Study Notes: Sensory Receptors, Nervous System, Special Senses, and Endocrine System

Sensory Receptors and Reflexes

Types of Sensory Receptors

Sensory receptors are specialized cells or structures that detect changes in the environment and transmit information to the nervous system. They are classified based on the type of stimulus they detect.

• Mechanoreceptors: Detect mechanical forces such as pressure, vibration, touch, and stretch.

• Thermoreceptors: Respond to changes in temperature.

• Photoreceptors: Found in the retina; respond to light (e.g., rods and cones).

• Chemoreceptors: Detect chemical stimuli, such as odors, tastes, or changes in blood chemistry.

• Nociceptors: Respond to potentially damaging stimuli that result in pain.

• Exteroceptors: Located near the body surface; detect external stimuli (e.g., touch, temperature).

• Interoceptors: Detect internal stimuli from organs and blood vessels.

• Proprioceptors: Found in muscles, tendons, and joints; provide information about body position and movement.

• Kinesthetic receptors: Subtype of proprioceptors; involved in sensing movement and position of body parts.

Additional info: Sensory receptors are essential for the perception of the environment and for initiating reflexes that protect the body.

Nerve Structure

Nerves are bundles of axons in the peripheral nervous system. Each nerve is organized into layers of connective tissue:

• Epineurium: Outermost layer; surrounds the entire nerve.

• Perineurium: Surrounds bundles of axons (fascicles).

• Endoneurium: Surrounds individual axons.

Spinal Nerves

• Ventral root: Contains motor (efferent) fibers that transmit impulses from the spinal cord to effectors (muscles and glands).

• Dorsal root: Contains sensory (afferent) fibers that transmit impulses from receptors to the spinal cord.

Reflexes

Reflexes are rapid, automatic responses to stimuli. They can be classified as:

• Intrinsic (innate) reflexes: Inborn, such as the knee-jerk reflex.

• Acquired (learned) reflexes: Developed through experience, such as driving skills.

Components of a Reflex Arc:

1. Receptor

2. Sensory neuron

3. Integration center

4. Motor neuron

5. Effector

Autonomic Nervous System (ANS)

Effects and Dual Innervation

The ANS regulates involuntary functions and consists of the sympathetic and parasympathetic divisions, which often have opposing effects on target organs (dual innervation).

• Sympathetic division: Prepares the body for 'fight or flight' responses.

• Parasympathetic division: Promotes 'rest and digest' activities.

Neurotransmitters of the ANS

• Acetylcholine (ACh): Released by all preganglionic neurons and parasympathetic postganglionic neurons.

• Epinephrine and Norepinephrine: Released by sympathetic postganglionic neurons and the adrenal medulla.

Hypothalamic Control of the ANS

The hypothalamus is the main integration center for autonomic functions, regulating heart rate, blood pressure, body temperature, and more.

Special Senses: Eye and Ear

• Fibrous (sclera, cornea), vascular (choroid, ciliary body, iris), and sensory (retina).

• Photoreceptors: Rods (dim light, peripheral visStructure of the Eyeball

  • Layers:ion) and cones (color vision, sharpness).

• Photoreceptors

1. Cornea

2. Aqueous humor

3. Pupil

4. Lens

5. Vitreous humor

6. Retina

Focusing for Close Vision

• Accommodation of the lens

• Pupil constriction

• Convergence of the eyeballs

Vision Conditions

• Emmetropic: Normal vision

• Myopic: Nearsightedness

• Hyperopic: Farsightedness

Ear Structure

• Outer ear: Auricle, external auditory canal

• Middle ear: Tympanic membrane, ossicles (malleus, incus, stapes)

• Inner ear: Cochlea, vestibule, semicircular canals

Properties of Sound

• Frequency (pitch)

• Amplitude (loudness)

Transmission of Sound to the Internal Ear

1. Sound waves enter the external auditory canal

2. Vibrate the tympanic membrane

3. Ossicles transmit vibrations to the oval window

4. Fluid movement in cochlea stimulates hair cells

Equilibrium

• Static equilibrium: Sensed by the vestibule; detects head position relative to gravity.

• Dynamic equilibrium: Sensed by semicircular canals; detects rotational movements.

Homeostatic Imbalances

• Cataracts

• Glaucoma

• Macular degeneration

• Deafness

Endocrine System

Pituitary Gland

• Posterior pituitary:

  • Antidiuretic hormone (ADH): Causes kidneys to retain water, inhibiting urine formation; causes vasoconstriction, raising blood pressure.

  • Oxytocin: Stimulates uterine contractions during childbirth; triggers milk ejection in lactating women.

• Anterior pituitary:

  • Growth hormone (GH): Stimulates most cells, especially bone and skeletal muscle growth.

  • Thyroid-stimulating hormone (TSH): Stimulates normal development and secretory activity of the thyroid gland.

  • Adrenocorticotropic hormone (ACTH): Stimulates the adrenal cortex to release corticosteroids.

  • Follicle-stimulating hormone (FSH): Stimulates gamete (egg and sperm) production.

  • Luteinizing hormone (LH): Promotes production of gonadal hormones.

  • Prolactin (PRL): Stimulates milk production; blood levels rise toward the end of pregnancy; suckling stimulates PRL release and promotes continued milk production.

Thyroid Gland

• Calcitonin: Antagonist to parathyroid hormone (PTH); inhibits osteoclast activity; stimulates calcium uptake and incorporation into bone matrix.

• T3/T4 (Thyroid hormones): Major metabolic hormones; increase metabolic rate and heat production; play roles in maintenance of blood pressure, regulation of tissue growth, development of skeletal and nervous systems, and reproductive capabilities.

Parathyroid Glands

• Parathyroid hormone (PTH): Most important hormone in calcium homeostasis; stimulates osteoclasts to digest bone matrix; enhances reabsorption of calcium and secretion of phosphate by the kidneys; promotes activation of vitamin D (by the kidneys), increasing absorption of calcium by intestinal mucosa.

Adrenal Glands

• Adrenal medulla: Secretes epinephrine and norepinephrine; raises blood glucose levels, constricts blood vessels, raises heart rate, and diverts blood to the brain, heart, and skeletal muscle.

• Adrenal cortex:

  • Aldosterone: Stimulates sodium reabsorption and water retention by the kidneys; maintains blood pressure by increasing the action of vasoconstrictors.

  • Glucocorticoids (e.g., cortisol): Keep blood sugar levels relatively constant; involved in stress response.

  • Gonadocorticoids (sex hormones): Most are androgens that are converted into testosterone or estrogen; may contribute to the onset of puberty, appearance of secondary sex characteristics, and sex drive.

Pineal Gland

• Melatonin: Regulates day/night cycles and physiological processes that show rhythmic variations (body temperature, sleep, appetite).

Pancreas

• Pancreatic islet cells:

  • Glucagon: Raises blood glucose levels.

  • Insulin: Lowers blood glucose levels.

Gonads

• Ovaries:

  • Estrogen and Progesterone: Maturation of female reproductive organs; appearance of female secondary sexual characteristics; breast development and cyclic changes in the uterine mucosa.

• Testes:

  • Testosterone: Initiates maturation of male reproductive organs; causes appearance of male secondary sexual characteristics and sex drive; necessary for normal sperm production; maintains reproductive organs in their functional state.

Other Hormones

• Leptin: Involved in appetite regulation.

• Resistin: Involved in insulin resistance.

• Adiponectin: Modulates glucose regulation and fatty acid oxidation.

• Gastrin: Stimulates secretion of gastric acid.

• Secretin: Stimulates secretion of bicarbonate-rich pancreatic juice.

• Cholecystokinin: Stimulates release of bile and pancreatic enzymes.

• Atrial natriuretic peptide (ANP): Reduces blood volume, blood pressure, and sodium concentration.

• Erythropoietin: Stimulates red blood cell production.

• Renin: Regulates blood pressure and fluid balance.

• Cholecalciferol: Precursor of vitamin D.

• Thymosin, thymopoietin, thymulin: Involved in T cell development and immune response.

Homeostatic Imbalances

• Growth hormone (GH):

  • Hypersecretion: Gigantism in children, acromegaly in adults.

  • Hyposecretion: Pituitary dwarfism in children.

• Antidiuretic hormone (ADH):

  • Deficiency: Diabetes insipidus.

• Thyroid hormone:

  • Hypersecretion: Graves' disease.

  • Hyposecretion: Cretinism in infants, myxedema and goiter in adults.

• Glucocorticoids:

  • Hypersecretion: Cushing's syndrome.

  • Hyposecretion: Addison's disease.

• Insulin:

  • Hyposecretion (Type 1) or hypoactivity (Type 2): Diabetes mellitus.