Neurotransmitters and Hormonal Coordination in Animals
Neurotransmitters
Neurotransmitters are biomolecules synthesized by neurons and stored in small, concentrated vesicles within the axon terminals.
Nerve Reflex Pathways
A nerve pathway is the route followed by nerve impulses from their origin at a dendrite to another body area.
Reflex Arc
A reflex arc consists of:
- Receptors: Structures responsible for capturing a specific internal or external stimulus and transforming it into nerve impulses.
- Sensory Neuron: Conducts the nerve impulse from the receptor to an association neuron.
- Association Neuron: Integrates the incoming information and formulates a response.
- Motor Neuron: Carries the generated nerve impulses from the association neuron to the effector.
- Effector: The part of the body that responds to the nerve impulses, such as muscles or glands.
Reflex: A response to a stimulus transported by a reflex arc. Reflexes can be innate or conditioned.
Nervous System in Animals with Radial Symmetry
Cnidarians
Cnidarians possess the most primitive nervous system, a nerve plexus, which is a network of neurons extending throughout the body.
Nervous System in Animals with Bilateral Symmetry
Evolutionary Trends
- Increase in the number and concentration of neurons in neuronal ganglia (clusters of neuron cell bodies).
- Increase in the number of axon bundles, allowing for greater complexity in reflex arcs and increased capacity to integrate information through interneurons.
- Accumulation of neurons in the central nervous system (CNS).
- Hierarchy in nerve centers, with some controlling others.
Cephalization
Cephalization is the concentration of sensory organs and nerve cells in the animal’s head, which integrate the information these organs provide.
Vertebrate Nervous System
Central Nervous System (CNS)
The CNS is formed by the brain and spinal cord, surrounded by meninges: dura mater, arachnoid mater, and pia mater.
Peripheral Nervous System (PNS)
The PNS is composed of sensory and motor neurons, forming the cranial and spinal nerves.
Brain
White Matter
Formed by myelinated axons.
Gray Matter
Contains neuronal cell bodies, dendrites, and glia.
Hindbrain
Composed of the myelencephalon (medulla oblongata), metencephalon (pons and cerebellum).
- Medulla Oblongata: Controls heart rate, respiratory rate, blood pressure, and swallowing.
- Cerebellum: Integrates information from sight, hearing, muscles, and joints, and from other brain areas.
Midbrain
In fish, amphibians, and reptiles, the midbrain controls more complex behaviors.
Forebrain
Formed by the diencephalon (thalamus and hypothalamus), telencephalon (cerebrum).
- Thalamus: Principal communication center between the brainstem and higher brain centers.
- Hypothalamus: Involved in homeostasis and integration of the endocrine system.
- Limbic System: Involved in emotions, motivation, and memory.
Brain Functions
Memory, reasoning, language, interpretation, and sensations.
Spinal Cord
A nerve cord extending within the vertebral column from the medulla oblongata to the second lumbar vertebra, with a dorsal (posterior) and ventral (anterior) groove.
Peripheral Nervous System (PNS)
Formed by the somatic and autonomic nervous systems.
Autonomic Visceral Reflexes
Comprises:
- Receptor: Formed by the dendrites of sensory neurons.
- Sensory Neuron: Conducts sensory information to integration centers.
- Association Neuron: Located in the brainstem and spinal cord.
Hormonal Coordination
The nervous and hormonal systems continuously integrate internal and external environmental information to produce appropriate responses for homeostasis.
Hormones
Hormones can be proteins, amino acid derivatives, peptides, and steroids.
Endocrine Cells
Endocrine cells are often clustered in glands and their products are transported by the blood.
Neurosecretory Cells
Specialized neurons that produce and release neurohormones.
Hormone Mechanism of Action
Activation of membrane receptors or intracellular receptors.
Hormones in Invertebrates
Regulate growth, molting, reproduction, etc. Flatworms control development and changes in coloration. Crustacean movements are hormonally regulated, and chromatophores control color by dispersing pigment granules.
Pheromones
Chemical substances produced by exocrine glands that influence the behavior of other individuals. Some pheromones induce immediate behavioral responses, while others induce physiological changes.
Glands and Hormones
- Pituitary Gland: The master gland.
- Thyroid Gland: Thyroxine activates metabolism; calcitonin lowers blood calcium.
- Parathyroid Glands: Parathyroid hormone increases blood calcium.
- Adrenal Glands: Glucocorticoids control glucose metabolism; mineralocorticoids control salt metabolism; adrenaline and noradrenaline prepare the body for stress.
- Pancreas: Insulin decreases blood glucose; glucagon increases blood glucose.
- Testes: Androgens determine male sexual characteristics.
- Ovaries: Estrogens determine female sexual characteristics; progesterone prepares for pregnancy.
Eye
- Choroid: Black layer located between the sclera and the retina.
- Sclera: White protective layer surrounding the eyeball.
- Fovea: Most sensitive part of the retina.
- Retina: Layer of light-sensitive cells that send messages to the CNS.
- Blind Spot: Where the optic nerve enters the eye.
- Optic Nerve: Transmits information to the brain.
- Vitreous Humor: Gelatinous substance filling the eyeball.
- Conjunctiva: Transparent membrane protecting the front of the eye.
- Cornea: Allows light to enter the eye.
- Aqueous Humor: Liquid between the cornea and lens.
- Iris: Controls the amount of light entering the eye.
- Lens: Transparent and can change shape to focus light.