Nervous and Digestive Systems: Invertebrates & Vertebrates
Nervous System
Function: The nervous system receives, integrates, and transmits information from the external and internal environment, as well as coordinating and monitoring responses. Receptors, sensory nerve pathways, modulators, motor nerve pathways, and effectors are involved.
Nerve Impulse
A nerve impulse is the transmission of signals coming from neurons. It consists of a resting potential, depolarization, action potential, and repolarization. The excitability threshold is the minimum intensity for the stimulus to be effective. The refractory period is the time it takes to start a momentum.
The nerve impulse moves through the plasma membrane of the neuron, and when it reaches the end of the axon, it can be transmitted to another neuron. The plasma membrane is polarized because the electric charges are distributed differently within and outside the cell. Inside, there is a predominance of negative charges, and outside, Na+. This potential difference is called the resting potential. If a certain stimulus in a neuron is effective, it causes an alteration in the permeability of the membrane that allows the influx of Na+ ions at that point and reverses the polarity: positive inside and negative outside. This process is called depolarization, and the potential variation is called the action potential. Depolarization electrically disturbs areas adjacent to the point where the stimulus was applied and is spread throughout the neuron. The action of enzymes that exist in the conveyor extract membrane Na+ causes it to recover the initial state point by point.
Nerve Impulse Transmission
Nerve impulse transmission occurs across the synapse, involving neurotransmitters that are stored in synaptic vesicles.
Nervous System in Invertebrates
- Coelenterates: The nervous system consists of a diffuse nerve plexus.
- Flatworms: They have a primitive brain with two nerve cords.
- Annelids: They have lymph node chains that form the knot and head periesophageal lymph cerebroideos.
- Molluscs: They are very similar to annelids.
- Arthropods: They are similar to annelids but more evolved.
- Echinoderms: They present a periesophageal ring that connects to the radial nerve cords.
Vertebrate Nervous System
Vertebrates present a neural tube that forms the brain and spinal cord. The central nervous system is composed of the brain and spinal cord. The peripheral nervous system consists of the somatic system (cranial and spinal nerves) and the autonomic nervous system (sympathetic and parasympathetic).
Nervous Integration
The simplest mechanism is the reflex arc, which produces a specific response.
Receptors
Receptors are cells that capture and transform stimuli (sensory organs) into nerve impulses. Receptors have several characteristics: differential sensitivity, range of duration, and adaptation.
Effectors
Effectors are the active part of the movement (muscles) and run that through your contractions behavioral motor responses. These, along with the skeleton (passive part), form the musculoskeletal system.
Digestive System
Function
The digestive system converts food molecules into cells by food capture, ingestion, digestion (breakdown of complex compounds to simple) that can be extracellular, intracellular, or mixed, and absorption of nutrients.
Types of Digestive Systems in Invertebrates
- With a single opening:
- Sponges: Intracellular digestion in the choanocytes and subsequent transport of nutrients by the amebocyte.
- Cnidarians: Mixed digestion, and they present cnidoblasts.
- With two openings and extracellular digestion along the tube:
- Annelids: Consists of mouth, pharynx, crop, gizzard, intestine, and anus.
- Molluscs: Radula grinding mouth, esophagus, stomach, hepatopancreas, intestine, and anus.
- Arthropods: They exhibit a great variety of structures to capture food and show different oral appliances.
- Echinoderms: Sea urchins have a masticator device called Aristotle’s lantern.
Digestive System in Vertebrates
- Oral cavity: Includes the lips, teeth, tongue, and salivary glands. Both mechanical and chemical digestion occur.
- Pharynx: Muscular tube that participates in the process of swallowing.
- Esophagus: Through rectum and muscles, whose contractions propel the food bolus. In birds, there is a dilation called a crop.
- Stomach: Digestive tract dilation, which produces chemical digestion by the action of gastric juice. In birds, a gizzard appears to grind food.
- Intestine: The small intestine, where chemical digestion and absorption of nutrients occur, and the large intestine, where water absorption takes place to form stool.