Animals-Nervous System: Definition, Structure & Function

Animals Nervous System: Introduction, Classification, Spinal Cord, Neurons

Edited By Irshad Anwar | Updated on Jul 02, 2025 05:11 PM IST

The nervous system is a complex network responsible for coordinating and controlling body activities. It consists of the brain, spinal cord, and nerves, forming the central and peripheral nervous systems. This system processes information from the environment and directs appropriate responses. In this article, nervous system, structure of the nervous system and nervous system parts and functions are discussed. Animal Nervous System is a topic of the chapter Neural Control and Coordination in Biology.

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Nervous System Definition
Structure of the Nervous System
Nervous System Parts and Functions

Animals Nervous System: Introduction, Classification, Spinal Cord, Neurons

Nervous System Definition

The nervous system is the network of nerves and cells that integrates and transmits action and signals between parts of an animal's body. It is an important part of the organ that serves a principal function in controlling and coordinating body activities, from simple to very complex, sensory perception, and maintaining homeostasis. As such, it enables animals to monitor, adapt, and react to both sets of internal and external stimulations.

Structure of the Nervous System

The broad division of the nervous system is classified into two parts: the Central Nervous System and the Peripheral Nervous System.

Central Nervous System (CNS)

The Central Nervous System consists of two major divisions:

Brain: Structure and Functions

The structure and functions of the cerebrum include higher brain functions such as thought, action, and sensory processing.
The structure and functions of the cerebellum include control over balance, coordination, and fine muscle control.
The structure and functions of the brainstem include regulating vital functions of the body such as heartbeat, breathing, and sleep cycles.

Spinal Cord: Structure and Functions

A cylindrical bundle of nerve fibres extending from the brainstem down the vertebral column.
It transmits information between the brain and the rest of the body and mediates reflex actions.

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Peripheral Nervous System (PNS)

The Peripheral Nervous System has the following components:

Somatic Nervous System

It is responsible for controlling voluntary movements and transmitting sensory information towards the CNS.
This comprises nerves connecting to muscles and sensory receptors.

Autonomic Nervous System

Sympathetic Division: This prepares the body for 'fight or flight' responses.
Parasympathetic Division: Serves 'rest and digest' activities.

Neurons: The Functional Units

Neurons are the simplest units of nervous systems that carry nerve impulses.

Types of Neurons

Sensory Neurons: Transmit impulses from sensory receptors to the CNS.
Motor Neurons: Carry instructions from the CNS to muscles and glands.
Interneurons: Integrate neurons within the CNS and interconnect sensory and motor functions.

Structure of Neurons

Cell Body (Soma): Nucleus and organelles.
Dendrite: Receive signals from other neurons.
Axon: Conducts the electrical impulse away from the cell body.
Synapse: Gap between neurons, where neurotransmitters are released.

Structure of Neuron

Neuroglia: The Supporting Cells

Astrocytes: Provide mechanical support and protection for neurons, and maintain the blood-brain barrier.
Oligodendrocytes: Generate myelin in the CNS.
Microglia: Serve as immune cells within the CNS.
Ependymal Cells: Line ventricles and produce CSF.

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Nervous System Parts and Functions

The nervous system includes-

Human Brain

Cerebral Cortex: Among its activities are voluntary movement, perception, language, logic, and thought.

Cerebellum: The Cerebellum has a role in preserving posture, mobility, and balance. The cortex envelops each of the two hemispheres that make up the cerebellum.

Hypothalamus: is the area in charge of controlling the body's temperature, hunger, emotions, circadian cycles, and other functions. It functions as a thermostat, detecting body temperature and sending forth signals to regulate it.

Brain stem: also known as the medulla oblongata, is made up of the reticular formation, tectum, tegmentum, pons, and medulla. It regulates blood pressure, breathing, and heart rate.

Thalamus: Through the integration of motor and sensory information, it gathers sensory data and transmits it to the cerebral cortex. It transmits the data it receives from the cerebral cortex to the other parts of the brain and spinal cord.

Limbic system: It consists of the hippocampus, cingulate gyrus, and mammillary bodies, which help control how one reacts to emotions. The hippocampus is essential for learning and memory.

Basal ganglia: The caudate nucleus, globus pallidus, putamen, substantia nigra, and subthalamic nucleus are all part of the basal ganglia, which control movement and balance.

Midbrain: It regulates vision, hearing, eye and body movement. It is made up of inferior and superior colliculi as well as the red nucleus.

Cerebrospinal Nervous System

It is made up of twelve pairs of cranial nerves, each of which has a specific function and is related to the brain. The nerves and their corresponding roles are as follows:

Optic: Sight
Oculomotor: Movement of the eyeball, pupils, and lens
Olfactory: The sense of smell
Trochlear: Eye muscle movement (superior oblique)
Trigeminal: Provides nerves to the cheeks, mouth, eyes, and controls chewing
Abducens: Human lateral rectus muscle action and outward vision
Facial: controls salivary gland function, facial muscle movement, and anterior tongue taste perception
Glossopharyngeal: A gustatory experience
Acoustic: Preserves hearing and balance
Vagus: Provides the organs in the chest and belly with nerve supply
Spinal accessory: Movement of the head and shoulders
Hypoglossal: Controls the tongue's muscles

Types of Nerves

A nerve is created when many axon fibres are bundled together. There are three kinds of nerves:

Sensory
The nerve fibres are referred to as sensory when the impulse travels from the receptor to the brain or spinal cord. The nerves in the ears, eyes, and nerves, for instance.

Motor
A motor neuron is what happens when an impulse travels from the brain or spinal cord to a gland or muscle.

Mixed
Both the motor and sensory nerves are found in a mixed nerve. For instance, the spinal nerves

Parts of the Human Brain

The sense organs send messages to the human brain, which then relays those signals back to the nerves. To shield the brain from mechanical shocks, it is housed inside the skull, which also houses the cerebrospinal fluid. It is separated into three areas:

Forebrain
The receptors send the impulses to it. It contains distinct sections for analysing the various signals, such as smell, hearing, etc. This is also where the thought process occurs. After being analysed, the incoming signals are sent to the appropriate areas.

Midbrain
Numerous voluntary and involuntary processes are carried out by our bodies. Humans are able to manage voluntary movements like pushing and running. Blinking and breathing are examples of involuntary behaviours that are automatic and beyond human control.

Hindbrain
The cerebellum and medulla are located in the hindbrain. These regulate blood, saliva, and respiration.

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Neuron	Nerve Impulse
Diagram of Neuron	Nerve Fibres
Neuron and Nerves	Action Potential

Frequently Asked Questions (FAQs)

1. What are the primary functions of the nervous system in animals?

The main function of the nervous system is to sense changes, integrate information, and provoke suitable responses to maintain homeostasis and coordinate activities.

2. What is the mode of transmission of nerve impulses by neurons?

Neurons transmit through electrical signals called action potentials and through a chemical signal in the form of neurotransmitters at synapses.

3. How is CNS different from PNS?

CNS includes the brain and spinal cord that processes and connects the information. PNS consists of all peripheral nerves that connect the CNS to the rest of the body code.

4. What is the mechanism of reflex actions?

The reflex action is a rapid, involuntary response to a stimulus having its pathway as the simple reflex arc.

5. What are some common neurological disorders and their symptoms?

The common ones include Parkinson's characterised by tremors and rigidity; Alzheimer's by amnesia; and Multiple Sclerosis by muscle weakness.

6. How do neurodegenerative diseases affect the nervous system?

Neurodegenerative diseases, such as Alzheimer's and Parkinson's, involve the progressive loss of structure or function of neurons. This can lead to problems with movement (ataxia), mental functioning (dementia), or both. These diseases often involve the accumulation of abnormal proteins, disruption of cellular processes, and eventual cell death. The specific symptoms depend on which regions of the nervous system are affected.

7. What is neuroplasticity, and why is it significant?

Neuroplasticity refers to the brain's ability to change and reorganize itself by forming new neural connections throughout life. It's significant because it allows the nervous system to adapt to new experiences, recover from injuries, and learn new skills. This plasticity is the basis for learning, memory, and the brain's ability to compensate for damage.

8. How does the blood-brain barrier protect the central nervous system?

The blood-brain barrier is a highly selective semipermeable border of endothelial cells that prevents many substances in the blood from entering the brain. It protects the central nervous system by restricting the passage of potentially harmful substances, pathogens, and even many beneficial drugs. This barrier is crucial for maintaining a stable environment for proper brain function but also presents challenges in treating certain neurological disorders.

9. How does the nervous system process pain, and what is the purpose of pain perception?

Pain is processed through specialized sensory neurons called nociceptors, which detect potentially damaging stimuli. These signals are transmitted through the spinal cord to various brain regions, including the thalamus and cortex, where they're interpreted as pain. The purpose of pain perception is protective – it alerts the body to potential or actual damage, prompting actions to avoid or minimize harm. Chronic pain, however, can persist beyond its protective function and become a disorder in itself.

10. What is the function of neurotransmitter reuptake, and why is it important?

Neurotransmitter reuptake is the process by which neurotransmitters are removed from the synaptic cleft after signaling. This process is crucial for terminating the signal and preparing the synapse for the next transmission. It helps maintain the proper balance of neurotransmitters, prevents overstimulation of receptors, and allows for the recycling of neurotransmitters, which is energetically efficient for the neuron.

11. What is the primary function of the nervous system in animals?

The primary function of the nervous system is to coordinate and control various activities of the body. It receives sensory input from the environment, processes this information, and generates appropriate responses. This system allows animals to interact with their surroundings, maintain homeostasis, and adapt to changes.

12. How does the nervous system differ between invertebrates and vertebrates?

The main difference lies in the complexity and organization. Invertebrates generally have a simpler nervous system, often consisting of nerve cords and ganglia. Vertebrates have a more complex system with a centralized brain and spinal cord, known as the central nervous system (CNS), and a peripheral nervous system (PNS) that connects the CNS to the rest of the body.

13. What is the role of glial cells in the nervous system?

Glial cells, often called neuroglia, play crucial supporting roles in the nervous system. They provide physical support and insulation for neurons, supply nutrients and oxygen, remove dead neurons and pathogens, and maintain the proper chemical environment for neural signaling. Some types of glial cells, like oligodendrocytes and Schwann cells, produce the myelin sheath that insulates axons.

14. What is the role of astrocytes in supporting neuronal function?

Astrocytes, a type of glial cell, play multiple crucial roles in supporting neuronal function. They help maintain the blood-brain barrier, regulate the chemical environment around neurons by taking up excess neurotransmitters and ions, provide nutrients to neurons, and help control blood flow in the brain. Recent research has also shown that astrocytes can release signaling molecules that influence neuronal activity and synaptic transmission, suggesting they play a more active role in information processing than previously thought.

15. How does the nervous system adapt to injury or damage?

The nervous system can adapt to injury or damage through several mechanisms. Neuroplasticity allows for the reorganization of neural pathways, where undamaged areas can sometimes take over functions of damaged regions. Axonal sprouting can occur, where undamaged axons grow new nerve endings to compensate for damaged ones. In some cases, neurogenesis can produce new neurons. The brain can also alter its processing strategies to compensate for lost functions. However, the extent of recovery depends on the severity and location of the damage.

16. What are neurons, and why are they considered the basic functional units of the nervous system?

Neurons are specialized cells that transmit electrical and chemical signals in the nervous system. They're considered the basic functional units because they're responsible for receiving, processing, and transmitting information throughout the body. Neurons have unique structures like dendrites, axons, and synapses that allow them to communicate with other cells effectively.

17. What are the main types of neurons, and how do they differ in function?

The main types of neurons are sensory neurons, motor neurons, and interneurons. Sensory neurons carry information from sensory receptors to the CNS. Motor neurons transmit signals from the CNS to muscles and glands. Interneurons connect and integrate information between other neurons within the CNS.

18. How do neurotrophic factors contribute to neural development and maintenance?

Neurotrophic factors are proteins that promote the survival, development, and function of neurons. They play crucial roles in neural development by guiding the growth of axons and dendrites, promoting synapse formation, and regulating neuron survival. In the adult nervous system, they continue to support neuron health, promote plasticity, and can even stimulate the growth of new neurons in certain brain regions.

19. What is neurogenesis, and does it occur in the adult brain?

Neurogenesis is the process of generating new neurons from neural stem cells. While it was once thought to occur only during embryonic development, we now know that neurogenesis continues in certain regions of the adult brain, particularly the hippocampus (important for memory) and the subventricular zone. Adult neurogenesis is believed to play roles in learning, memory, and mood regulation, though its extent and significance in humans are still subjects of ongoing research.

20. How does long-term potentiation (LTP) contribute to learning and memory?

Long-term potentiation is a persistent strengthening of synapses based on recent patterns of activity. It's believed to be one of the main cellular mechanisms underlying learning and memory. When neurons repeatedly fire together, the connections between them are strengthened, making future communication more efficient. This process involves changes in receptor density, neurotransmitter release, and even structural modifications of synapses.

21. What is the role of the spinal cord in the nervous system?

The spinal cord serves as a conduit for nerve signals between the brain and the rest of the body. It also contains neural circuits that can independently control certain reflexes and simple movements without direct input from the brain. Additionally, the spinal cord plays a crucial role in processing and modulating sensory information before it reaches the brain.

22. How does the reflex arc work, and why is it important?

A reflex arc is a neural pathway that produces a rapid, automatic response to a stimulus without involving the brain. It typically involves a sensory neuron, an interneuron in the spinal cord, and a motor neuron. Reflex arcs are important because they allow for quick responses to potentially harmful stimuli, enhancing survival and preventing injury.

23. How does the nervous system maintain homeostasis in the body?

The nervous system maintains homeostasis by constantly monitoring the body's internal environment through various sensors and receptors. When deviations from the normal state are detected, the nervous system initiates appropriate responses. For example, it can adjust heart rate, breathing rate, or hormone release to maintain stable conditions. This involves complex feedback loops and integration of information from multiple body systems.

24. How does the nervous system interact with the endocrine system to regulate body functions?

The nervous and endocrine systems work together in a complex interplay called the neuroendocrine system. The hypothalamus, a part of the brain, acts as a major link between these systems. It can produce hormones and also regulate the pituitary gland, often called the "master gland" of the endocrine system. This interaction allows for both rapid (nervous) and sustained (endocrine) responses to environmental changes and internal needs.

25. How does the nervous system process and integrate information from multiple sensory modalities?

The nervous system processes and integrates information from multiple sensory modalities through a process called multisensory integration. This occurs at various levels, from early sensory processing areas to higher-order association cortices. Neurons in these areas can respond to inputs from multiple senses, combining this information to create a coherent perception of the environment. This integration allows for more accurate and robust representations of the world, enhancing our ability to interact with our surroundings effectively.

26. What is the difference between the central nervous system (CNS) and the peripheral nervous system (PNS)?

The CNS consists of the brain and spinal cord, which are the main processing and control centers. The PNS includes all the nerves that connect the CNS to the rest of the body. The PNS is further divided into the somatic nervous system (voluntary control) and the autonomic nervous system (involuntary control).

27. What is the difference between the somatic and autonomic nervous systems?

The somatic nervous system controls voluntary movements and carries sensory information from the body to the CNS. It includes motor neurons that innervate skeletal muscles. The autonomic nervous system, on the other hand, regulates involuntary functions like heart rate, digestion, and respiration. It's further divided into the sympathetic ("fight or flight") and parasympathetic ("rest and digest") systems.

28. How do different regions of the brain specialize in various functions?

Different brain regions specialize in various functions through a combination of evolutionary development and experience-dependent plasticity. For example, the occipital lobe primarily processes visual information, while the frontal lobe is involved in executive functions like planning and decision-making. This specialization allows for efficient processing of complex information and behaviors. However, it's important to note that most complex functions involve networks spanning multiple brain regions.

29. What is the role of the cerebellum in motor control and learning?

The cerebellum plays a crucial role in motor control, balance, and motor learning. It receives input from sensory systems and other parts of the brain and integrates these inputs to fine-tune motor activity. The cerebellum is involved in the timing and precision of movements, allowing for smooth, coordinated actions. It's also important for motor learning, helping to adjust movements based on past experiences and errors.

30. What is the difference between the sympathetic and parasympathetic nervous systems?

The sympathetic and parasympathetic systems are two branches of the autonomic nervous system. The sympathetic system prepares the body for "fight or flight" responses, increasing heart rate, dilating pupils, and diverting blood flow to muscles. The parasympathetic system promotes "rest and digest" functions, slowing heart rate, stimulating digestion, and promoting relaxation. These systems often have opposing effects on the same organs, allowing for fine-tuned control of bodily functions.

31. How do neurons communicate with each other?

Neurons communicate through a process called synaptic transmission. When a neuron is stimulated, it generates an electrical impulse (action potential) that travels along its axon. At the synapse (the junction between two neurons), this electrical signal triggers the release of chemical messengers called neurotransmitters. These neurotransmitters cross the synaptic cleft and bind to receptors on the receiving neuron, potentially triggering a new electrical signal.

32. How does myelination enhance nerve signal transmission?

Myelination is the process where certain nerve fibers are covered with a fatty substance called myelin. This myelin sheath acts as an insulator, allowing electrical impulses to travel much faster along the axon through saltatory conduction. This speed increase is crucial for efficient neural communication, especially over longer distances in the body.

33. How do neurotransmitters and their receptors contribute to synaptic specificity?

Neurotransmitters and their receptors contribute to synaptic specificity through a "lock and key" mechanism. Each type of neurotransmitter has a specific shape that fits only certain receptors. This ensures that signals are transmitted to the appropriate target cells, allowing for precise control of neural communication and preventing inappropriate responses.

34. What is the difference between excitatory and inhibitory synapses?

Excitatory synapses increase the likelihood of the postsynaptic neuron firing an action potential, while inhibitory synapses decrease this likelihood. Excitatory synapses typically use neurotransmitters like glutamate, which depolarize the postsynaptic membrane. Inhibitory synapses often use neurotransmitters like GABA, which hyperpolarize the membrane, making it harder for the neuron to reach its firing threshold.

35. What is the role of ion channels in generating and propagating action potentials?

Ion channels are protein structures in the neuron's membrane that allow specific ions to pass through. Voltage-gated sodium and potassium channels are particularly important for action potentials. When a neuron is stimulated, sodium channels open, allowing sodium ions to rush in, depolarizing the membrane. This triggers more sodium channels to open, propagating the signal. Potassium channels then open, allowing potassium to flow out, repolarizing the membrane and resetting the neuron for the next signal.

36. What is the role of neurotransmitter receptors in synaptic transmission?

Neurotransmitter receptors are proteins on the postsynaptic neuron that bind specific neurotransmitters. When a neurotransmitter binds to its receptor, it can cause changes in the postsynaptic neuron, such as opening or closing ion channels or activating signaling cascades. This is how the chemical signal of the neurotransmitter is converted back into an electrical signal in the receiving neuron, allowing for the continuation of information flow.

37. How do neurotoxins affect the nervous system?

Neurotoxins are substances that can damage or destroy nerve tissue. They can work in various ways, such as blocking neurotransmitter receptors, interfering with ion channels, or disrupting the synthesis or breakdown of neurotransmitters. Some neurotoxins, like tetrodotoxin, block sodium channels, preventing the generation of action potentials. Others, like botulinum toxin, prevent the release of neurotransmitters. The effects of neurotoxins can range from mild symptoms to paralysis and death, depending on the toxin and exposure.

38. What is the role of calcium ions in synaptic transmission?

Calcium ions play a crucial role in synaptic transmission. When an action potential reaches the axon terminal, it causes voltage-gated calcium channels to open. The influx of calcium ions triggers the fusion of synaptic vesicles (containing neurotransmitters) with the presynaptic membrane, releasing the neurotransmitters into the synaptic cleft. Calcium is thus essential for the process of neurotransmitter release and, consequently, for all synaptic communication in the nervous system.

39. What is the difference between fast and slow synaptic transmission?

Fast synaptic transmission occurs within milliseconds and typically involves ionotropic receptors, which are ligand-gated ion channels. When neurotransmitters bind to these receptors, they directly open ion channels, quickly changing the membrane potential of the postsynaptic cell. Slow synaptic transmission, on the other hand, can last from seconds to minutes and often involves metabotropic receptors. These receptors activate second messenger systems within the cell, leading to more prolonged and diverse effects, including changes in gene expression.

40. How do neurotransmitters and neuromodulators differ in their effects on neural activity?

Neurotransmitters are chemicals that transmit signals across synapses, typically having rapid, short-lived effects on the postsynaptic neuron. They usually act on ionotropic receptors, causing immediate changes in the neuron's electrical state. Neuromodulators, while also released by neurons, have broader, more diffuse effects. They often act on metabotropic receptors, altering the properties of neurons or groups of neurons over longer time scales. Neuromodulators can change how neurons respond to neurotransmitters, effectively "tuning" neural circuits.

41. What is the function of inhibitory interneurons in neural circuits?

Inhibitory interneurons play crucial roles in shaping and regulating neural activity. They release inhibitory neurotransmitters like GABA, which decrease the likelihood of the postsynaptic neuron firing. This inhibition is important for creating contrast in sensory processing, sharpening the timing and specificity of neural responses, and preventing over-excitation in the nervous system. Inhibitory interneurons are also key components in generating rhythmic activity in neural circuits, which is important for various brain functions.