Anatomy of the Penis and Nervous System

Prostate Resection

  • Follow-up on prostate resection.
  • Regrowth is common, which is why cauterization is favored.
  • Cauterization burns tissue, creating scar tissue that inhibits regrowth.
  • Prostates may regrow and clog ducts/urethra, but most stay open for a few years.

Comparative Penis Anatomy

  • Diversity in penis anatomy across the animal kingdom.
  • Duck and reptile penises are particularly unusual.
    • Some reptile penises are forked or have barbs.
    • Some act like raccoon traps, easy to insert but hard to withdraw.
  • These adaptations likely increase reproductive fitness by improving the chances of passing on genes.
  • The presence of a penis is a strongly conserved trait in animals, despite variations.

Personal Anecdote: Reptile Penises

  • Personal experience working with reptiles in college.
    • Snakes and lizards sometimes stick out their penises when angry.
    • Evolutionary purpose is unclear, as it seems counterproductive to gene preservation.

Human Penis Anatomy

  • In males, the genitals and urinary system share structures after the merging of the ejaculatory duct and urethra.
  • Anterior, frontal plane cross-sectional view of the penis:
    • Urinary bladder.
    • Prostate gland.
    • Urethra runs through the abdomen and exits through the penis.
    • Spongy urethra runs through the center.
    • Corpus cavernosum surrounds the spongy urethra. Same tissue as in the clitoris, functions as erectile tissue.
    • Glans (head) of the penis, similar to the glands of the clitoris, contains neurosensory tissue.
    • Prepuce (foreskin) covers and protects the glands.
    • External urethral orifice is where the urethra ends.

Erectile Tissue

  • Transverse plane view of the penis:
    • Spongy urethra.
    • Two corpus cavernosa.
    • Corpus spongiosum surrounds the urethra, maintaining its openness during erections to allow ejaculate to pass through.

Erection Physiology

  • Erection is driven by changes in blood flow and the nervous system.
  • Flaccid penis:
    • Equal blood flow in through penile arteries and out through penile veins (net zero).
  • Erect penis:
    • Penile artery dilates, increasing blood flow into the corpus cavernosum.
    • Increased pressure in the corpus cavernosum compresses the penile veins.
    • More blood flows in than out, causing the penis to enlarge.
    • Afterwards the nervous system constricts the penile artery, decreasing pressure and allowing blood to flow out through the penile veins.

Clinical Question: UTIs

  • Question: Can UTIs affect the reproductive tract in males?
  • Answer: It can happen, but it is less common.
    • Infections can affect the prostate and seminal vesicles.
    • It is rare for infections to reach the testes.

Ejaculation

  • Ejaculation is the release of sperm and ejaculate out of the penis.

Nervous System Control

  • Erections are controlled by the parasympathetic nervous system (rest and digest).
    • Random erections often occur in the early morning when parasympathetic activity is high and sympathetic activity is low.
  • Ejaculation is driven by the sympathetic nervous system.
    • Ejaculation often leads to a quick loss of erection.
  • Analogy: Relaxed situations (making out) promote erections, while fearful situations (parents walking in) cause them to subside due to sympathetic nervous system activation.

Muscular Events

  • Ejaculation involves a series of muscular events that propel ejaculate through the reproductive and urinary tracts.
    • Ischio cavernosus and bulbospongiosis muscles are the 2 primary muscles responsible for generating the force during ejaculation to expel ejaculate.

Evolutionary Perspective on Intercourse and Pleasure

  • Intercourse is linked to pleasure.
  • Pleasure is evolutionarily linked to activities such as eating and exercise.
  • Eating is pleasurable to enforce the need to sustain life.
  • Runner's high and other pleasurable responses are linked to behaviors that promotes fitness.
  • Modern society provides easy access to pleasure, which can lead to unhealthy habits such as overeating or drug use.
  • The brain seeks pleasure, even if easy access leads to addiction or health risks.
  • All things in moderation.

Practical Advice

  • Be careful with drugs due to the brain's hardwiring for pleasure and potential for addiction.
  • Have a friend look out for your best interest if experimenting.
  • Avoid IV drugs due to the risk of infection and stroke from reusing needles.

Needle Exchanges

  • Acknowledge the potential to reduce healthcare burdens and prevent debilitating strokes by providing clean needles.

Introduction to Neurons

  • Neurons are the primary cells in the nervous system that transmit information.
  • Neurons vary by structure.
  • All neurons share structural components.

Structural Elements of a Neuron

  • Cell body: Contains the nucleus and other organelles for metabolic functions.
  • Dendrites: Projections from the cell body that receive communications from neighboring cells.
  • Axon: A long dendrite that carries the signal to a specific region/target cell in the body.
  • Motor neuron: A motor neuron is responsible for controlling your muscles by connecting to a muscle cell.
  • Terminal arborization: Branching at the end of the axon which allows a single axon to communicate with multiple cells at the terminal.
    • Synaptic terminal: Structure where an axon connects and communicates with another cell.
  • Axon hillock: Regulates neuron function.
    • Detects whether signals are strong enough to generate an action potential and transmit information down the axon.

Action Potential

  • Axons have a relative negative charge inside compared to a positive charge outside.
  • Depolarization: When a neuron is stimulated, the charges normalize or switch, causing an electrochemical gradient.
    • This quick change from negative to positive causes events that creates energy that will travel down the axon.
    • Depolarization occurs in one location, then triggers it in the adjacent section causing action potential.

Synapse

  • Synapse: Point where a neuron connects and communicates with another cell.
  • Synaptic cleft: The gap between the synaptic terminal and the target cell.
  • Neurotransmitters: Chemicals released at the synaptic terminal that float across the synaptic cleft to bind to the postsynaptic target cell, allowing communication between cells.
  • Synaptic vesicles: Structures inside the synaptic terminal where neurotransmitters are stored.
    • When action potential reaches synaptic terminal, synaptic vesicles move to the edge of the synaptic cleft and release the neurotransmitters.
    • Mitochondria present in synaptic terminals fuel this process.

Glial Cells

  • Cells within the nervous system that are not neurons.
  • Support/improve the function of neurons.

Nerve Structure

  • Nerves consist of bundles of neurons.
  • Layers of tissue making up a nerve:
    • Endoneurium: Connective tissue surrounding each individual axon.
    • Fascicle: Bundles of axons.
    • Perineurium: Connective tissue surrounding each fascicle.
    • Epineurium: Connective tissue surrounding the entire nerve (bundles of fascicles).
  • Nerves have their own blood supply.

Myelin

  • A protein structure that wraps around the axon of some neurons, creating a protective barrier.
  • Made by Schwann cells and oligodendrocytes.
  • Improves the speed at which action potentials travel down the axon.
  • Generates stronger signals.

Nodes

  • Parts of the axon that don't have myelin.
  • Allow movement of charged particles in and out of the axon.
  • Enable regeneration and strengthening of the action potential.

Nerve Conduction

  • Continuous propagation: Action potential regenerated at every step along the axon.
  • Saltatory propagation: The movement of an action potential down the length of an axon that does have myelin.

Multiple Sclerosis

  • Autoimmune disease where the body attacks its own myelin.
  • Impairs neuron function.
  • Potentially fatal due to affecting neurons controlling vital functions.

Types of Multiple Sclerosis

  • Primary Progressive MS:
    • The body's immune system continually attacks the myelin for an extended amount of time.
    • Disability from the lack of myelin worsens over time until death.
  • Relapsing Remitting MS:
    • Life expectancy is longer than primary progressive MS.
    • The body attacks the myelin but has periods of remission where the attack stops before attacking again.
    • Disability worsens with each relapse.
    • Each time the body goes into remission a greater disability level is noted than the previous remission event.