
Presynaptic neuron – conducts impulses toward the synapse
Postsynaptic neuron – Cell that receive the impulse
Most are axodendritic or axosomatic
Electrical Synapses:Are gap junctions that allow
ion flow between adjacent cells by protein channels called Connexons
Not common in CNS
Found in cardiac muscle and many types of smooth muscle Action potential of one cell causes action potential in next cell
Chemical SynapsesMost are this type
Neurotransmitter released from synaptic vesicles of presynaptic neuron
Neurotransmitter binds to receptors on postsynaptic membrane
Binding of neurotransmitter to receptor permeability change in postsynaptic membrane
Released at chemical synapses
In response to AP Voltage-regulated calcium channels open
Ca2+ diffuse into presynaptic terminal
And causes synaptic vesicles to fuse with presynaptic membrane
This fusion releases neurotransmitter into the synaptic cleft via exocytosis
When bound to receptors on postsynaptic neuron, the neurotransmitter can either excite or inhibit the postsynaptic neuron
Resting neurons maintain a difference in electrical charge inside and outside cell membrane = RESTING MEMBRANE POTENTIAL (RMP)
The inside of the resting neuron is negatively charged, the outside is positively charged.
Concentration of K+ higher inside than outside cell
Na+ higher outside than inside
RMPs vary from -40 to -90mV in different neuron types
When bound to receptors on the postsynpatic neuron membrane: Causes the opening of positive
ion channels
Sodium ions enter rapidly
RMP becomes more positive
This positive change in the RMP is called depolarization
This brings the neuron closer to firing
• A positive change in the RMP– Caused by influx of
positive ions
– Causes the inside of the cell membrane to become less negative
– Depolarization spreads to adjacent areas
When bound to receptors on the postsynaptic membrane: Make the membrane more permeable
to negative ions (usually Cl-)
As negative ions rush into the neuron, the RMP becomes more negative
The negative change in the RMP = hyperpolarization
Brings the neuron farther from firing
• A negative change in RMP
• Usually caused by influx of chloride ions
• Decreases the likelihood of the neuron firing
• Short changes in the RMP in small regions of the membrane
• Can be positive or negative (depolarize or hyperpolarize the membrane)
• Alone, not strong enough to initiate an impulse
• summate or add onto each other
• Together, can trigger a nerve impulse (action potential)
EPSP (Excitatory Postsynaptic Potential) When depolarization occurs, response is
stimulatory & graded potential is called EPSP
Binding of a neurotransmitter on the postsynaptic membrane more positive RMP, reaches threshold (depolarization occurs)
producing an action potential and cell response
IPSP (Inhibitory Postsynaptic Potential) When hyperpolarization occurs,
response is inhibitory & graded potential is called IPSP
Binding of the neurotransmitter on the postsynaptic membrane more negative RMP (hyperpolarization)
Decrease action potentials by moving membrane potential farther from threshold
40 to 50 Known NeurotransmittersAcetylcholine (ACh) Norepinephrine (NE)GABADopamineSerotonin
Action Potential = Nerve Impulse
Consists of: Depolarization Propagation Repolarization
If depolarization reaches threshold (usually a positive change of 15 to 20 mV or more), an action potential is triggered
The positive RMP change causes electrical gates in the axon hillock to open
Sudden large influx of sodium ions causes a reversal in the membrane potential (becomes approx. 100mV more positive)
Begins at the axon hillock and travels down the axon
Chemically gated channels – open with binding of a specific neurotransmitter
Voltage-gated channels – open and close in response to membrane potential
Chemically Gated(on dendrite or soma)
Voltage Gated(on axon hillock and axon)
Movement of the action potential down the axolemma
voltage-gated sodium channels open in response to positive RMP change
Restoration of the RMP back to it’s negative state
A repolarization wave follows the depolarization wave
3 factors contribute to restoring the negative RMP: Sodium (Na+) gates close (it no longer
enters) Potassium (K+) gates open, potassium
rushes out Sodium/potassium pump kicks in
An active process: requires cellular energy
Actively pumps 3 sodium (Na+)
ions out of the cell and 2 potassium (K+) ions in
Potassium leaks back out
Period of time when electrical sodium gates are open
From beginning of action potential until near end of repolarization
No matter how large the stimulus, a second action potential cannot be produced
The interval following the absolute refractory period when:
Sodium gates are closedPotassium gates are openRepolarization is occurring
A stronger-than-threshold stimulus can initiate another action potential
A single EPSP cannot induce an action potential
EPSP’s can add together or SUMMATE to initiate an action potential
Spatial Summation Large numbers of axon
terminals stimulate the postsynaptic neurons simultaneously
Temporal SummationOne or more presynaptic neurons transmit impulses in rapid fire succession
An action potential is an “all or none” phenomenon
When threshold is reached, the action potential will occur completely
If threshold is not reached, the action potential will not occur at all
Occurs only in myelinated axons
Depolarization wave jumps from one node of Ranvier to the next
Results in faster nerve impulse transmission
A nerve impulse in the presynaptic neruon causes release of neurotransmitter into synaptic cleft
Neurotransmitter binding to receptors on postsynaptic neuron dendrite or soma cause certain chemically gated ion channels to open
If Na+ channels open: Rapid influx of Na+ ions (depolarization) A small positive graded potential occurs (EPSP) If RMP changes in a positive direction by 20mV (or reaches the
threshold), voltage gated sodium channels in the axon hillock open Sodium rushes in at the axon hillock resulting in an action potential As the positive ions get pushed down the axon, more voltage gated
sodium channels open and the depolarization continues down the axon (propagation)
The process of restoring the negative RMP begins immediately following the depolarization wave (repolarization)
The larger the axon diameter, the faster the impulse travels
Myelinated axons conduct impulses more rapidly
Fiber Types: Type A fibers
Large diameter axon with thick myelin sheath
Impulse travels at 15 to 150 m/sec. Sensory and motor fibers serving skin,
muscles, joints Type B fibers
Intermediate diameter axon, lightly myelinated
Impulse travels at 3 to 15 m/sec, Part of ANS
Type C fibers Small axon diameter, unmyelinated Slow impulse conduction (1 m/sec. or less) Part of ANS
Organization of neurons in CNS varies in complexity Convergent pathways: many
converge and synapse with smaller number of neurons. E.g., synthesis of data in brain
Divergent pathways: small number of presynaptic neurons synapse with large number of postsynaptic neurons. E.g., important information can be transmitted to many parts of the brain
Oscillating circuit: outputs cause reciprocal activation