Chapter Notes

Neural Control and Coordination

To maintain a stable internal environment, a state known as homeostasis, all the organs and systems in our body must work together. Coordination is the process where two or more organs interact and complement each other's functions.

In our bodies, two systems are responsible for this coordination and integration:

• The Neural System: Provides a fast, point-to-point network for quick coordination.
• The Endocrine System: Provides slower, chemical integration using hormones.

This chapter focuses on the human neural system, how nerve impulses are transmitted, and how they travel across junctions called synapses.

Neural System

The neural system in all animals is made of highly specialised cells called neurons. These cells are unique because they can detect, receive, and transmit different kinds of stimuli.

The complexity of the neural system varies across the animal kingdom.

• In lower invertebrates like Hydra, the system is a simple network of neurons.
• In insects, it is more organised, featuring a brain, ganglia, and neural tissues.
• Vertebrates possess the most developed neural system.

Human Neural System

The human neural system is divided into two main parts:

1. The Central Neural System (CNS): This includes the brain and the spinal cord. It is the primary site for information processing and control.
2. The Peripheral Neural System (PNS): This consists of all the nerves that connect the CNS to the rest of the body.

The nerve fibres of the PNS are classified into two types based on the direction of impulse transmission:

• (a) Afferent fibres: These transmit impulses from tissues and organs to the CNS. Think of them as sensory pathways.
• (b) Efferent fibres: These transmit regulatory impulses from the CNS to the peripheral tissues and organs. Think of them as motor pathways.

The PNS is further divided into two main divisions:

• Somatic Neural System: Relays impulses from the CNS to the skeletal muscles, which are under voluntary control.
• Autonomic Neural System: Transmits impulses from the CNS to involuntary organs and smooth muscles. This system is further divided into:
  • Sympathetic Neural System
  • Parasympathetic Neural System

Another part of the PNS is the Visceral Nervous System, which is a complex network of nerves, fibres, and ganglia that carries impulses from the CNS to the internal organs (viscera) and back from the viscera to the CNS.

Neuron as Structural and Functional Unit of Neural System

A neuron is the microscopic, fundamental unit of the nervous system. It is composed of three main parts: the cell body, dendrites, and an axon. (See Figure 18.1 in the source).

• Cell Body: Contains the cytoplasm, typical cell organelles, and special granular bodies called Nissl's granules.
• Dendrites: These are short, branching fibres that project out from the cell body. They also contain Nissl's granules. Their primary function is to transmit electrical impulses towards the cell body.
• Axon: This is a single, long fibre that branches at its end. Each branch ends in a bulb-like structure called a synaptic knob. These knobs contain synaptic vesicles filled with chemicals known as neurotransmitters. The axon's job is to carry nerve impulses away from the cell body to a synapse or a neuro-muscular junction.

Types of Neurons

Based on the number of axons and dendrites, neurons are classified into three types:

• Multipolar: Have one axon and two or more dendrites. Found in the cerebral cortex.
• Bipolar: Have one axon and one dendrite. Found in the retina of the eye.
• Unipolar: Have a cell body with only one axon. Typically found in the embryonic stage.

Types of Axons

Axons can be either myelinated or non-myelinated.

• Myelinated Axons: These nerve fibres are wrapped by Schwann cells, which form a fatty layer called the myelin sheath around the axon. This sheath is not continuous; it has gaps called the nodes of Ranvier. Myelinated fibres are found in spinal and cranial nerves.
• Non-myelinated Axons: These fibres are also enclosed by a Schwann cell, but the cell does not form a myelin sheath. These are commonly found in the autonomic and somatic neural systems.

Generation and Conduction of Nerve Impulse

Neurons are considered excitable cells because their membranes exist in a polarised state. This polarisation is key to their ability to conduct electrical signals.

The Resting Potential (Polarised State)

When a neuron is not conducting an impulse (it is at rest), its membrane has a specific electrical charge difference across it. This is called the resting potential. Here’s why it exists:

• Selective Permeability: The neural membrane contains ion channels that are selectively permeable. At rest, the membrane is more permeable to potassium ions ($K^+$) and almost impermeable to sodium ions ($Na^+$). It is also impermeable to negatively charged proteins found inside the neuron's cytoplasm (axoplasm).
• Ion Concentration Gradient: As a result, the inside of the axon has a high concentration of $K^+$ and negatively charged proteins, but a low concentration of $Na^+$. The fluid outside the axon has the opposite: a low concentration of $K^+$ and a high concentration of $Na^+$.
• Sodium-Potassium Pump: To maintain this gradient, a mechanism called the sodium-potassium pump actively transports ions across the membrane. It pumps 3 $Na^+$ ions outwards for every 2 $K^+$ ions it brings into the cell.

This entire process results in the outer surface of the axonal membrane having a positive charge, while the inner surface has a negative charge. This difference in charge is the resting potential.

The Action Potential (Depolarised State)

When a stimulus is applied to the polarised membrane (at a point we'll call site A):

1. Depolarisation: The membrane at site A suddenly becomes freely permeable to $Na^+$ ions.
2. Sodium Influx: This leads to a rapid flow of $Na^+$ ions into the axon.
3. Polarity Reversal: The influx of positive ions reverses the polarity at site A. The outer surface of the membrane becomes negatively charged, and the inner surface becomes positively charged.
4. Action Potential: This electrical potential difference across the membrane at site A is called the action potential. This is, in fact, the nerve impulse.

Conduction of the Nerve Impulse

The action potential doesn't stay in one place; it travels along the axon.

• At the site immediately ahead of the impulse (site B), the membrane is still in its resting state (positive outside, negative inside).
• A current flows on the inner surface of the axon from site A (now positive) to site B (still negative).
• On the outer surface, a current flows from site B (positive) to site A (now negative), completing the circuit.
• This flow of current causes the polarity at site B to reverse, generating a new action potential there.
• This sequence repeats along the entire length of the axon, causing the impulse to be conducted from one end to the other.

Repolarisation

The change in permeability to $Na^+$ is extremely brief.

• Immediately after the influx of $Na^+$, the membrane's permeability to $K^+$ increases.
• $K^+$ ions diffuse out of the membrane, which restores the resting potential (positive outside, negative inside).
• The membrane is now repolarised and ready to respond to another stimulus.

Transmission of Impulses

A nerve impulse is transmitted from one neuron to the next at junctions called synapses. A synapse is formed by the membrane of a pre-synaptic neuron (the one sending the signal) and a post-synaptic neuron (the one receiving it).

There are two main types of synapses:

Electrical Synapses

• The membranes of the pre- and post-synaptic neurons are very close to each other.
• Electrical current can flow directly from one neuron to the next.
• Transmission is extremely fast, similar to impulse conduction along a single axon.
• Electrical synapses are rare in the human system.

Chemical Synapses

• The membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called the synaptic cleft.
• Chemicals called neurotransmitters are used to transmit the impulse across this gap.
• The process occurs in the following steps:
  1. An impulse (action potential) arrives at the axon terminal of the pre-synaptic neuron.
  2. This stimulates synaptic vesicles, which are filled with neurotransmitters, to move towards the pre-synaptic membrane.
  3. The vesicles fuse with the membrane and release their neurotransmitters into the synaptic cleft.
  4. The neurotransmitters diffuse across the cleft and bind to specific receptors on the post-synaptic membrane.
  5. This binding opens ion channels, allowing ions to enter the post-synaptic neuron.
  6. This generates a new potential in the post-synaptic neuron. This new potential can be either excitatory (encouraging an impulse) or inhibitory (discouraging an impulse).

Central Neural System

The brain is the central information processing organ of our body, acting as the 'command and control system'.

Key Functions of the Brain:

• Controls voluntary movements and body balance.
• Regulates the functioning of vital involuntary organs like the lungs, heart, and kidneys.
• Controls body temperature (thermoregulation), hunger, and thirst.
• Manages circadian (24-hour) rhythms.
• Controls the activities of many endocrine glands.
• Governs human behaviour.
• Serves as the site for processing vision, hearing, speech, memory, intelligence, emotions, and thoughts.

Protection of the Brain

The brain is well-protected by the skull. Inside the skull, it is covered by three protective layers called cranial meninges:

• Dura mater: The tough outer layer.
• Arachnoid: A very thin middle layer.
• Pia mater: The delicate inner layer that is in direct contact with the brain tissue.

The brain can be divided into three major parts: the forebrain, midbrain, and hindbrain.

Forebrain

The forebrain is composed of the cerebrum, thalamus, and hypothalamus.

• Cerebrum: This is the largest part of the human brain.

  • It is divided longitudinally by a deep cleft into the left and right cerebral hemispheres.
  • These hemispheres are connected by a tract of nerve fibres called the corpus callosum.
  • The outer layer of the cerebrum is the cerebral cortex, which is folded into prominent ridges.
  • Grey Matter: The cerebral cortex has a greyish appearance because it is concentrated with neuron cell bodies. This layer is called the grey matter. It contains motor areas, sensory areas, and large association areas responsible for complex functions like memory, communication, and intersensory associations.
  • White Matter: The inner part of the cerebral hemisphere is made of nerve fibres (tracts) covered with a myelin sheath, giving it an opaque white appearance. This is called the white matter.

• Thalamus: The cerebrum wraps around the thalamus, which serves as a major coordinating centre for sensory and motor signaling.

• Hypothalamus: Located at the base of the thalamus.

  • It contains centres that control body temperature and the urge for eating and drinking.
  • It also has groups of neurosecretory cells that secrete hypothalamic hormones.

• Limbic System (or Limbic Lobe): This is a complex structure formed by the inner parts of the cerebral hemispheres and associated deep structures like the amygdala and hippocampus. Working along with the hypothalamus, it is involved in:

  • Regulation of sexual behaviour.
  • Expression of emotional reactions (e.g., excitement, pleasure, rage, and fear).
  • Motivation.

Midbrain

The midbrain is situated between the thalamus/hypothalamus of the forebrain and the pons of the hindbrain.

• A canal called the cerebral aqueduct passes through the midbrain.
• The dorsal (back) portion of the midbrain consists mainly of four round swellings (lobes) called the corpora quadrigemina.

Hindbrain

The hindbrain is composed of the pons, cerebellum, and medulla (also called the medulla oblongata).

• Pons: Consists of fibre tracts that interconnect different regions of the brain.
• Cerebellum: Has a highly convoluted (wrinkled) surface to provide additional space for a large number of neurons.
• Medulla: This part of the brain connects to the spinal cord. It contains vital centres that control:
  • Respiration
  • Cardiovascular reflexes
  • Gastric secretions

Brain Stem

The midbrain, pons, and medulla oblongata together form the brain stem. The brain stem acts as the connection between the brain and the spinal cord.