Differences Between Upper Motor Neuron (UMN) and Lower Motor Neuron (LMN) Lesion

Q3. Define Arterial Blood Pressure. What is the Normal Blood Pressure? Explain the Regulation of Blood Pressure in Detail.

Definition of Arterial Blood Pressure

Normal Blood Pressure

• Hypertension: BP > 140/90 mmHg
• Hypotension: BP < 90/60 mmHg

Regulation of Blood Pressure

A. SHORT-TERM (RAPID) REGULATION

1. Nervous System Mechanisms (Most Rapid - seconds to minutes)

• Baroreceptors (pressoreceptors) are stretch receptors located in the walls of:
  • Carotid sinus (at bifurcation of common carotid - CN IX)
  • Aortic arch (CN X)
• Mechanism:
  • Rise in BP → baroreceptors stretched → increased afferent firing → vasomotor center inhibited → decreased sympathetic + increased parasympathetic output → vasodilation + decreased heart rate/contractility → BP falls back to normal
  • Fall in BP → less stretch → decreased firing → vasomotor center activated → increased sympathetic → vasoconstriction + increased HR/contractility → BP rises back to normal
• This is a negative feedback mechanism
• Responds to rapid changes; not effective for long-term BP regulation (baroreceptors reset over 1-2 days)

• When cerebral blood flow falls dangerously, the vasomotor center is strongly excited
• Produces massive sympathetic vasoconstriction - can raise BP to 250 mmHg
• Acts as an "emergency last resort" mechanism

• Peripheral chemoreceptors (carotid and aortic bodies) respond to hypoxia, hypercapnia, and acidosis
• Stimulate the vasomotor center → increase BP
• Primary role is respiratory control; BP effect is secondary

• Volume receptors in atria detect low cardiac filling pressure
• Activate sympathetic system via the Bainbridge reflex
• Increase heart rate when atrial pressure rises (to compensate)

2. Hormonal Mechanisms (Minutes to hours)

• Fall in renal perfusion pressure → kidneys release renin
• Renin converts angiotensinogen → Angiotensin I → (ACE) → Angiotensin II
• Angiotensin II: potent vasoconstrictor + stimulates aldosterone release
• Aldosterone: promotes Na+ and water reabsorption in kidney → increases blood volume → increases BP
• This is also an important long-term mechanism

• Released during sympathetic activation / stress
• Increase heart rate, cardiac output, and cause vasoconstriction
• Raise blood pressure quickly

• Released from posterior pituitary in response to decreased BP / increased plasma osmolality
• Causes vasoconstriction + water reabsorption in kidneys → increases blood volume and BP

B. LONG-TERM REGULATION

Renal Fluid Volume Mechanism (Most Powerful Long-Term Regulator)

• BP rises → kidneys excrete more Na+ and water (pressure diuresis and natriuresis) → blood volume falls → BP returns to normal
• BP falls → kidneys retain Na+ and water → blood volume rises → BP returns to normal

• High Angiotensin II → shifts the renal function curve to higher pressures (promotes hypertension)
• ACE inhibitors / ARBs block this, restoring BP toward normal

Summary Table: BP Regulation Mechanisms

• Guyton and Hall Textbook of Medical Physiology - Corticospinal tract (Ch. 56), Arterial pressure regulation (Ch. 18)
• Costanzo Physiology - Negative feedback in arterial pressure regulation
• Medical Physiology - Neural and baroreceptor regulation of arterial pressure

Fasciculus Answers (10 Marks Each)

Q1. List the Hormones of the Thyroid Gland. Briefly Explain Functions of Thyroid Hormone. What is Myxedema?

Hormones of the Thyroid Gland

Functions of Thyroid Hormones (T3/T4)

• Increase Basal Metabolic Rate (BMR) - stimulate O₂ consumption in almost all tissues
• Increase heat production (thermogenesis)
• Stimulate Na⁺/K⁺ ATPase activity → increased ATP utilization

• Increase glucose absorption from the gut
• Promote glycogenolysis and gluconeogenesis
• Increase insulin-dependent glucose uptake

• Stimulate lipolysis - mobilize fatty acids
• Decrease plasma cholesterol (increase LDL receptor expression)
• Deficiency → hypercholesterolemia

• Physiological levels → protein synthesis (anabolic)
• Excess levels → protein catabolism (muscle wasting)

• Increase heart rate (positive chronotropy)
• Increase cardiac output and stroke volume
• Increase systolic BP; decrease peripheral vascular resistance

• Essential for fetal/neonatal brain development (absence → cretinism)
• Adults: maintain alertness, memory, cognition
• Hyperthyroidism → anxiety, restlessness
• Hypothyroidism → sluggishness, memory loss

• Essential for normal skeletal growth and maturation
• Works synergistically with Growth Hormone (GH)

• Required for normal menstrual cycles and fertility
• Hypothyroidism → menorrhagia, anovulation, infertility

• Stimulate gut motility
• Hyperthyroidism → diarrhea; Hypothyroidism → constipation

Myxedema

• Hashimoto's thyroiditis (most common - autoimmune)
• Thyroidectomy / radioiodine treatment
• Iodine deficiency
• Pituitary/hypothalamic failure (secondary/tertiary)

• TSH: High (primary hypothyroidism)
• Free T4: Low
• Elevated cholesterol; normocytic or macrocytic anemia

Q2. Explain Origin, Course, and Termination of the Corticospinal Tract with a Neat Labelled Diagram. Tabulate Differences Between UMN and LMN Lesion.

Corticospinal (Pyramidal) Tract

Origin

• 30% - Primary motor cortex (Area 4 - precentral gyrus) - from Betz cells (giant pyramidal cells)
• 30% - Premotor and supplementary motor areas (Area 6)
• 40% - Somatosensory cortex (Areas 3, 1, 2 - postcentral gyrus)

There are ~34,000 large Betz cell fibers per tract (diameter ~16 µm), out of >1 million total fibers. Betz cell fibers conduct at ~70 m/sec - the fastest signals from brain to cord.

Course

1. Fibers converge → pass through posterior limb of internal capsule (between caudate nucleus and putamen)
2. Descend through crus cerebri (basis pedunculi) of midbrain
3. Pass through longitudinal fascicles of pons
4. Form the pyramids of medulla oblongata (hence "pyramidal tract")
5. At lower medulla - ~85-90% of fibers cross at the pyramidal (motor) decussation
6. Crossed fibers → descend as lateral corticospinal tract (lateral funiculus of spinal cord)
7. ~10-15% uncrossed fibers → descend as anterior (ventral) corticospinal tract - cross at spinal cord level later

Termination

• Primarily on interneurons in the intermediate zone (Rexed laminae V-VIII) of spinal cord gray matter
• Small number on sensory relay neurons in the dorsal horn
• Very few directly on anterior horn motor neurons (alpha motor neurons) - for fine skilled movements of distal limbs
• Anterior corticospinal tract terminates bilaterally - controls axial/postural muscles

Labelled Diagram of Corticospinal Tract

Differences: UMN vs LMN Lesion

Q3. Define Arterial Blood Pressure. What is Normal Blood Pressure? Explain the Regulation of Blood Pressure in Detail.

Definition

Normal Blood Pressure

Regulation of Blood Pressure

A. SHORT-TERM (RAPID) REGULATION

1. Nervous System Mechanisms (Seconds)

• Baroreceptors (stretch receptors/pressoreceptors) located in:
  • Carotid sinus - signals via Glossopharyngeal nerve (CN IX)
  • Aortic arch - signals via Vagus nerve (CN X)
• Mechanism:

BP rises → Baroreceptors stretched → Increased afferent firing
→ Vasomotor center inhibited → Decreased sympathetic + Increased parasympathetic
→ Vasodilation + Decreased HR + Decreased contractility
→ BP falls back to normal (Negative Feedback)

BP falls → Less stretch → Decreased firing
→ Vasomotor center activated → Increased sympathetic
→ Vasoconstriction + Increased HR/contractility → BP rises

• Responds to rapid, moment-to-moment changes
• Baroreceptors reset over 1-2 days, so not effective for long-term control

• When cerebral perfusion falls dangerously → vasomotor center strongly excited
• Produces powerful sympathetic vasoconstriction → BP may rise to 250 mmHg
• Acts as an emergency "last resort" mechanism

• Peripheral chemoreceptors (carotid and aortic bodies) respond to hypoxia, hypercapnia, acidosis
• Stimulate vasomotor center → increase BP
• Primary role is respiratory; BP effect is secondary

2. Hormonal Mechanisms (Minutes to Hours)

Fall in renal perfusion pressure
        ↓
Kidney releases RENIN
        ↓
Angiotensinogen → Angiotensin I → (ACE) → Angiotensin II
        ↓                                        ↓
  Potent vasoconstriction              Stimulates Aldosterone
        ↓                                        ↓
   BP rises                    Na⁺ + H₂O retention → Blood volume ↑ → BP ↑

• Released during stress/sympathetic activation
• Increase HR, cardiac output, and cause vasoconstriction → raise BP rapidly

• Released from posterior pituitary in response to decreased BP or increased plasma osmolality
• Causes vasoconstriction + water reabsorption in kidneys → increases blood volume and BP

B. LONG-TERM REGULATION

Renal Fluid Volume Mechanism (Most Powerful Long-Term Regulator)

• BP rises → kidneys excrete more Na⁺ and water (pressure diuresis) → blood volume falls → BP normalizes
• BP falls → kidneys retain Na⁺ and water → blood volume rises → BP normalizes

This mechanism has "infinite gain" - given enough time, it can correct almost any sustained BP disturbance. It is modulated strongly by RAAS.

Summary Table: BP Regulation Mechanisms