The Nervous System
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Transcript of The Nervous System
The Nervous System
Neurons - Chapter 7
The Nervous System
• Rapid Communication and Control
– Sensation • receives info. on environmental changes
– Integration
• interprets the changes, integrates signals from multiple signals
– Response
• induces action from of muscles or glands
Nervous System Organization:General Anatomy
• Central Nervous System (CNS)
– Brain + Spinal Cord
– control center (integration)
• Peripheral Nervous System (PNS) – cranial nerves and spinal nerves
– connects CNS to sensory receptors, muscles and glands
Cell Types
• Neurons – conduct electrical signals
• Neuroglia
– 80% of all NS cells– support neurons
Neurons• Cell Body
– nucleus and organelles• Dendrites
– receive information• Axon
– conduct electrical signals (action potentials)
– axon hillock - site where AP’s originate
– axon terminals - where chemical signals are released
Types of Neurons
• Sensory (Afferent) Neuron - input– part of the PNS
– transmit electrical signals from tissues and organs to CNS
• detect changes in environment
Types of Neurons• Motor (Efferent) neuron - output
– part of the PNS
– transmit signals from CNS to effector tissues (muscle, gland cells)
– somatic motor neurons • skeletal muscle contraction
• both voluntary and reflexive
– autonomic motor neurons• smooth muscle, cardiac muscle, and glands
• involuntary
Types of Neurons
• Interneurons = processors & integrators – 99% of all neurons
– connect afferent to efferent
– located entirely in the CNS
Types of Neuroglia
• Schwann Cells – surround axons of all PNS
neurons
– form myelin sheath around axons
• gaps = Nodes of Ranvier
– presence increases the speed nerves conduct signals
Types of Neuroglia
• Oligodendrocytes– form myelin sheaths around
CNS axons – white matter
• area of CNS with high density of axons
– grey matter • area of CNS with mostly cell
bodies and dendrites (no myelin)
Types of Neuroglia
• Microglia – phagocytose (eat)
foreign and degenerated material
Types of Neuroglia• Astrocytes
– cover capillaries within the brain
– control permeability of capillaries
• regulate exchange of material between blood and cerebrospinal fluid
• “blood-brain barrier”
– Deliver nutrients directly to neurons
– Uptake excess neurotransmitter and control ion concentrations at synapses.
Types of Neuroglia
• Ependymal cells – form epithelial lining
of brain and spinal cord cavities
– produce cerebrospinal fluid
Types of Neuroglia
• Satellite Cells (Ganglionic Gliocytes)– Form capsules around cell
neuron cell bodies in ganglia
– Support and protect cell bodies
Electrical Activity of Neurons:Resting Potential
• Due to differences in permeability of membrane to charged particles
– completely impermeable to A-
– relatively permeable to K+
– relatively impermeable to Na+
• Inside of cell negative relative to the outside (-70 mV)
• At resting potential, neither K+ nor Na+ are in equilibrium
Electrical Activity of Neurons:Electrical Signals
• Electrical signals – changes in membrane potential
– due to changes in membrane permeability and increased flow of charged particles
– changes in permeability are due to increased number of open membrane channels.
• Allows ions to flow along electrochemical gradient
Depolarization and Hyperpolarization
• Depolarize – reduce charge
difference
• Hyperpolarize – increase charge
difference
Membrane Proteins Involved in Electrical Signals
• Non-gated ion channels – Always open
– specific for a particular ion
Membrane Proteins Involved in Electrical Signals
• Gated Ion channels– open only under particular conditions (stimulus)
– voltage gated – changes in membrane potential
– chemically gated – binding of a chemical messenger
– physically gated – stretching/distortion of the membrane
Membrane Proteins Involved in Electrical Signals
• Na+/K+ pump – active (require ATP)
– Na+ pumped out, K+ pumped in (3 Na+ per 2 K+)
Types of Electric Signals: Graded Potentials
• occur in dendrites and cell body
• small, localized change in membrane potential
– change of only a few mV– opening of chemically-gated or
physically-gated ion channels • changes permeability of membrane
– travels only a short distance (mm)
Types of Electric Signals: Graded Potentials
• a triggered event (requires stimulus)– e.g. - light, touch, chemical messengers
• graded stimulus intensity → change in membrane
potential
Types of Electric Signals:Action Potentials
• begins at the axon hillock, travels down axon
• brief, rapid reversal of membrane potential– Large change (~70-100 mV)– Opening of voltage-gated Na+
and K+ channels – self-propagating - strength of
signal maintained– transmits electrical signals over
long distances
Types of Electric Signals:Action Potentials
• triggered – membrane depolarization at axon hillock
• not graded = "All or none" – axon hillock must be depolarized a minimum amount
(threshold)
– if depolarized to threshold, AP will occur at maximum strength
– if threshold not reached, no AP will occur
Action Potential: Depolarization Phase
• Triggering event causes membrane to depolarize
• slow increase until threshold is reached
• voltage-gated Na+ channels open quickly (K+ channels slowly)– Na+ enters cell– further depolarization– more channels open– further depolarization
• membrane depolarizes to 0 mV, but continued flow of Na+ in leads to reversed polarity (+30 mV)
Action Potential: Repolarization Phase
• At +30 mV, voltage-gated Na+ channels close
• Slow opening of voltage-gated K+ channels– reach peak K+ permeability
as Na+ channels close• K+ rushes out of the cell
– membrane potential restored• K+ channels close @
threshold• [Na+] and [K+] restored by
the Na+-K+ pump
Action Potentials
• response of the nerve cell to the stimulus is “all or none”
– Amt of depolarization (amplitude) always the same
– differences in stimulus intensity are detected by
• The number of neurons undergoing AP in response to the stimulus
• The frequency of action potential generation
Refractory Period
• time that must pass before the neuron segment can undergo a second action potential
• absolute refractory period
– neuron segment is undergoing AP– cannot respond to a second stimulus – channels enter an inactive state
• relative refractory period
– neuron segment is repolarizing– action potential may be produced if
a stronger stimulus is applied
Action Potential Propagation
• Na+ moving into one segment of the neuron quickly moves laterally inside the cell
• Depolarizes adjacent segment to threshold
Action Potential Propagation:Myelinated Axons
• Saltatory conduction - increased speed of the AP produced by myelination of the axon – myelin = lipid insulator (PM of
Schwann cells or oligodendrocytes) – nodes of Ranvier =contain lots of
Na+ channels
• signals “jump” from one node to the next AP conduction speed
Synapses
• Synapse – functional connection between a neuron and
either an effector cell or another neuron
– allow information to pass from one cell to the next
Electrical Synapses (Gap Junctions)
• Present in cardiac and smooth muscle, and some neurons
• Series of channels crossing membranes of both cells
• Allow flow of ions from one cell to the next
• Electrical signals move quickly from one cell to the next
Chemical Synapses• Unidirectional info. flow• presynaptic neuron
– synaptic terminal bouton – contains synaptic vesicles filled
with neurotransmitter
• synaptic cleft – space in-between cells
• postsynaptic neuron – Subsynaptic membrane– Receptor proteins for
neurotransmitter
Chemical Synapses
• Many voltage-gated Ca2+ channels in the terminal bouton – Ca2+ is in higher conc. in the ECF than
the ICF – AP in bouton opens Ca2+ channels – Ca2+ rushes in.
• Ca2+ causes vesicles to fuse to plasma membrane and release contents
• Transmitter diffuses across synaptic cleft and binds to receptors on subsynaptic membrane
Chemical Synapses
• Specific ion channels in subsynaptic membrane open – chemically-gated ion channels
• Ions enter postsynaptic cell
– graded potential forms
• If depolarizing graded potential is strong enough to reach threshold, action potential generated in postsynaptic cell
Types of Chemical Synapse
• Excitatory chemical synapse: – excitatory postsynaptic
potentials (EPSPs)
– Transmitter binding opens Na+ channels in the postsynaptic membrane
– Small depolarization of postsynaptic neuron
• More positive inside the cell • closer to threshold
Types of Chemical Synapse
• Inhibitory chemical synapse: – inhibitory postsynaptic
potentials (IPSPs) – Transmitter binding opens
K+ or Cl- ion channels – K+ flows out or Cl- flows in
down gradients – Small hyperpolarization of
postsynaptic neuron • More negative inside cell• further from threshold
Neurotransmitters
• Chemicals that carries the message of the A.P. from one cell to the next
• Acetylcholine – somatic MNs – skeletal muscle contraction – autonomic MNs – slow HR, gland secretion etc.
• Norepinephrine – autonomic MNs – mental alertness, increases blood
pressure and HR, etc.
• Seratonin + Dopamine – interneurons – behavioral effects
Neurotransmitters
• types vary between synapses
• response depends on postsynaptic membrane
• e.g. acetylcholine – produces EPSPs when applied to
skeletal muscle – produced IPSPs when applied to
cardiac muscle
Synaptic Integration
• Multiple synaptic events have an additive effect on membrane potential
• Sum of inputs determines whether axon hillock depolarized enough for AP to form.
Spatial Summation
• numerous presynaptic fibers may converge on a single postsynaptic neuron
• additive effects of numerous neurons inducing EPSPs and IPSPs on the postsyn. neuron
Temporal Summation
• additive effects of EPSPs and IPSPs occurring in rapid succession
• next synaptic event occurs before membrane recovers from previous event