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