Endocrine System Lecture 2/13

Hormones and the Nervous System

Hormonal Production

• No such thing as “male hormone” or “female hormone.” Both men and women produce androgens and estrogens.

• Adult women generally produce more estrogens and progesterone.

• Hormone levels fluctuate significantly:

  • Estradiol levels in women vary 10-fold during the menstrual cycle.

  • Progesterone levels in women vary 100-fold during a menstrual cycle.

  • Post-menopausal women have hormone levels similar to men.

• Women have 2 to 3 fold lower testosterone than men, especially after age 50, when androgens in men decrease by about 50%.

• Androgens and estrogens act antagonistically, decreasing each other's effectiveness.

Steroidogenesis

Steroidogenesis is the biological process where steroids are generated from cholesterol and transformed into steroid hormones, which include androgens, estrogens, and corticosteroids. The key steps include:

1. Cholesterol Uptake: Cholesterol is taken up by the cells and is transported to the mitochondria.

2. Conversion to Pregnenolone: In the mitochondria, cholesterol is converted to pregnenolone by the enzyme CYP11A1. This step is crucial as pregnenolone serves as the precursor for all steroid hormones.

3. Progesterone Production: Pregnenolone can be converted to progesterone, which is a key steroid in the synthesis of other hormones.

4. Formation of Androgens and Estrogens:

   • In the ovaries and testes, progesterone can be further converted to androgens (like testosterone) through a series of enzyme-catalyzed reactions involving various cytochrome P450 enzymes.

   • Androgens can then be converted to estrogens (such as estradiol) by the enzyme aromatase.

5. Release into Circulation: Once synthesized, these steroid hormones are released into the bloodstream, where they exert effects on various target tissues in the body.

Nervous System

• The nervous system is an electrochemical system that receives and transmits stimuli (information) across the body.

• It connects organs and various systems, controlling and being regulated by hormones.

• Neurons: The human brain consists of hundreds of billions of neurons and 100 trillion synapses.

• The Central Nervous System (CNS) includes the brain and spinal cord, while the Peripheral Nervous System (PNS) carries information to and from the CNS.

Human Brain Structure

• The brain is composed of two hemispheres divided by the longitudinal fissure:

  • Left Hemisphere: Controls right side movement, sensations, language, and linear reasoning.

  • Right Hemisphere: Controls left side movement, sensations, intuitive reasoning, tactile estimations, and language modulation.

• Corpus Callosum: The structure connecting the two hemispheres, exhibiting sexual dimorphism.

Key Brain Areas

• Frontal Lobe: Movement, reasoning, self-control, can alter personality.

• Temporal Lobe: Some memory functions, object recognition, facial recognition, aspects of language.

• Parietal Lobe: Processes sensory information related to touch.

• Occipital Lobe: Responsible for vision.

• Cerebellum & Postcentral Gyrus: Contains a “somatosensory map” for processing sensations like touch, temperature, and pain.

Peripheral Nervous System Components

• Sensory neurons and motor neurons are fundamental components of the PNS. The autonomic nervous system, divided into:

  • Sympathetic Nervous System: Controls the fight, flight, or fright responses.

  • Parasympathetic Nervous System: Manages rest and digest activities.

Spinal Anatomy Functionality

• Cervical Vertebrae: Responsible for breathing, heart rate, shoulder, arm, and wrist movement.

• Thoracic Vertebrae: Control finger movement, abdominal movement, and ejaculation.

• Lumbar Vertebrae: Manage hip and knee movement, also ejaculation.

• Sacrum: Engages with bowel and bladder control and vaginal lubrication.

Sexual Response Mechanisms

Two elements contribute to sexual arousal and response:

1. Directional Stimulation: Reflexogenic response through stimulation of the clitoris or penis.

2. Sexual Thoughts and Fantasies: Psychogenic response involving cognitive stimulation.

The role of neurosecretory cells is crucial in hormone release, involving the hypothalamus (HPT) and pituitary gland (PT), which are regulated by regular neurons and are integral to the functioning of the endocrine system and the production of neurohormones.