FUNCTIONS
Sensing changes (INPUT) Processing information (INTEGRATING) Responding (OUTPUT) SEAT OF ALL MENTAL ACTIVITY
• CONSCIOUSNESS• MEMORY• THINKING
Central Nervous System
Involves the brain and spinal cord Part of the dorsal body cavity Contains gray and white matter, and
has fluid filled spaces
Peripheral Nervous System
Cranial and Spinal nerves Afferent Nervous System - input Efferent Nervous System - output
•somatic (voluntary) NS• autonomic (involuntary) NS
•sympathetic and parasympathetic divisions
Neuroglial Cells Found in CNS
•Astrocytes•Star-shaped; most
abundant; anchor to blood vessels
•Microglia – •Protect
Neuroglial Cells Found in CNS
•Oligodendrocytes•Produce insulating
myelin sheath in the CNS
•Ependymal •Line cavities•Circulate
cerebrospinal fluid
Neuroglial Cells
Found in PNS•Schwann cell
•Form myelin sheath in PNS cells
•Satellite cell•Surround cell•Function relatively unknown
Neurons by structure
Pseudounipolar (unipolar) – one process
Bipolar – one dendrite & one axon Multipolar
•3 or more processes; most common
•1 axon and 2 or more dendrites
Neurons by function
Afferent neurons - sensory neurons Efferent neurons - motor or secretory
neurons Association neurons - interneurons or
intermediate neurons
Neurons by the area served
Visceral neurons •serving the internal organs•visceral afferent, visceral efferent
Somatic neurons •serving the body wall•somatic afferent, somatic efferent
Neuron Characteristics
Excitable Conductive Transmits
information Cannot reproduce
Regeneration • cell body intact• presence of
neurolemma sheath• regeneration tunnel
aligned• very, little scar
tissue
Nerves
Bundles of nerve cell processes in the PNS
Link the PNS to the CNS Sensory, motor, or mixed Connective tissue
• epineurium, perineurium, endoneurium Tract - bundles of nerve fibers in the
CNS
White / Gray Matter
White matter contains myelinated processes• PNS - myelinated nerves• CNS - myelinated tracts
Gray matter- consists of cell bodies and unmyelinated processes• Nuclei- collection of cell bodies (CNS)• Ganglia - cell bodies outside CNS
Neurophysiology
Basic electrical principles• Voltage – measures potential difference
between to charges• Resistance – hindrance to charge flow• Insulators – resist charge flow• Conductors – allow charge flow• Ohm's law: Current = Voltage/Resistance• Ions – charged atoms flow across membranes
Ion channels
Passive channels – always open Active (gated) channels.
• Chemically-gates channels – open when combined with appropriate neurotransmitter
• Voltage-gates channels – open in response to changes in membrane potential
Establishing the resting membrane potential.
The neuron membrane is positively charged on the outside and negativelycharged on the inside.
Inequality of charged particles occurs only at membrane. The total numberof positive and negative ions and molecules inside and outside the cell are equal.
Distribution of charged particles
Sodium ions (Na+) Potassium ions (K+) Chloride ions (Cl-) Negatively charged
proteins (A-)
Factors that help establish the RMP.
Differences in the resting permeability of the plasma membrane to Na+ and K+ ions –
ATPase, Na+ ion/K+ ion pump
Impermeability of the plasma membrane to proteins –
Membrane potentials that act as signals
Communication in neurons and muscle cells involves changing the membrane potential.
Factors that change membrane potential.
Changing the permeability of the plasma membrane to any ion.
Changing the concentration of ions across the plasma membrane.
Depolarization - membrane potential decreases
Hyperpolarization – membrane potential increases
Graded membrane potential.
Short-lived depolarization or hyperpolarization of the plasma membrane.
Caused by opening of gated ion channels in the plasma membrane.
Magnitude of the change in potential is directly related to intensity of the stimulus.
Action potential
Rapid reversal of membrane potential.
Occurs only in neurons and muscle cells.
Also called a nerve impulse in a neuron.
Only axons can generate an action potential.
Steps in generating an action potential.
Resting membrane potential. • Voltage-gated Na+ channels closed. • Voltage-gated K+ channels closed.
Depolarization. • Opening voltage-gated Na+ channel in the
axon. • Influx of Na+ ions results in depolarization of
the axonal membrane. • Closing of Na+ channels stops the influx of
Na+ ions.
Repolarization.
• Open voltage-gated K+ channel in the axon.
• Efflux of K+ ions results in repolarization of the axonal membrane.
• Membrane potential moves back to RMP.
• Na+/K+ ATP pump helps re-establish the Na+ and K+ ion concentrations inside and outside the neuron.
Propagation of the action potential.
An action potential travels away from its point of origin.
An action potential is self-propagating. (domino effect)• Changes in membrane potential in one
section stimulates depolarization in next section of the membrane
Threshold and the all-or-none phenomenon -
All action potentials are the same regardless of the strength of stimulus.
Strong stimuli lead to more action potentials during a time frame.
Weak stimuli lead to fewer action potentials during a time frame.
Refractory period.
Absolute refractory period - • Time period in which no impulse can
be generated Relative refractory period –
• Time period in which only a strong stimulus will generate an impulse
Conduction velocities of axons -
Speeds up to 100 m/s or more Influenced by
• Axon diameter: larger = faster• Myelinated versus unmyelinated axons
•Saltatory Conduction = impulse leaps from one node of Ranvier to the next
Synapse = junctions between neurons
Axodendritic – axon to dendrite Axosomatic - axon to cell body Axoaxonic - axon to axon Dendrodendritic – dendrite to dendrite Presynaptic neurons – conducts to the
synapse Postsynaptic neuron – conducts away
from the synapse
Synapses
Electrical synapse - direct connections allow current to flow
from one cell to the next
Chemical synapse – use chemical neurotransmitters to
conduct impulses across the synapse
Parts of a chemical synapse
Presynaptic neuron's axon terminal contains synaptic vesicles containing a neurotransmitter.
Synaptic clefts - Fluid-filled space between the pre- and postsynaptic neurons.
Postsynaptic neuron has receptors for neurotransmitters released from the synaptic vesicle.
Information flow across a chemical synapse.
Action potential opens calcium channels in the presynaptic membrane.
Synaptic vesicles fuse with the membrane of the axon terminal and neurotransmitter is released into the synaptic cleft.
Neurotransmitter binds with receptors on the postsynaptic membrane.
Ions channels open leading to the depolarization or hyperpolarization of the postsynaptic membrane.
Termination of the neurotransmitter effect.
Neurotransmitter degraded by an enzyme.
Neurotransmitter taken up by the presynaptic terminal.
Diffusion of the neurotransmitter from the synaptic cleft.
Postsynaptic potentials. Excitatory postsynaptic potential (EPSP).
• Causes depolarization of postsynaptic
membrane.• Opens channels allowing Na+ and K+ ions to
cross the membrane. Inhibitory postsynaptic potential (IPSP).
• Causes hyperpolarization of postsynaptic membrane.
• Opens K+ or chloride channels allowing one of both of these ions to cross the membrane.
Modification of synaptic events.
Summation = effects add up • Temporal summation – rapid
stimulation• Spatial summation – stimulation from
mutiple presynaptic terminals Synaptic potentials – continued use
increases ability to excite the postsynaptic membrane
Neurotransmitters - Acetylcholine - Biogenic amines.
•Catecholamines.•Dopamine -•Norepinephrine -•Epinephrine -•Indolamines.•Serotonin -•Histamine -
Neurotransmitters Amino acids.
• Gamma amino butyric acid -
• Glutamate -• Glycine -• Aspartate -
Peptides.• Substance P -• Endorphins -• Enkephalins -
Novel messengers • ATP• NO• CO
Neurotransmitters by function.
Excitatory - cause depolarization
Inhibitory – cause hyperpolarization
Levels of Sensation
Sensation - is the arrival of a sensory impulse to the brain
Perception - is the interpretation of the sensation
Sensory Receptors
Simple receptors - General senses Complex receptors - Special senses Selectivity Types by location
• Exteroceptors - outside• Interoceptors – visceral (more general)• Proprioceptors – musculoskeletal (more
specific
Receptors classified by stimulus
Mechanoreceptor – touch, pressure, vibration, etc.
Photoreceptor - light Thermoceptor - temperature Chemoreceptor - chemicals Nociceptor – damage / pain
Cutaneous Sensation
Tactile sensations • touch • pressure• vibration• cold, heat• pain
Crude / Discriminative Touch
Types of Tactile Receptors
Meissner’s corpuscles – light touch Hair root plexuses – light touch Merkel discs – light touch Pacinian corpuscles – deep pressure Itch/tickle Thermoreceptors - heat Nociceptors - pain