Introduction
Learning Outcomes
* Relate the CNS and PNS to afferent and efferent signals
* Illustrate input, integration and output signals of the nervous system
* Compare and contrast the somatic and visceral nervous systems
Subtopic: PNS & CNS Overview
Lecture Video Part 1
* The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS)
* CNS >
* brain (cerebrum, cerebellum, brainstem)
* spinal cord
* PNS >
* 12 pairs of cranial nerves
* 31 pairs of spinal nerves with sensory (afferent) and motor (efferent) components
* ganglia (small masses of tissues)
* sensory receptors (monitors change in internal/external environment)
* PNS is subdivided into somatic and autonomic divisions.
* Somatic Nervous System: Voluntary (skeletal muscles)
* Autonomic Nervous system: Involuntary (smooth muscle, cardiac, glands)
* Sympathetic Nervous System: Increase Heart Rate, supports exercise, emergency situations, 'flight or flight response'
* Parasympathetic Nervous System: slow heart rate; 'rest and digest' response/activities.
* Enteric nervous System: neurons that extend the GI Tract.; Sensory neurons of ENS monitor stretching of GI Tract walls.
Functions of the Nervous System
Video: Overview of the nervous system
1. Sensory Function (detection of stimuli):
1. sense changes through sensory receptors (changes in the internal/ external environment)
2. process begins in the PNS and sent to the CNS
3. Sensory neurons serve this function.
2. Integration function (of information):
1. Decision making; analyzing incoming sensory information
2. The CNS determines the response to stimuli
3. Association and interneurons serve this function.
3. Motor Function (response to stimuli): Respond to stimuli by initiating action
1. From the CNS, motor output signals target effectors (muscles and glands) in the PNS.
2. stimulate tissues to control body movement and gland secretion.
3. Motor neurons serve this function.
4. Homeostasis: coordination between the CNS and PNS enables the body to maintain a healthy physiology.
Subtopic: Sensory and Motor Neurons
1. Sensory Neurons (afferent) send signals towards the CNS.
* sensory receptors initiate signals sending to the brain/ spinal cord.
* Includes sensory input or sensation.
2. Integration takes place in the CNS perform
3. Motor (efferent) neurons must send signals away from the CNS to target organs in the PNS.
Result: contraction of muscles and/or secretion from glands.
Cells of the Nervous System
Learning Outcomes
* Detail the structure and function of neurons
* Differentiate the various neuroglia and neuron types
* Describe the process of myelination
Subtopic: Neuron Cell parts
Video: Neuron cell parts
Video: How neurons communicate
* Cell body: contains nucleus, lysosomes, mitochondria, golgi complex, nissl bodies, neurofibrils.
* Nissl Bodies: Rough ER; constantly replaces the cell membrane (normal process of growth and repair).
* Neurofibrils: form Cytoskeleton; give structure to the cell.
* Dendrites: receiving or input of information
* Axon: conducts the nerve impulses from neuron to dendrites or to an effector organ (muscle /gland).
* Axon Hillock: connects the cell body with the axon: initiation of electrical impulse (action potential).
Synapse: site of functional contact between two neurons. (or neuron and effector organ)
Axonal transport: intracellular transport in neurons.
Subtopic: Classification of Neurons (See Figure 12.9 in your Textbook)
Structural Classification: On the number of processes extending from the cell body. (Unipolar, bipolar, multipolar)
1. Unipolar
* single process extending from the cell body
* Several dendrites; One axon fused together (pseudounipolar neuron)
* Sensory receptors
2. Bipolar
* two processes attached to the cell body
* One dendrite; One axon
* Retina, eye, inner ear
3. Multipolar
* Brain & Spinal cord
* Several dendrites extending from the cell body, one axon
Functional Classification
* Sensory, association, or motor
Subtopic: Neuroglia
* specialized tissue cells that support the neuron.
* attach neurons to blood vessels; "acts as glue"
* produce myelin sheath around the axon
* carries out phagocytosis
Types of Neurolgia
Found in the CNS
1. Astrocytes
* aid in the development of the blood-brain barrier (BBB).
* have numerous processes (extensions) making them appear like a star.
* These extensions wrap around capillaries in the brain to regulate which blood compounds can enter the CNS.
* Another important function of astrocytes is their role as a potassium “buffer”. By absorbing or releasing potassium, astrocytes help to keep the concentration constant to maintain the electrical properties of neurons.
* Astrocytes also help remove excess neurotransmitters.
2. Oligodendocytes
* large cells in the CNS with extensions that wrap around neuron axons forming a myelin sheath.
* Similar to the rubber coating on electrical wires, myelin helps insulate nervous signals.
* The glossy-white appearance of myelin is due to the high lipid content.
3. Microglial cells
* Microglia are small cells that wander around the CNS and replicate when there is inflammation caused by infection or damaged tissue.
* Similar to macrophages of the immune system, they remove debris and pathogens by engulfing and digesting substances in a process called phagocytosis.
4. Ependymal cells
* Ependymal cells help to cushion and nourish the brain and spinal cord by producing a cerebrospinal fluid (CSF) that circulates throughout the CNS.
* Adjacent to capillaries, these cells are able to exchange waste products and nutrients to maintain CSF homeostasis.
Found in the PNS
1. Schwann cells
* wrap around neuron axons in the PNS forming myelin sheaths
2. Satellite cells
* wrap around the neuron cell body
Subtopic: Clinical Application
Multiple sclerosis
* condition with progressive demyelination of neurons in the CNS (loss of oligodendrocytes).
* Disruption of conduction of nervous signals causing impairment of sensory and motor function.
* progressive disorder (it gets worse over time)
Guillain-Barré Syndrome
* loss of myelin on neurons in the PNS
* characterized by impaired sensation and muscle weakness.
* Unlike multiple sclerosis, this disorder often resolves spontaneously.
Subtopic: Myelin Sheath
Video: Myelin Sheath
Video: Nerve Damage & Regeneration
* Oligodendrocytes (CNS) and Schwann cells (PNS) produce myelin sheath
* Myelin is a multilayered lipid and protein covering
* Myelination: process of wrapping around axons to form layers of myelin.
* Myelination (insulation) allow neural signals to be transmitted more quickly.
* The myelin sheath is not continuous along the axon.
* There are gaps between sheaths where the neuron plasma membrane is exposed.
* These regions are called nodes of Ranvier (aka neurofibril nodes).
* Some axons are heavily myelinated, others are unmyelinated.
Membrane Potentials
Learning Outcomes
* Distinguish between the various pumps and ion channels
* Employ Ohm’s law to explain polarization/ depolarization of the neuron membrane
* Relate the resting membrane potential to depolarization and hyperpolarization
* Differentiate graded and action potentials
* Compare and contrast continuous versus saltatory conduction and the fiber types
Subtopic: Pumps and Channels
* membrane protein pumps concentrate ions by moving them in or out of the cell.
* Sodium-potassium (Na+/K+) and calcium (Ca++) pumps are the most common ion pumps.
* Na+/K+ pumps move three Na+ ions out for every two K+ ions brought in whereas Ca++ pumps simply move Ca++ ions out.
* Pumps move ions in the direction of greatest concentration, it is often described as moving ions up a concentration gradient.
1. Leakage
* passive channel; always open for continuous diffusion.
2. ligand-gated
* Known as chemical channels; usually closed
* Open when a chemical messenger, such as a neurotransmitter binds to them.
3. voltage-gated
* Voltage-gated channels are typically closed as well.
* Open when the electrical conditions of the membrane change.
4. Mechanically-gated ion channels.
* Open when some physical force is applied such as those on neurons involved in the sensation of touch.
Subtopic: Resting Membrane
Video: Resting Membrane Potential
* The membrane of a nonconducting (hence, it’s resting!) neuron is positive outside & negative inside.
* The resting membrane potential is determined by several factors:
* unequal distribution of ions across the plasma membrane
* most anions cannot leave the interior of the cell
* the sodium potassium pumps compensate for slow leakage of Na+ into the cell by pumping it back out.
* The difference in electrical charge is voltage and across the neuron plasma membrane this measures around -70mV
* Resting Membrane Potential is about -70mV
* Ohm’s Law: I=V/R (where I = current, V = voltage and R = resistance)
Subtopic: Grade & Action potential
Video: Action Potential
Video: How myelin sheath speed up action potentials
Video: The Schwann cell and action potential
* A graded potential can either depolarize (less polarized) or hyperpolarize (more polarized).
* Graded potentials occur most often in the dendrites and cell body of a neuron.
* Graded potentials occur as the result of opening of ligand gated or mechanically gated channels.
* Graded potentials initiated by sensory receptors are called receptor or generator potentials. When graded potentials are strong enough to depolarize the membrane beyond a certain level, it may open voltage-gated Na+ channels in the axon hillock. If this occurs, we have created an action potential!
* An action potential is a sequence of rapidly occurring events that decrease and eventually REVERSE the membrane potential (depolarization) and eventually restore it to the resting state (repolarization).
Propagation:
* An action potential is generated at the axon hillock and is conducted along the axon towards the terminus in a process called propagation.
* unmyelinated axons: result of numerous sodium and potassium voltage-gated channels close enough to one another so that if one opens nearby gates will also open. (continuous conduction)
* myelinated axons: the voltage-gated channels for sodium and potassium open are only present where myelin is absent (the areas known as the nodes of Ranvier)
* In these axons, action potential propagation is known as saltatory conduction.
* Action potential in myelinated axons is much faster than in unmyelinated axons (don’t rely on the opening and closing of ion gates)
Nerve fibers are bundles of axons classified according to their diameter and degree of myelination.
Factors that affect Propagation:
* axon diameter
* amount of myelination
* Temperature
Types of fibers:
1. Type A
* Motor neurons that innervate skeletal muscles
* large diameters = highly myelinated
* Action potential > very fast conduction
2. Type B:
* fibers are lightly myelinated; intermediate diameters.
3. Type C: unmyelinated; smallest diameters: slowest conduction
Synapses and Neurotransmitters
Learning Outcomes
* Outline the events of signal transmission at chemical synapses
* List the structures and functions of various neurotransmitters
* Compare and contrast ionotropic and metabotropic receptors
* Describe neurotransmitter degradation and reuptake
Subtopic: Synapse
Video: Synaptic Transmission
The area where two neurons come together is a synapse.
* A synapse is the junction between neurons (or a neuron and effector)
* electrical synapse: gap junctions connect cells and allow transfer of information to synchronize activity
* chemical synapse: one way transfer of information
* Postsynaptic Potentials: can receive many signals at once.
* Excitatory postsynaptic potential
* postsynaptic neuron can receive many signals at once.
Subtopic: Neurotransmitters
Neurotransmitters at chemical synapses cause either an excitatory or inhibitory graded potential
Neurotransmitter receptors: Ionotropic and metabotropic receptors.
Small Molecule neurotransmitters:
* acetycholine
* amino acids
* biogenic amines
* ATP and other purines
* Nitric oxide
* carbon monoxide
Learning Outcomes
* Explain neuronal integration
* Relate ganglia, nerves, nuclei, tracts and gray and white matter
* Illustrate the various types of neural circuits
Subtopic: Integration
When neurotransmitters bind a receptor, ion gates open causing the cell to depolarize or hyperpolarize.
* Depolarization signals may generate action potentials, therefore these are called excitatory postsynaptic potentials (EPSPs).
* Hyperpolarizing signals are called inhibitory postsynaptic potentials (IPSPs).
Graded potentials weaken as they travel away from the receptor.
Net effect is due to the summation of all of them at the axon hillock (which causes an action potential)
Subtopic: Circuits
A neural circuit is a functional group of neurons that process specific types of information.
The sequence of neuron interaction is referred to as a circuit:
* Converging circuits: involve many sources of input which then act upon a single output neuron. For example, the rate and depth of breathing is influenced by many sensory inputs such as blood pH, emotions, and pain among others.
* Diverging circuits:
* single nucleus could send signals to many output targets.
* Example, the “fight or flight” sympathetic nervous system nuclei can send signals to increase heart and breathing rate and dilate the airways, pupils of the eye, and blood vessels.
* Reverberating circuits:
* rhythmic functions that will continue until there is an inhibitory signal.
* Example: maintaining breathing patterns even while sleeping.
* Parallel-after-discharge circuits:
* combination of converging and diverging circuits
* a signal can diverge from a nucleus with multiple pathways but then converge onto another nucleus.
* Example: more complex functions (such as higher order thinking)
Clinical Application:
Neuropathy- any disorder of cranial or spinal nerves.
* Note: Not all these disorders are found in your textbook
1. Parkinson's Disease:
* neurons in CNS break down or die. Decrease in dopamine levels (neurotransmitter) in the substantia nigra
* Tremors, affects movements
2. Alzheimer's Disease
* plaques found in the brain. Atrophy of brain/ breakdown of energy production within the cells.
* memory loss
3. Multiple Sclerosis:
* Myelin around neurons is loss in the CNS (demyelination occurs)
* difficulties walking, visual problems, pain.
4. Stroke:
* blood supply to brain is stopped. Neurons die (lack of oxygen)
* sudden weakness, loss of speech
5. Guilaine Barre Syndrome (GBS):
* Immune system attacks nerves (neurons), caused by bacteria. Myelin sheath of PNS is affected. (demyelination occurs)
* Muscle weakness in extremities
* Most people recover from GBS.
6. Amyotrophic Lateral Sclerosis (ALS)
* motor neurons from brain and spinal cord (CNS) to the voluntary muscles; motor neurons stop functioning.
* muscle weakness
7. Lyme Disease
* Bull's eye rash
* bacteria spread by ticks; bacteria enters bloodstream via tick attachment.
* chills, fever, neurological problems (facial palsy, stiff neck)
8. Spina Bifida:
* failure of the spinal cord to develop properly
* paralysis, bladder & bowel difficulties; spine disorder.