## 5.3 NEURON The neuron is the basic structural and functional unit of the nervous system, specialized for transmitting electrical and chemical signals for nervous coordination. ### Structure of a Neuron Although neurons vary in size and shape, they consist of three fundamental components: the *cell body*, *dendrites*, and an *axon*. - **Cell Body (Soma)** - The main part of the neuron, containing the nucleus and most organelles. - **Nucleus:** Large, centrally located with a prominent nucleolus. - **Cytoplasm:** Contains standard organelles (Golgi apparatus, mitochondria) and specialized **Nissl's granules** (groups of ribosomes and rough ER involved in protein synthesis). - **Dendrites** - Short, thin, highly branched cytoplasmic extensions. - *Function:* To receive stimuli and conduct impulses *towards* the cell body. - They form synapses (connections) with the axons of other neurons. - **Axon** - A single, long, thick nerve fiber with a constant diameter. - Can range from a few millimeters to over a meter in length. - Terminates in small, branched extensions with enlarged ends called **presynaptic terminals**. - *Function:* To conduct action potentials (nerve impulses) *away* from the cell body to other neurons, muscles, or glands. ### Types of Neurons (Based on Function) Neurons are classified into three types based on their function in the nervous system. 1. ***Sensory Neurons (Afferent Neurons)*** - **Function:** Transmit sensory information from sensory receptors (e.g., in skin, eyes) to the Central Nervous System (CNS) (brain and spinal cord). - **Structure:** Typically *unipolar* or *pseudo-unipolar*. The cell body is located in the **dorsal root ganglion** just outside the spinal cord. They have a long axon that extends from the sensory receptor to the CNS. - Related Concept: 2. ***Motor Neurons (Efferent Neurons)*** - **Function:** Transmit motor commands from the CNS to **effectors** (muscles or glands), causing a response like contraction or secretion. - **Structure:** Typically *multipolar*. The cell body is located within the CNS. They have a long, often myelinated axon that extends to the effector. 3. ***Interneurons (Association/Relay Neurons)*** - **Function:** Connect sensory and motor neurons within the CNS. They are crucial for processing information, reflexes, and complex functions like learning. - **Structure:** Typically *multipolar* with short axons and highly branched dendrites, allowing them to connect with many other neurons. They are located entirely within the CNS. #### Summary of Neuron Types | Neuron Type | Function | Structure | Location | | :--- | :--- | :--- | :--- | | **Sensory Neuron** | Transmit sensory information to the CNS. | Unipolar, with a long axon; cell body in dorsal root ganglia. | Sensory organs to CNS. | | **Motor Neuron** | Transmit motor commands from CNS to effectors. | Multipolar, with a long axon connecting to a muscle or gland. | CNS to muscles/glands. | | **Interneuron** | Relay signals between sensory and motor neurons; process information. | Multipolar, with a short axon and highly branched dendrites. | Entirely within the CNS. | ### Myelinated vs. Non-myelinated Neurons Axons can be covered by a fatty layer called the **myelin sheath**, which affects the speed of nerve impulse transmission. | Myelinated Neuron | Non-myelinated Neuron | | :--- | :--- | | Possess a myelin sheath. | Do not possess a myelin sheath. | | Schwann cells wrap tightly around the axon. | Schwann cells are present but do not wrap to form a sheath. | | Nerve impulses are transferred very fast (saltatory conduction). | Nerve impulses are transferred slowly. | | Appear white in fresh tissue (form "white matter"). | Appear grey in fresh tissue (form "grey matter"). | | Have **Nodes of Ranvier** (gaps in the myelin sheath). | Do not have Nodes of Ranvier. | | Found in the white matter of the brain and spinal cord. | Found in the autonomic nervous system. | ### Possible Questions/Answers - **Q:** What is a neuron? **A:** A neuron is the basic structural and functional unit of the nervous system, specialized for transmitting nerve impulses. - **Q:** What are the three main parts of a neuron and their functions? **A:** 1. **Cell Body (Soma):** Contains the nucleus and organelles; the metabolic center of the neuron. 2. **Dendrites:** Receive signals from other neurons and transmit them *towards* the cell body. 3. **Axon:** Transmits signals *away* from the cell body to other neurons, muscles, or glands. - **Q:** How do sensory and motor neurons differ in function? **A:** Sensory (afferent) neurons carry signals *from* sensory receptors *to* the central nervous system. Motor (efferent) neurons carry signals *from* the central nervous system *to* effectors like muscles and glands. - **Q:** What is the function of the myelin sheath? **A:** The myelin sheath acts as an electrical insulator for the axon, increasing the speed at which nerve impulses travel.