Chapter 5.5: Neural Control and Coordination

1. Neuron and Nerves

• Neuron (Nerve Cell): A neuron is a microscopic structure composed of three major parts, namely, the cell body, dendrites and axon. It is the structural and functional unit of the nervous system.

  • Cell Body (Soma or Cyton): Contains the cytoplasm with typical cell organelles and certain granular bodies called Nissl’s granules.
  • Dendrites: Short fibres which branch repeatedly and project out of the cell body. They also contain Nissl’s granules. Dendrites transmit electrical impulses towards the cell body.
  • Axon: A long fibre, the distal end of which is branched. Each branch terminates as a bulb-like structure called a synaptic knob which possesses synaptic vesicles containing chemicals called neurotransmitters. The axon transmits nerve impulses away from the cell body to a synapse or to a neuromuscular junction.

• Nerves: In the peripheral nervous system, the axons of neurons are bundled together to form a nerve. Nerves are covered by connective tissue sheaths.

Types of Neurons

Based on the number of axon and dendrites, the neurons are divided into four types:

• Unipolar Neuron: Cell body with one axon only. Found usually in the embryonic stage.
• Bipolar Neuron: With one axon and one dendrite. Found in the retina of the eye and the olfactory epithelium.
• Multipolar Neuron: With one axon and two or more dendrites. Found in the cerebral cortex.
• Pseudounipolar Neuron: A single process arises from the cell body and then divides into an axon and a dendrite. Found in the dorsal root ganglia of the spinal cord.

Types of Axons

• Myelinated Nerve Fibre: The axon is enveloped with Schwann cells, which form a myelin sheath around the axon. The gaps between two adjacent myelin sheaths are called nodes of Ranvier. Found in spinal and cranial nerves.
• Non-myelinated Nerve Fibre: The axon is enclosed by a Schwann cell that does not form a myelin sheath around the axon. Found in autonomous and the somatic neural systems.

2. Generation and Conduction of Nerve Impulse

A nerve impulse is a wave of electrochemical change that travels along a neuron.

a) Resting Potential (Polarized State)

• In a resting neuron (a neuron not conducting any impulse), the axonal membrane is more permeable to potassium ions (K+) and nearly impermeable to sodium ions (Na+). Similarly, the membrane is impermeable to negatively charged proteins present in the axoplasm.
• Consequently, the axoplasm inside the axon contains a high concentration of K+ and negatively charged proteins and a low concentration of Na+.
• In contrast, the fluid outside the axon contains a low concentration of K+ and a high concentration of Na+ and thus forms a concentration gradient.
• This ionic gradient across the resting membrane is maintained by the active transport of ions by the sodium-potassium pump (Na+-K+ Pump) which transports 3 Na+ outwards for 2 K+ into the cell.
• As a result, the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged. The electrical potential difference across the resting plasma membrane is called as the resting potential. The membrane is said to be polarized.

b) Action Potential (Depolarized State)

• When a stimulus is applied, the membrane at the site becomes freely permeable to Na+.
• This leads to a rapid influx of Na+ followed by the reversal of the polarity at that site, i.e., the outer surface of the membrane becomes negatively charged and the inner side becomes positively charged. This reversal of polarity is called depolarization.
• The electrical potential difference across the plasma membrane at the site of the action potential is called the action potential, which is in fact termed as a nerve impulse.

c) Repolarization

• At the peak of the action potential, the permeability to Na+ decreases, and the membrane becomes more permeable to K+.
• K+ ions diffuse outwards, and the membrane potential returns towards the resting potential. This is called repolarization.

d) Conduction of Nerve Impulse

• At sites immediately ahead, the axon membrane has a positive charge on the outer surface and a negative charge on its inner surface. As a result, a current flows on the inner surface from the site of the action potential to the next site. On the outer surface, a current flows from the next site to the site of the action potential to complete the circuit.
• Hence, the polarity at the site is reversed, and an action potential is generated at the next site. Thus, the impulse (action potential) generated at one point arrives at the next point. This sequence is repeated along the length of the axon and consequently, the impulse is conducted.
• Saltatory Conduction: In myelinated nerve fibres, the action potential jumps from one node of Ranvier to the next. This type of conduction is much faster than in non-myelinated fibres.

3. Synaptic Transmission

A nerve impulse is transmitted from one neuron to another through junctions called synapses.

• Synapse: A synapse is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a gap called the synaptic cleft.
• Types of Synapses:
  • Electrical Synapse: The membranes of pre- and post-synaptic neurons are in very close proximity. Electrical current can flow directly from one neuron into the other across these synapses. Transmission of an impulse across electrical synapses is very similar to impulse conduction along a single axon. Impulse transmission across an electrical synapse is always faster than that across a chemical synapse.
  • Chemical Synapse: The membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called the synaptic cleft. Chemicals called neurotransmitters are involved in the transmission of impulses at these synapses.

Mechanism of Chemical Synaptic Transmission

1. When an action potential arrives at the axon terminal (synaptic knob), it stimulates the movement of the synaptic vesicles towards the membrane where they fuse with the plasma membrane and release their neurotransmitters into the synaptic cleft.
2. The released neurotransmitters bind to their specific receptors, present on the post-synaptic membrane.
3. This binding opens ion channels allowing the entry of ions which can generate a new potential in the post-synaptic neuron. The new potential developed may be either excitatory or inhibitory.

• Neurotransmitters: These are chemicals that transmit signals across a chemical synapse. Examples include Acetylcholine, Dopamine, Serotonin, GABA (Gamma-Aminobutyric Acid), etc.

4. Human Nervous System

a) Central Nervous System (CNS)

The CNS includes the brain and the spinal cord and is the site of information processing and control.

i) Brain

• Forebrain (Prosencephalon):

  • Cerebrum: Forms the major part of the human brain. A deep cleft divides the cerebrum longitudinally into two halves, which are termed as the left and right cerebral hemispheres. The hemispheres are connected by a tract of nerve fibres called corpus callosum. The layer of cells which covers the cerebral hemisphere is called the cerebral cortex and is thrown into prominent folds. The cerebral cortex is referred to as the grey matter due to its greyish appearance. The cerebral cortex contains motor areas, sensory areas and large regions that are neither clearly sensory nor motor in function. These regions called as the association areas are responsible for complex functions like intersensory associations, memory and communication. The inner part of the cerebral hemisphere consists of fibres of the tracts covered with the myelin sheath, which constitute the white matter.
  • Thalamus: A major coordinating centre for sensory and motor signaling.
  • Hypothalamus: Lies at the base of the thalamus. It contains a number of centres which control body temperature, urge for eating and drinking. It also contains several groups of neurosecretory cells, which secrete hormones called hypothalamic hormones.

• Midbrain (Mesencephalon): Located between the thalamus/hypothalamus of the forebrain and pons of the hindbrain. A canal called the cerebral aqueduct passes through the midbrain. The dorsal portion of the midbrain consists mainly of four round swellings (lobes) called corpora quadrigemina.

• Hindbrain (Rhombencephalon):

  • Pons: Consists of fibre tracts that interconnect different regions of the brain.
  • Cerebellum: Has a very convoluted surface in order to provide the additional space for many more neurons. It is responsible for the regulation of balance and coordination of voluntary movements.
  • Medulla Oblongata: Connected to the spinal cord. The medulla contains centres which control respiration, cardiovascular reflexes and gastric secretions.

ii) Spinal Cord

• Structure: A long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column. The spinal cord is covered by the same three meninges as the brain. The grey matter is in the centre (H-shaped) and the white matter is on the outside.
• Functions:
  1. Conducts sensory and motor impulses to and from the brain.
  2. Acts as a centre for reflex actions.

b) Peripheral Nervous System (PNS)

The PNS comprises all the nerves of the body associated with the CNS (brain and spinal cord).

• Somatic Nervous System: Relays impulses from the CNS to skeletal muscles.
• Autonomic Nervous System (ANS): Transmits impulses from the CNS to the involuntary organs and smooth muscles of the body. The ANS is further classified into the sympathetic and parasympathetic nervous systems.

c) Visceral Nervous System

• The part of the peripheral nervous system that comprises the whole complex of nerves, fibres, ganglia, and plexuses by which impulses travel from the central nervous system to the viscera and from the viscera to the central nervous system. It is a part of the autonomic nervous system.