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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. - PowerPoint PPT Presentation
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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