Homeostasis and Key Components of Homeostatic Mechanisms definition - the body’s maintenance of a relatively stable internal environment. [blood glucose] ~ 100 mg/ml blood pH ~ 7.4 blood pressure ~ 120/80 mmHg body temperature ~ 37˚C homeostatic mechanisms exist to ensure that all body systems function within a range that will sustain life despite fluctuations in the external environment (called dynamic equilibrium). consider thermoregulation, which controls body temperature: when cold, you get “goose bumps” - an attempt to fluff up body hair and conserve heat. shivering generates heat by involuntary contraction of muscles. vasoconstriction cuts down on the circulation of blood to the extremities (fingers, toes) in an effort to keep the major organs functioning (can lead to hypothermia). to get rid of excess heat, sweating occurs, because evaporation of sweat gives off heat. vasodilation causes more blood to flow to the skin where it releases heat. thermoregulation is an example of a negative feedback loop, where the mechanism is activated to reverse a change (similar to a furnace thermostat). sensory receptors are found throughout every body organ and tissue and their job is to send nerve impulses (stimuli) to the brain (hypothalamus) in response to environmental information. (p. 339 - figure 2). the brain is the integrator (sends instructions) to effectors (which cause a change in internal conditions). positive feedback loops (amplifying a small effect) exist, but aren’t common. (during birth, the baby’s head pushing to get out increases the release of oxytocin, which increases contractions of the uterus).

Human Nervous System The Neuron

A Reflex Response Resting Potential in Neurons

Structure and Function of the Nervous System There are 2 major parts in the human nervous system: CNS (central nervous system) - brain and spinal cord. PNS (peripheral nervous system) - nerves that lead in and out of CNS PNS is comprised of: autonomic NS relays info to the internal organs (not consciously controlled). It is made of the sympathetic NS (controls organs in times of stress - fight or flight) and the parasympathetic NS (controls organs when at rest). somatic NS is made up of sensory nerves that carry impulses from the body’s sense organs to the CNS and it also contains motor nerves which carry commands from the CNS to muscles. It is somewhat under your control. a reflex response (reflex arc) is one which doesn’t require a conscious decision by the somatic NS (being poked in the eye - automatic blink) the structural unit of the NS is the neuron, both PNS and CNS are made of interconnected neurons. nerves are bundles of neurons held by connective tissue - found in PNS structure of a neuron (see diagram): cell body - large nucleus, lots of mitochondria, do not divide after puberty. dendrites - primary site for receiving signals from other neurons. axon - long extension of the cell body (1mm to 1 m), it transmits a wave of depolarization along its length when the cell receives a sufficient stimulus. The end of the axon has structures which release chemicals (neurotransmitters) which stimulate surrounding neurons or muscle cells.

Poked with a Pin… your hand instinctively pulls away before you know you’ve been stabbed. this is a reflex that involves your spinal cord, not your brain. what happens? the pain triggers dendrites of a sensory neuron in your finger. an impulse is sent up the axon to the spinal cord. interneurons (nerve cells which link neurons) in the spinal cord pass the message to a motor nerve. the stimulated motor nerve sends a signal along its axon to the muscles in your hand and finger which triggers contraction.

How the Neuron Works at rest - the outside of the cell membrane is + charged, there are high [Na+], low [K+] and some Cl-. inside the cell are low [Na+], high [K+] and negative charge from larger anions (proteins, amino acids). the membrane has channels for K, Na and Cl, but the larger anions remain inside the cell. at rest, far more K+ moves out of the cell than Na+ moves in, thus, the negative interior. the Na+/K+ pump (a membrane protein) actively (using energy) pumps out 3 Na+ in exchange for 2 K+ the difference in charge between inside and outside is -70 mV (called the resting potential). All or None Principle When a neuron is sufficiently stimulated, a wave of depolarization is sent down the axon. sensory neurons can be stimulated by chemicals, light, heat or mechanical distortion. motor neurons are usually stimulated by neurotransmitters. the firing of a nerve depends on the stimulus to be of a certain intensity (called the threshold level) the axon works on the “all or none” principle, it will only trigger an impulse if the threshold level for stimulus is met. Depolarization if the threshold level is reached, Na+ flood into the axon, changing the interior charge to positive (called the action potential). this depolarization causes neighbouring Na+ channels to open and propagates the wave along the axon. Repolarization The depolarization of any region of the axon lasts for only a split second because right after the sodium channels open to allow Na+ into the cell, K+ channels open to let K+ out.

the sodium channels close at the same time and with the help of the Na+/K+ pump, the resting membrane potential is restored quickly. the brief time between the triggering of an impulse in an axon and when it will next be available is called the refractory period. (usually about 0.001 s) the wave of depolarization moves along an axon at about 2 m/s and maintains the same strength the entire way. the speed of transmission can be increased by coating the axon with a myelin sheath made of Schwann Cells. the sheath blocks action potentials, so the impulse must jump between nodes and can thus travel at up to 120 m/s. myelinated nerve fibres are found in the CNS and the PNS wherever speed is important.