Neural System

The **neural system** is composed of highly specialized cells called **neurons**. Neurons are capable of detecting, receiving, and transmitting different kinds of stimuli. This forms an organised network for rapid communication and control.

The complexity of the neural organisation varies across the animal kingdom:

• **Lower invertebrates** (e.g., Hydra): Have a simple network of neurons.
• **Insects:** Have a better-organized neural system with a brain, ganglia, and neural tissues.
• **Vertebrates:** Possess a more highly developed neural system, reaching its peak complexity in humans.

Human Neural System

The human neural system is divided into two main parts:

1. **Central Neural System (CNS):** Includes the **brain** and the **spinal cord**. This is the site for information processing, control, and decision-making.
2. **Peripheral Neural System (PNS):** Comprises all the nerves of the body that are associated with the CNS. Nerves are bundles of nerve fibres (axons).

Nerve fibres of the PNS are of two types based on the direction of impulse transmission relative to the CNS:

• **Afferent fibres:** Transmit impulses **from tissues/organs towards the CNS** (carrying sensory information).
• **Efferent fibres:** Transmit impulses **from the CNS to peripheral tissues/organs** (carrying motor commands or regulatory signals).

The PNS is further divided based on the type of target tissue:

• **Somatic neural system:** Relays impulses from the CNS to **skeletal muscles** (controls voluntary actions).
• **Autonomic neural system:** Transmits impulses from the CNS to **involuntary organs** (smooth muscles, glands, cardiac muscle). This system regulates involuntary actions. The autonomic neural system is further classified into **sympathetic** and **parasympathetic** neural systems, which often have opposing effects on target organs.

**Visceral nervous system** is part of the PNS that comprises nerves, fibres, ganglia, and plexuses that transmit impulses between the CNS and the viscera (internal organs).

Neuron As Structural And Functional Unit Of Neural System

The **neuron** (nerve cell) is the structural and functional unit of the neural system. It is a microscopic cell specialized for receiving, transmitting, and processing information in the form of electrical and chemical signals (nerve impulses).

A neuron is composed of three major parts:

1. **Cell body (Soma):** Contains the cytoplasm with typical cell organelles (nucleus, ER, Golgi, mitochondria) and granular bodies called **Nissl's granules** (involved in protein synthesis).
2. **Dendrites:** Short fibres branching repeatedly from the cell body. They also contain Nissl's granules. Dendrites transmit impulses **towards the cell body** (receive signals from other neurons or receptors).
3. **Axon:** A long fibre extending from the cell body. The distal end is branched. Each branch terminates as a bulblike structure called a **synaptic knob**, which contains **synaptic vesicles** filled with **neurotransmitters** (chemicals for transmitting signals across synapses). The axon transmits nerve impulses **away from the cell body** towards a synapse or a neuromuscular junction.

Neurons are classified based on the number of axons and dendrites:

• **Multipolar:** One axon and two or more dendrites (e.g., in cerebral cortex).
• **Bipolar:** One axon and one dendrite (e.g., in retina of eye).
• **Unipolar:** Cell body with only one axon (often branched); dendrites are absent (e.g., found in embryonic stage).

Axons can be:

• **Myelinated:** Axons enveloped by Schwann cells (in PNS) or oligodendrocytes (in CNS) that form a lipid-rich myelin sheath. The myelin sheath is interrupted at regular intervals by gaps called **Nodes of Ranvier**. Myelinated fibres are found in spinal and cranial nerves. Myelination allows for faster impulse conduction (saltatory conduction).
• **Non-myelinated:** Axons enclosed by Schwann cells that do not form a myelin sheath. Commonly found in autonomic and somatic neural systems. Impulse conduction is slower.

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Generation And Conduction Of Nerve Impulse

Neurons are **excitable cells** because their membranes are in a **polarised state**. This polarisation is due to a difference in the electrical potential across the membrane, maintained by differing concentrations of ions (Na$^+$, K$^+$, Cl$^-$, negatively charged proteins) on the inside and outside of the cell.

In a **resting neuron** (not conducting an impulse):

• The axonal membrane is more permeable to K$^+$ and impermeable to Na$^+$ and negatively charged proteins inside the axon (axoplasm).
• Axoplasm has high K$^+$ and negatively charged proteins, low Na$^+$.
• Fluid outside axon has low K$^+$ and high Na$^+$.

These gradients are maintained by the **sodium-potassium pump** using ATP energy, transporting 3 Na$^+$ outwards for every 2 K$^+$ inwards. This results in the outer surface having a positive charge and the inner surface a negative charge (polarised membrane). The electrical potential difference across the resting membrane is the **resting potential** (typically around -70 mV inside relative to outside).

When a **stimulus** is applied at a site on the polarised membrane (e.g., site A), it increases the membrane's permeability to Na$^+$. This causes a rapid influx of Na$^+$ ions into the cell. The influx of positive charge reverses the polarity at that site: the outer surface becomes negative, and the inner surface becomes positive. This reversal of polarity is called **depolarisation**, and the electrical potential difference across the membrane at this point is the **action potential** or **nerve impulse**.

The action potential generated at site A triggers a current flow on the inner surface towards the adjacent polarised site (site B) and a current flow on the outer surface in the opposite direction. This flow causes depolarisation at site B, generating a new action potential there. The process repeats along the axon, conducting the impulse as a wave of depolarisation. Immediately after, the membrane at the excited site becomes permeable to K$^+$, which diffuses outwards, restoring the resting potential (repolarisation). This allows the fibre to be ready for a new stimulation.

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Transmission Of Impulses (Synaptic Transmission)

A nerve impulse is transmitted from one neuron to another or to an effector cell (muscle, gland) at specialized junctions called **synapses**. A synapse is formed by the membranes of the presynaptic neuron (sending the signal) and the postsynaptic neuron or cell (receiving the signal), separated by a synaptic cleft (a fluid-filled gap) or in close proximity.

There are two types of synapses:

• **Electrical synapses:** Membranes of pre- and postsynaptic neurons are in very close proximity, allowing direct flow of electrical current (ions) from one neuron to the other through gap junctions. Impulse transmission is very fast. Rare in the human system.
• **Chemical synapses:** Pre- and postsynaptic membranes are separated by a synaptic cleft (Figure 18.3 in textbook). Impulse transmission involves **neurotransmitters** (chemicals).

Mechanism of transmission at a chemical synapse:

1. An impulse (action potential) arrives at the axon terminal of the presynaptic neuron.
2. This stimulates synaptic vesicles (containing neurotransmitters) to move towards the presynaptic membrane.
3. Vesicles fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft (exocytosis).
4. Neurotransmitters diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic membrane.
5. Binding of neurotransmitters opens ion channels in the postsynaptic membrane, allowing specific ions to enter or exit the postsynaptic neuron.
6. Movement of ions generates a new potential in the postsynaptic neuron. This potential can be excitatory (promoting the generation of an action potential) or inhibitory (suppressing the generation of an action potential).

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Central Neural System (CNS)

The **CNS** includes the **brain** and the **spinal cord**. It is the primary site for processing information, making decisions, and controlling body activities. The brain is well-protected by the skull and covered by cranial meninges (dura mater, arachnoid, pia mater). The spinal cord is protected by the vertebral column. The brain is divided into three major parts: forebrain, midbrain, and hindbrain.

The **brain stem** is a structure formed by the midbrain, pons, and medulla oblongata, connecting the forebrain to the spinal cord. It controls many basic involuntary functions necessary for survival.

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Forebrain

The forebrain is the most complex and largest part of the human brain. It consists of:

• **Cerebrum:** Forms the major part. Divided longitudinally into two cerebral hemispheres connected by the **corpus callosum** (nerve fibre tract). The outer layer (cerebral cortex) is the **grey matter** (due to neuron cell bodies), highly folded (gyri and sulci) for increased surface area. Inner part is **white matter** (myelinated nerve fibres). Cortex contains motor areas (control voluntary movement), sensory areas (receive sensory input), and association areas (for complex functions like memory, learning, communication, intersensory associations).
• **Thalamus:** Wraps around by the cerebrum. A major coordinating centre for sensory and motor signaling (relays information to and from the cerebral cortex).
• **Hypothalamus:** Lies at the base of the thalamus. Contains centers that control body temperature, hunger, thirst. Also contains neurosecretory cells producing hypothalamic hormones that regulate the pituitary gland.
• **Limbic lobe or limbic system:** Formed by inner parts of cerebral hemispheres and associated structures (amygdala, hippocampus). Involved in regulating sexual behaviour, emotional reactions (excitement, pleasure, rage, fear), and motivation.

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Midbrain

Located between the thalamus/hypothalamus (forebrain) and pons (hindbrain). A canal (cerebral aqueduct) passes through it. Dorsal portion has four round swellings (lobes) called **corpora quadrigemina**, involved in processing visual and auditory information.

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Hindbrain

Comprises the pons, cerebellum, and medulla (medulla oblongata).

• **Pons:** Consists of fibre tracts that interconnect different regions of the brain (e.g., cerebellum to cerebrum, cerebellum to medulla).
• **Cerebellum:** Located below the cerebrum, behind the pons and medulla. Has a very convoluted surface. Responsible for coordinating voluntary movements, maintaining posture and balance, and learning motor skills.
• **Medulla (Medulla Oblongata):** Connected to the spinal cord. Contains vital centers that control essential involuntary functions like respiration, cardiovascular reflexes (heart rate, blood pressure), and gastric secretions.

The **brain stem** is formed by the midbrain, pons, and medulla oblongata. It connects the brain (cerebrum and cerebellum) to the spinal cord and is essential for transferring information and controlling basic life functions.

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