UNIT 4 - Introduction to the nervous system Flashcards
Division of the nervous system
Central nervous sytem - barain & spinal cord
Peripheral Nervous system - cranial nerves , spinal nerves & ganglia
Subdivisions of the PNS ( peripheral nervous system )
1.Sensory (Afferent) Division – carries impulses toward CNS from receptors.
2.Motor (Efferent) Division – carries impulses away from CNS to effectors.
a. Somatic Nervous System – carries impulses to skeletal muscle
b. Autonomic Nervous System – carries impulses to cardiac muscle, smooth muscle, and glands
Neuroglia = glial cells
▪ supporting cells that aid the function of neurons
▪ more abundant than neurons
▪ mitotic (can divide)
▪ four types in CNS and two types in PNS
Neuroglia of CNS
Astrocytes – help with exchanges between neurons & capillaries; control chemical environment around neurons.
Microglial cells – defensive cells; can become phagocytic.
Ependymal cells – line cavities of the brain where they help to circulate CSF (cerebrospinal fluid).
Oligodendrocytes – form myelin sheaths around axons in the CNS.
Neuroglia of PNS
satellite cells - similar to astrocytes of the CNS.
Schwann cells - surround nerve fibers in the PNS and form myeline sheaths.
Neurons - RGCRHLA
- Respond to stimuli
- generate & conduct nerve impulses
- release chemical regulators ( neurotransmitters)
- high metabolic rate
- long - lived
- amitotic ( don’t divide)
Structure of neurons - three main parts
- Cell body - large central portion containing nucleus .
- Dendrites - cellular processes that carry impulses towards the body (usually numerous)
- Axon - cellular process that carries impulses away from the body ( usually 1 )
Terms associated with neuron parts :
- Nuclei = clusters of cell bodies in the CNS
- Ganglia = clusters of cell bodies in the PNS
- Tract = bundle of axons in CNS
- Nerve = bundle of axon in the PNS
- Nerve fiber = any long axon
Special features of neurons
▪ Axon hillock – cone-shaped region of the cell body from which the axon emerges.
→ This is the area where nerve impulses are first generated.
▪ Axon terminals – bulb-like distal endings of axon that contain many synaptic vesicles.
Unmyelinated vs Myelinated Fibers - 1UN
Unmyelinated – thin nerve fibers with no myelin sheath.
Myelinated Fibers
Myelinated – axons with myelin sheath surrounding them.
▪ Myelin sheath – formed (in the PNS) from Schwann cells that wrap repeatedly around an axon.
◦ Myelin is a whitish lipoprotein in the
Schwann cell membrane.
◦ Insulates axon.
Myelinated Fibers (cont)
▪ Neurilemma – outer part of Schwann cell surrounding the myelin sheath.
▪ Myelin sheath gap (nodes of Ranvier) – gaps in the myelin sheath between adjacent Schwann cells.
what is the Myelin sheath?
▪ Myelin sheath – formed (in the PNS) from Schwann cells that wrap repeatedly around an axon.
◦ Myelin is a whitish lipoprotein in the
Schwann cell membrane.
◦ Insulates axon.
FUNCTIONAL classification of neurons - 3 main
1 . Sensory (afferent) neurons – carry impulses from sensory receptors into the CNS.
Functional classification of neurons
- Motor (efferent) neurons – carry impulses out of the CNS to effector organs (muscles and glands
Functional Classification of neurons
- Association neurons (interneurons) – connect sensory & motor neurons.▪ located entirely within the CNS.
▪ provide the integrative functions of the nervous system.
STRUCTURAL classification of neurons . 3
- Multipolar Neurons – have many dendrites and one axon.
- all association neurons and most motor neurons are of this type
- most common
STRUCTURAL classification of neurons
- Bipolar Neurons – one dendrite and one axon
- found in retina of eye and cochlea of ear
- rare
STRUCTURAL classification of neurons
- Unipolar Neurons – short single process emerges from the cell body and divides (like a T) into two branches.
- most found in ganglia in the PNS functioning as sensory neurons
Membrane Ions Channels
▪ Proteins in membrane that transport one or more specific ions across the plasma membrane.
▪ Crucial to establishing the resting membrane potential (RMP) and to generating an action potential.
TYPES of membrane Ions Channels
a. Leakage (nongated) channels – always open; allow substances to move according to concentration gradients
TYPES of membrane Ion Channels
- Gated channels – open and close in response to specific signals.
- Chemically - gated ion chennels -
ex. a neutransmitter binds causing channel to open.
-Voltage- gated ion channels
ex. an ion enters the cell, altering its membrane potential.
Definition of RMP ( resting membrane potential)
charge difference that exists across the plasma membrane of a cell at rest.
Neuron’s RMP - Facts
▪ Neurons establish a resting membrane potential (membrane is “polarized”).
→ There are more positive charges on the outer surface of the membrane relative to the inner surface of the membrane.
Neuron’s RMP - Facts
▪ The RMP in a neuron is about -70 millivolts (mV).
▪ Neurons use changes in their membrane potential as communication for receiving, sending, and integrating information.
What causes a neuron’s RMP?
a. The concentrations of Na+ and K+ on each side of the membrane are different.
a. The membrane is 25x more permeable to K+ than it is to Na+. Therefore, much K+ diffuses out of cell and only a small amount of Na+ diffuses in (through leakage channels).
→ The net loss of positive charges leaves the inside of the membrane negatively charged relative to the outside.
Sodium- Potassium pum’ps role
▪ Pumps Na+ out of cell and pumps K+ in.
→ This maintains the Na+ and K+ concentration gradients across the
membrane (stabilizing the RMP).
Initiation of an Action Potential : 1
- Stimulus occurs – something in the environment causes depolarization of the neuron’s membrane
- ex. neurotransmitter binds
Initiation of an action potential 1
- Stimulus (cont.)
To initiate an action potential, stimulus must be strong enough to cause the membrane potential to reach threshold (-55 mV).
Initiation of an Action Potential 2
Depolarization – a reduction in charge
difference across the membrane; the inside of the cell becomes less negative
(or more positive →→ moves toward zero).
Initiation of an Action Potential - 2
Depolarization
▪ A threshold stimulus prompts voltage- gated Na+ channels in the membrane to open, allowing Na+ to move into the cell.
- Na + moves in until membrane potential reaches = + 300mV
Initiation of an Action potential - 3
- Repolarization – restores RMP
(returns cell to resting electrical state)
▪ Voltage-gated Na+ channels in the membrane close, and entry of Na+ into the cell stops.
▪ Voltage-gated K+ channels in the membrane open allowing K+ to move out of the cell.
▪ K+ moves out until membrane potential reaches: - 90mV
Initiation of an Action potential - 4
- Reestablishing normal resting conditions
Na+/K+ pump restores proper ionic concentrations by:
▪ pumping Na+ out of the cell▪ pumping K+ back into the cell
Action Potential Propagation:
- The initiated action potential (AP) causes depolarization of adjacent membrane areas which causes the action potential to be transmitted (propagated) down the entire length of an axon.
Two types of propagation : continuous & saltatory.
Continuous conduction – depolarization occurs at every point along an axon
→ occurs along unmyelinated axons
Two types of propagation : 2
- Saltatory conduction– depolarization occurs only at nodes of Ranvier
→ impulse “jumps” from node-to-node
→ occurs along myelinated axons
Characteristics of AP propagation:
speed propagation , all or none principle & refractory periods (2)
Characteristics of AP propagation - 1
- Speed of Propagation:
Faster in axons that:
▪ have larger diameters
▪ are myelinated
Characteristics AP propagation 2
- All or None Principle:
▪ If a stimulus is strong enough to cause threshold to be reached, an action potential will be generated (all).
◦ This will be conducted at a constant and maximum strength to the end of the axon.
▪ Weaker stimuli do not trigger an action potential (none).
Characteristics AP propagation 3
- Refractory Periods:
During depolarization & repolarization, a neuron cannot respond as it would normally to a second stimulus.
→ prevents AP from
traveling backwards
characteristics about AP propagation 3
Refractory Periods (cont.):
▪ Absolute refractory period — neuron cannot respond to any stimuli; no action potential can be generated.
▪ Relative refractory period — only strong stimuli can provoke an action potential (AP)
Therefore, strong stimuli provoke more APs per
second.
INTRODUCTION to a synapse
- Neurons can receive information from other neurons and send information to other neurons or to effectors across chemical synapses.
INTRODUCTION TO SYNAPSE 2
Let’s consider a chemical synapse between two neurons:
◦ The “sender” is referred to as the presynaptic neuron.
◦ The “receiver” is referred to as the postsynaptic neuron.
STEPS in tranmission
- Action potential arrives at axon terminal of presynaptic neuron which triggers Ca2+ channels in membrane to open.
STEPS in transmission
- Ca2+ flows into axon terminal.
STEPS in transmission
- Ca2+ triggers fusion of synaptic vesicles with membrane and exocytosis (release) of neurotransmitter into synaptic cleft occurs.
STEPS in transmission
- Neurotransmitter diffuses across synaptic cleft and binds to receptors on postsynaptic neuron’s membrane and opens ion channels—which alters membrane potential
→ causes either excitation or inhibition
Excitation - steps in transmission
- If opening of ion channels
causes depolarization, this is
called an excitatory
postsynaptic potential (EPSP). - An EPSP increases a neuron’s ability to initiate an action potential.
Inhibition - steps in transmission
- If opening of ion channels causes hyperpolarization (inside of cell becomes more negative), this is called an inhibitory postsynaptic potential (IPSP).
- An IPSP decreases a neuron’s ability to initiate an action potential.
Steps in Transmission
- Neurotransmitter must be removed from synaptic cleft to discontinue activity at synapse.
a. Enzymatic break down
b. Reuptake by presynaptic neuron
c. Diffusion away from synaptic cleft
Common Neurotransmitters
▪ Acetylcholine (ACh)
▪ Serotonin
▪ Dopamine
▪ Epinephrine
▪ Norepinephrine
