Animal Hormones and Glands

• The final review covers units 14-17.

Signaling Around the Body

• Animals use two systems for internal communication: the nervous system and the endocrine system.
• The nervous system discussion will be in the following lecture.

Hormones

• Hormones are specific molecules, either polypeptide or steroid, synthesised and secreted by specialized cells.
• Hormones are released into body fluids and travel to target cells to elicit biological responses.

Glands

• Endocrine glands are ductless and secrete hormones directly into the bloodstream for distribution throughout the body.
• Exocrine glands produce substances like sweat, mucus, and digestive enzymes, conveying them through ducts and not part of the endocrine system.

Non-Steroid Hormone Action

• Hydrophilic protein hormones bind to receptors on the target cell's surface.
• Hormone binding triggers chemical events, altering cell metabolism, and always involves a second messenger within the cell.

Steroid Hormone Action

• Steroids, being hydrophobic, easily pass through the plasma membrane.
• Receptors are located within the cytoplasm or the cell's nucleus.

Human Endocrine Glands

• The hypothalamus receives nerve signals from the body and brain.
• This also initiates appropriate endocrine signals.

Pituitary Gland

• It is located at the base of the brain and is known as the "Master Gland".
• The pituitary gland's two lobes (posterior and anterior) release hormones to regulate other endocrine glands and the body's general metabolism.

Endocrine Regulation of the Kidneys

• Osmoreceptors in the hypothalamus sense increased solute concentration in the blood, or loss of H2O due to severe sweating.

Antidiuretic Hormone (ADH) or Vasopressin

• The hypothalamus produces ADH, which is transported to the posterior pituitary for release into the blood.
• ADH acts on the kidneys, increasing the permeability of the collecting ducts' cells to water.

ADH Loop

• More water osmotically passes out of the filtrate, decreasing urine production, which consequently lowers solute concentration.
• Osmoreceptors in the hypothalamus sense reduced concentration, leading to decreased ADH production through negative feedback.

Neuron Structure

• The cell body is the main part and this contains the nucleus and all other organelles.
• Dendrites conduct messages to the cell body.
• The axon carries impulses away from the cell body.

Neuron Structure Continued

• Many neurons have a single, potentially very long axon.
• Neurons in vertebrate systems have a myelin sheath, often formed by Schwann cells.
• Schwann cells are glial cells that wrap their plasma membrane around an axon, creating a myelin insulating layer that facilitates rapid signal conduction.
• Axons may be branched, with branches ending in synaptic terminals releasing chemical messengers to other cells.
• Neurotransmitters are chemical messengers between nerve cells or to effector cells.
• The site of contact between a synaptic terminal and a target cell is called the synapse.

Glial Cells

• Glial cells are vital for the nervous system's structural integrity and outnumber neurons 10x to 50x.

Glial Cells -- Types

• Astrocytes provide structural support to neurons in the CNS.
• Astrocytes also regulates the extracellular concentrations of neurotransmitters.
• A layer of cells joined by tight junctions provide the structural basis of the blood-brain barrier.
• Schwann cells form the myelin sheath around axons of neurons in the PNS.
• Oligodendrocytes produces myelin sheath around axons of CNS.
• Myelin contains lipids and provides an electrical insulation for the axon

Neuron Types

• Sensory neurons communicate information from sensory receptors (nose, eyes, ears, tongue, skin) about the external environment to the central nervous system.
• Interneurons are in the CNS, integrate sensory input and motor output, and synapse with other neurons only.
• Motor neurons convey impulse (motor outputs) from the CNS to effector cells.
• A ganglion is a cluster of nerve cell bodies, often of similar function, in the PNS.

Neural Circuits

• A population of neurons is interconnected by synapses to carry out a specific function when activated.
• The simplest circuits synapse sensory and motor neurons.
• Results in an automatic reflex response.

Knee-Jerk Response

• This is an example of a reflex.
• A tap on the knee exerts a pull on the tendon in the thigh muscle, which stretches receptors in the muscle.
• The muscle detects the pull, sending signals to synapses with motor neurons in the spinal cord.

Nervous Signals

• Neurons have a voltage gradient across the plasma membrane called membrane potential.
• This is due to ionic composition differences in intracellular and extracellular fluids.

Membrane Potential Basis

• The Na+/K+ pump actively maintains differing concentrations on sides of membrane, despite permeability to K+ and Na+.
• It pumps out more Na+ than K+ (3:2), creating a net negative charge inside the cell.
• Some are anions pumped into the cell.

Action Potential

• When a stimulus is received by a neuron, it causes depolarization of the membrane at that site.
• When a depolarization of the membrane occurs that shift the membrane potential significantly, that results in a massive change in the membrane potential.

Propagation of Action Potential

• At rest, the inside of the cell is net negative due to the membrane potential and Na+/K+ pump action.
• When a stimulus is received by a neuron, it causes depolarization at that site.
• Depolarization reduces the electrical gradient across the membrane when channels open for Na+.
• This causes Na+ to travel down its concentration gradient, becoming more positive.
• Then the membrane reverses it electrical gradient, becoming positive on the inside and negative on the outside as Na+ rushes in.
• As the membrane is depolarized, an action potential is reached, causing adjacent Na+ gates to open and further depolarization.
• The process continues down the neuron from the dendrites through the axon to the synaptic terminal.
• Transmission speeds vary from centimeters per second to 1000 m/sec in the giant axons of squids.

Saltatory Conduction

• Myelin sheaths in vertebrate axons allow for much faster action potential propagation.
• Channels are concentrated in the Nodes of Ranvier.
• Extracellular fluid is in contact with the axon here
• Action potential jumps from node to node, which is why it does not propagate over the length of the axon.
• This increases the transmission speed of neurons.

Chemical Synapse

• When an action potential reaches the synaptic terminal, it stimulates vesicles release by exocytosis.
• The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the post-synaptic membrane.
• Upon binding, ion channels open, allowing Na+ ions into the neuron.
• This influx of ions depolarizes the membrane, generating a new action potential that travels down the neuron.

Sensory Reception

• Action potentials that reach the brain from sensory neurons are called sensations.
• The interpretation of sensations by the brain leads to perception of stimuli.
• There are two basic types of sensory receptors: exteroreceptors and interoreceptors.
  • Exteroreceptors detect stimuli outside the body like light, heat, or pressure.
  • Interoreceptors detect stimuli inside the body like body position and blood pressure.

Skeletons

• Skeletons have three main functions: support, protection, and providing attachment for muscles to facilitate movement.
• All skeletons do at least one of these, or sometimes all three.

Vertebrate Skeletal Muscle

• Vertebrate skeletal muscle is known as striated muscle because of lines when viewed under microscope.
• Skeletal muscle is made of a bundle of long fibers running the length of the muscle.
• Each fiber is a single cell with multiple nuclei and is made up of smaller myofibrils.
• Myofibrils contain two types of filaments: thin filaments (actin) and thick filaments (myosin).

Locomotion

• All types of locomotion, require the animal to overcome the forces of friction and gravity
• Swimming organism must overcome the friction (resistance) produced by water.
• Running organism must be able to support itself and overcome gravity to jump or run
• Flying organism is affected by gravity. Appendages create lift to overcome gravity

Types of Skeletons

• There are fluid skeletons
• Hydrostatic skeletons containing fluid under pressure in a closed body compartment, the most common fluid skeleton for different groups.
• These functions in locomotion and provides a small amount of protection for soft parts.
• Fluids can be coelomic, GVC, or pseudocoelomic fluids.

Hydrostatic Skeleton - Hydra

• In Hydra, the hydrostatic skeleton utilizes the gastrovascular cavity (GVC).
• The mouth must be closed for the skeleton to function.
• Water intake causes GVC to expand.
• Longitudinal fibers in epidermis contract the animal making it become short and fat.
• Otherwise, if the mouth is open, water is expelled.

Hydrostatic Skeleton - Earthworms

• Earthworms are associated with the body wall acting on fluid of the skeleton cause movement.
• They occur because the coelomic cavity is divided by septa between the segments.
• Alternate contraction and relaxation in alternate segments of longitudinal and circular muscles causes thinning and fattening for burrowing.
• The setae on the ventral surface is used to anchor the worm as it extends.

Exoskeletons

• An exoskeleton is a hard encasement deposited on the surface of the animal as seen in arthropods.
• The exoskeleton, cuticle and attached to the inside is composed of chitin, a polysaccharide similar to cellulose.
• Attaching to knobs and plates that extend to the inside, muscles enable movement by waxy epicuticle on the outside that to prevent water loss from the body.
• As arthropods, it support movement with attachment, and provide protection as internal support muscles.

Fluid Skeletons vs Hard Skeletons

• Fluid Skeletons:*
  • Difficulty to move on land
  • Restriction of size
  • The muscles associated with the body wall cause changes to the shape of body
    • there is no articulation and no precise control, however.
  • There is little protection
• Hard Skeletons:*
  • the extensions, limbs for articulated movements (precise control) support and provide protection
  • muscles support as a function of limbs, less important diminishing importance of body wall muscles

Exoskeleton vs Endoskeleton

• Exoskeleton*
  • Advantage: Protection against injury and desiccation on land
  • Disadvantages:
    • It must be shed periodically for growth, the land sizes limit because exoskeleton heavy as size increases. Not a problem in water.
      • There is not room for many muscles
• Endoskeleton*
  • Advantage:
  • Living material that grows with animal.
  • Able to have a greater variety of movements.
    • Some protection is always available in cranium.
  • Disadvantage: No overall protection on land, only support