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Neuronal Structure, Function, and
Supporting Cells
The Nervous System
Why are neurons amitotic?
a. Neurons contain a nucleus and all other cytoplasmic organelles EXCEPT centrioles, which are needed for cells to divide. Because centrioles function in cell division, the fact that neurons lack these organelles is consistent with the amitotic nature of the cell.
Do primary brain tumors originate from neurons in the
brain or the supporting cells (neuroglial cells)? Explain
your answer.
b. Primary brain tumors originate from the supporting neuroglial cells, not the neurons. Neurons are amitotic, meaning they cannot divide or replicate. Therefore, they cannot proliferate uncontrollably to form a tumor. The supporting neuroglial cells, on the other hand, have the capability to proliferate and form primary brain tumors. The only way for a neuron to be involved in a brain tumor would be through metastasis of cancer from another part of the body, which would result in a secondary brain tumor.
Which part of a neuron is known as the:
a. biosynthetic center and why? i. The cell body is the biosynthetic center because all biosynthetic activities occur in the cell body.
b. receptive center and why? i. The dendrites are the receptive center because they receive and convey electrical signals towards the cell body.
c. conducting region and why? i. The axon is the conducting region because it generates and transmits action potentials away from the cell body.
d. How many axons can each neuron have? i. Each neuron has only ONE axon.
e. secretory region and why? i. The axon terminals are the secretory region because they release neurotransmitters into the extracellular space.
Define the following:
a. Nissl body : Well-developed rough endoplasmic reticulum, involved in metabolic activity and oxygen supply for neurons. b. Ganglion : A cluster of neuron cell bodies in the peripheral nervous system (PNS). c. Nucleus : A
cluster of neuron cell bodies in the central nervous system (CNS). d. Tract : A bundle of axons in the CNS. e. Nerve : A bundle of axons in the PNS. f. Neurilemma : The outermost nucleated cytoplasmic layer of Schwann cells that surrounds the axon of neurons in the PNS. g. Nodes of Ranvier : The spaces between adjacent myelin sheaths (covering each Schwann cell) that facilitate rapid electric conduction. h. Endoneurium : The connective tissue that covers each axon. i. Perineurium : The connective tissue that covers a fascicle. j. Epineurium : The connective tissue that covers a bundle of perineurium-covered fascicles.
Describe the structural organization of a tract or nerve.
The structural organization from innermost to outermost is: Neuron → Endoneurium → Fascicle → Perineurium → Epineurium → Tract or Nerve.
Name and describe the function of all 6 types of supporting
cells.
a. CNS (Central Nervous System) : i. Astrocytes : Involved in forming the blood-brain barrier and regulating brain function, support and repair neurons. ii. Microglia : Weak phagocytes, clean up cell debris, less active than macrophages. iii. Ependymal cells : Line the ventricles and beat cilia to move cerebrospinal fluid. iv. Oligodendrocytes : Myelinate axons in the brain and spinal cord.
b. PNS (Peripheral Nervous System) : i. Schwann cells : Myelinate axons in the PNS, have a neurilemma, and are divided along the axon by nodes of Ranvier. ii. Satellite cells : Supporting cells that act as a chemical barrier.
c. Which type of supporting cells are involved in the formation of the Blood- Brain Barrier (BBB)? Astrocytes are involved in the formation of the Blood- Brain Barrier.
d. What is the function of the BBB? The BBB prevents blood and nerve/nerve cells from coming into direct contact, which can cause conditions like schizophrenia. It also prevents harmful substances from crossing into the vicinity of neurons in the brain to stop unwanted excitation.
Explain why myelinated axons in the CNS do not
regenerate when severed.
Myelinated axons in the CNS do not regenerate when severed because they lack a regeneration tube formed by the neurilemma from Schwann cells, as is present in the PNS. Additionally, CNS neurons are amitotic and cannot replicate properly, and the microglia cells clean up damaged areas poorly. The presence of growth-inhibiting proteins also prevents overgrowth of the brain and spinal cord, which are confined by the cranium and vertebral column.
How do you tell the difference between a strong stimulus
(such as an intense pain) and a weak stimulus (such as a
mild pain) that caused action potential generation?
If there are more frequent action potentials (more waves on the graph), that indicates a strong stimulus. If there are fewer action potentials (less/fewer waves on the graph), that indicates a weaker stimulus.
Name and describe the 3 structural classes of neurons.
a. Unipolar : A single process extending from the cell body, divided into peripheral and central processes (conduct impulses toward the CNS). b. Bipolar : Two processes attached to the cell body, one dendrite and one axon (found only in the eye, ear, and olfactory mucosa). c. Multipolar : Many processes issued from the cell body, many dendrites and one axon (most abundant type of neurons in the body).
b. The most abundant structural class of neurons is the multipolar type.
Name and describe the 3 functional classes of neurons.
a. Sensory (Afferent) Neurons : Carry impulses from sensory receptors in the skin, internal organs, muscles, and special sense organs toward the CNS. Motor (Efferent) Neurons: Carry impulses away from the CNS to organs, muscles, and glands. Association Neurons (Interneurons): Conduct impulses within the CNS.
b. The most abundant functional class of neurons is the association/ interneurons, which make up 99% of the neurons in the human body.
Name and describe the 3 types of nerve fibers.
a. Group A : Have the largest diameter and are heavily myelinated, transmitting impulses at a rate of 150 m/s (e.g., motor neurons). b. Group B : Have an intermediate diameter and are lightly myelinated, transmitting impulses at a rate of 15 m/s. c. Group C : Have the smallest diameter and are unmyelinated, transmitting impulses at a rate of 1 m/s.
d. Group A fibers have the fastest conduction velocity due to their large diameter and heavy myelination, which reduces resistance and allows for faster impulse transmission.
e. Group C fibers have the slowest conduction velocity due to their small diameter and lack of myelination, which increases resistance and slows down impulse transmission.
f. When myelinated axons become demyelinated, the conduction velocity decreases. This is because myelin insulates the axonal membrane, reducing the ability of current to leak out of the axon and increasing the distance along the axon that a given local current can flow passively. The lack of
ability to "jump" from node to node also contributes to the slower conduction velocity.
CNS: Brain
Name the 4 structures protecting the brain. a. Cranium, Meninges, Cerebrospinal Fluid (CSF), and Blood-Brain Barrier (BBB).
b. The significance of the Blood-Brain Barrier is that it protects the brain and nerves/nerve cells from direct contact with the blood. It also serves as additional shock absorption and insulation within/around the cranial structure, and provides nutrients and oxygen to the brain.
Name and describe the 3 types of meninges. a. Outermost meninx is the dura mater, which is a tough and inflexible double layer that forms structures separating the cranial cavity into compartments. The middle meninx is the arachnoid mater, and the innermost meninx is the pia mater. The latter two aid in the transfer and production of CSF.
a. The areas where CSF is located are the subarachnoid space outside the brain and spinal cord, the two lateral ventricles, the third ventricle, and the fourth ventricle.
b. The lateral ventricles are located in the cerebral cortex (spanning the occipital, frontal, and parietal lobes), the third ventricle is in the diencephalon of the forebrain between the right and left thalamus, and the fourth ventricle is located at the back of the pons and upper half of the medulla oblongata of the hindbrain. The ventricles aid in the production and circulation of cerebrospinal fluid.
c. The interventricular foramen connects the paired lateral ventricles with the third ventricle at the midline of the brain. The cerebral aqueduct is within the midbrain and contains the cerebrospinal fluid, connecting the third ventricle in the diencephalon to the fourth ventricle.
The ependymal cells line the ventricles and circulate the cerebrospinal fluid.
The 4 regions of the adult brain are the Cerebrum, Cerebellum, Diencephalon, and Brainstem.
Cerebrum
Definitions: a. Gyrus: Raised areas/ridges on the external surface of the cerebrum. b. Sulcus: Grooves on the external surface of the cerebrum. c. Fissure: Deeper sulci. d. Corpus callosum: Holds the two cerebral hemispheres (left and right) together medially. e. Gray matter: Cell bodies outside. f. White matter: Axons inside. g. Commissural tracts: Connect corresponding areas in the two cerebral hemispheres. h. Projection tracts: Connect the cerebral cortex to lower brain areas and the spinal cord. i. Association tracts: Connect areas within the same cerebral hemisphere.
Broca's Area and Cerebral Hemisphere
Damage
Broca's Area and Right-Side Paralysis
Broca's area is located only in the left frontal lobe of the cerebral hemisphere. Right-side paralysis is associated with left-side cerebral brain damage. Right-side brainstem/spinal cord damage can also cause right-side paralysis. This is due to the principle of "contralateral control," where the left side of the brain controls the right side of the body, and vice versa.
Left-Side Paralysis and Broca's Aphasia
No, left-side paralysis is not necessarily accompanied by Broca's aphasia. Broca's area is located only in the left frontal lobe of the brain. If the left frontal lobe is damaged, it will include Broca's area, but Broca's area is absent on the right frontal lobe.
Damage to the Postcentral Gyrus
Consequence of Damage to the Right Postcentral Gyrus
Damage to the right postcentral gyrus results in somatosensory damage (homunculus) and left-side loss of sensation.
Cerebral Basal Nuclei
The Three Main Cerebral Basal Nuclei and Their Functions
Caudate nucleus:
Involved in motor and behavioral functions.
Putamen:
Involved in motor function and physical movement.
Responsible for skeletal muscle innervation.
Globus pallidus:
Involved in the regulation of voluntary movement.
Parkinson's Disease
Pathogenesis of Parkinson's Disease
Parkinson's disease is caused by damage to the dopaminergic neurons.
Use of L-DOPA in Parkinson's Disease Management
L-DOPA is a non-polar precursor to dopamine that can cross the blood- brain barrier. Once in the brain, L-DOPA is converted into dopamine, which can then be used to manage the symptoms of Parkinson's disease. Dopamine itself is not used directly because it is a polar molecule and cannot cross the blood-brain barrier.
Diencephalon
The Thalamus as the "Gateway to the Cerebral Cortex"
The thalamus is referred to as the "Gateway to the Cerebral Cortex" because all sensory input must stop in the thalamus before projecting to the respective cerebral cortex.
Endocrine Functions of the Diencephalon
The epithalamus and hypothalamus are two areas in the diencephalon with endocrine functions.
Brainstem
Structural Similarities between the Brainstem and Spinal
Cord
The brainstem is continuous with the spinal cord. Both the brainstem and spinal cord contain tracts (bundles of nerves).
The Three Parts of the Brainstem
The brainstem is composed of the midbrain, pons, and medulla oblongata.
The Corpora Quadrigemina and Their Functions
The superior colliculus is the visual reflex center. The inferior colliculus is the auditory reflex center.
The Tandem Walk Test and the Cerebellum
The inability to maintain balance during a tandem walk (walking in a straight line with one foot immediately in front of the other) due to alcohol intoxication is indicative of impairment of the cerebellum, which coordinates and balances skeletal muscle movements.
Central Nervous System: Spinal Cord
Arrangement of Gray and White Matter
In the cerebellum and cerebrum, the inner layer is white matter, and the outer layer is gray matter. In the brainstem and spinal cord, the inner layer is gray matter, and the outer layer is white matter.
Structures Surrounding the Spinal Cord
Spinal dural sheath: A single-layered dura mater with an epidural space between the internal surface of the vertebral column and the dura mater. Filum terminale: Part of the pia mater that supports the spinal cord vertically. Denticulate ligaments: Part of the pia mater that supports the spinal cord laterally. Epidural space: Supports the area underneath the dura mater layer. Subarachnoid space: Supports the area underneath the arachnoid mater layer and contains cerebrospinal fluid (CSF). Central canal: Runs the length of the spinal cord and contains CSF.
Differences between Brain and Spinal Cord Coverings
The dura mater surrounding the brain is double-layered, while the dura mater surrounding the spinal cord is single-layered. The pia mater surrounding the brain is a delicate membrane attached to the surface of the brain, following all its involutions, while the pia mater surrounding the spinal cord forms two extensions that anchor the spinal cord vertically and laterally.
Amyotrophic Lateral Sclerosis (ALS)
ALS, also known as Lou Gehrig's disease, results from the degeneration of nuclei in the ventral horns of the spinal cord.
Paraplegia and Quadriplegia
Paraplegia results from damage below the cervical and lumbar enlargements of the spinal cord, where only the lower limbs are affected.
Quadriplegia results from damage above the cervical enlargement of the spinal cord, where both the upper and lower limbs are affected.
Differentiating Paralysis Caused by Damage at the
Precentral Gyrus vs. Spinal Cord
Hemiparesis (one-sided paralysis) is caused by damage at the level of the precentral gyrus, reflecting the contralateral control of the motor areas. Limb paralysis is caused by damage to the enlargements of the spinal cord, resulting in paralysis of the affected limbs.
Peripheral Nervous System (PNS)
Divisions of the PNS
Afferent (sensory) division: Transmits information from the PNS and body to the central nervous system (CNS). Efferent (motor) division: Transmits information from the CNS to the PNS and body. Autonomic nervous system (ANS): A division of the PNS that is further divided into the sympathetic and parasympathetic systems. Somatic nervous system: Responsible for transmitting information to skeletal muscles (voluntary organs).
Sensory Receptors
Sensory receptors include somatosensory, mechanoreceptors, chemoreceptors, and photoreceptors.
Autonomic Nervous System Functions
Sympathetic division causes dilation of the pupils. Parasympathetic division causes constriction of the pupils.
Sensory Organs: The Eye
The Three Major Parts of the Eye
Tri-layered eye wall Humors (fluids) Lens
Function of the Lens
The lens reflects and refracts light from the cornea at the front of the eyeball to the photoreceptors at the back of the eyeball (the retina).
Visual Pathway
Cornea → Aqueous humor → Pupil → Lens → Vitreous humor → Retina → Photoreceptors → Optic nerve → Occipital lobe The ganglion cells in the retina form the optic nerve and generate/ transmit action potentials.
Myopia and Hyperopia
Myopia (nearsightedness) is corrected with concave lenses. Hyperopia (farsightedness) is corrected with convex lenses.
The Ear
The Three Major Parts of the Ear
External ear Middle ear Internal/labyrinth ear
Function of the Pinna (Auricle)
The pinna collects and amplifies sound, directing it to the external auditory canal.
The Auditory Ossicles
The three auditory ossicles in the middle ear, in order from the tympanic membrane, are the malleus, incus, and stapes.
The Bony and Membranous Labyrinths
The bony labyrinth consists of the vestibule, semicircular canals, and cochlea, each containing endolymph. The membranous labyrinth within the bony labyrinth contains perilymph. The membranous labyrinth includes the utricle and saccule in the vestibule, the semicircular ducts in the semicircular canals, and the cochlear duct in the cochlea.
Equilibrium Receptors
Maculae: Located in the vestibule. Cristae ampullares: Located in the anterior, posterior, and lateral semicircular canals.
The Organ of Corti
The organ of Corti is the receptor organ for hearing in the cochlea, containing hair cells that transduce auditory signals into nerve impulses. It is composed of the basilar membrane, supporting cells, and the tectorial membrane.
Auditory Pathway
Sound waves travel through the ear, ultimately reaching the hair cells in the organ of Corti, which transduce the signals into nerve impulses.
Auditory Relay Centers
Medial geniculate nucleus: Auditory relay center in the thalamus. Inferior colliculus: Auditory reflex control center in the lower part of the corpora quadrigemina. Primary auditory cortex: Located in the temporal lobe and responsible for hearing.
Consequences of Auditory Cortex Damage
Damage to the left primary auditory cortex results in deafness or partial deafness in the left ear.
Olfaction (Sense of Smell)
Olfactory Cells
Olfactory cells are epithelial cells that exhibit longevity, making them unique in humans.
Criteria for Olfactory Processing
The chemical must dissolve in the thin layer of mucus. The chemical must bind to olfactory receptors.
Olfactory Nerve (Sense of Smell)
Axons of which neurons form the olfactory nerve?
The axons of mitral neurons/cells form the olfactory nerve.
For example, a high level of cortisol in the blood can inhibit the further secretion of cortisol-releasing hormone (CRH) and adrenocorticotropic hormone (ACTH), which in turn reduces the secretion of cortisol. The less these hormones are released, the less cortisol is secreted, and vice versa.
Classes of Hormones
Biogenic Amine Hormones
Biogenic amine hormones are derived from the amino acid tyrosine, such as dopamine and catecholamines. These hormones are generally polar chemicals and bind to membrane receptors.
Peptide/Protein/Glycoprotein Hormones
Peptide/protein/glycoprotein hormones are composed of a sequence of amino acids, attaining structural complexity. They can be proteins with a carbohydrate moiety attached, such as oxytocin, insulin, and follicle- stimulating hormone. These hormones are polar chemicals and bind to membrane receptors.
Steroid Hormones
Steroid hormones are derived from cholesterol. They are non-polar chemicals and bind to intracellular receptors.
Target Cells for Hormones
Target cells for a hormone are those that respond to the hormone because they have the specific receptors for that hormone. These cells can be affected by that hormone and no other hormone unless they have the appropriate receptors.
Hormone Receptors
There are two types of hormone receptors:
Membrane receptors: These are located on the plasma membrane surface and typically bind to biogenic amine and peptide hormones. Intracellular receptors: These are located within the cytoplasm and typically bind to steroid hormones.
Hormone receptors are specific, meaning they can only bind to their corresponding hormones.
Hormone Interactions
Classical Endocrine Interaction
An endocrine gland releases a hormone into the bloodstream, which then transports the hormone to its target cells.
Paracrine Interaction
Endocrine cells release a hormone into the interstitial fluid surrounding the neighboring target cells. The endocrine cells and the target cells are typically located in the same gland or organ.
Juxtacrine Interaction
Endocrine cells are juxtaposed (in close proximity) to the target cells in the same organ, and the released hormone interacts directly with its target cells.
Autocrine Interaction
The endocrine cells releasing the hormone act as the target cells for the hormone.
Hormonal Interrelationships
Agonism
A hormone binds to the receptors of another hormone and mimics the biological effects of that hormone.
Antagonism
A hormone binds to the receptors of another hormone, blocking the hormone from binding to its own receptors, resulting in no biological effects of that hormone.
Permissiveness
The biological effects of a hormone (bound to its own receptors) increase the levels of another hormone and/or increase the number of receptors of that hormone, resulting in an overall increase in the biological effects of that hormone.
Cooperativity
Hormones work in tandem on the same target tissue to bring about a desired biological effect.
Effects of Calcium Channel Blockers
Smooth Muscle Contraction
Calcium channel blockers affect smooth muscle contraction by blocking the entry of Ca2+ ions into the smooth muscle cells. This inhibits the formation of the calcium-calmodulin complex, which in turn prevents the activation of MLCK and the subsequent formation of cross-bridges between the myosin and actin filaments. As a result, smooth muscle contraction is inhibited.
Cardiac Muscle Contraction
Calcium channel blockers also affect cardiac muscle contraction by blocking the entry of Ca2+ ions into the cardiac muscle cells. This inhibits the binding of Ca2+ ions to the TnC subunit of the troponin complex, preventing the conformational change that allows the myosin globular heads to attach to the actin filaments. Consequently, the formation of cross-bridges and the power stroke are inhibited, leading to a reduction in cardiac muscle contraction.
Skeletal Muscle Contraction
Calcium channel blockers have no effect on skeletal muscle contraction, as the mechanism of contraction in skeletal muscle does not rely on the influx of Ca2+ ions from the extracellular fluid.
Effect of Acetylcholinesterase Inhibitors on
Skeletal Muscle Contraction
Acetylcholinesterase inhibitors affect skeletal muscle contraction by preventing the breakdown of acetylcholine (ACh) in the neuromuscular cleft. Acetylcholinesterase is the enzyme responsible for the degradation of ACh, which is the neurotransmitter that activates the motor neurons and triggers the contraction of skeletal muscle. By inhibiting acetylcholinesterase, the acetylcholine remains in the neuromuscular cleft for a longer period, leading to a prolonged activation of the motor neurons and, consequently, a sustained contraction of the skeletal muscle.
