Integumentary & Nervous System Overview

• Study notes on the integumentary system, thermoregulation, membrane potentials, action potentials, and the central and peripheral nervous systems

Integumentary System

• The stratum basale contains melanocytes.
• The stratum lucidum is found only in thick skin.
• Four functions of the integumentary system are:
  • Protection from dehydration
  • Protection from foreign invaders causing infection
  • Synthesis of vitamin D
  • Sensory feedback about the external environment and assisting in thermoregulation
• If all melanocytes die, melanin cannot be created, increasing the risk of UV damage
• The epidermis layer in contact with the environment is the stratum corneum, which is made of dead stratified squamous cells.
• A third-degree burn damages or destroys both the epidermis and dermis layers.
• Hair typically grows for 2-7 years.
• Merkel's cells are sensory cells in the epidermis that detect light touch.
• Cells in the stratum spinosum begin linking up with desmosomes.
• Fibroblasts are the predominant cell type in the dermis and secrete collagen, elastic, and reticular fibers.
• Other cell types in the dermis include mast cells, white blood (immune) cells, adipose (fat) cells, and sensory cells.
• Eccrine, apocrine, and sebaceous glands all secrete a substance that makes its way to the skin's surface.
• Eccrine glands are found all over the skin, especially in the palms of the hands and soles of the feet, and secrete sweat to cool the body.
• Armpits tend to have body odor due to a high concentration of apocrine glands, which secrete a sweat with cell pieces containing lipids, proteins, and carbohydrates, which bacteria consume the waste, making the odor.
• Sebaceous glands act as a moisturizing and waterproofing agent for skin and hair.
• Pacinian corpuscles are found deep in the dermis and are used to detect deep pressure on the skin.

Thermoregulation

• If body temperature is 89.5 °F, effectors that warm the body would activate, including:
  • The thyroid gland releasing more thyroid hormone to increase metabolism and heat production
  • Muscles shivering to increase heat production
  • Blood vessels in the skin constricting to reduce blood flow to the surface to help retain heat
  • Arrector pili muscles contracting to pull hairs upright to increase the air-insulation layer around the skin
  • Behavioral centers of the brain increasing the motivation to seek out warmth
• The hypothalamus is the brain area that integrates for thermoregulation.
• Three effectors used for both hypothermia and hyperthermia (with opposite effects):
  • Thyroid gland: decreases thyroid hormone production when hot, increases when cold
  • Blood vessels in the skin: dilate when hot, constrict when cold
  • Behavioral centers in the brain: seek cold when hot, seek warmth when cold
• A tumor on the thyroid gland causing excessive thyroid hormone secretion would increase metabolism, generating more body heat, potentially causing hyperthermia and triggering cooling effectors.

Membrane Potentials / Action Potentials

• If a channel for a cation opened and crossed the cell membrane, the positive ion into the cell would cause a depolarization. If the positive ion gets pushed out of the cell, it would cause a hyperpolarization.
• If a cell has a positive membrane potential, the electrical force of a negative ion would be into the cell because negative ions would be attracted to the positivity inside the cell.
• For a typical cell, with a higher potassium concentration inside the cell, if the potassium concentration outside the cell is doubled, the concentration gradient decreases.
• The Na+/K+-ATPase (Na+/K+ pump) causes the concentration of sodium (Na+) to be high outside the cell and the concentration of potassium to be high inside the cell.
• For an ion at equilibrium, all forces acting on the ion, including concentration and electrical forces, have canceled each other out, resulting in no net force acting on the ion.
• The Nernst equation calculates an ion's equilibrium potential, which represents the membrane potential at which the ion is at equilibrium and has the charge needed to create an electrical force to counteract the concentration force.
• An ion channel for X²- opens, this negatively charged ion flows out of the cell, making the inside of the cell less negative (or, more positive), moving the membrane potential from -70mV toward -30.5mV.

Membrane Potential Determination

• The relative permeabilities of sodium and potassium determine the actual membrane potential. The more permeable ion has a greater influence.

Neuron Ion Channels

• Chemically gated ion channels are found on the dendrites and, to a lesser degree, the cell body.
• If all voltage-gated Na+ channel activation gates are stuck in the closed position, a graded potential reaching -55 mV would not cause the cell to fire an action potential, halting neuronal communication.
• If all voltage-gated K+ channel activation gates are stuck in the closed position:
  • The membrane potential would stay positive longer after an action potential due to blocked K+ channels. This can be reversed with the leak channels and Na+/K+ pump working very slowly.
  • Neurons would release lots of neurotransmitter from just a single action potential.
• If all voltage-gated Ca2+ channel activation gates were stuck in the closed position:
  • The cell could reach threshold potential and fire action potentials
  • The axon terminals would not release any neurotransmitter once action potentials arrived there, stopping all neuronal communication.
• If all chemically gated channel gates were stuck in the closed position:
  • Neurons could not receive input from other neurons, preventing graded potentials from triggering action potentials, stopping all neuronal communication.

Action Potential and Graded Potential

• An action potential is a brief membrane polarity switch (negative to positive, then back) that travels down a neuron's axon, starting at the spike initiation zone.
• The threshold potential is the membrane potential at which voltage-gated channels are triggered to switch positions and is -55mV.
• A graded potential is a variable change to the cell's membrane potential due to chemically gated channels on the dendrites, and can trigger an action potential if it gets the spike initiation zone to reach -55mV.

Ion Channel Action and Refractory Periods

• During an action potential, Na⁺ rushes in while K+ efflux is delayed due to the time delay of the K+ activation gate.
• The absolute refractory period is the time when another action potential cannot be generated because the Na⁺ inactivation gate has closed.
• The relative refractory period is when generating another action potential is possible but harder because the membrane potential is farther from the threshold potential.
• Exocytosis of neurotransmitter from synaptic vesicles is triggered by increased calcium concentration in the axon terminal cytosol.
• A chemically gated channel allowing Cl- to enter a postsynaptic neuron would cause an IPSP, moving the spike initiation zone farther from the -55mV threshold.
• Spatial summation is when the effects of two different postsynaptic neurons overlap and sum together in a postsynaptic neuron.

Central Nervous System (CNS)

• Sensory neurons in the skin are unipolar neurons, classified by shape.
• Interneurons are the most common type of neuron (classified by function), shuttling signals between two other neurons.
• Glutamate is the most abundant excitatory neurotransmitter, while GABA is the most abundant inhibitory neurotransmitter.
• The brain stem is part of the central nervous system and controls vital functions like breathing and blood pressure.
• The cerebellum maintains balance and creates/stores/modifies complex motor programs.
• The primary somatosensory cortex is in the anterior portion of the parietal lobe, behind the central sulcus, and receives information from the skin, muscles, and joints about touch, pain, itch, temperature, muscle length, and muscle tension.
• The visual association area is in the occipital lobe and puts visual information into context.
• The pre-motor cortex sits anterior to the central sulcus in the frontal lobe and plans conscious movements.
• The thalamus lies near the center of the cerebrum, acts as a sensory relay station, and is a sensory filter.

Peripheral Nervous System (PNS)

• A nerve is a bundle of neurons covered with a connective tissue sheath.
• There are 12 thoracic spinal nerves on each side of the body.
• In the spinal cord, gray matter is mostly cell bodies of neurons, while white matter is mostly axons of neurons.
• Dorsal root ganglia contain a cluster of cell bodies of sensory neurons in the skin, muscles, and joints.
• Reflexes are hardwired, unlearned, involuntary motor responses, while reactions are voluntary actions.
• Myotatic stretch reflexes are faster than withdrawal reflexes because they lack interneurons and have faster sensory neurons.
• During a withdrawal reflex, the inhibitory interneuron releases its neurotransmitter on the dendrites of alpha-motor neurons going to the extensors of the limb.

Neuron Conduction Velocity and Coding

• Neuron conduction velocity is affected by the diameter of the neuron and myelination.
• Frequency coding is how a neuron conveys the magnitude of the signal it's sending.

Cranial Nerves

• Cranial nerve VII is the Facial nerve, carrying sensory and motor information to/from the face, controlling facial muscles and taste from the anterior 2/3rds of the tongue.
• Damage to cranial nerve XII (Hypoglossal nerve) will severely impact chewing and talking because it controls the muscles of the tongue.
• The movement of the eyes is controlled by three separate cranial nerves: oculomotor (III), trochlear (IV), and abducens (VI).