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Questions and Answers
What is the primary function of the cerebellum?
Which condition is caused by damage to the cerebellum, resulting in the inability to perform rapid alternating muscle movements?
What is the primary function of the brainstem?
Where does the cerebrospinal fluid (CSF) primarily flow after exiting the 4th ventricle?
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Which layers surround the brain beginning from the outermost layer?
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How is the blood-CSF barrier maintained?
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What type of information do dorsal roots carry in the spinal cord?
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What is the approximate volume of cerebrospinal fluid present in the central nervous system at any given time?
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What part of the central nervous system acts as the primary conduit for motor signals?
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Which of the following cranial nerves is primarily responsible for sight?
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What is the primary role of the hypothalamus in integrative brain functions?
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Which type of reflex involves rapidly removing a limb from an injurious stimulus?
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Which statement correctly describes upper motor neurons?
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What type of sensory information do muscle spindles primarily provide?
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Which of the following is a characteristic of Golgi tendon organs?
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Which reflex arc primarily uses type 1a sensory neurons?
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Which structure primarily contributes to muscle spindle sensitivity?
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What is a common sign of damage to lower motor neurons?
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What type of neuron is primarily responsible for carrying sensory information concerning proprioception?
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What function do reticulospinal pathways serve in motor control?
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What significant effect does damage to upper motor neurons have on muscle control?
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Which part of the reflex arc is responsible for connecting sensory and motor pathways?
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How does the flexor withdrawal reflex affect opposing muscles?
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Study Notes
Brain Structure
- Enclosed in skull, increased pressure leads to reduced blood flow and potential herniation
- Brain has a highly selective blood-brain barrier (BBB)
- Small lipophilic substances like oxygen, carbon dioxide, and ethanol cross freely
- Glucose enters via facilitated diffusion through GLUT-1 protein
- BBB can be disrupted by factors like hypertension, hyperosmolality, infection, or trauma
- Neurons are the functional units of the brain with a soma (body), dendrites (receiving signals), and an axon (transmitting signals)
- Unipolar: one neurite
- Bipolar: two neurites
- Multipolar: three or more neurites, the most common type
- Dendrites can be spiny or aspinous
- Axons are classified as Golgi type I (long, projecting to different parts of the system) or Golgi type II (short, involved in local circuits)
- Neurons can be sensory (afferent), motor (efferent), or interneurons
Glial Cells
- Glial cells outnumber neurons in the brain by a factor of 10
- They provide support and are classified into four types:
- Ependymal cells: line ventricles, produce cerebrospinal fluid (CSF)
- Astrocytes: maintain structural integrity, store glycogen and provide lactate to neurons, regulate potassium levels, remove neurotransmitters, synthesize neurotransmitter precursors, and contribute to the BBB
- Microglia: immune cells, act as phagocytes
- Oligodendrocytes (CNS) and Schwann cells (PNS): produce myelin sheaths that insulate axons
Neuronal Injury
- Necrosis: cell lysis and inflammation, caused by acute trauma or stroke
- Apoptosis: programmed cell death without inflammation
- Gliosis: proliferation of astrocytes after CNS injury, leading to scarring and impaired function
- Gliosis is a common consequence of stroke, multiple sclerosis, and Alzheimer's disease
Neuronal Signaling
- Action potential (AP) generation:
- Triggered when threshold potential is reached
- Sodium channels open, causing depolarization (sodium influx)
- Absolute refractory period: neuron cannot fire again
- Potassium channels open, causing repolarization (potassium efflux)
- Relative refractory period: neuron is less likely to fire
- Membrane potential drops below resting potential (hyperpolarization)
- Synapse: communication between neurons
- Neurotransmitters (NTs) are synthesized and packaged into vesicles
- Vesicles fuse with the presynaptic membrane, releasing NTs into the synaptic cleft
- NTs bind to ligand-gated channels on the postsynaptic neuron, leading to excitatory or inhibitory postsynaptic potentials
- Postsynaptic potentials are graded:
- Excitatory potentials cause depolarization, increasing the likelihood of an AP
- Inhibitory potentials cause hyperpolarization, decreasing the likelihood of an AP
- AP firing occurs when the sum of excitatory and inhibitory potentials surpasses the threshold
Neurotransmitters
- Classified as small molecule (amino acids, monoamines, acetylcholine) or large molecule (neuropeptides)
-
Small
- Glutamate: excitatory, acts through ionotropic (AMPA, NMDA, Kainate) and metabotropic receptors
- GABA: inhibitory in the brain, acts through ionotropic (GABA A) and metabotropic (GABA B) receptors
- Glycine: inhibitory in the spinal cord, acts through ionotropic receptors
- Monoamines:
- Serotonin (5HT): involved in limbic function, reuptake dysfunction linked to depression, acts through ionotropic (5HT3) and metabotropic (5HT1-7) receptors
- Catecholamines: act as both neurotransmitters and hormones
- Dopamine: produced in the substantia nigra, acts through metabotropic receptors (D1: excitatory, D2: inhibitory)
-
Large/Neuropeptides:
- Opioids: bind to mu, kappa, and delta receptors
- Beta-endorphins bind to mu receptors, inhibiting substance P (pain transmission) in the PNS and GABA release (excess dopamine) in the CNS, reuptake dysfunction related to Parkinson's disease
- Opioids: bind to mu, kappa, and delta receptors
Sensory Neurophysiology
- Stimulus: distinct type of energy characterized by modality, intensity, location, and duration
- Receptors: convert stimuli into electrical signals
- Receptor adaptation: decline in action potential generation with continuous stimulus
- The somatosensory system processes cutaneous sensations (touch, vibration, pain, temperature), proprioception, and discriminative fine touch
- Merkel's disks: slowly adapting, sense steady pressure
- Meissner's corpuscles: rapidly adapting, sense light pressure
- Ruffini endings: large receptive fields, sense skin stretching, slowly adapting
- Pacinian corpuscles: very rapidly adapting, sense vibration
- Hair follicles: detect hair displacement
- Ascending tracts:
- Dorsal column-medial lemniscus: carries discriminative touch, vibration, proprioception, and pressure information, decussation occurs in the medulla
- Spinothalamic tract: carries temperature, pain, and localizing touch information, decussation occurs in the spinal cord
- Primary somatosensory cortex: area in the cortex dedicated to sensory processing, size reflects the amount of sensation (e.g., large areas for hands, mouth, tongue, lips, ears)
- Secondary somatosensory cortex: posterior to primary somatosensory cortex, integrates touch with other sensations, damage can cause agnosia (inability to recognize objects)
- Visceral pain is poorly localized:
- Myocardial infarction: left chest wall, neck or jaw, shoulder, arm pain
- Diaphragm irritation: right shoulder pain (cholecystitis, hepatitis) or left shoulder pain (ruptured spleen)
- Stomach ulcer or cancer: epigastrium, midback pain
- Lower lobe pneumonia: upper quadrant pain on the same side as the pneumonia
- Appendicitis: periumbilical area pain
- Kidney stone: flank pain radiating to groin, testicle, or labia majora
- Pain gating:
- Touch fibers synapse on inhibitory interneurons, releasing enkephalins (opioids) to inhibit pain transmission
Motor Neurophysiology
- Alpha motor neurons control muscle contraction
- Three inputs influence alpha motor neurons:
- Upper motor neurons: regulate voluntary movement, inhibitory
- Spinal interneurons: excitatory or inhibitory, complex circuitry
- Sensory neurons from muscle proprioceptors: provide feedback on muscle length and tension
- Muscle spindles:
- Sensory neurons (Type 1a: fast, convey stretch and speed information)
- Intrafusal muscle fibers innervated by gamma motor neurons (regulate spindle sensitivity)
- Golgi tendon organs:
- Sensory nerve fibers in tendons (Type 1b), convey muscle force information
- Myotatic (muscle stretch) reflex:
- Tapping on tendon stretches the muscle, activating Type 1a neurons
- Agonist muscle contracts, antagonist muscle inhibits
- Flexor withdrawal reflex:
- Rapidly removes limb from harmful stimuli
- Crossed extensor reflex supports the body using the opposite limb
- Descending motor pathways:
- Lateral pathways: voluntary movement
- Corticospinal (pyramidal) tract: upper motor neurons originate in the motor cortex, descend through the internal capsule and brainstem, cross at the medullary pyramids, lesions above the cross cause contralateral weakness, lesions below the cross cause ipsilateral weakness
- Rubrospinal tract: originates in the red nucleus of the midbrain, crosses immediately, controls muscle tone in flexor groups, decorticate posture (elbow flexion) indicates a lesion rostral to the red nucleus, decerebrate posture (elbow extension) indicates a lesion caudal to the red nucleus
- Ventromedial pathways: body position and balance
- Vestibulospinal tract: posture and equilibrium
- Tectospinal tract: head and eye movements
- Reticulospinal tract: posture
- Lateral pathways: voluntary movement
Upper Motor Neuron (UMN) Damage
- Cell bodies located in the brain or brainstem
- Signs:
- Spastic paralysis
- Hyperreflexia
- Positive Babinski reflex
- Increased muscle tone
- Example: stroke
Lower Motor Neuron (LMN) Damage
- Cell bodies located in the anterior horn of the spinal cord
- Signs:
- Flaccid paralysis
- Areflexia
- Decreased muscle tone
- Atrophy
- Fasciculations (muscle twitch)
- Example: polio
Motor Loop
- Direct pathway: putamen → internal segment of globus pallidus (inhibitory via GABAergic neurons)
- Indirect pathway: putamen → external segment of globus pallidus and subthalamic nucleus (excitatory)
Integrative and Behavioral Functions
- Not directly involved in sensory or motor pathways
- Include emotions, motivated behavior, consciousness, language, memory, cognition
- Hypothalamus: controls homeostatic functions and motivated behaviors, receives input from visceral and somatic sensory pathways, frontal lobe and limbic system, outputs to the endocrine system, regulates temperature, eating, circadian rhythms, and sex drive
- Temperature regulation: normal range 98.6 +/- 1 degree Fahrenheit, fever above 100.4 degree Fahrenheit, cytokines stimulate prostaglandin E2 (PGE2) release, PGE2 raises set point temperature in the hypothalamus, negative feedback loop involving peripheral and central thermoreceptors, effectors include skin circulation, metabolic rate, sweating, and behavioral changes
- Reticular formation: columns of neurons throughout the brainstem, influences wakefulness (reticular activating system), serotonergic (wakefulness, sleep, emotions, mood), noradrenergic (general arousal), cholinergic (attention, memory, learning), dopaminergic (voluntary movement, rewards)
- Sleep: non-REM (reduced heart rate and respiration, relaxed muscles, stage 3 most restful) and REM (dreaming, awake brain in a paralyzed body)
- Limbic system:
- Cingulate cortex: involved in higher-level cognition, perception of emotions
- Hippocampus: converts short-term memory to long-term memory, declarative (facts, events) and non-declarative (procedural, learned emotions, conditioned reflexes)
- Amygdala: strong emotions (fear, aggression), links emotions to memories, cognitive fear (learned response) and reactive fear (direct threat), hippocampus exerts negative feedback on fear response in response to increased cortisol levels
- Language and speech:
- Broca's area: controls motor movements for speech production, Broca's aphasia (impaired speech production but intact comprehension)
- Wernicke's area: involved in language comprehension and processing, Wernicke's aphasia (preserved speech production but impaired comprehension)
Brain Regions
- Cerebrum: generates motor function, processes sensory information, high-order thinking
- Cerebellum: equilibrium, coordination, functions include movement coordination, muscle tone maintenance, and motor learning, damage can lead to ataxia (lack of muscle coordination), hypotonia (decreased muscle tone), dysmetria (under/over-shooting intended positions), intention tremor (tremor during movement toward target), and dysdiadochokinesia (inability to perform rapid alternating movements)
- Brainstem: conduit for information flow between the cerebrum and spinal cord, responsible for breathing, consciousness, blood pressure, heart rate, and sleep
- Cortex: outer layer of gray matter
- Gyri: folds or bumps
- Sulci: fissures, major sulci divide the cortex into frontal, parietal, occipital, and temporal lobes
- Spinal cord: continuous with the brainstem, terminates at the conus medullaris, conduit for sensory and motor information between the brain and periphery, each spinal nerve attaches via dorsal and ventral roots, axons in white matter are organized into tracts carrying information to and from the brain (dorsal: sensory, ascending; ventral and lateral: sensory and motor, ascending and descending)
Peripheral Nervous System (PNS)
- 12 cranial nerve pairs
- 31 spinal nerve pairs (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal)
Meninges
- Dura mater: outermost layer
- Arachnoid mater: middle layer
- Subarachnoid space: contains CSF
- Pia mater: innermost layer
Extracellular Fluid Compartments of the CNS
- Blood plasma: within the vascular system
- Interstitial fluid: between neural and glial cells
- CSF: in ventricular system and subarachnoid space
- Choroid plexus: network of blood vessels in each ventricle, derived from pia mater, connective tissue, and epithelial cells, filters blood plasma to produce CSF, maintains blood-CSF barrier through tight junctions
- CSF flow: lateral ventricles → interventricular foramen → 3rd ventricle → cerebral aqueduct → 4th ventricle → central canal of the spinal cord, continuously secreted by the choroid plexus, exits the 4th ventricle into the subarachnoid space, drains into the venous system through arachnoid granulations
- Lumbar puncture: performed at the L3-L4 level to obtain CSF
- Volume: ~150 mL in CNS, 400-600 mL produced daily
- Functions: protection, homeostasis, waste removal
Importance of Brain Fluid Regulation
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