Physio Midterm II

One nerve cell, includes dendrites, cell body (soma), axon to conduct nerve impulses

Axons, peripheral and central processes

A bundle of nerve fibers

A collection of neuron cell bodies located outside the CNS

Central Nervous System

Peripheral Nervous System

Clusters of neuron cell bodies in CNS

Neuron cell bodies

myelinated nerve fibers, form tracts in CNS

Interconnecting circuits of neurons

Brain and Spinal Cord

Nerves and Ganglia

• Somatic: supplies motor impulses to skeletal muscles (voluntary)
• Autonomic (visceral) system: supplies motor impulses to cardiac, smooth muscle, glands (involuntary)

Sympathetic and Parasympathetic-dual innervation of effectors.

• General (body wide) and special (localized)
• Somatic (body wall, cutaneous) and autonomic (visceral)
• Visceral-pertaining to internal organs
• Somatic-pertaining to the body's framework-skin, muscle, tendon, joints

• Do not conduct impulses, are supporting cells.
• Serve to nourish and protect neurons.
• Some form blood-brain barrier with capillary cell walls.

• Form myelin sheath around axons in PNS
• Myelin acts as insulator and speeds nerve impulse conduction
• Breaks between Schwann cells are Nodes of Ranvier-where ions can cross membranes in APs

Transmit information from receptors to CNS

Connecting, located entirely in CNS

Transmit info from CNS to effector cells

Dendrites and soma

Axon, peripheral process (nerve impulse-a propagated action potential)

Neurotransmitters cross a synapse; may include signal transduction

Sensory recpetors

• A voltage difference across a cell membrane
• The adjectives membrane, resting, graded, action, and post synaptic all define conditions under which a potential is measured or the way it develops in cells; all are used tp define potentials

• Another series of terms is used to describe the direction of membrane potential changes relative to the resting state (-70mV)
• -Hyperpolarize-inside of cell is more negative than usual
• -Depolarize-positive charge is added to inside of cell, inside of cell becomes positive
• -Repolarize-membrane potential returns to resting value after a change

Ion channels are gated 3 ways: voltage (electrical), chemical (receptor), mechanically (stretch)

• Neurons are polarized, Na outside, K and protein inside, inside is negative (-70mV)
• (Neurons are polarized, can be depolarized, repolarized, hyperpolarized)
• All cells have resting membrane potential, only muscle and nerve cells are excitable:can change membrane potential in response to a stimulus

• An action potential is a transient, all or none reversal of membrane polarity
• They constitute an electrical signal which can be propagated by nerve and muscle cells.
• APs are initiated by graded potentials. If the incoming stimulus is strong enough (reaches threshold)voltage gated Na channels will open-membrane becomes highly permeable to Na

• Are opened by Na from graded potential, as more Na enters cell, more channels open=a positive feedback cycle which causes the cell to depolarize (+50 mV)
• This localized reversal of membrane polarity is an action potential.
• The voltage gated channels are found initially at axon hillock, and all along axon.

• Along a neuron
• Action potentials=brief reversal of membrane polarity in one place; how is message conducted?

• One AP results in Na entry, which is the trigger for voltage gated channels on adjacent membrane to open
• A series of APs os omitiated down the axon membrane=propagation
• The propagated action potential is called a nerve impulse

• time during which neuron cannot respond to a second stimulus.
• It is due to nature of gated channel, ensures one way conduction of impulses

• If threshold is reached, an AP is stimulated
• APs are all the same size=all or none

• AP jumps from one node to next in myelinated fibers
• Increases speed of conduction

• The information that is transmitted is in the frequency of APs generated
• APs are frequency modulated; graded potentials are amplitude modulated

• 1. occur only in excitable membranes (they are only ones with voltage gated channels)
• 2. requires a minimal (threshold) potential for initiation
• 3. upstroke of AP is result of a positive feedback loop, is explosive, self-perpetuating
• 4. APs are all-or-none phenomena; amplitude of response is independent of stimulus intensity
• 5. APs are propagated without a decline in amplitude
• 6. Refractory period- membrane has diminished excitability immediately after an AP; this prevents fushion or impulses, permits propagation of discrete impulses
• 7. APs rapidly conduct information over long distances

• A stimulus is a physical, chemical or electrical event that alters neuron membrane permeability, makes neuron permeable to ions which diffuse in (depolarization) or out (hyperpolarization)
• The event that alters the membrane permeability is gating of an ion channel (stretch, chemical, voltage)
• This event occurs in sensory receptors, dendrites, soma
• The change in membrane permeability is a graded potential

• Changes in membrane potential confined to a small region of the membrane.
• They are graded in magnitude, the size of the graded potential reflects size of stimulus
• Gated channels open to initiate GPs, the resulting current flow spreads passively
• GPs are important in signaling over short distances

Generated by sensory stimuli

Generated in dendrites and soma by neurotransmitters; can be excitatory (EPSP); depolaraization or inhibitory (IPSP); hyperpolarization

Spontaneous oscillations in membrane potentials that trigger APs (heart)

repeated stimuli over time=larger graded potentials

GPs initiated at many sites in a neuron=larger graded potential

• Initiate info flow in sensory cells
• Conduct info from dendrites to axons
• Represent the entire information transfer in neurons with very short axons (CNS)
• Initiate action potentials in cardiac muscle cells

Conduction across a synapse

• Region of communication between two neurons
• Consists of synaptic knob, synaptic cleft, postsynaptic membrane

• Ions pass through gap junction between cells
• Useful to synchronize electrical events rapidly, found in heart and smooth muscle

A chemical called a neurotransmitter is released from presynaptic neuron, influences postsynaptic neuron, initiates a GP via chemical gating of ion channels

• Propagated APs open Ca voltage gated channels at axon terminals
• Ca enters axon and effects exocytotic release of neurotransmitter from synaptic vesicles
• Release is quantal-number of vesicles released relates to frequency of APs

• Neurotransmitters bind to specific receptors in post-synaptic membranes; there are 2 possible effects:
• 1. the binding gates (opens or closes) an ion channel which is an integral part of receptor protein, ions move driven by concentration gradients, either into or out of cells
• 2. the binding of NT to receptor initiates a complex mechanism called signal transduction; the off switch is removal of neurotransmitter molecules-by enzymatic degradation or reuptake

• Can be excitatory or inhibitory
• Depends on nature of ion channel, not neurotransmitter
• EPSP-excitatory= Na/K channels open, Na ions will enter cell and depolarize it
• IPSP-inhibitory=Cl or K channels will hyperpolarize cell (Cl- flows in, K+ flows out)
• A particular synapse is always either excitatory or inhibitory
• Each presynaptic neuron makes only one kind of neurotransmitter

• Chemicals released from presynaptic membrane
• They bind to postsynaptic membrane receptors and usually gate ion channels
• Quickly inactivated by enzymes or removed by presynaptic reuptake pumps
• Acetylcholine and norepinephrine are usual PNS neurotransmitters
• Others in CNS include dopamine, seratonin, endorphins, GABA, glutamate, glycine

• Block Na voltage gated channels in axons-tetrodotoxin
• Stimulate release of NT-amphetamine, black widow venom
• Prevent NT release-botulism toxin prevents ACH release, tetanus toxin blocks release of inhib NT
• Block enzymatic degradation-malathion, sarin, flea collar, MAO inhibitors

• An automatic, involuntary repsonse to a stimulus; it is mediated by a reflex arc
• Reflexes include knee jerks, withdrawal, homeostatic adjustments, postural adjustments
• (The reflex is the observed action; the reflex arc is the wiring that accomplishes the reflex)

• A conduction pathway, the most basic neural pathway
• The neuron is the structural unit of the NS, the reflex arc is the functional unit

• Sensor, sensory neuron, interneuron, motor neuron, effector
• Note that in some relfex arcs there is no interneuron, in some there are more than one
• Information flow has a specific directionality in reflex arcs
• This is ensured by refractory period in axons and postsynaptic receptors in synapses