Nervous and Endocrine Systems in Vertebrates
Nervous System
The nervous system collects various stimuli from both inside and outside the body through specialized neurons organized in receptors. External stimuli are detected by exteroceptors, while internal stimuli are detected by interoceptors. This information is transmitted to the central nervous system (CNS), which processes the information and generates a response. Organs that execute commands from the CNS are called effectors.
Invertebrate Nervous Systems
- Nerve Net: Found in primitive animals like cnidarians, this system consists of a diffuse network of nerve cells.
- Nerve Cord: Characteristic of flatworms, this system features a pair of ganglia in the anterior region (a primitive brain) and two nerve cords running through the body.
- Radial Nerve Cord: Found in echinoderms, this system consists of a nerve ring connected to radial nerves.
- Nodal Nervous System: Observed in arthropods, this system exhibits a tendency towards the concentration of nerve cells in ganglia.
Central Nervous System (CNS)
The CNS comprises the brain and spinal cord. Both structures are delicate and protected by:
- Bones of the skull and vertebral column.
- Meninges: Three membranes (dura mater, arachnoid, and pia mater) that encase the nervous tissue. Cerebrospinal fluid circulates between the arachnoid and pia mater, providing further protection.
Brain
The brain receives, processes, and orders responses to information. It controls the operation of the entire CNS and is responsible for voluntary actions and consciousness. The brain consumes a significant amount of energy (20% of the body’s glucose) despite its small size (2% of total body weight).
The brain has two hemispheres connected by the corpus callosum. Each hemisphere governs one side of the body. The right hemisphere is associated with artistic skills and intuition, while the left hemisphere is associated with logic, analysis, and reasoning.
The cerebral cortex contains gray matter (neuron cell bodies) and white matter (myelinated axons). Folds (gyri) and grooves (sulci) divide the cortex into four lobes:
- Frontal Lobe: Reasoning, emotions, and personality.
- Parietal Lobe: Sensory processing (touch, temperature, pain).
- Temporal Lobe: Hearing, smell, and memory.
- Occipital Lobe: Vision.
Other brain regions include:
- Diencephalon: Contains the thalamus (relay center for sensory information) and hypothalamus (regulates thirst, hunger, and body temperature).
- Midbrain: Involved in eye movement and auditory processing.
- Hindbrain: Contains the cerebellum (coordinates movement and balance) and medulla oblongata (controls involuntary functions like breathing and heart rate).
Spinal Cord
The spinal cord transmits information between the brain and the rest of the body. Unlike the brain, gray matter is located internally (butterfly-shaped) and white matter is external. The central canal (ependyma) contains cerebrospinal fluid. Sensory nerves enter the spinal cord through the dorsal horns, and motor nerves exit through the ventral horns.
Peripheral Nervous System
The peripheral nervous system (PNS) consists of nerves that connect the CNS to the rest of the body. Cranial nerves (12 pairs) innervate the head and neck, while spinal nerves (31 pairs) innervate the rest of the body. Most nerves are mixed, containing both sensory and motor fibers.
Autonomic Nervous System
The autonomic nervous system controls involuntary functions. It consists of two branches:
- Sympathetic Nervous System: Prepares the body for “fight or flight” responses.
- Parasympathetic Nervous System: Promotes “rest and digest” functions.
Endocrine System
The endocrine system consists of glands that produce hormones. Hormones are molecules that regulate various bodily functions. Hormone regulation is achieved through negative feedback mechanisms.
Hormones in Vertebrates
- Steroid Hormones: Small, lipid-soluble hormones that can pass through cell membranes and bind to intracellular receptors.
- Protein Hormones: Large, water-soluble hormones that bind to receptors on the cell surface.
Hypothalamus-Pituitary Axis
The hypothalamus and pituitary gland work together to control many endocrine functions. The hypothalamus releases neurohormones that regulate the pituitary gland, which in turn releases hormones that act on target organs.
Thyroid Gland
The thyroid gland produces thyroxine (T4) and triiodothyronine (T3), which regulate metabolism. It also produces calcitonin, which lowers blood calcium levels.