Biology: Class 11 — Chapter 18: Neural Control and Coordination

The nerve fibres of the PNS are of two types:

• Afferent fibres: Transmit impulses from tissues/organs to the CNS.
• Efferent fibres: Transmit regulatory impulses from the CNS to the concerned peripheral tissues/organs.

• Somatic neural system: Relays impulses from the CNS to skeletal muscles (voluntary).
• Autonomic neural system: Transmits impulses from the CNS to involuntary organs and smooth muscles. It is further classified into:
  • Sympathetic neural system
  • Parasympathetic neural system
• Visceral nervous system: A part of the PNS comprising the complex of nerves, fibres, ganglia, and plexuses by which impulses travel between the CNS and the viscera.

Based on the number of axons and dendrites, neurons are classified as:

• Multipolar: One axon and two or more dendrites (found in the cerebral cortex).
• Bipolar: One axon and one dendrite (found in the retina of the eye).
• Unipolar: Cell body with one axon only (found usually in the embryonic stage).

• Myelinated nerve fibres: Enveloped with Schwann cells, which form a myelin sheath around the axon. Gaps between adjacent myelin sheaths are called nodes of Ranvier. Found in spinal and cranial nerves.
• Non-myelinated nerve fibres: Enclosed by a Schwann cell that does not form a myelin sheath. Commonly found in autonomous and somatic neural systems.

Neurons are excitable cells because their membranes are in a polarised state due to selective permeability to different ions.

• Resting State (Polarised):
  • The axonal membrane is comparatively more permeable to potassium ions ($K^+$) and nearly impermeable to sodium ions ($Na^+$) and negatively charged proteins in the axoplasm.
  • Consequently, the axoplasm contains a high concentration of $K^+$ and negatively charged proteins, and a low concentration of $Na^+$.
  • The fluid outside the axon contains a low concentration of $K^+$ and a high concentration of $Na^+$, establishing a concentration gradient.
  • These ionic gradients are maintained by the active transport of ions by the sodium-potassium pump, which transports $3\text{ Na}^+$ outwards for $2\text{ K}^+$ into the cell.
  • This results in a positive charge on the outer surface and a negative charge on the inner surface, creating a potential difference called the resting potential.
• Action Potential (Depolarised State):
  • When a stimulus is applied at a site (e.g., point A), the membrane becomes freely permeable to $Na^+$, leading to a rapid influx of $Na^+$.
  • This reverses the polarity at that site (outer surface becomes negative, inner surface becomes positive), depolarising the membrane.
  • The electrical potential difference across the plasma membrane at site A is called the action potential (nerve impulse).
  • At adjacent sites (e.g., point B), the outer surface remains positive and the inner surface negative. Current flows on the inner surface from A to B, and on the outer surface from B to A to complete the circuit.
  • This generates an action potential at site B, conducting the impulse along the axon.
  • The rise in permeability to $Na^+$ is extremely short-lived, quickly followed by a rise in permeability to $K^+$, which diffuses outside to restore the resting potential.

Nerve impulses are transmitted from one neuron to another through junctions called synapses, formed by pre-synaptic and post-synaptic membranes, which may or may not be separated by a synaptic cleft.

• Electrical Synapses:
  • Membranes of pre- and post-synaptic neurons are in very close proximity.
  • Electrical current flows directly from one neuron to another.
  • Transmission is faster than chemical synapses and is rare in the human system.
• Chemical Synapses:
  • Membranes are separated by a fluid-filled space called the synaptic cleft.
  • Uses chemicals called neurotransmitters to transmit impulses.
  • Mechanism: When an impulse reaches the axon terminal, it stimulates synaptic vesicles to fuse with the plasma membrane and release neurotransmitters into the synaptic cleft. These chemicals bind to specific receptors on the post-synaptic membrane, opening ion channels and generating a new potential (either excitatory or inhibitory).

Consists of the cerebrum, thalamus, and hypothalamus.

• Cerebrum: Forms the major part of the human brain. A deep cleft divides it longitudinally into left and right cerebral hemispheres, connected by a tract of nerve fibres called the corpus callosum.
  • Cerebral cortex: The outer layer of cells covering the hemispheres, referred to as grey matter. It contains motor areas, sensory areas, and large association areas (responsible for complex functions like intersensory associations, memory, and communication).
  • The inner part of the cerebral hemisphere consists of myelinated fibres, giving it an opaque white appearance (white matter).
• Thalamus: A structure wrapped by the cerebrum; acts as a major coordinating centre for sensory and motor signaling.
• Hypothalamus: Lies at the base of the thalamus. It contains centres controlling body temperature, urge for eating and drinking, and secretes hypothalamic hormones.
• Limbic System: Formed by the inner parts of cerebral hemispheres and associated deep structures (amygdala, hippocampus, etc.). Along with the hypothalamus, it regulates sexual behaviour, emotional expressions (excitement, pleasure, rage, fear), and motivation.

Located 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 portion consists mainly of four round swellings (lobes) called corpora quadrigemina.

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

• Pons: Consists of fibre tracts that interconnect different regions of the brain.
• Cerebellum: Has a highly convoluted surface to provide additional space for many more neurons.
• Medulla: Connected to the spinal cord. It contains vital centres controlling respiration, cardiovascular reflexes, and gastric secretions.
• Brain Stem: Formed by the midbrain, pons, and medulla oblongata. It forms the connection between the brain and the spinal cord.