Anatomy ll

Spinal Reflexes

Sensory and Motor Neurons

• Sensory (afferent) neurons: Carry information from peripheral receptors to the Central Nervous System (CNS).

• Motor (efferent) neurons: Carry commands from the CNS to peripheral effectors.

The Simple Reflex

1. Stimulus Detection & Receptor Activation

   • A stimulus alters the internal or external environment.

   • Receptors:

     • Specialized sensory cells or dendritic endings.

     • Respond to chemical or physical changes.

2. Sensory Neuron Activation

   • Receptor stimulation produces graded potentials.

   • If threshold is reached, an action potential occurs.

   • The action potential enters the spinal cord via the dorsal root.

3. Central Processing

   • The sensory neuron releases excitatory neurotransmitters.

   • Neurotransmitters bind to interneurons.

4. Motor Neuron Stimulation

   • Interneurons excite motor neurons.

   • Motor neurons send action potentials to effectors.

5. Effector Response

   • Neurotransmitter release at the neuromuscular junction.

   • Response removes or counteracts the stimulus, demonstrating a form of negative feedback.

Reflex Categories Classification

—Developmental Origin—

• Innate reflexes:

  • Inborn, automatic reflexes present at birth.

  • Genetically programmed; appear in predictable developmental order.

  • Consistent and stereotyped responses essential for early survival.

  • Examples: Withdrawal reflex, suckling reflex.

• Acquired reflexes:

  • Learned reflexes developed after birth through experience.

  • Formed through repetition and practice; strengthened and refined over time.

  • Can improve in speed and accuracy; adaptable.

  • Examples: Learned motor skills, conditioned responses.

—Type of Motor Response—

• Somatic reflexes:

  • Reflexes that activate skeletal muscles.

  • Produce body movement or changes in posture; initially involuntary but may later be voluntarily modified.

  • Often protective.

  • Examples: Withdrawal reflex, stretch (knee-jerk) reflex.

• Visceral (autonomic) reflexes:

  • Reflexes that regulate internal organs.

  • Control smooth muscle, cardiac muscle, glands, adipose tissue; fully involuntary; main role is to maintain homeostasis.

  • Examples: Pupil constriction, heart rate regulation, digestion, gland secretion.

—Neural Circuit Complexity—

• Monosynaptic reflexes:

  • Simplest reflex arc with one synapse.

  • Sensory neuron directly synapses with motor neuron; no interneurons involved.

  • Fastest response time; minimal processing.

  • Example: Stretch (patellar) reflex.

• Polysynaptic reflexes:

  • Reflex arc with multiple synapses.

  • Includes sensory neurons, interneurons, and motor neurons; slower but more versatile.

  • Allows integration and coordination of multiple muscles.

  • Examples: Withdrawal reflex, crossed-extensor reflex.

—Processing Location—

• Spinal reflexes:

  • Reflexes integrated in the spinal cord.

  • Rapid responses; often protective or postural; may be segmental (one spinal level) or intersegmental (multiple levels).

  • Examples: Withdrawal reflex, stretch reflex.

• Cranial reflexes:

  • Reflexes integrated in brain nuclei.

  • Controlled by brainstem centers; regulate head, facial, and visceral activities; involve cranial nerves.

  • Examples: Pupillary light reflex, corneal blink reflex, gag reflex.

Monosynaptic Reflexes

Stretch Reflexes

• Maintain muscle length and tone.

• Operate automatically.

• Example: Patellar (knee-jerk) reflex.

Patellar Reflex: Steps

1. Stimulus: Tap stretches quadriceps tendon.

2. Receptor: Stretch receptors activated.

3. Sensory neuron: Direct synapse with motor neuron.

4. Integration: Occurs in motor neuron cell body.

5. Response: Quadriceps contract leading to leg extension.

Muscle Spindles

• Primary stretch receptors made of intrafusal fibers; embedded in extrafusal muscle fibers.

• Continuously send sensory information to CNS.

Postural Reflexes

• Subset of stretch reflexes.

• Maintain balance and posture.

• Require antagonist muscle coordination.

• Postural muscles possess high baseline tone; extremely sensitive stretch receptors.

Polysynaptic Reflexes

• Control automatic parts of complex movements.

• Essential for walking and running.

• Can be:

  • Ipsilateral: Same side.

  • Contralateral: Opposite side.

Withdrawal Reflexes

• Protect the body from harmful stimuli.

• Initiated by pain, highly adaptable due to interneurons.

• Response depends on stimulus intensity and location.

Flexor Reflex

• Triggered by painful stimulus.

• Nociceptors activate sensory neurons.

• Interneurons:

  • Excite flexor muscles.

  • Inhibit extensor muscles (reciprocal inhibition).

• Stronger stimuli recruit more muscle fibers.

Crossed Extensor Reflex

• Occurs alongside the flexor reflex.

• For the injured limb:

  • Flexors contract, extensors relax.

• For the uninjured limb:

  • Extensors contract to support body weight, flexors inhibited.

Reflexes in Neurological Assessment

Babinski Reflex

• Stroke lateral plantar surface of the foot.

• Positive Result: Toes extend and fan outward.

  • Normal in infants, but abnormal in adults indicating possible CNS damage.

• Negative Result: Plantar flexion (normal adult response).

Abdominal Reflex

• Light stroking of the skin results in muscle contraction.

• Requires intact descending motor pathways; absence suggests spinal or brain damage.

General Sensory Modalities

• Types include pain, temperature, touch, pressure, vibration, proprioception.

• Receptors have specific receptive fields:

  • Large (~7 cm): trunk, limbs.

  • Small (~1 mm): fingertips, tongue.

• Smaller fields provide better spatial discrimination.

Sensation vs Perception

• Sensation: Neural transmission of sensory input.

• Perception: Conscious interpretation of sensory information.

• Not all sensory input reaches our awareness.

Motor Pathway Outcomes

• Immediate involuntary responses:

  • Processed in the spinal cord or brainstem; occur before awareness.

• Voluntary responses:

  • Require perception, can modify reflexes.

Functional Categories of Receptors

Nociceptors

• Free nerve endings, large receptive fields; detect pain.

Pain Fiber Types

• Type A fibers:

  • Fast, sharp, well-localized pain, rapid conduction.

• Type C fibers:

  • Slow, dull, burning pain, poor localization.

Thermoreceptors

• Detect temperature changes.

• Found in dermis, muscles, liver, hypothalamus; cold receptors are more numerous.

Chemoreceptors

• Respond to dissolved chemicals; located in blood, interstitial fluid, CSF.

Mechanoreceptors

• Activated by mechanical deformation using mechanically gated ion channels.

• Subclasses include:

  • Proprioceptors: Measure muscle length, joint position, tension.

  • Baroreceptors: Detect pressure changes.

  • Tactile receptors: Involved in touch, pressure, and vibration detection.

Receptor Response Patterns

• Tonic receptors:

  • Always active, slow to adapt.

• Phasic receptors:

  • Normally silent; respond to changes only and adapt quickly.

Major Somatic Sensory Pathways

Spinothalamic Pathway

• First-order neurons synapse in posterior gray horn.

• Second-order neurons cross the spinal cord; third-order neurons project to sensory cortex.

• Tracts:

  • Anterior: cruder touch & pressure.

  • Lateral: pain & temperature.

Posterior Column Pathway

• Responsible for fine touch, vibration, pressure, proprioception.

• Incorporates fasciculi gracilis & cuneatus.

• Medial lemniscus pathway leads to the thalamus.

Spinocerebellar Pathway

• Carries proprioceptive information to the cerebellum.

• Posterior tracts: Ipsilateral connections.

• Anterior tracts: Cross before ascending.

Somatic Motor Pathways

Upper and Lower Motor Neurons

• Upper Motor Neuron:

  • Cell body located in CNS.

• Lower Motor Neuron:

  • Cell body in spinal cord or brainstem; directly innervates muscle.

Corticospinal (Pyramidal) Pathway

• Main voluntary motor pathway, originating in the primary motor cortex.

• Upper motor neurons descend from the cortex and synapse with lower motor neurons.

• Tracts:

  • Corticobulbar: Controls face, jaw, eyes, neck, pharynx.

  • Corticospinal: Controls trunk and limb movements.

Lateral Pathway

• Facilitates precise limb movement and muscle tone.

• Primary nucleus is the red nucleus.

• Involves the rubrospinal tracts.

Medial Pathway

• Controls posture and gross movements.

• Origins from:

  • Colliculi (visual/auditory reflexes).

  • Reticular formation (tone and reflex sensitivity).

  • Vestibular nuclei (head position).

Somatic vs Autonomic Nervous System (SNS vs ANS)

Autonomic Nervous System (ANS) General Characteristics

• Controls visceral functions automatically.

• Activity usually occurs below the level of consciousness.

• Regulates:

  • Heart rate (HR), blood pressure (BP), respiration, digestion, temperature, metabolism.

Structure of ANS

1. Preganglionic neuron:

   • Cell body located in CNS, myelinated axon, releases Acetylcholine (ACh), synapses in autonomic ganglion.

2. Postganglionic neuron:

   • Cell body located in the ganglion, unmyelinated axon, innervates effector organ.

   • The neurotransmitter released depends on the division of the ANS.

Major Integrative Centers

• Hypothalamus: Master coordinator of the ANS.

• Brainstem nuclei: Regulate reflexes.

• Spinal cord: Handle local reflexes.

DIVISIONS OF THE ANS

Sympathetic Division (Thoracolumbar)

• Anatomical Origin: From the lateral horns of T1 to L2.

• Structural Pattern: Short preganglionic fibers and long postganglionic fibers; exhibits extensive branching (divergence leading to mass activation).

• Functional Role: Fight-or-Flight response, mobilizes energy; prepares body for stress or exertion.

• Increases blood flow to muscles/heart/brain, decreases nonessential functions like digestion and urination.

Parasympathetic Division (Craniosacral)

• Origin: Brainstem (Cranial Nerves III, VII, IX, X) and sacral S2–S4.

• Structural Pattern: Long preganglionic fibers and short postganglionic fibers; minimal branching for localized effects.

• Functional Role: Rest-and-Digest response, conserves energy, promotes digestion and storage, supports maintenance and repair.

Enteric Nervous System (ENS)

• “Brain of the Gut.”

• Contains approximately 100 million neurons (similar count as spinal cord neurons).

• Controls peristalsis, enzyme secretion, and local blood flow.

• Functions independently but is modulated by the ANS.

AUTONOMIC GANGLIA & PATHWAYS

Sympathetic Structure

• Characterized by short pre- and long post-ganglionic fibers; significant divergence leads to coordinated systemic responses.

Types of Sympathetic Ganglia

1. Sympathetic chain (paravertebral) ganglia:

   • Located along the vertebral column, widely distributes fibers.

2. Collateral (prevertebral) ganglia:

   • Includes celiac, superior mesenteric, and inferior mesenteric ganglia, serving the abdominopelvic organs.

3. Adrenal medulla:

   • Specialized modified ganglion with chromaffin cells, releasing norepinephrine (NE) and epinephrine (E) directly into the bloodstream.

Parasympathetic Structure

• Characterized by long pre- and short post-ganglionic fibers; low divergence results in organ-specific responses.

Types of Parasympathetic Ganglia

• Terminal: Located near the organ.

• Intramural: Located within the organ wall.

PHYSIOLOGICAL EFFECTS OF THE ANS

Sympathetic (“Fight/Flight”) Effects

• Increased alertness.

• Increased heart rate (HR) and contractility.

• Increased blood pressure (BP) via vasoconstriction of skin/viscera.

• Bronchodilation.

• Glycogenolysis, lipolysis.

• Decreased gastrointestinal (GI) motility/secretion.

• Urinary retention and sweating.

• Pupil dilation (mydriasis).

Parasympathetic (“Rest/Digest”) Effects

• Decreased heart rate (HR).

• Increased GI motility/secretion.

• Enhanced absorption/storage.

• Facilitated urination and defecation.

• Bronchoconstriction.

• Pupillary constriction (miosis).

• Near-vision accommodation.

Autonomic Receptors

Sympathetic Adrenergic Receptors

• Alpha (α) receptors:

  • α1 → smooth muscle contraction (vasoconstriction).

  • α2 → inhibitory actions.

• Beta (β) receptors:

  • β1 → heart stimulation.

  • β2 → smooth muscle relaxation.

  • β3 → fat breakdown.

Neurotransmitters in ANS

• Epinephrine (E): Circulates systemically with longer duration effects.

• Norepinephrine (NE): Released locally at synapses with shorter duration; maintains baseline vascular tone; important for resting BP.

Parasympathetic Neurotransmitter: Acetylcholine (ACh)

Receptor Types

• Nicotinic receptors: Located at ganglia; excitatory.

• Muscarinic receptors: Found in target organs; can be excitatory or inhibitory, involved in GPCR pathways.

Autonomic Tone

• Continuous baseline firing of autonomic pathways.

• Allows for increases or decreases of function from the baseline; enables fine control.

Dual Innervation

• Most organs receive innervation from both divisions of the ANS.

Effects of Dual Innervation

• Antagonistic effects:

  • Heart rate regulation (ex. Sympathetic increases HR while parasympathetic decreases it).

• Cooperative effects:

  • Salivary gland secretion.

MONITORING VISCERAL FUNCTION

Baroreceptors

• Stretch-sensitive receptors that monitor pressure and volume.

• Trigger mechanisms to adjust blood pressure reflexes.

Chemoreceptors

• Monitor levels of blood gases and pH.

• Regulate respiration and cardiovascular output.

Locations of Receptors

• Medulla: Central regulation of cardiovascular functions.

• Carotid bodies: Respond to blood gas levels in arteries.

• Aortic bodies: Monitor gas levels returning to the heart.

ENDCRINE SYSTEM

• Functions to maintain homeostasis by regulating:

  • Growth and development.

  • Metabolism and energy balance.

  • Fluid and electrolyte balance.

  • Reproduction and stress responses.

  • Circadian rhythms.

Key Characteristics of Endocrine Signals

• Slower onset than neural control.

• Longer-lasting effects (from minutes to hours to days).

• More widespread/systemic responses via bloodborne signals rather than synapses.

NERVOUS VS ENDOCRINE CONTROL

Terminology

• Neural chemical: refers to neurotransmitters.

• Endocrine chemical: refers to hormones.

• Some molecules function as both (e.g., epinephrine).

HORMONE CLASSES (BY CHEMICAL STRUCTURE)

1. Amino Acid–Derived Hormones

• Origin: Synthesized from tyrosine or tryptophan.

• Examples:

  • Thyroid hormones (T₃, T₄),

  • Catecholamines (E, NE, dopamine),

  • Melatonin.

• Properties:

  • Small molecules.

  • Some are lipid-soluble (thyroid hormones); others are water-soluble (catecholamines).

2. Peptide Hormones (largest group)

• Synthesis:

  • Created as preprohormones → prohormones → active hormones.

  • Stored in vesicles, released by exocytosis.

• Size:

  • Ranges from small peptides to large proteins and glycoproteins.

• Examples:

  • Hypothalamus, pituitary, pancreas, thymus, heart, GI tract.

• Properties:

  • Water-soluble; act through membrane receptors.

  • Often involved in second messenger systems (commonly cAMP); possess short half-lives.

3. Lipid-Derived Hormones

Types A. Eicosanoids

• Derived from fatty acids, act locally (paracrine/autocrine).

• Examples: Prostaglandins, leukotrienes.

• Regulate inflammation, blood clotting, and smooth muscle function.

Types B. Steroid Hormones

• Derived from cholesterol, secreted by adrenal cortex, gonads, kidneys.

• Properties: Lipid-soluble; bind to plasma proteins; possess long-lasting effects; interact with intracellular receptors; alter gene transcription mechanisms.

ENDOCRINE ORGANS

Primary Endocrine Organs

• Main function includes hormone secretion:

  • Hypothalamus.

  • Pituitary gland.

  • Pineal gland.

  • Thyroid gland.

  • Parathyroid glands.

  • Adrenal glands.

  • Thymus.

Secondary Endocrine Organs

• Perform other primary functions but also secrete hormones:

  • Heart.

  • Kidneys.

  • Intestines.

  • Reproductive organs.

HYPOTHALAMUS

• Master Integrator:

  • Highest level of endocrine control.

• Functions include:

  • Links the nervous system with the endocrine system.

  • Controls the pituitary.

  • Regulates fluid balance, smooth muscle contraction, and stress responses.

• Hormones secreted:

  • Antidiuretic hormone (ADH).

  • Oxytocin.

  • Releasing and inhibiting hormones (control anterior pituitary).

PITUITARY GLAND (Master Gland)

Anterior Pituitary (Adenohypophysis)

• Tropic Hormones:

  • Thyroid-stimulating hormone (TSH): Stimulates thyroid hormone release.

  • Adrenocorticotropic hormone (ACTH): Stimulates glucocorticoids from the adrenal cortex.

  • Follicle-stimulating hormone (FSH):

    • In females: follicle growth + estrogen.

    • In males: sperm production; inhibited by inhibin.

  • Luteinizing hormone (LH):

    • In females: ovulation + progesterone secretion.

    • In males: testosterone production.

  • Growth hormone (GH): Affects growth in children and metabolism in adults including protein synthesis and lipolysis.

  • Prolactin: Development of mammary glands, milk production.

  • Melanocyte-stimulating hormone (MSH): Stimulates melanin production.

Posterior Pituitary (Neurohypophysis)

• Stores/releases hormones from the hypothalamus:

  • Antidiuretic hormone (ADH): Water conservation, increases blood pressure.

  • Oxytocin: Uterine contractions, milk ejection in lactation.

THYROID GLAND

• Structure: Composed of follicles with colloid, follicular cells produce T₃ and T₄, and C cells release calcitonin.

• Hormones:

  • T₃ and T₄:

    • Increase basal metabolic rate.

    • Increase oxygen use and heat production.

    • Required for normal growth and development.

  • Calcitonin:

    • Lowers blood calcium (Ca²⁺) levels.

PARATHYROID GLANDS

• Hormone: Parathyroid hormone (PTH) is the primary calcium regulator.

• Actions of PTH (raises Ca²⁺ levels):

  • Decrease activity of osteoblasts.

  • Increase activity of osteoclasts.

  • Increase kidney reabsorption of Ca²⁺.

  • Stimulate calcitriol synthesis.

ADRENAL GLANDS

• Structure: Consists of capsule, cortex, and medulla.

Cortex Layers

• Zona glomerulosa:

  • Produces aldosterone which promotes Na⁺ retention and K⁺ excretion, increasing BP.

• Zona fasciculata:

  • Produces cortisol which regulates glucose metabolism, has anti-inflammatory effects, and is involved in stress responses.

• Zona reticularis:

  • Produces androgens.

Medulla

• Hormones:

  • Epinephrine and norepinephrine released, primarily responsible for mediating fight-or-flight response.

  • Results in increased heart rate, blood pressure, and glucose levels.

PANCREAS

• Composed of islets of Langerhans which include:

  • Alpha cells: Produce glucagon.

  • Beta cells: Produce insulin.

  • Delta cells: Produce somatostatin.

  • F cells: Secrete pancreatic polypeptide.

Glucose Homeostasis

• Insulin: Decreases blood glucose levels by promoting uptake and storage.

• Glucagon: Increases blood glucose levels by promoting glycogen breakdown into glucose.

PINEAL GLAND

• Hormone: Melatonin.

• Functions include:

  • Regulation of circadian rhythms.

  • Sleep cycle regulation.

  • Reproductive timing.

  • Antioxidant protection.

  • Light–dark synchronization.

DIABETES MELLITUS

Definition

• A chronic hyperglycemia condition.

Effects

• Can cause glycosuria, polyuria, and dehydration.

Types of Diabetes

1. Type 1 diabetes:

   • No insulin production (autoimmune destruction of beta cells).

   • Usually occurs in childhood/adolescence.

   • Requires insulin therapy.

2. Type 2 diabetes:

   • Insulin resistance often associated with obesity; lifestyle changes and medications can help.

Complications of Diabetes Mellitus

• Retinopathy, cardiovascular disease (CVD), myocardial infarction (MI), nephropathy, neuropathy, ulcers, amputation.

HORMONAL INTERACTIONS

• Antagonistic interactions: Opposite effects (e.g., Insulin vs. glucagon; PTH vs. calcitonin).

• Synergistic interactions: Greater effect together (e.g., Growth hormone + glucocorticoids).

• Permissive interactions: One hormone enables another’s action (e.g., Thyroid hormone + epinephrine).

BLOOD PRESSURE REGULATION

Natriuretic Peptides

• Promote Na⁺/water loss; lower renin, ADH, and aldosterone levels; decrease blood volume/pressure.

Renin-Angiotensin System

• Triggered by renin release influencing fluid retention, thirst, and blood pressure elevation.

STRESS RESPONSE

• Phases:

  • Alarm phase: Characterized by sympathetic activation releasing epinephrine.

  • Resistance phase: Involves cortisol release and glucose conservation.

  • Exhaustion phase: Energy depletion leading to potential organ failure.

ENDOCRINE DISORDERS