Neural Control and Coordination

[Image of Human Brain Sagittal Section]

The brain is the central information processing organ of our body. It is protected by the skull and three cranial meninges. It is divided into three major parts:

1. Forebrain: Consists of Cerebrum, Thalamus, and Hypothalamus. The cerebrum forms the major part of the human brain, divided into two hemispheres connected by the Corpus Callosum. It controls voluntary actions, memory, and communication.
2. Midbrain: Located between the thalamus/hypothalamus and pons. It contains the Corpora Quadrigemina (four swellings) involved in visual and auditory reflexes.
3. Hindbrain: Consists of Pons, Cerebellum, and Medulla Oblongata. The cerebellum maintains balance and posture. The medulla controls involuntary functions like respiration and heart rate.

CNS	PNS
Consists of the Brain and Spinal Cord.	Consists of all nerves arising from the CNS (Cranial and Spinal nerves).
Site of information processing and control.	Connects the CNS to different parts of the body.

Resting Potential	Action Potential
Potential difference across the membrane at rest (approx -70mV).	Potential difference during impulse conduction (approx +30mV).
Membrane is more permeable to $K^+$ than $Na^+$.	Membrane becomes highly permeable to $Na^+$.
Maintained by $Na^+-K^+$ pump.	Generated by rapid influx of $Na^+$.

• (a) Polarisation: In a resting nerve fibre, the axoplasm contains high $K^+$ and low $Na^+$. The membrane is permeable to $K^+$ but impermeable to $Na^+$. The $Na^+-K^+$ pump actively transports 3 $Na^+$ out and 2 $K^+$ in. This creates a positive charge outside and negative charge inside.
• (b) Depolarisation: When a stimulus is applied, the membrane becomes freely permeable to $Na^+$. A rapid influx of $Na^+$ reverses the polarity at that site (inner side becomes positive). This reversal is called depolarisation.
• (c) Transmission at Chemical Synapse:
  1. Impulse reaches the axon terminal.
  2. Synaptic vesicles fuse with the pre-synaptic membrane.
  3. Neurotransmitters (e.g., Acetylcholine) are released into the synaptic cleft.
  4. They bind to receptors on the post-synaptic membrane, opening ion channels and generating a new potential.

[Image of Neuron Structure]

(a) Neuron: Labels should include Dendrites, Cell Body (Soma), Nucleus, Axon, Myelin Sheath, Schwann Cell, Node of Ranvier, Axon Terminal.

[Image of Human Brain Anatomy]

(b) Brain: Labels should include Cerebrum, Corpus Callosum, Thalamus, Hypothalamus, Midbrain, Pons, Cerebellum, Medulla, Spinal Cord.

• (a) Neural Coordination: The process by which two or more organs interact and complement the functions of one another via the neural system to maintain homeostasis.
• (b) Forebrain: The anterior part, consisting of the cerebrum (thinking part), thalamus (coordination center), and hypothalamus (temperature/urge control).
• (c) Midbrain: Located between the forebrain and hindbrain. Contains the cerebral aqueduct and corpora quadrigemina.
• (d) Hindbrain: The posterior part consisting of pons, cerebellum, and medulla. Regulates involuntary functions and balance.
• (e) Synapse: A junction between two neurons involving a pre-synaptic neuron, a synaptic cleft (optional), and a post-synaptic neuron.

[Image of Synaptic Transmission Diagram]

When an action potential arrives at the axon terminal, it stimulates the movement of synaptic vesicles towards the membrane. They fuse with the plasma membrane and release neurotransmitters into the synaptic cleft. These neurotransmitters bind to specific receptors on the post-synaptic membrane. This binding opens ion channels allowing the entry of ions ($Na^+$), generating a new potential (excitatory or inhibitory) in the receiving neuron.

Sodium ions ($Na^+$) play a critical role in Depolarisation. Upon stimulation, voltage-gated $Na^+$ channels open, allowing a rapid influx of $Na^+$ from the extracellular fluid into the axoplasm. This influx reverses the membrane polarity from negative to positive inside, generating the action potential (nerve impulse).

(a) Myelinated Axon	Non-myelinated Axon
Enveloped by Schwann cells forming myelin sheath.	Enclosed by Schwann cells but no myelin sheath.
Conduction is fast (Saltatory).	Conduction is slow.
Found in Spinal/Cranial nerves.	Found in Autonomous/Somatic systems.

(b) Dendrites	Axons
Short, branching fibres projecting from cell body.	Long, single fibre arising from cell body.
Transmit impulse towards cell body.	Transmit impulse away from cell body.

(c) Thalamus	Hypothalamus
Major coordinating center for sensory/motor signaling.	Regulates temperature, hunger, thirst, and hormones.
Dorsal part of diencephalon.	Ventral part of diencephalon.

(d) Cerebrum	Cerebellum
Largest part of brain (Forebrain).	Part of Hindbrain.
Controls voluntary actions, intelligence, memory.	Controls balance, posture, equilibrium.

(a) Most developed part of human brain: Cerebrum.

(b) Master clock of CNS: Hypothalamus (Specifically the Suprachiasmatic Nucleus, which controls circadian rhythms).

• Afferent (Sensory): Transmit impulses from tissues/organs to the CNS.
• Efferent (Motor): Transmit impulses from the CNS to peripheral tissues/organs.

• Myelinated: Impulse jumps from one Node of Ranvier to the next (Saltatory conduction). It is very fast.
• Unmyelinated: Impulse travels continuously along the entire length of the nerve fibre. It is slower.

• Cranial Nerves: Arise directly from the Brain. (12 pairs in humans).
• Spinal Nerves: Arise from the Spinal Cord. (31 pairs in humans).