Neuroscience Study Guide PDF

Chapter 1 Cellular Components of the Nervous System -​ Neurons: Neurons are fundamental to the nervous system. -​ They include: Functional organization, including the soma, dendrites, and axon. The axon can be myelinated. -​ Types such as multipolar, pseudounipo...

Chapter 1 Cellular Components of the Nervous System -​ Neurons: Neurons are fundamental to the nervous system. -​ They include: Functional organization, including the soma, dendrites, and axon. The axon can be myelinated. -​ Types such as multipolar, pseudounipolar, and bipolar neurons. Synapses, which can be axodendritic, axosomatic, or axoaxonic. Axodendritic synapses form connections between an axon and a dendrite, and contribute to temporal spatial summation. -​ Glia: Glial cells include: -​ Astroglia: Consists of astrocytes and Müller cells and contributes to the blood-brain barrier and tripartite synapses. Astrocytes can be fibrous (white matter) or protoplasmic (gray matter). -​ Oligodendroglia: Myelinating cells found in the central nervous system. -​ Schwann cells: Myelinating cells in the peripheral nervous system. -​ Microglia: Immune cells of the monocyte-macrophage lineage that remove cell debris and present antigens. -​ Polydendrocytes: Stem cells that can generate both glia and neurons. They are implicated in demyelinating disorders and cross-talk between neuronal and glial networks. -​ Ependymal cells: Epithelial cells that line the ventricles, separating cerebrospinal fluid (CSF) from the neuropil, and are involved in the production of CSF by the choroid plexus]. -​ Blood-brain barrier: The blood-brain barrier's components include endothelial cells with tight junctions and astrocyte processes, which maintain homeostasis and transport substances via diffusion or active transport. Basic Neurophysiology Neurophysiology involves the study of ion movements across membranes to initiate signal transduction and action potentials, as well as the action of neurotransmitters. -​ Ion Movements Ion movement across a membrane is critical to neurophysiology. -​ Action Potential An action potential is a wave of depolarization that moves down the axon. -​ It is generated when the membrane voltage declines to a threshold level, opening voltage-activated ion channels and allowing Na+ ions to flow into the neuron. -​ The action potential is an all-or-none response with no variation in strength and is self-propagating. -​ Repolarization occurs behind the action potential as it moves down the axon.  -​ During depolarization, the axon enters an absolute refractory period, preventing another action potential from being transmitted. After enough Na+ channels reset, the neuron enters a relative refractory period with a higher threshold. -​ Action potential velocity depends on axon diameter and myelination. -​ In continuous conduction, the entire axon plasma membrane is involved, as seen in unmyelinated neurons. Saltatory conduction, occurring in myelinated neurons, involves depolarization skipping along the axon from one node of Ranvier to the next, making it more rapid -​ Synaptic Transmission Synaptic transmission can be electrical or chemical Synaptic Transmission A synapse is a functional junction between two neurons. -​ Electrical Synapse: uses gap junctions -​ Chemical Synapse: Chemical synapses use vesicles. -​ Synaptic Signal Transduction: Synaptic signal transduction involves inhibitory postsynaptic potentials (IPSP) and excitatory postsynaptic potentials (EPSP). ​ Temporospatial summation refers to the synaptically evoked potential. ​ Receptors can be ionotropic or metabotropic. Main Neurotransmitters: Neurotransmitters include glutamate, GABA, glycine, acetylcholine, biogenic amines, ATP, and neuropeptides. -​ Glutamate: Glutamate is the most common excitatory neurotransmitter in the central nervous system (CNS) and acts on NMDA, AMPA, kainate, and mGluRs receptors. Glutamine, supplied by astrocytes, is its precursor. -​ GABA and Glycine: GABA and glycine are key inhibitory neurotransmitters in the CNS. GABA can bind to ionotropic GABAA and GABAC receptors, causing Cl- influx and hyperpolarization. The metabotropic GABAB receptor activates K+ channels and blocks Ca2+ channels, also leading to hyperpolarization. -​ Acetylcholine: Acetylcholine is present in the peripheral nervous system (PNS), CNS, and neuromuscular junctions. Nicotinic acetylcholine receptors are ionotropic, while muscarinic acetylcholine receptors are metabotropic. Acetylcholinesterase deactivates acetylcholine. -​ Biogenic Amines: Biogenic amines include catecholamines (dopamine, norepinephrine, epinephrine), histamine, and serotonin. -​ ATP: ATP functions as an energy carrier, cotransmitter, and neuromodulator. -​ Neuropeptides: Neuropeptides include substance P, metenkephalin, opioids, and corticotropin-releasing hormone]. Chapter 2 General Organization and Major Anatomical Structures of the CNS -​ Brain The brain consists of the forebrain, brainstem, and hindbrain. -​ Forebrain The forebrain includes the telencephalon (cerebrum, basal ganglia) and the diencephalon (thalamus, hypothalamus, subthalamus). The thalamus serves as a  gatekeeper to the cerebral cortex. The hypothalamus regulates autonomic and endocrine functions and influences behavior, while the subthalamus modulates voluntary motor activity. -​ Brainstem The brainstem includes the midbrain. -​ Hindbrain The hindbrain consists of the medulla, pons, and cerebellum. -​ Spinal Cord The spinal cord is divided into cervical, thoracic, lumbar, sacral, and coccygeal levels. -​ Meninges The brain and spinal cord are covered by three layers of meninges: the dura mater, arachnoid mater, and pia mater. Lobes of the Brain and Separating Fissures -​ The major lobes of the brain are the frontal, parietal, temporal, occipital, and limbic lobes. -​ The longitudinal fissure separates the two cerebral hemispheres. -​ The central sulcus separates the frontal and parietal lobes. -​ The lateral fissure (fissure of Sylvius) separates the frontal and parietal lobes from the temporal lobe -​ The parieto-occipital sulcus, on the medial surface, separates the occipital lobe from the parietal and temporal lobes -​ The calcarine fissure is on the medial surface of the occipital lobe Important Gyri and Sulci -​ Gyrus A gyrus is a ridge on the cerebral cortex -​ Sulcus A sulcus is a groove between ridges on the cerebral cortex; a deep sulcus is called a fissure -​ Precentral gyrus Located anterior to the central sulcus, it is the primary motor area. -​ Postcentral gyrus Located posterior to the central sulcus, it is the primary somatosensory area. -​ Superior temporal gyrus Involved in higher-level auditory processing, including language processing and sound localization. -​ Parahippocampal gyrus Plays a role in memory formation, spatial processing, and scene recognition. -​ Fusiform gyrus Involved in facial recognition. -​ Cingulate gyrus Implicated in motivation, error detection, and decision-making. Functions of the Different Lobes -​ Frontal Lobe The frontal lobe is responsible for control of voluntary muscle movements throughout the body, planning and coordination of complex movements, language production and speech formation, higher-order cognitive functions, emotional processing, decision-making, and social behavior -​ Parietal Lobe The parietal lobe processes sensory information related to touch, pressure, temperature, and pain. It is involved in language processing and reading  -​ Temporal Lobe The temporal lobe processes auditory information, is critical for language comprehension and memory formation, and is specialized for facial and object recognition. -​ Occipital Lobe The occipital lobe is the primary visual cortex. -​ Limbic Lobe The limbic lobe is involved in emotional regulation. Arrangement of Gray and White Matter -​ Gray Matter Gray matter consists of nerve cell bodies and is found in the cortical layer and deep nuclei. -​ White Matter White matter consists of fiber tracts (axons) that connect different parts of the CNS and is mostly myelinated. Vessels That Supply the CNS -​ Internal Carotid Artery (ICA) The internal carotid artery has cervical, petrous, cavernous, and cerebral parts. -​ Vertebral-Basilar Arterial System -​ Anterior Spinal Artery The anterior spinal artery is formed by branches of the vertebral arteries and supplies the anterior two-thirds of the spinal cord -​ Posterior Spinal Arteries The posterior spinal arteries originate from the vertebral arteries or the posterior inferior cerebellar arteries (PICAs) and supply the posterior one-third of the spinal cord. Anatomy and Function of the Choroid Plexus -​ The choroid plexus produces cerebrospinal fluid (CSF) and is located in all four ventricles. It consists of ependymal cells that become cuboidal epithelium, known as the choroid epithelium, and capillaries. Circulation of Cerebral Spinal Fluid (CSF) -​ CSF is secreted by the choroid plexus in the ventricles. It moves from the lateral ventricles to the third ventricle, then to the fourth ventricle. CSF flows into the central canal of the spinal cord and exits the fourth ventricle through the foramina of Luschka and Magendie into the subarachnoid space. CSF moves through the subarachnoid space to the arachnoid granulations or villi, which protrude into the superior sagittal sinus, allowing CSF to be reabsorbed into venous blood. Role of Cerebral Spinal Fluid (CSF) in Normal and Pathological Conditions -​ In normal conditions, CSF cushions the brain and spinal cord, removes waste products, and helps maintain a stable chemical environment. -​ In pathological conditions, disruptions in CSF circulation or absorption can lead to hydrocephalus, increased intracranial pressure, and other neurological disorders. Internal Carotid Arterial System -​ The internal carotid artery (ICA) gives rise to the ophthalmic artery, anterior cerebral artery (ACA), middle cerebral artery (MCA), anterior choroidal artery, and posterior communicating artery.  -​ Anterior Cerebral Artery (ACA) supplies the medial portions of the frontal and parietal lobes. -​ Middle Cerebral Artery (MCA) supplies a large portion of the lateral cerebral cortex, including the temporal lobe, insula, and basal ganglia. -​ Anterior Choroidal Artery supplies the posterior limb of the internal capsule, optic tract, lateral geniculate nucleus, globus pallidus, amygdala, and hippocampus. Vertebral-Basilar Arterial System -​ The vertebral arteries merge to form the basilar artery, which supplies the brainstem, cerebellum, and posterior cerebrum -​ The basilar artery gives rise to the posterior cerebral arteries (PCA), which supply the occipital lobe and inferior temporal lobe. Somatotopy and Blood Supply Occlusions -​ Somatotopy refers to the point-for-point correspondence of an area of the body to a specific point on the central nervous system. Occlusions in blood supply can lead to specific deficits based on the affected area's somatotopic organization. For example, lesions in the primary motor cortex (precentral gyrus) typically result in contralateral weakness or paralysis. Similarly, lesions in the primary sensory cortex (postcentral gyrus) often lead to contralateral sensory deficits Circle of Willis -​ The Circle of Willis is an arterial anastomosis that connects the internal carotid and vertebral-basilar systems. It is formed by the anterior communicating artery, which connects the two ACAs, and the posterior communicating arteries, which connect the ICAs to the PCAs. This configuration provides redundancy in blood supply to the brain. Blood Supply Syndromes -​ Occlusion of specific arteries can result in predictable patterns of neurological deficits. -​ Anterior Spinal Artery Syndrome Damage to the anterior spinal artery can cause motor deficits, pain and temperature sensory loss, and autonomic dysfunction -​ Posterior Spinal Artery Syndrome -​ Middle Cerebral Artery (MCA) Syndrome Venous Drainage of the Brain and Major Sinuses -​ The venous drainage of the brain involves superficial and deep veins that drain into the dural venous sinuses. Major sinuses include the superior sagittal sinus, inferior sagittal sinus, straight sinus, transverse sinuses, sigmoid sinuses, and internal jugular vein. Structure of the Blood-Brain Barrier (BBB) and Its Importance -​ The blood-brain barrier (BBB) is a selective barrier that protects the brain from harmful substances while allowing essential nutrients to enter. It is formed by tight junctions between endothelial cells, astrocyte end-feet surrounding blood vessels, and a thick basement membrane. The BBB restricts the passage of large molecules, pathogens, and toxins, maintaining a stable environment for optimal neuronal function. Spinal Cord Functional Anatomy Spinal Cord Structure -​ Surface Features: -​ Spinal Meninges: Similar to the brain, the spinal cord is protected by three layers of meninges. -​ The dura mater is the outermost layer. -​ The arachnoid mater is the middle layer. In life, the subarachnoid space contains cerebrospinal fluid. -​ The pia mater is the innermost layer that adheres tightly to the surface of the spinal cord. -​ White Matter: Consists of axons organized into tracts or columns and is located outside the gray matter area -​ Gray Matter: Located in the center of the spinal cord, the gray matter consists of somas (cell bodies) surrounding the central canal. -​ Rexed's Laminae: The gray matter is organized into distinct layers known as Rexed's laminae -​ Lamina II: Also called the substantia gelatinosa, involved in pain modulation. -​ Laminae III & IV: Form the nucleus proprius, which is the first synapse of the spinothalamic tract. -​ Lamina VII: Contains Clarke's column (nucleus thoracicus dorsalis), which is the origin of the spinocerebellar tract. -​ Lamina IX: Contains alpha and gamma motor neurons. -​ Blood Supply: -​ Anterior Spinal Artery: Supplies the anterior two-thirds of the spinal cord. -​ Posterior Spinal Arteries: Supply the posterior one-third of the spinal cord. -​ Artery of Adamkiewicz: Injury to this artery can lead to paraplegia. Spinal Cord Regional Characteristics -​ The spinal cord consists of 31 segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal Spinal Nerves -​ Functional Components: Not specified in the provided text. -​ Sensory Components: -​ Sensory neurons are pseudounipolar, with cell bodies located in the dorsal root ganglia. -​ These are the first neurons of the spinothalamic tract and dorsal column-medial lemniscus (DC-ML) pathway -​ Neurotransmitters of Primary Sensory Neurons: Primary sensory neurons have their cell bodies in a spinal ganglion -​ Posterior Root Entry Zone: Sensory information enters the spinal cord through the posterior roots. As the posterior root approaches the spinal cord, it separates into rootlets  that enter the spinal cord in the posterolateral sulcus. Fibers carrying pain and temperature enter the spinal cord in the posterior horn where they then ascend or descend several spinal levels in the posterolateral funiculus or Lissauer tract -​ Motor Components: -​ Motor information leaves the spinal cord through the anterior roots -​ Lower motor neurons (LMNs) are located in the anterior horn at each spinal level. -​ Primary efferent autonomic fibers also have their cell bodies in the lateral horn and leave the spinal cord through the anterior root. -​ The anterior corticospinal tract carries motor information from the cortex to LMNs concerned with proximal trunk musculature. -​ The lateral corticospinal tract descends from the forebrain, having crossed in the brainstem, to reach the LMNs at each spinal cord level. -​ Neurotransmitters of Spinal Motor Neurons: -​ The source states that the neurotransmitter used at the neuromuscular junction is acetylcholine (ACh). -​ When an action potential reaches the axon terminal, Ca2+ channels open and the influx of Ca2+ causes vesicles filled with neurotransmitters to fuse with the membrane releasing neurotransmitters into the synaptic cleft. The neurotransmitter binds to postsynaptic receptors, and ion channels open Spinal Reflexes -​ Reflex Arc: A simple neuronal circuit in which a sensory stimulus initiates a motor response directly. -​ Muscle Stretch Reflex (Myotatic Reflex): Initiated by stretching a muscle, causing contraction of the same muscle. -​ Sensory signals are carried by Ia nerve afferents to the spinal cord, where they synapse with α-motor neurons innervating the same muscle. -​ Reciprocal Innervation: Ia interneurons inhibit α-motor neurons that innervate antagonist muscles. -​ Can be monosynaptic. -​ Flexor Reflex: Withdraws a limb from a painful stimulus. -​ Polysynaptic -​ Intensely painful stimuli could cause flexor muscles to become tetanic if the reflex circuit were unregulated. Regulation comes in the form of Renshaw cells, which are a special class of spinal inhibitory interneuron that are excited by α-motor neuron collaterals. -​ Crossed Extension Reflex: Braces the opposing limb for weight transfer during withdrawal from a painful stimulus -​ Polysynaptic Pathways and Tracts of the Spinal Cord -​ Ascending Tracts:  -​ Dorsal Column-Medial Lemniscus System (DC-ML): Carries information about discriminative touch, vibration, pressure, and proprioception. -​ Pathway crosses in the medulla -​ Anterolateral System (Spinothalamic Tract): Carries information about pain, temperature, and non-discriminative touch. -​ Fibers cross to the contralateral spinal cord via the anterior white commissure. -​ Spinocerebellar Tracts: Carry proprioceptive information to the cerebellum. -​ Descending Tracts: -​ Lateral Corticospinal Tract: Controls motor function. -​ Most fibers originate in the primary motor cortex (Brodmann’s area 4) of the precentral gyrus. -​ Anterior Corticospinal Tract: Important for ipsilateral postural adjustments during contralateral extremity movements. -​ Rubrospinal Tract: Motor control of the upper limbs. Deficits Characteristic of Spinal Cord Lesions -​ General Symptoms: -​ Plegia: Complete lesion resulting in paralysis. -​ Paresis: Some muscle strength is preserved. -​ Tetraplegia (Quadriplegia): Injury of the cervical spinal cord. -​ Paraplegia: Injury of the thoracic or lumbo-sacral cord or cauda equina. -​ Hemiplegia: Paralysis of one half of the body, usually in brain injuries. Spinal cord Syndromes Lesions of the Anterior Column -​ The main tract in the anterior column is the anterior corticospinal tract, which carries motor information from the cortex to lower motor neurons (LMNs) concerned with proximal trunk musculature. -​ Damage to the anterior horn cells at a specific spinal cord level would cause weakness in muscles innervated by the lower motor neurons originating from that level, as well as reduced or absent reflexes. Lesions of the Lateral Column -​ The lateral corticospinal tract, which is the main motor tract to the spinal cord, descends from the forebrain and crosses in the brainstem to reach the LMNs. Therefore, a lesion in the lateral corticospinal tract would result in ipsilateral loss of motor function. This could manifest as paresis (partial weakness) or paralysis (complete weakness), along with spasticity and brisk deep tendon reflexes. -​ The spinothalamic tract is located in the anterior part of the lateral column and carries pain and temperature information from the contralateral side of the body. Therefore, a lesion would cause loss of pain and temperature sensation on the contralateral side.  ​ Spinocerebellar tracts carry proprioceptive information to the ipsilateral cerebellum. Lesions of the Posterior Column -​ The posterior column contains sensory information concerning discriminative touch and proprioception from the ipsilateral side of the body. -​ A lesion in the posterior column would cause a loss of discriminative touch and proprioception on the ipsilateral side of the body. Information from the lower part of the body (T6 and below) ascends in the fasciculus gracilis, while information from the upper part of the body (above T6) ascends in the fasciculus cuneatus. Anterior Spinal Cord Syndrome -​ Anterior cord syndrome results from ischemia to the anterior two-thirds of the spinal cord due to disruption of the anterior spinal artery. -​ This leads to loss of pinprick/temperature sensation (spinothalamic tract) and flaccid paraplegia with absent deep tendon reflexes (DTRs) in the legs (anterior horn cells). -​ It can also affect the corticospinal tract (UMN fibers), causing flaccid paraplegia with reduced reflexes initially, followed by spasticity and brisk reflexes with upgoing toes. Syringomyelia (central canal lesion) -​ A late consequence of trauma to the spinal cord can be the development of a syrinx -- a widening of the central canal within the spinal cord. Brown-Séquard Syndrome (Hemisection of the Spinal Cord) -​ Hemicord injury results in a specific array of symptoms due to the tracts transected on one side. -​ Ipsilateral loss of motor function due to transection of the lateral corticospinal tract. -​ Ipsilateral loss of discriminative touch and proprioception due to transection of the posterior columns. -​ Contralateral loss of pain and temperature sensation. Chapter 3 Peripheral Nerve Anatomy -​ The peripheral nervous system (PNS) consists of cranial and spinal nerves that link the brain and spinal cord to the periphery and visceral tissues. -​ Peripheral nerves contain both somatic and visceral information. -​ Peripheral nerves are arranged in bundles called fasciculi, surrounded by connective tissue sheaths. -​ The layers of connective tissue sheaths include: -​ Epineurium: The external, vascular connective tissue layer around the nerve fascicles. -​ Perineurium: Connective tissue covering each individual fascicle. -​ Endoneurium: A thin layer of collagenous fibers that covers individual axons. Classification of Peripheral Nerve Fibers (Nerve Anatomy) -​ Peripheral nerve fibers are classified based on conduction velocity or axonal diameter.  -​ Conduction velocity depends on axon diameter and myelination. -​ Classification based on conduction velocity uses letters A, B, and C: -​ A fibers: The fastest, further divided into Aα, Aβ, Aδ, and Aγ -​ B fibers: Smaller, myelinated, preganglionic visceral motor (autonomic) fibers -​ C fibers: Small diameter, unmyelinated, postganglionic visceral motor (autonomic) and some sensory fibers. -​ Classification based on axon diameter is used for sensory fibers, using Roman numerals I, II, III, and IV, with I being the largest. -​ Sensory fibers can be further divided using "a" and "b" (e.g., Ia, Ib fibers). Sensory Transduction -​ Sensory receptors detect information from the environment or from within the body. -​ Sensory receptors act as transducers, transforming physical or chemical stimuli into electrical impulses. -​ They translate stimuli into receptor potentials, which are electrical signals caused by the opening and closing of ion channels. -​ Each sensory receptor has a receptive field, allowing discrimination of the location of the sensory stimulus. -​ Sensory receptors can be classified by the source of the stimulus or the mode of detection: -​ Exteroceptors: Located superficially in the skin, respond to external stimuli like pain, temperature, touch, and pressure. -​ Proprioceptors: In muscles, tendons, and joints, signal body position and movement. -​ Enteroceptors: Monitor events within the body, like movement through the gut. Neuromuscular Junction -​ The neuromuscular junction (NMJ), or motor endplate, is a chemical synapse between motor nerve fibers and muscle fibers. -​ A motor unit consists of a motor neuron and the muscle fibers it innervates. -​ The more precise the required movement, the smaller the motor unit. -​ The NMJ has three components: -​ Axonal endings from the motor neuron. -​ Postsynaptic skeletal muscle membrane. -​ Associated Schwann cells -​ Action potentials travel along the motor neuron and depolarize the axon terminal, causing an influx of Ca2+ through voltage-gated channels. -​ The increase in intracellular Ca2+ causes synaptic vesicles to fuse with the membrane and release the neurotransmitter acetylcholine (ACh) into the synaptic cleft. -​ ACh binds to ACh receptors on the skeletal muscle membrane, causing an influx of Na+ and generating an excitatory postsynaptic potential, or endplate potential. -​ This change in muscle membrane potential triggers the opening of voltage-gated Ca2+ channels in the sarcoplasmic reticulum, leading to muscle contraction Neuromuscular Junction (In greater detail) Overview -​ The neuromuscular junction (NMJ), also known as the motor endplate, is a specialized chemical synapse between a motor nerve fiber and a muscle fiber. -​ It is the site where a motor neuron communicates with a skeletal muscle fiber to initiate muscle contraction. Components of the NMJ -​ The NMJ has three main components: -​ Axonal endings of the motor neuron. As the motor nerve axon approaches its target muscle, it branches extensively, with each axonal process innervating one muscle fiber. -​ Postsynaptic skeletal muscle membrane. -​ Associated Schwann cell. Signal Transduction at the NMJ ​ The following sequence of events leads to signal transduction at the NMJ: 1.​ Action potential arrival: An action potential travels along the motor neuron and reaches the axon terminal. 2.​ Calcium influx: The depolarization of the axon terminal causes voltage-gated Ca2+ (calcium) channels to open, leading to an influx of Ca2+ into the axon terminal. 3.​ Neurotransmitter release: The increase in intracellular Ca2+ triggers the synaptic vesicles to fuse with the presynaptic membrane and release the neurotransmitter acetylcholine (ACh) into the synaptic cleft. 4.​ Receptor binding: ACh diffuses across the synaptic cleft and binds to ACh receptors on the skeletal muscle membrane. These receptors are ligand-gated ion channels. 5.​ Endplate potential generation: The binding of ACh to its receptors causes the opening of ion channels, resulting in an influx of Na+ (sodium) into the muscle fiber [4, 5]. This influx of positive charge leads to the generation of an excitatory postsynaptic potential (EPSP), also known as an endplate potential. 6.​ Muscle contraction: If the endplate potential is large enough to reach the threshold, it triggers the opening of voltage-gated Ca2+ channels in the sarcoplasmic reticulum (the endoplasmic reticulum within the muscle fiber). The subsequent influx of Ca2+ into the muscle fiber initiates muscle contraction. 7.​ Signal Termination: Acetylcholinesterase hydrolyzes acetylcholine, clearing it from the synaptic cleft. Motor Unit -​ The motor neuron and all the muscle fibers it innervates are collectively referred to as a motor unit. -​ The size of a motor unit varies depending on the precision of movement required. Muscles requiring fine control (e.g., eye muscles) have smaller motor units, with one nerve fiber controlling very few muscle fibers. Clinical Significance -​ Diseases like myasthenia gravis can affect the NMJ, leading to muscle weakness. -​ Myasthenia gravis is an autoimmune disease that attacks the acetylcholine receptors -​ Guillain-Barré syndrome is an inflammatory disorder of peripheral nerves where demyelination leads to a conduction block. Both sensory and motor signaling are affected. -​ Guillain-Barre syndrome is an autoimmune disease of the PNS that affects Shwann cells -​ Note: MS is an autoimmune disease of the CNS that affects the oligodendrocytes -​ Understanding the NMJ is crucial for diagnosing and treating various neuromuscular disorders Tracts: lateral and anterior corticospinal, spinothalamic, DC-medial lemniscal path, rubrospinal tract (5 tracts in total) Lateral Corticospinal Tract (Descending) -​ Function: Mediates voluntary motor activity, particularly to the limbs. -​ Origin: Cerebral cortex. -​ Decussation: Fibers cross in the medulla. -​ Location in spinal cord: Located medially in the lateral column, adjacent to the gray matter. Fibers to the upper body are located most medially. -​ Termination: Anterior horn cells (lower motor neurons) at appropriate spinal levels. -​ Lesions: -​ Ipsilateral loss of motor function. -​ May result in paresis (partial weakness) or paralysis (complete weakness), as well as spasticity and brisk deep tendon reflexes. Anterior Corticospinal Tract (Descending) -​ Function: Carries motor information to lower motor neurons concerned with proximal trunk musculature. -​ Origin: Cerebral cortex. -​ Decussation: Fibers cross over at the level of the spinal cord at which they innervate the LMNs. -​ Location in spinal cord: Lies medially in the anterior column. -​ Termination: Lower motor neurons. -​ Innervation: Mostly bilateral to allow for maintaining posture during upright gait. Spinothalamic Tract (Ascending) -​ Function: Carries pain and temperature information. -​ Origin: Posterior horn. -​ Decussation: Fibers cross the midline in the anterior white commissure. -​ Location in spinal cord: Located in the anterior part of the lateral column.  -​ Termination: Thalamus. -​ Lesions: -​ Loss of pain and temperature sensation on the contralateral side of the body. -​ The loss usually occurs a few segments below or above the level of the lesion because pain and temperature fibers travel a few segments in the Lissauer tract before they synapse. DC-Medial Lemniscal Pathway (Posterior Columns) (Ascending) -​ Function: Carries sensory information concerning discriminative (fine) touch, vibration, and proprioception. -​ Origin: Sensory receptors. -​ Decussation: Fibers cross the midline in the caudal medulla. -​ Location in spinal cord: Ascends in the ipsilateral posterior columns. -​ Termination: Thalamus. -​ Organization: -​ Sensory information from the lower part of the body (T6 and below) ascends in the fasciculus gracilis, which is located medially. -​ Information from the upper part of the body (above T6) ascends in the fasciculus cuneatus, which is wedged in laterally between the fasciculus gracilis and the posterior horn. -​ Lesions: Loss of discriminative touch and proprioception on the ipsilateral side of the body. ​ Rubrospinal Tract (Descending) -​ Location in spinal cord: Lateral column. -​ Function: Coordination of movement between the LMN circuits and the cerebellum and cortex -​ The rubrospinal tract is a descending motor pathway that can be found in the lateral column of the spinal cord -​ Originates from the red nucleus in the midbrain and primarily facilitates flexor muscle activity in the upper limbs while inhibiting extensors, playing a role in motor coordination Lateral & Anterior Corticospinal Tracts: 2 neurons (Upper Motor Neuron from motor cortex → Lower Motor Neuron in spinal cord). Spinothalamic Tract: 3 neurons (Primary sensory neuron → Secondary neuron in spinal cord → Tertiary neuron in thalamus). Dorsal Column-Medial Lemniscal Pathway: 3 neurons (Primary sensory neuron → Secondary neuron in medulla → Tertiary neuron in thalamus). Rubrospinal Tract: 2 neurons (Neuron from red nucleus → Lower Motor Neuron in spinal cord) The alpha motor neuron is the lower motor neuron (LMN) located in the ventral horn of the spinal cord, responsible for directly innervating and contracting skeletal muscle fibers. It is the final output neuron in motor pathways like the corticospinal and rubrospinal tracts. The myotatic reflex, also known as a stretch reflex or deep-tendon reflex, is initiated by stretching a muscle, which causes the contraction of the same muscle. It is designed to resist inappropriate changes in muscle length and is important for maintaining posture. -​ Initiation: The reflex is initiated by tapping a tendon, such as the patellar ligament, which stretches the associated muscle (e.g., quadriceps). -​ Muscle Spindles: Stretching the muscle activates muscle spindles, which are sensory receptors within the muscle body. These spindles contain intrafusal fibers that monitor muscle length and changes in length. -​ Sensory Signals: Sensory signals are carried by Ia nerve afferents to the spinal cord. These afferent fibers are wide and myelinated to maximize signal conduction velocity. -​ Synaptic Connection: Within the spinal cord, the Ia nerve afferents synapse with and excite α-motor neurons that innervate the same muscle. -​ Muscle Contraction: The α-motor neurons cause the muscle to contract reflexively. For example, tapping the patellar ligament stretches the quadriceps, leading to a reflexive contraction that extends the leg and causes the foot to jerk forward. -​ Reciprocal Innervation: The Ia interneuron is activated by the same Ia afferent signal that caused the quadriceps to contract. The interneuron synapses with and inhibits the α-motor neurons that innervate the hamstring muscles and, thereby, allows the leg to extend without resistance. This circuitry is referred to as reciprocal innervation and is used commonly in situations in which two or more sets of muscles oppose each other around a joint. -​ Monosynaptic: The myotatic reflex involves a direct synapse between the sensory neuron and the motor neuron, making it a monosynaptic reflex. The inverse myotatic reflex, also referred to as the Golgi tendon reflex, is activated when a muscle contracts and the Golgi tendon organs (GTOs) are stretched. -​ Location of GTOs: GTOs are located at the musculotendinous junction and monitor the amount of tension that develops in a muscle when stretched. -​ Activation: Stretching or contraction of a muscle stretches the GTO. Collagen fibers within the GTO tighten and compress nerve endings, opening mechanosensitive channels and increasing nerve-firing rates. -​ Type Ib afferents: Type Ib afferents from GTOs synapse with Ib inhibitory interneurons upon entering the spinal cord. -​ Action: When activated, they inhibit α-motor output to the homonymous muscle, while excitatory interneurons simultaneously activate α-motor output to the heteronymous muscle. -​ Function: The Golgi tendon reflex is believed to be important for fine motor control and for maintaining posture, acting synergistically with the myotatic reflex.