Table of Contents
The nervous system of a frog is a complex structure that controls and coordinates the various functions and activities of the frog’s body. It can be broadly divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS).
Central Nervous System (CNS):
Brain: The frog’s brain is divided into several parts:
- Olfactory Lobes: Responsible for the sense of smell.
- Cerebrum: Although not as developed as in mammals, it is associated with sensory functions and voluntary movements.
- Optic Lobes: Responsible for vision.
- Cerebellum: Controls balance and coordination.
- Medulla Oblongata: Connects the brain to the spinal cord and controls involuntary functions like heartbeat and respiration.
- Spinal Cord: It extends from the medulla oblongata and runs down the back, transmitting nerve impulses to and from the brain and the rest of the body.
- Brain: The frog’s brain is divided into several parts:
Peripheral Nervous System (PNS):
- Cranial Nerves: There are 10 pairs of cranial nerves in frogs that arise from the brain and cater to the head region.
- Spinal Nerves: These arise from the spinal cord and are distributed to the rest of the body. There are 10 pairs of spinal nerves in frogs.
- Sympathetic Nervous System: This is a part of the autonomic nervous system and controls involuntary functions like the dilation of the pupil.
- Eyes: Frogs have well-developed eyes with a nictitating membrane that helps protect them. They also have a unique feature called the “rods and cones” in their retina which helps them see in low light.
- Ears: Frogs have an external eardrum or tympanum which is visible. The inner ear contains semicircular canals that help in maintaining balance.
- Nasal Openings: These are present on the snout and lead to the olfactory chambers for the sense of smell.
- Taste Buds: Found on the tongue and mouth for the sense of taste.
- Lateral Line System: This is present in aquatic frogs and tadpoles. It’s a series of sensory organs that detect water movements and vibrations.
Endocrine System (related but distinct from the nervous system):
- Frogs have various glands like the pituitary, thyroid, parathyroid, adrenal, and pancreatic glands which secrete hormones. These hormones help in the regulation of various body functions and processes.
The nervous system of the frog is adapted for its amphibious lifestyle. For example, the presence of the lateral line system in aquatic stages helps them detect prey and predators in the water. Similarly, their well-developed eyes and ears are crucial for both aquatic and terrestrial environments.
Central Nervous system of frog
1. Brain of Frog
The frog’s brain, an intricate organ, is situated securely within the cranial cavity of the skull. This elongated, somewhat flattened structure, which is white in hue, is enveloped by two protective meninges. These meninges are specialized connective tissue membranes. The innermost layer, known as the piamater, is delicate, pigmented, and vascular, adhering closely to the brain’s surface. In contrast, the outermost layer, the duramater, is robust and fibrous, lining the cranial cavity. Between these membranes and the brain’s inner cavities, a clear, watery, and lymphatic substance called cerebrospinal fluid circulates. This fluid plays a pivotal role in safeguarding and nourishing the brain.
The brain’s architecture can be delineated into three primary segments:
- Olfactory Lobes: These are the foremost, spherical lobes. Each lobe dispatches an olfactory nerve to its respective nasal chamber. Internally, they house a distinct cavity known as the olfactory ventricle or rhinocoel. Their primary function is related to olfaction, which is relatively underdeveloped in frogs.
- Cerebral Hemispheres: Situated posterior to the olfactory lobes, these elongated, oval structures form the cerebrum. They are characterized by a deep mid-longitudinal fissure that separates them. Their internal cavities, termed I, II, or lateral ventricles, are interconnected and also link with the diencephalon’s III ventricle through the foramen of Monro. The cerebral hemispheres are integral to memory, intelligence, consciousness, and voluntary actions.
- Diencephalon: Positioned posteriorly to the cerebrum, this rhomboid region encompasses the third ventricle or diocoel. It plays a crucial role in vision and balance.
- Midbrain: This section is the most expansive part of the brain. It houses the iter or aqueduct of Sylvius, which connects the III and IV ventricles. The midbrain also features two prominent optic lobes, known as the corpora bigemina. These lobes are instrumental in inhibiting spinal reflexes.
- Cerebellum: Located dorsally behind the optic lobes, the cerebellum is a relatively underdeveloped structure in frogs. It is responsible for equilibrium and muscular coordination.
- Medulla Oblongata: This terminal segment of the brain seamlessly transitions into the spinal cord. It houses the IV ventricle or metacoel, which is linked to the central cavity of the spinal cord. The medulla is indispensable for regulating involuntary functions such as heartbeat and respiration. Its removal invariably results in the organism’s demise.
In summary, the frog’s brain, with its intricate structure and divisions, plays a pivotal role in regulating both voluntary and involuntary functions, ensuring the amphibian’s survival and adaptation to its environment.
2. Spinal cord of frog
The spinal cord of the frog is a vital component of its central nervous system. Originating posteriorly from the medulla oblongata, it traverses through the foramen magnum. Ensconced securely within the neural canal of the vertebral column, this cylindrical structure, which is slightly flattened, exhibits a white hue. Its morphology is reminiscent of its anterior counterpart, the brain, in that it too is enveloped by two protective meninges: the piamater and the duramater. These meninges harbor the cerebrospinal fluid, which serves the dual purpose of protection and nourishment for the spinal cord.
The terminal end of the spinal cord culminates into a slender, non-neural filament known as the filum terminale, located within the urostyle. Notably, the spinal cord exhibits two distinct swellings along its length: the brachial enlargement, positioned between the forelimbs, and the sciatic or lumbar swelling, situated anterior to the filum terminale.
Two salient longitudinal grooves traverse the entirety of the spinal cord. The dorsal fissure, located mid-dorsally, and the ventral fissure, positioned mid-ventrally, demarcate its surface. Internally, the spinal cord encompasses a slender central canal, which seamlessly continues from the brain’s ventricles.
Structurally, the spinal cord can be dichotomized into:
- Outer White Matter: This layer is predominantly constituted of nerve fibers.
- Inner Gray Matter: This central region is largely made up of nerve cells.
Functionally, the spinal cord plays an instrumental role in mediating reflex actions, facilitating rapid responses to stimuli without the direct involvement of the brain.
In essence, the frog’s spinal cord, with its intricate structure and pivotal functional role, underscores its significance in the amphibian’s neural framework.
Functions of Different Parts of the Brain
The brain, a central organ of the nervous system, plays a pivotal role in orchestrating a myriad of vital activities. It functions as the primary control center, processing sensory information received from various body parts and subsequently directing appropriate responses through motor fibers. Here’s an overview of the functions associated with different regions of the brain:
- Pallium: This region of the cerebral hemispheres is responsible for managing the activities of olfactory (smell), tactile (touch), and optic (vision) organs.
- Cerebral Hemispheres: Beyond coordinating neuromuscular activities, these structures are believed to be the locus of intelligence and voluntary control, especially in higher animals.
- This brain region plays a crucial role in metabolic processes, particularly those involving fats and carbohydrates. Additionally, it oversees the regulation of genital functions.
Optic Lobes and Optic Thalami:
- These structures are primarily associated with visual sensations. They are integral to the perception of sight and also govern the coordinated movement of eye muscles.
- The cerebellum is instrumental in managing automatic movements. It ensures coordination in locomotive activities and works in tandem with the medulla oblongata to regulate intricate muscular movements throughout the body.
- As a vital nerve center, the medulla oblongata houses the nerve centers for numerous reflex functions. It predominantly governs involuntary bodily functions that are not directly influenced by conscious will. These include heart rate regulation, respiratory processes, swallowing, taste perception, auditory sensations, sound production, and the secretion of various digestive enzymes.
Peripheral nervous system of frog
The peripheral nervous system (PNS) of the frog serves as a communication bridge between the central nervous system (CNS) and the rest of the body. It comprises nerves that emanate from both the brain and the spinal cord.
Classification of Nerves:
- Cranial Nerves: These nerves originate directly from the brain.
- Spinal Nerves: These nerves emerge from the spinal cord.
Structure of a Nerve:
- Nerves appear as solid, white thread-like structures. Each nerve is an assembly of multiple bundles known as funiculi. An outer sheath of loose connective tissue, termed the epineurium, envelopes each nerve.
- Every funiculus is encased by a dense tissue layer called the perineurium. Within each funiculus, individual nerve fibers are further shielded by a connective tissue sheath named the endoneurium, which seamlessly integrates with the neurilemma of nerve fibers.
Types of Nerves Based on Function:
- Afferent or Sensory Nerves: These carry impulses from receptors to the CNS.
- Efferent or Motor Nerves: These transmit orders from the CNS to effector organs.
- Mixed Nerves: These contain both sensory and motor nerve fibers.
Classification of Fibers:
- Somatic Sensory: These fibers relay impulses from receptors such as the skin, eyes, nose, body wall, muscles, and joints to the CNS. Their dendrons, originating from receptors, extend to the dorsal root ganglion where their cell body resides.
- Somatic Motor: These fibers convey impulses from the CNS to effector organs, primarily muscles. Their dendrites and cell bodies are located in the grey matter, while their axons traverse through the ventral roots to the muscles.
- Visceral Sensory: These fibers transmit sensations from receptors in the alimentary canal wall to the CNS. Their dendrites initiate from these receptors and lead to cell bodies in the dorsal root ganglia, from which axons proceed into the grey matter.
- Visceral Motor: These fibers relay impulses from the CNS to the involuntary muscles of the alimentary canal, glands, and other visceral organs. Their dendrites and cell bodies are situated in the grey matter. Axons exit through the ventral root, terminating in autonomic ganglia. From these ganglia, a second neuron emerges, extending its axon to the involuntary muscles or glands.
i. Cranial nerves
The cranial nerves are a set of nerves that emanate directly from the brain, bypassing the spinal cord. In the frog, there are typically 10 pairs of these nerves, although some sources suggest the presence of an additional pair, the terminal nerves, bringing the total to 11 pairs. Each of these nerves has a specific designation, origin, distribution, and functional nature. Here is a detailed overview:
Terminal (O) Nerve:
- Origin: Forebrain
- Distribution: Lining of the nose
- Nature: Sensory (presumed)
Olfactory (I) Nerve:
- Origin: Olfactory lobe
- Distribution: Lining of the nose
- Nature: Sensory (olfaction)
Optic (II) Nerve:
- Origin: Diencephalon
- Distribution: Retina of the eye
- Nature: Sensory (vision)
Oculomotor (III) Nerve:
- Origin: Ventral midbrain
- Distribution: Four eye muscles
- Nature: Motor
Trochlear (IV) Nerve:
- Origin: Dorsal midbrain
- Distribution: Superior oblique muscle of the eye
- Nature: Motor
Trigeminal (V) Nerve: Comprising of Ophthalmic, Maxillary, and Mandibular branches.
- Origin: Lateral medulla
- Distribution: Skin of the snout, upper jaw, muscles of the lower jaw, and tongue
- Nature: Mixed; somatic sensory for the first two branches and visceral motor for the mandibular branch
Abducens (VI) Nerve:
- Origin: Ventral medulla
- Distribution: External rectus of the eye
- Nature: Motor
Facial (VII) Nerve: Including Palatinus and Hyomandibularis branches.
- Origin: Lateral medulla
- Distribution: Roof of the buccal cavity, tympanum, skin of the lower jaw, and tongue
- Nature: Mixed; visceral sensory for the palatinus branch and visceral motor for the hyomandibularis branch
Auditory (VIII) Nerve:
- Origin: Lateral medulla
- Distribution: Internal ear
- Nature: Sensory (auditory)
- Glossopharyngeal (IX) Nerve:
- Origin: Lateral medulla
- Distribution: Tongue, hyoid, and pharynx
- Nature: Mixed
- Vagus (X) Nerve: Comprising of Laryngeal, Gastric, Pulmonary, and Cardiac branches.
- Origin: Lateral medulla
- Distribution: Laryngotracheal chamber, stomach, lungs, and heart
- Nature: Mixed
ii. Spinal nerves
Spinal nerves are integral components of the peripheral nervous system, serving as conduits for transmitting sensory and motor information between the spinal cord and various body parts. In the frog, there are typically 10 pairs of spinal nerves, although occasionally this count may be reduced to 9 pairs. This reduced number aligns with the frog’s relatively short spinal cord.
Each spinal nerve emerges from the spinal cord through two roots, which converge as the nerve exits the neural canal via an intervertebral foramen. The dorsal root is characterized by the presence of a ganglion housing nerve cells and is exclusively composed of afferent or sensory nerve fibers. These fibers relay impulses from different body regions to the spinal cord. Conversely, the ventral root is solely made up of efferent or motor nerve fibers, which transmit impulses from the spinal cord to various body tissues. Consequently, all spinal nerves exhibit a mixed nature, encompassing both sensory and motor fibers.
A unique feature in frogs is the presence of calcareous bodies or Swammerdam glands, which envelop the dorsal root ganglia. These white, chalk-like masses are postulated to serve as reservoirs of calcium.
Upon its emergence, each spinal nerve bifurcates into three branches:
- Ramus Dorsalis: A brief branch catering to the dorsal skin and muscles.
- Ramus Ventralis: A more substantial branch serving the ventral skin and muscles.
- Ramus Communicans: A diminutive branch that connects to the nearest sympathetic ganglion.
- The inaugural spinal nerve, termed the hypoglossal, exits the neural canal between the first two vertebrae, directing anteriorly to cater to the tongue muscles.
- The second spinal nerve, robust in nature, collaborates with the third spinal nerve and a minor branch from the hypoglossal to form the brachial plexus. This subsequently gives rise to the brachial nerve, which innervates the forelimb.
- The fourth to sixth spinal nerves are relatively smaller, diagonally supplying the abdominal skin and muscles.
- The seventh to ninth spinal nerves, more substantial in size, retrogressively form the sciatic plexus. This plexus then branches into the prominent sciatic nerve and several minor nerves, all of which serve the hind limb.
- The tenth spinal nerve, often absent in the species Rana tigrina, may occasionally be present unilaterally. It emerges through an aperture in the urostyle near its anterior extremity. In addition to contributing to the sciatic plexus, it innervates the urinary bladder, cloaca, and other associated structures.
III. Sympathetic nervous system of frog
- The sympathetic nervous system in frogs is a crucial component of their autonomic nervous system, which governs involuntary bodily functions. This system is characterized by two slender, delicate, thread-like nerve cords known as the sympathetic trunks. Positioned beneath the vertebral column, these trunks run parallel on either side of the dorsal aorta.
- Integral to these trunks are a series of 10 sympathetic ganglia. These ganglia establish connections with the spinal nerves through small nerve branches termed as rami communicantes. Additionally, corresponding ganglia from both sympathetic cords are interconnected via small transverse commissures, ensuring coordinated function.
- Anteriorly, each sympathetic cord extends into the skull. Initially, it merges with the vagus ganglion of the X nerve. Subsequently, it progresses forward to associate with the gasserian ganglion of the V or trigeminal nerve, marking its terminal point.
- The primary role of the sympathetic ganglia is to distribute nerves predominantly to the circulatory system, the digestive tract, and glandular organs. This system orchestrates a myriad of involuntary activities. For instance, it modulates the heartbeat rate, regulates the muscular tone of blood vessels, and oversees the secretion of digestive enzymes. Furthermore, it governs muscular movements associated with the stomach and intestines.
- In essence, the frog’s sympathetic nervous system is pivotal in managing and coordinating a range of involuntary physiological processes, ensuring the amphibian’s homeostasis and adaptability to its environment.
Autonomic Nervous System
The autonomic nervous system (ANS) operates autonomously, orchestrating involuntary physiological functions without conscious intervention. Although it functions independently, it remains intricately linked with the central nervous system (CNS) and is associated with certain spinal and cranial nerves. The ANS primarily focuses on the internal regulation of the body, while the CNS, in conjunction with spinal and cranial nerves, manages external interactions.
Structure of the ANS: The ANS comprises two delicate longitudinal chains of ganglia, positioned on either side of the backbone and dorsal aorta, extending from the brain to the urostyle’s end. Each chain features ten metamerically arranged sympathetic ganglia. These ganglia connect to corresponding spinal nerves via a branch known as the ramus communicans. Additionally, corresponding ganglia from both chains are interconnected through small transverse commissures. Notably, between the first and second ganglia, the sympathetic chain on each side forms a loop around the subclavian artery, termed the annulus of Vieussens. Each chain penetrates the skull alongside the Xth cranial nerve, merging with the vagus ganglion before advancing to the Gasserian ganglion of the fifth cranial nerve. Posteriorly, each chain concludes by connecting with branches of the 9th spinal nerve. These ganglia dispatch nerves to the viscera, forming plexuses like the solar plexus near the coeliaco-mesenteric artery and a cardiac plexus on the heart.
Types of Autonomic Nervous System:
Sympathetic System: Comprising both visceral sensory and visceral motor fibers, the sympathetic system releases a chemical called sympathin upon stimulation, generally activating organs. The system includes:
- Visceral Sensory Fibers: Originating from organs not under voluntary control, these fibers relay information to the CNS.
Visceral Motor Fibers: These fibers, with cell bodies in the spinal cord, form synapses with neurons in the sympathetic ganglia. They are further categorized into:
- Preganglionic Fibers: Originating from the spinal cord’s grey matter, these medullated fibers connect to corresponding sympathetic ganglia.
- Postganglionic Fibers: Non-medullated fibers with cell bodies in sympathetic ganglia that extend to target organs.
Parasympathetic System: This system encompasses parasympathetic nerves and ganglia. The fibers of these nerves emerge from cranial and spinal nerves, while the ganglia are located in the organs they innervate. Notably, upon stimulation, they release acetylcholine, which inhibits organ activity. The system includes:
- Preganglionic Fibers: These long fibers extend from the CNS to a ganglion near or within the target organ.
- Postganglionic Fibers: Short fibers connecting to preganglionic fibers within the ganglia.
The parasympathetic nerve fibers are part of specific cranial nerves, including the oculomotor, trigeminal, facial, and vagus nerves. Interestingly, both sympathetic and parasympathetic systems innervate many organs, often producing opposing effects. While the sympathetic system’s sympathin induces action, the parasympathetic system’s acetylcholine retards it.
In essence, the ANS plays a pivotal role in modulating involuntary organ functions. Although it operates autonomously, it remains closely intertwined with the CNS and peripheral nervous system, both structurally and functionally.