Endocrinology Flashcards

Question

What is the main stimulant for glucagon release?

Answer 1

Sympathetic stimulation

Answer 2

1. Decrease in glycogen synthesis 2. Increase in glycogenolysis 3. Increase in gluconeogenesis

Answer 3

1. Increase in lipolysis 2. Decrease triglyceride synthesis 3. Increase circulating free fatty acids

Answer 4

Largely caused by a autoimmune attack of the beta cells.

Answer 5

Strong genetic influence. May develop as a result of other endocrine diseases

Answer 6

Glucotoxicity and lipotoxicity

Answer 7

Glucotoxicity may cause damage to the small blood vessels. This may affect the eye, the kidney etc.

Answer 8

Diabetes causes a reduced blood supply to the gums, decrease salivary flow, decreased pH and lower salivary calcium concentration.

Answer 9

The main function is to provide the linkage between nervous and endocrine systems

Answer 10

The hypothalamus is comprised of nerves. It regulates hormone production and release, contraction of the uterus during labour, milk production and release, kidney function and growth and development.

Answer 11

Anterior and posterior (anterior being more vascular and posterior being more neural). They are two independent structures that develop independently embryologically.

Answer 12

The paraventricular nucleus and the supraoptic nucleus

Answer 13

ADH & oxytocin. They are produced in the supraoptic nucleus and paraventricular nucleus and then move into the pituitary gland which distributes it into the blood stream

Answer 14

1. Increase in osmolarity 2. Decrease in blood pressure 3. Fight of Flight response

Answer 15

1. Increase in aquaporins to increase water reabsorption 2. Vasocontriction 3. Increase in aldosterone secretion

Answer 16

The hypophysiotropic hormones coming from the hypothalamus

Answer 17

It is the nonspecific response of the body to any demand made upon it

Answer 18

1. Alarm reaction: Flight or Fight 2. Resistance stage 3. Allostatic overload – stress mediators can have both protective and damaging effects

Answer 19

1. Release of noradrenaline from sympathetic nerve terminals 2. Secretion of adrenaline and noradrenaline from the adrenal medulla

Answer 20

The catecholamines – adrenaline and noradrenaline

Answer 21

We can describe it as a modified sympathetic ganglion. Meaning when it receives sympathetic stimulation, it releases hormones.

Answer 22

A hypophysiotropic hormone corticotrophin releasing hormone stimulate the secretion of adrenocorticotrophic hormone from anterior pituitary. It acts primarily on zona fasciculata to stimulate the release of glucocoricoids

Answer 23

It follow circadian diurnal rhythms

Answer 24

Primary role is increasing blood glucose at the expense of fats and protein. It stimulates hepatic gluconeogenesis and inhibits glucose uptake in tissues

Answer 25

Cortisol increases the sensitivity of the heart and vasculate to adrenaline, noradrenaline and angiotensin II.

Answer 26

The main function of aldosterone is to increase the reabsorption of sodium whilst increasing the excretion of potassium

Answer 27

Phaeochromocytoma – adrenal medulla tumour. This could lead to elevated heart rate, systemic hypertension, anxiety, hyperglycemia and more.

Answer 28

We can separate them into 2 categories: 1. Effects on metabolic pathways – increase of basal metabolic rate 2. Effects on cellular differentiation and development

Answer 29

A thyroid follicle which secretes thyroid hormones (T3 and T4) as well as a C cell which secretes calcitonin

Answer 30

It is two iodine-containing hormones derived from the amino acid tyrosine that are produced by the thyroid follicles

Answer 31

1. Synthesis of thyroglobulin in the cell of the thyroid follicle 2. Thyroglobulin is moved into the follicular space 3. Iodine is moved into the cells from the blood stream via Sodium-Iodine transporter 4. Iodine is move into the follicular space and is oxidised by thyroperoxidase 5. Iodination of thyroglobulin occurs which lead to conjugation when iodine is bound to tyrosine residues 6. The resulted T3 andT4 precursors is than move back into the cell via endocytosis 7. Inside the vesicle, proteolysis occurs and complete forms of T3 and T4 are produced 8. T3 and T4 are move into the blood stream

Answer 32

Iodine needs to be taken from the diet

Answer 33

Only 10% of the T3 comes from the thyroid, most of it comes from conversion of T4 into T3 in other tissues

Answer 34

Hypothalamus releases the hormone TRH which triggers the release of hormone TSH which binds with receptors on the thyroid gland and causes the release of T3 and T4

Answer 35

1. Non-genomic effects – enhances iodine pump, increase iodination, increases proteolysis of thyroglobulin 2. Genomic effects – promoting general gene transcription for proteins related to production of T3 and T4

Answer 36

1. Hypothyroidism | 2. Hyperthyroidism

Answer 37

1. Increase susceptibility to caries and periodontal disease 2. Accelerated dental eruption 3. Burning mouth syndrome 4. Maxillary or mandibular osteoporosis 5. Increased levels of anxiety

Answer 38

1. Salivary gland enlargement 2. Macroglossia 3. Glossitis 4. Delayed dental eruption 5. Compromised periodontal health 6. Dysgeusia

Answer 39

1. Muscle contractility 2. Neurotransmitter release 3. Blood clotting 4. Maintenanc of cellular integrity 5. Bone and teeth structure and strength

Answer 40

1. Absorption/excretion involving intestines and kidneys | 2. Exchange between fixed and free pools

Answer 41

1. Parathyroid hormone 2. Calcitonin 3. Vitamin D

Answer 42

1. it is the main hormone in regulation of plasma calcium levels 2. The receptors on the parathyroid gland sense the levels of calcium and release parathyroid hormone 3. Parathyroid hormone bind with osteoblasts and cause an increase in production RANKL 4. RANKL mobilises osteoclasts which breakdown bone in order to release more into the plasma

Answer 43

1. Conservation of calcium | 2. Enhances action of vitamin D which enhance responsiveness of bones to PTH

Answer 44

Decrease the movement of calcium from the labile pool and inhibits osteoclast activity in bones

Answer 45

Vitamin D binds to VDR and increases DNA transcription

Answer 46

The sympathetic pathway through use of adrenaline and noradrenaline and parasympathetic system

Answer 47

Beta receptors interaction between adrenaline and noradrenaline are more sensitive than the ones with alpha receptors.

Answer 48

1. Blod pressure drop detected by baroreceptors on the aortic arch and carotid sinus 2. This is relayed to CNS 3. Autnomus nervous system causes the release the upregulation of sympathetic dive 4. Release of adrenaline and noradrenaline

Answer 49

When low blood pressure is detected in afferent arteriole, it constricts and reduced GFR

Answer 50

1. Incresin sodium retention 2. Increasin water reabsorption 3. Increasin potassium secretion

Answer 51

1. Low blood pressure 2. Dehydration 3. Low salt 4. High potassium

Answer 52

1. Aldsteron combines with cytoplasmic receptors 2. Hormone-receptr complex initiates transcription 3. New protein channels and pumps are synthesised 4. Aldosterone induced proteins modification 5. Result - increase sodium channel insertion, increased sodium reabsorption and potassium secretion

Answer 53

1. Increase insertion of aquaporins | 2. Increased water intake by inducing thirst

Answer 54

1. Decrease GFR due to decrease of number of nephrons 2. Decrease in secretion of RAAS hormones 3. Decrease sensativity of the baroreceptors

Answer 55

You get vascular remodelling which increase the stiffness of the arterioles which makes it harder to regulate blood pressure

Answer 56

They inhibit ACE and reduce the conversion of Angiotensin I into Angiotensin II

Answer 57

1. Dehydration 2. ELectrolyte imbalance 3. Excesive vasodilation 4. Orthostatic hypotension 5. Dry cough – build up of bradykinin

Answer 58

They promote relaxation of vascular smooth muscle as interfere with calcium entry

Answer 59

It is the organized addition of new tissue that occurs normally in development.

Answer 60

1. Genetics 2. Nutrition 3. Growth hormone 4. Chronic stress and disease

Answer 61

It is a polypeptide hormone produced in anterior pituitary. It is regulate by GHRH and GHIH. It targets many tissues. It alters gene transcription. It promotes using fat stores and promote protein synthesis.

Answer 62

IGFs are Insulin-like growth factors. They are peptide hormones that have strong mitogenic properties – meaning they promote cell division. They are mainly produced in the liver. There are released by presence of the growth hormone.

Answer 63

1. It stimulate proliferation of epiphyseal cartilage 2. It increases conversion of cartilage to new bone 3. It increase the proliferation of periosteal osteoblasts 4. It increase bone remodelling

Answer 64

Ovarian secretion of oestrogen decreases and secretion of FSH and LH increase rather abruptly in bout sixth decade in women

Answer 65

It causes a decrease in bone mineral density. Oestrogen decrease the production of cytokines by bone marrow and immune cells. Without oestrogen, the cytokines are able to accumulate, osteoclasts are able to proliferate, secretion of osteoprotegrin decreases

Answer 66

Oestrogen aids in production of nitric oxide which causes dilation of blood vessels. With lack of oestrogen, lower levels of nitric oxide result in stiffer blood vessels

Answer 67

1. Receptor - site of stimulus transduction 2. Sensory neuron - transmits afferent impulses to CNS 3. Integration centre - in the CNS 4. Motor neuron - conducts efferent impulses from the integration centre to an effector organ 5. Effector - muscle fibre or gland cell that produces response (muscle contraction or gland secretion.

Answer 68

They are the largest reflex systems in the body for balance and coordination. There are two sensory receptors located within muscle that provide rapid feedback signals to the nervous system: msucle spindle and tendon organ.

Answer 69

The muscle spindle detects responds to, and modulates changes in the length of skeletal (extrafusal) muscle fibres to provide an important regulatory function for movemnt and maintenance of posture. It consists of: - 2 sensory afferent fibres: Type 1a (primary, annulospiral endings) and type II (secondary, flower-spray endings) detect length changes - 1 motor efferent fibre: Gamma motor neuron innervates contractile ends of intrafusal fibres - 1 intra-fusal muscle fibre: Specialised fibres within the spindle.

Answer 70

The Golgi Tendon Organ detects changes in muscle force

Answer 71

1. afferent input from sensory endings of muscle spindle fibre 2. alpha motor neuron output to regular skeletal muscle fibre 1->2. Stretch reflex pathway 3. Gamma motor-neuron output to contractile end portions of spindle fibre. 4. Descending pathways co-activating alpha and gama motor neurons.

Answer 72

- The stretch reflex (e.g., knee jerk) is a monosynaptic reflex (one synapse between sensory and motor neurons).

Answer 73

- sustained muscle tension causes steady firing of 1a afferent fibres, maintaining sensitivity. - Gamma motor neurons keep the spindle taut (pulled tight), ensuring it can deetect length changes even during contraction. - Without gamma innervation, the spindle would slack and stop firing. -> Thus gamma activation ensure continuous spindle feedback during muscle shortening -> this is critical for smooth, controlled movements.

Answer 74

- Voluntary contractions involve simultaneous activation of alpha and gama motor neurons by the cerebral cortex. - This is called alpha-gamma coactivation, ensuring spindle sensitivity during movement. - Coactivation is essential for precise voluntary movements.

Answer 75

- Tapping the patellar tendon stretches quadriceps muscle spindles. - This triggers a monosynaptic reflex, contracting the quadriceps (knee jerk). - Used to assess nervous system function; abnormal response indicates pathway issues.

Answer 76

Occurs when a muscle is suddenly unloaded (e.g., dropping a load), causing a rapid decrease in muscle spindle activity, leading to reduced muscle contraction. Involves muscle spindles and is a response to decreased tension.

Answer 77

Triggered by excessive muscle tension detected by Golgi tendon organs (GTOs), causing reflex inhibition (relaxation) of the muscle to prevent injury. Involves GTOs and Ib afferents. - GTOs connect to ~25 extrafusal fibers near the muscle-tendon junction. - Detect muscle tension to protect against excessive load. - When activated (via Ib afferents), they inhibit the muscle, causing relaxation. - GTOs are safety mechanisms, preventing muscle/tendon damage. - They trigger the inverse myotatic reflex (reflex inhibition).

Answer 78

- Triggered by a painful stimulus (e.g., stepping on a tack). - Causes coordinated excitation of flexors and inhibition of extensors in the stimulated limb to withdraw it. - Polysynaptic (multiple synapses), so slower than the stretch reflex. - Polysynaptic pathway allows variability based on stimulus type. - Protects by rapidly removing the limb from harm.

Answer 79

A complex reflex combining: - Ipsilateral withdrawal: Flexor activation in the stimulated limb. - Contralateral extension: Extensor activation in the opposite limb. - Helps maintain balance (e.g., when withdrawing one foot while standing). - Can be modified by voluntary commands.

Answer 80

- ~300 mechanoreceptive afferents per tooth, directionally sensitive. Reflexes: - Tapping/rapid load increase: Inhibits jaw-closer muscles (disynaptic, like withdrawal reflex). - Weak pressure: Excites jaw-closer muscles (pathway unclear). - Functions: Guide teeth into occlusion, coordinate chewing, crush food.

Answer 81

- Jaw jerk reflex is a stretch reflex in jaw-closers, similar to the knee jerk, mediated by muscle spindles. - Unloading reflex in the jaw occurs when resistance drops (e.g., food breaks), reducing jaw-closer activity. - Antagonist muscles (jaw-openers) activate during unloading to stabilize the jaw. - Many muscle spindles in jaw-closer muscles (e.g., masseter, temporalis), very few in jaw-openers (e.g., digastric). - Stretch (or increased resistance) of jaw-closer muscle spindles mediates a monosynaptic reflex excitation of the muscle (jaw jerk reflex). - Plays a role in maintaining rest position (e.g., jogging). - During chewing, stretch reflexes adjust jaw-closer muscle activity based on food texture (e.g., biting a nut causes reflex decrease due to spindle inactivation, known as the unloading reflex). - When unloading occurs, antagonist muscles (digastric, mylohyoid) contract to stiffen the jaws and minimize movement (like a seatbelt mechanism).

Answer 82

- Jaw-opening reflex protects by inhibiting jaw-closers in response to pain/stimuli. - Disynaptic (two synapses), involving inhibition of jaw-closers but not excitation of jaw-openers in humans. - Triggered by various sensory inputs (e.g., periodontal, gingival, mucosal). - Painful oral/perioral stimuli (mechanical or electrical) inhibit jaw-closing muscles. - High-threshold mechanoreceptors and nociceptors in the periodontal ligament, gingiva, oral mucosa, tongue, lips, facial skin, and tooth pulp trigger this reflex. - Disynaptic inhibition of jaw-closer muscles (similar to the flexor withdrawal reflex). - No disynaptic excitation of jaw-openers in humans (only in animals). - Plays a protective role (e.g., fishbone in gingiva triggers jaw opening to prevent injury).

Answer 83

It includes both the awareness of position of body segments in space and the sense of movement. It allows us to: - Perform accurate movements without contiuous visual control - Adjust motor control patterns - Perform motor tasks that require multi-limb coordination.

Answer 84

- Muscle Spindles: Abundant in jaw-closers (e.g., masseter), few in openers; key for jaw position sense and jaw jerk reflex; dominate proprioception in chewing - Periodontal Mechanoreceptors: Ruffini-like, Aβ axons in periodontal ligament; directionally sensitive, aid fine motor control and tactile perception during chewing - Golgi Tendon Organs: Few or absent in jaw muscles, minimal role in masticatory proprioception - Joint Receptors (TMJ): Unclear role in jaw movement control, signal extreme conditions Spindles compensate for reduced periodontal input (e.g., with anesthesia/dentures) for interdental size detection

Answer 85

- Dorsal Column-Medial Lemniscal Pathway: Carries proprioception, fine touch, vibration, pressure, and precise location/intensity from receptors to somatosensory cortex; involves parallel processing for texture/shape - Spinothalamic Pathway: Carries crude touch, pressure, pain, and temperature; less precise, crosses midline - Somatotopy: Organized mapping of body sensations onto the cortex via specific pathways

Answer 86

- Aα axons (fastest) carry proprioceptive signals from muscles. - Aβ axons handle touch; C fibers handle pain/temperature (not proprioception). - Axon diameter correlates with conduction speed and receptor type. - Aα (13-20 μm, 80-120 m/s): Proprioceptors of skeletal muscle. - Aβ: Touch sensations. - C fibers: Pain and temperature.

Answer 87

- Separate Receptor (a): Chemical messenger opens Na⁺ channels, initiating action potentials. - Specialized Ending (b): Local current flow opens voltage-gated Na⁺ channels. - Both produce graded potentials in afferent neurons, transducing stimuli into CNS signals (action potentials). - Receptors convert stimuli into electrical signals via graded potentials and action potentials. - Mechanism depends on receptor type (chemical vs. voltage-gated).

Answer 88

- A sensory unit = One afferent neuron + all its receptor endings. - Receptors in a unit share the same modality (e.g., all detect pressure). - Receptive Field: Area where stimulation generates action potentials in the sensory axon. - Sensory units are specialized for specific stimuli. - Receptive fields define the area a sensory unit monitors.

Answer 89

- Sensory information is coded via specific pathways, frequency, and spatial mapping. - Somatotopy ensures organized cortical representation. - Sensory coding answers: What (modality), how big (intensity), where (location)? - Modality: Determined by receptor type and pathway (labeled lines to cortex). - Intensity: Coded by action potential frequency and recruitment of more sensory units. - Location: Determined by activated receptive field, somatotopic organization, and field size/overlap.

Answer 90

- Spindles and GTOs complement each other: Spindles for length, GTOs for force. - GTOs are active during contraction; spindles are silent unless stretched. - Muscle spindles (parallel to extrafusal fibers) detect length; GTOs (in series) detect tension. - During contraction, GTOs fire (Ib axons) due to increased tension; spindles (Ia axons) are silent if muscle length doesn’t change.

Answer 91

- Muscle spindles are key for proprioception; vibration creates illusions by mimicking stretch. - This shows their dominance in position sense. - Muscle vibration activates spindle afferents, increasing action potential frequency. - Brain interprets this as muscle lengthening, causing a kinesthetic illusion (e.g., incorrect elbow angle matching). - Highlights the critical role of muscle spindles in position sense.

Answer 92

- Skin mechanoreceptors contribute to proprioception via touch and pressure. - Each receptor type has a specific role (detailed in later slides). - Skin receptors: Pacinian corpuscles, Ruffini endings, Meissner’s corpuscles, Merkel’s disks. - Found in hairy and glabrous skin, within dermal/epidermal layers. - Most have non-neural structures (except free nerve endings).

Answer 93

- Microneurography maps receptive fields, showing variability in size. - Small fields = Precise detection; large fields = Broader detection. - Microneurography: Records action potentials from a single sensory axon in the median nerve to map receptive fields. Results: - Small receptive fields: Meissner’s corpuscles, Merkel’s disks. - Large receptive fields: Pacinian corpuscles, Ruffini endings.

Answer 94

- Adaptation rate (fast/slow) and field size (small/large) define receptor function. - Fast-adapting = Detect changes; Slow-adapting = Detect sustained stimuli. Skin receptors vary in receptive field size and adaptation rate: - Rapid Adaptation, Small Field: Meissner’s corpuscle. - Rapid Adaptation, Large Field: Pacinian corpuscle. - Slow Adaptation, Small Field: Merkel’s disk. - Slow Adaptation, Large Field: Ruffini ending.

Answer 95

- Skin receptors contribute to proprioception by detecting movement-related stimuli. - Frequency sensitivity varies: Pacinian (high), Merkel (low). Receptor types and effective stimuli: - Merkel’s Disks (SA I): Sustained contact, pressure (<5 Hz). Meissner’s Corpuscles (FA I): Lateral movement, vibration (5-50 Hz). - Ruffini Endings (SA II): Sustained skin stretch/tension. - Pacinian Corpuscles (FA II): High-frequency vibration (40-400 Hz). - Joint movement activates most receptors: Ruffini (SA II), Pacinian (FA II), Merkel (66%), Meissner (57%).

Answer 96

Joint receptors are protective, signaling extreme conditions. Less reliable for precise joint angle detection. - Joint receptors (e.g., TMJ) include free nerve endings similar to GTOs. - Signal damaging states: High capsule tension, extreme joint angles, inflammation. - Respond to flexion, extension, compression, stress. - Limited ability to signal joint angle (minimal response to passive movement). - Role in jaw movement control (TMJ) is unclear.

Answer 97

Muscle spindles are the primary contributors to proprioception. Joint and cutaneous receptors play a secondary role. - Proprioception is best with all mechanoreceptors (Condition 1). - Muscle/tendon receptors (spindles, GTOs) contribute most, especially at slow speeds (Condition 2). - Joint + cutaneous receptors (Condition 4) outperform cutaneous only (Condition 5).

Answer 98

Dorsal column-medial lemniscal pathway is the main route for proprioception. It’s specialized for precise sensory processing. - Proprioceptive information travels via the dorsal column-medial lemniscal pathway to the somatosensory cortex. This pathway handles: - Fine touch, precise location/intensity. - Vibration, movement against skin. - Proprioception, pressure. - Includes parallel pathways (e.g., for texture, shape).

Answer 99

Proprioceptive signals enter the spinal cord, branch for reflexes, and ascend via the dorsal column-medial lemniscal pathway. Pain (nociceptors) signals cross the midline, ascend via the spinothalamic pathway (handles pain, temperature, crude touch/pressure, poorly localized). Some crude proprioceptive info travels via the spinothalamic pathway. - Proprioceptive signals enter the spinal cord, branch for reflexes, and ascend via the dorsal column-medial lemniscal pathway. - Pain (nociceptors) signals cross the midline, ascend via the spinothalamic pathway (handles pain, temperature, crude touch/pressure, poorly localized). - Some crude proprioceptive info travels via the spinothalamic pathway.

Answer 100

Somatotopy ensures organized sensory representation in the brain. Specific pathways link body regions to cortical areas. - Somatotopy: Mapping of body sensations onto the somatosensory cortex. - Sensory information excites specific cortical areas via defined pathways.

Answer 101

Sensitivity depends on receptor density, field size, and cortical representation. Fingers/face have finer discrimination due to these factors. - Two-point discrimination varies by body region (fingers/face = high sensitivity). Due to: - Higher receptor density. - Smaller receptive fields. - Larger cortical representation. - Two points felt if separate fields are activated; one point felt if fields overlap.

Answer 102

Proprioception integrates sensory and motor signals. Efference copy = Brain’s copy of motor commands, aiding perception.

Answer 103

Periodontal mechanoreceptors are crucial for jaw proprioception and chewing control. They detect force and direction, contributing to fine motor adjustments. - Periodontal ligament (attaches teeth to bone) is rich in low-threshold mechanoreceptors (Ruffini-like, Aβ axons). - Directionally sensitive to mechanical loads on teeth, aiding motor control during chewing. - Respond to low forces; 80% saturate at 3-4 N, 20% respond linearly to higher forces. - Key for tactile perception and proprioception (force amplitude, direction, teeth contact).

Answer 104

Proprioception allows fine detection of objects between teeth. Muscle spindles dominate jaw proprioception; periodontal receptors play a smaller role. Perception of interdental size (object thickness between teeth): - Natural dentition (unanesthetized): Threshold 0.008-0.060 mm; Discrimination 0.014 mm. - Natural dentition (anesthetized): Threshold 0.030-0.180 mm; Discrimination 0.040 mm. - Dentures (no anesthetic): Threshold 0.4-0.5 mm; Discrimination 0.18 mm. - Dentures (topical anesthetic): Discrimination 0.22 mm. - Normal subjects detect 20 μm objects; increases to 80 μm with anesthesia. - Minimal reduction in ability with anesthesia or dentures. - Muscle receptors (spindles) are key; few/no GTOs in jaw muscles.

Answer 105

Dentures reduce periodontal input, affecting chewing and texture perception. Proprioception (via muscle spindles) compensates for size/hardness detection. Dentures cause: - Loss of periodontal ligament receptors → Loss of related sensations/reflexes (e.g., glide into occlusion). - Oral mucosa covered → Impaired force perception, altered chewing/swallowing. - Decreased texture appreciation. - Size/hardness perception is preserved (proprioceptive, effort-related). - Motor learning required to adapt to dentures for speech/mastication.

Answer 106

Proprioception, the awareness of body segment position in space, enables accurate movements without visual control by relying on sensory coding of modality, intensity, and location through proprioceptors like muscle spindles, tendon organs, joint, and skin receptors, with muscle spindles having the greatest influence and joint receptors the least, as demonstrated by kinesthetic illusions from muscle vibration; this information ascends via the dorsal column-medial lemniscal pathway to the cortex, while in the masticatory system, periodontal ligament mechanoreceptors aid fine jaw motor control, and proprioceptive signals allow detection of object sizes between teeth.

Answer 107

The smallest functional unit of muscle control, consisting of a motor neuron and its innervated muscle fibers, with varying innervation ratios based on muscle function.

Answer 108

Muscle force is graded by recruiting more motor units and increasing their firing rate, following the Size Principle for efficient control.

Answer 109

Slow (Type I), fast fatigue-resistant (Type IIA), and fast fatigable (Type IIB) motor units are recruited based on task demands.

Answer 110

Jaw-closers have a high proportion of Type I and intermediate fibers for fatigue resistance, while jaw-openers (e.g., digastric) have more fast-twitch fibers for rapid movements. These differ from limb muscles in fiber size and composition.

Answer 111

Planning (cortex, basal ganglia, cerebellum), commanding (motor cortex, brainstem), and execution (spinal neurons) levels coordinate movement.

Answer 112

Bilateral projections from the cortex to brainstem motoneurons control jaw muscles, preserving mastication after unilateral stroke.

Answer 113

Chewing is initiated voluntarily but controlled automatically by a brainstem CPG, modulated by sensory feedback for efficiency and safety.

Answer 114

A motor unit is the functional unit of the neuromuscular system, consisting of a single motor neuron and all the muscle fibres it innervates. It represents the connection between the nervous system and skeletal muscle.

Answer 115

All muscle fibers within a single motor unit have similar physiological properties (e.g., contraction speed, force, fatigability).

Answer 116

- The number of muscle fibers per motor unit varies depending on the muscle's function, known as the innervation ratio: - Low innervation ratio (~20): Muscles involved in precise movements, such as those in the eye or hand. - High innervation ratio (~2000): Muscles involved in powerful movements, such as those in the arms or legs. - Intermediate innervation ratio: Jaw-closing muscles like the masseter (~640) and temporalis (~900) balance precision and power.

Answer 117

The motor unit is the smallest controllable element for producing muscle force, allowing the nervous system to fine-tune movement. E.g., When you blink (eye muscle, low innervation ratio), a small number of fibers are activated for precision. When you lift a heavy object (arm muscle, high innervation ratio), many fibers are activated for power.

Answer 118

- Muscle Force Modulation: The force a muscle produces is controlled by two mechanisms: 1. Recruitment: Activating more motor units to increase force. 2. Rate Coding: Increasing the frequency of action potentials (firing rate) in active motor units to produce stronger contractions.

Answer 119

- Recruitment: - Motor units are recruited in an orderly manner based on the Size Principle (explained in detail below). - Small motor units (producing less force) are activated first, followed by larger ones as more force is needed.

Answer 120

- Rate Coding: - Increasing the firing rate of a motor unit causes twitch summation, where muscle contractions overlap due to increased calcium availability in the muscle fiber's cytosol. - If the firing rate is high enough, the muscle reaches tetanus, a sustained contraction with no relaxation between stimuli, maximizing force output. - Example from Lecture: Figure 8-16 from Sherwood illustrates twitch summation and tetanus, showing how repeated stimuli before relaxation increase force.

Answer 121

Size Principle: - Small motor neurons have higher input resistance due to fewer ion channels and smaller surface area. According to **Ohm’s Law** (Vm=IR), a given synaptic current produces a larger excitatory postsynaptic potential (EPSP) in small motor neurons, causing them to reach the firing threshold first. Vm=IRV_m = IR - Advantage: This ensures precise, low-force movements are initiated by small, fatigue-resistant motor units, while powerful movements recruit larger, more fatigable units only when necessary, optimizing energy use and control.

Answer 122

- **Motor Unit Classification**: - Motor units are classified into three types based on three contractile properties: **contraction speed**, **maximal force**, and **fatigability**: 1. **Slow (S, Type I)**: - Slow contraction speed, low force, high fatigue resistance. - Used for endurance tasks (e.g., posture maintenance). 2. **Fast Fatigue-Resistant (FR, Type IIA)**: - Fast contraction speed, moderate force, resistant to fatigue. - Used for sustained, moderate-intensity tasks (e.g., walking). 3. **Fast Fatigable (FF, Type IIB)**: - Fast contraction speed, high force, low fatigue resistance. - Used for short, powerful movements (e.g., sprinting, lifting heavy weights).

Answer 123

- Different muscles have varying proportions of these fiber types based on their function: - **Endurance muscles** (e.g., soleus in the leg): High proportion of Type I fibers. - **Power muscles** (e.g., quadriceps): High proportion of Type IIB fibers.

Answer 124

- The central nervous system (CNS) recruits motor units in a task-dependent manner, following the Size Principle: - Low-force tasks (e.g., holding a cup) primarily use Type I motor units. - High-force tasks (e.g., jumping) recruit Type IIA and IIB units as needed. - This ensures efficiency, fatigue resistance, and precision tailored to the task.

Answer 125

- **Jaw-Closing Muscles (e.g., Masseter, Temporalis, Medial Pterygoid)**: - **Unique Features**: - **High proportion of Type I fibers**: Fatigue-resistant, suitable for sustained chewing. - **Low proportion of Type IIA fibers**: Less emphasis on fast, fatigue-resistant contractions compared to limb muscles. - **High proportion of intermediate fibers (Type IIC, IM)**: These fibers have properties between Type I and II, enhancing versatility. - **Fiber diameter**: Type I fibers are larger than Type II fibers, unlike limb muscles where Type II fibers are typically larger. Both types are smaller overall, possibly aiding fatigue resistance. - **Regional grouping**: Fibers of the same type are grouped within the muscle, likely reflecting functional specialization (e.g., different regions for biting vs. chewing). - **Functional Implications**: - The predominance of Type I and intermediate fibers supports prolonged, repetitive tasks like chewing, which require fatigue resistance. - Smaller fiber diameters and regional grouping may enhance precise control and endurance.

Answer 126

- **Jaw-Opening Muscles (e.g., Digastric, Lateral Pterygoid)**: - **Digastric**: - Mixed fiber composition, similar to limb muscles. - Higher proportion of Type IIA and IIB fibers, suitable for fast, unloaded contractions (e.g., during speech or jaw opening). - **Lateral Pterygoid**: - Similar to jaw-closers, with a high proportion of Type I and intermediate fibers, supporting sustained activity. - **Functional Implications**: - Digastric’s fast-twitch fibers enable rapid jaw opening for speech or swallowing, while lateral pterygoid’s composition supports chewing and jaw stabilization.

Answer 127

- **Comparison to Limb Muscles**: - Limb muscles (e.g., biceps brachii) typically have a more balanced mix of Type I, IIA, and IIB fibers, tailored to diverse functions (endurance, power, or both). - Jaw muscles prioritize fatigue resistance (Type I and intermediate fibers) for repetitive tasks like chewing, with less emphasis on high-force, fast-twitch fibers (Type IIB).

Answer 128

- **Hierarchy of Motor Control**: - Motor control is organized into three levels: 1. **Highest Level (Planning)**: - Involves cortical association areas, basal ganglia, and cerebellum. - Develops strategies and motor plans to achieve movement goals. 2. **Middle Level (Commanding)**: - Motor cortex and brainstem descending pathways integrate signals from planning areas and send specific motor commands to subcortical structures, spinal cord interneurons, and motoneurons. 3. **Lowest Level (Execution)**: - Spinal interneurons and motoneurons integrate sensory and descending inputs to activate muscles for movement.

Answer 129

- **Cerebral Cortex and Descending Pathways**: - The primary motor cortex integrates inputs from premotor and association areas and projects via the corticospinal and corticobulbar tracts. - **Basal Ganglia**: - Regulates movement indirectly through cortical areas, aiding in movement initiation and postural adjustments. - **Cerebellum**: - Compares planned and actual movements, correcting disparities for coordination. - **Brainstem**: - Integrates ascending and descending signals and originates descending pathways (except corticospinal tract). - **Motor Neurons and Interneurons**: - Execute stereotyped reflexes and integrate sensory/descending inputs for muscle activation.

Answer 130

- The corticobulbar pathway originates in the sensorimotor cortex and controls motoneurons in the brainstem for muscles of the eye, face, tongue, and throat. - Unlike the corticospinal tract (which controls spinal muscles), it is **bilateral**, innervating motoneuron pools on both sides of the brainstem. - **Trigeminal Nerve (Cranial Nerve V)**: Controls muscles of mastication (e.g., masseter, temporalis). - **Functional Advantage**: Bilateral innervation ensures mastication is preserved after a unilateral stroke, as the unaffected cortex can still control both sides. - **Example**: After a stroke affecting one side of the cortex, a patient may have arm weakness (corticospinal damage) but can still chew effectively due to bilateral corticobulbar projections.

Answer 131

- The motor cortex has a **somatotopic representation** (motor homunculus), with large areas dedicated to the hand and mouth due to their complex, precise movements. - This reflects the high number of motor cortical projections to these regions, enabling fine control.

Answer 132

- **Mastication Overview**: - Mastication is the rhythmic, coordinated activation of jaw muscles to break down food into a swallowable bolus. - It requires powerful yet precise muscle control, modulated by sensory feedback to optimize efficiency and prevent damage to oral structures (e.g., teeth, tongue). - **Example**: Chewing hard food (e.g., nuts) requires stronger, slower cycles, while soft food (e.g., bread) involves faster, lighter cycles, adjusted by sensory input.

Answer 133

- A CPG is a neural network in the brainstem that generates rhythmic motor patterns without requiring constant conscious input. - **Evidence for Chewing CPG**: - Repeated cortical stimuli can induce cyclical activity in masticatory motoneurons, even in paralyzed animals, indicating an autonomous rhythm generator. - Lesion studies suggest the CPG is located in the **brainstem reticular formation**. - Sensory inputs (e.g., food texture) modify the chewing pattern, such as altering the rate or force of cycles. - **Function**: The CPG drives the automatic, rhythmic jaw movements during chewing, freeing the cortex from continuous monitoring.

Answer 134

- **Voluntary Initiation**: Mastication begins with voluntary commands from the motor cortex. - **Automatic Continuation**: The brainstem CPG takes over, producing rhythmic jaw movements. - **Sensory Modulation**: - Sensory receptors in the oral mucosa, periodontal ligaments, masticatory muscles, and temporomandibular joint (TMJ) provide feedback to adjust the chewing rhythm and force. - **Example**: Encountering a hard piece of food increases sensory feedback, slowing, causing the CPG to slow chewing cycles and recruit more motor units for greater force. - **Swallowing Integration**: When the food bolus reaches the appropriate size and consistency, the swallowing center is activated, coordinating swallowing with chewing. - **Example**: Chewing a tough piece of meat involves sensory feedback adjusting the force and speed of jaw movements, followed by swallowing once the bolus is ready.

Endocrinology Flashcards

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