# Nervous communication

Nervous communication is a vital system that allows organisms to **detect changes** in their environment and **coordinate appropriate responses**. This complex process involves the rapid transmission of information, primarily through electrical impulses.

#### Components of the Nervous System

The mammalian nervous system is organized into two main parts:

* **Central Nervous System (CNS)**: Comprises the **brain and spinal cord**, serving as the coordinating center where information is processed and responses are formulated.
* **Peripheral Nervous System (PNS)**: Consists of **neurones (nerve cells)** that connect the CNS to the rest of the body, including sensory receptors and effectors.

Three main types of neurones are involved in nervous communication:

* **Sensory Neurones**: Transmit electrical impulses **from receptors to the CNS**. They have a single long dendron bringing impulses towards the cell body, and a single axon carrying impulses away.
* **Motor Neurones**: Transmit electrical impulses **from the CNS to effectors** (e.g., muscle cells and glands). Their cell body lies within the spinal cord or brain, and they have a single long axon.
* **Relay Neurones (Intermediate Neurones)**: Transmit electrical impulses **between sensory and motor neurones** within the CNS. They typically have numerous short fibres.

Neurones themselves have a cell body containing the nucleus, dendrites that receive impulses, and an **axon** that carries impulses away. Many neurones are covered by a **myelin sheath**, an electrical insulator made of **Schwann cells**, which speeds up impulse conduction. Gaps in the myelin sheath are called **nodes of Ranvier**.

#### Nerve Impulse (Action Potential)

Nervous communication relies on rapid changes in the electrical potential across the neurone membrane.

* **Resting Potential**: When a neurone is not stimulated, its membrane maintains a **resting potential** (typically **-70 mV** inside relative to outside). This is established and maintained by **sodium-potassium pumps**, which actively transport **three sodium ions out for every two potassium ions in**, and by the differential permeability of the membrane, which is more permeable to potassium ions diffusing out.
* **Action Potential**: A nerve impulse is an **action potential**, a brief reversal of the resting potential.
  1. **Depolarization**: A stimulus causes **voltage-gated sodium ion channels to open**, leading to a rapid influx of positively charged **sodium ions** into the neurone. This makes the inside of the membrane less negative, or even positive (e.g., to **+30 mV**), a process called depolarization.
  2. **Repolarization**: Sodium ion channels close, and **potassium ion channels open**, causing **potassium ions to diffuse out**, restoring the negative potential inside the neurone. This is repolarization. The sodium-potassium pump then works to restore the original ion concentrations.
* **All-or-Nothing Principle**: Action potentials follow an **all-or-nothing principle**; if the stimulus reaches a certain **threshold**, an action potential will always fire with the same change in voltage. If the threshold is not reached, no action potential occurs.
* **Propagation**: Action potentials are propagated along the axon.
  * In **unmyelinated neurones**, sodium ions diffusing sideways cause adjacent regions of the membrane to depolarize, creating a continuous **wave of depolarization**.
  * In **myelinated neurones**, the myelin sheath acts as an electrical insulator. Action potentials are forced to **"jump" from one node of Ranvier to the next**. This process, called **saltatory conduction**, significantly **increases the speed of transmission** (up to 50 times faster).
* **Refractory Period**: After an action potential, there is a brief **refractory period** during which the neurone cannot be excited again. This ensures action potentials are **discrete**, limits their **frequency**, and ensures **unidirectional** transmission.
* **Speed Factors**: Besides myelination, **axon diameter** (larger is faster) and **temperature** (higher is faster) also affect conduction speed.

#### Synaptic Transmission

Neurones communicate with each other, or with effectors, at specialized junctions called **synapses**.

* **Structure**: A synapse includes a tiny gap, the **synaptic cleft** (about 20 nm wide), separating the **presynaptic neurone** (before the synapse) from the **postsynaptic neurone** or effector cell (after the synapse). The presynaptic neurone has a **synaptic knob** containing **synaptic vesicles** filled with chemical messengers called **neurotransmitters**. The postsynaptic membrane has **specific receptors** for these neurotransmitters.
* **Mechanism (Cholinergic Synapse Example)**:
  1. An action potential arrives at the presynaptic knob.
  2. This stimulates **voltage-gated calcium ion channels to open**, and **calcium ions (Ca2+) diffuse into the synaptic knob**.
  3. The influx of calcium ions causes **synaptic vesicles to fuse with the presynaptic membrane** and release neurotransmitters (e.g., **acetylcholine/ACh**) into the synaptic cleft by **exocytosis**.
  4. ACh **diffuses across the synaptic cleft** and binds to specific cholinergic **receptors on the postsynaptic membrane**.
  5. This binding causes **sodium ion channels to open**, leading to an **influx of sodium ions into the postsynaptic neurone**, causing depolarization. If the depolarization reaches the threshold, an **action potential is generated** in the postsynaptic neurone.
  6. **ACh is quickly removed** from the synaptic cleft by the enzyme **acetylcholinesterase (AChE)**, which breaks it down into acetate and choline. These products are reabsorbed by the presynaptic neurone to synthesize more ACh, ensuring the response is short-lived and doesn't keep happening.
* **Unidirectionality**: Synapses ensure **impulses travel in only one direction** because neurotransmitters are released only from the presynaptic side and receptors are only on the postsynaptic side.
* **Summation**: Synapses can integrate information through summation. **Spatial summation** occurs when neurotransmitters from many neurones are added together. **Temporal summation** occurs when impulses arrive in quick succession from the same presynaptic neurone.
* **Neuromuscular Junctions**: These are specialized cholinergic synapses between a motor neurone and a muscle cell, always using ACh and always excitatory.
* **Inhibitory Synapses**: Some synapses are inhibitory, releasing neurotransmitters that hyperpolarize the postsynaptic membrane (make it more negative), preventing action potential generation and allowing for the creation of neural pathways.
* **Drugs**: Drugs can affect synaptic transmission by mimicking neurotransmitters, stimulating their release, opening/blocking channels, or inhibiting breakdown enzymes.

#### Reflex Arcs

A **reflex arc** is the pathway of neurones linking receptors to effectors in a simple reflex. A **simple reflex** is a **rapid, involuntary response** to a stimulus. This pathway typically involves a sensory neurone, a relay neurone (though some simple reflexes like the knee-jerk reflex do not involve one in the spinal cord), and a motor neurone. Reflexes are protective, allowing organisms to react quickly to avoid damage.

#### Comparison with Hormonal Communication

Nervous and hormonal communication are two complementary control systems in animals.

| Feature                     | Nervous Communication                               | Hormonal Communication                                                                 |
| --------------------------- | --------------------------------------------------- | -------------------------------------------------------------------------------------- |
| **Form of Transmission**    | **Electrical impulses**                             | **Chemical messengers (Hormones)**                                                     |
| **Pathway**                 | Along **neurones** (direct to target cells)         | In **blood** (plasma) throughout the whole body                                        |
| **Speed of Transmission**   | **Very fast** (milliseconds-seconds)                | **Slower** (minutes-days; "speed of blood")                                            |
| **Duration of Effects**     | **Short-lived** (neurotransmitters quickly removed) | **Long-lasting** (hormones not broken down quickly)                                    |
| **Target Area/Specificity** | **Localised** (neurones go to specific cells)       | **Widespread** (affect specific target cells/organs anywhere in body due to receptors) |
| **Energy Required**         | **Energy expensive** (e.g., sodium/potassium pumps) | **Energy cheap** (small quantities produced)                                           |

#### Plant Electrical Communication (Briefly)

Plants, despite lacking a nervous system and muscle tissue, also possess communication systems. They have **electrochemical gradients and resting potentials** across their cell membranes, similar to animals. Plant action potentials can be triggered by stimuli and involve ion movements (e.g., outflow of chloride ions for depolarization, potassium ions for repolarization). However, these electrical signals travel much **slower** and are generally **weaker** than those in animals. Responses are mainly growth movements, coordinated by plant growth regulators (hormones).