Neural System:

• The neural system of all animals is composed of highly specialized cells called neurons that can detect, receive and transmit different stimuli.

  • The neural organization is elementary in lower invertebrates.

    • For example, Hydra is composed of a network of neurons.

  • The neural system is better organized in insects, where a brain and a number of ganglia and neural tissues are present.

  • Vertebrates have a more developed neural system.

Human Neural System:

• The human neural system is divided into two parts

  • The central neural system (CNS)

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

  • The peripheral neural system (PNS)

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

    • The nerve fibers of the PNS are of two types:

      • The afferent nerve fibers transmit impulses from tissues/organs to the CNS

      • The efferent fibers transmit regulatory impulses from the CNS to the concerned peripheral tissues/organs.

    • The PNS is divided into two divisions called:

      • The Somatic neural system relays impulses from the CNS to skeletal muscles

      • The Autonomic neural system transmits impulses from the CNS to the involuntary organs and smooth muscles of the body.

        • The autonomic neural system is further classified into:

          • The sympathetic neural system

          • The parasympathetic neural system.

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

Neuron as Structural and Functional Unit of Neural System:

• A neuron is a microscopic structure composed of three major parts, namely, the cell body, dendrites, and axon.

  • The cell body contains cytoplasm with typical cell organelles and certain granular bodies called Nissl’s granules.

  • Short fibers which branch repeatedly and project out of the cell body also contain Nissl’s granules and are called dendrites.

    • These fibers transmit impulses toward the cell body.

  • The axon is a long fiber, 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 axons transmit nerve impulses away from the cell body to a synapse or to a neuro-muscular junction.

  • Based on the number of axons and dendrites, the neurons are divided into three types, i.e.

    • Multipolar (with one axon and two or more dendrites; found in the cerebral cortex)

    • Bipolar (with 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).

  • There are two types of axons, namely:

    • The myelinated nerve fibers are enveloped with Schwann cells, which form a myelin sheath around the axon.

      • The gaps between two adjacent myelin sheaths are called nodes of Ranvier.

      • Myelinated nerve fibers are found in spinal and cranial nerves.

    • Nonmyelinated nerve fiber is enclosed by a Schwann cell that does not form a myelin sheath around the axon and is commonly found in autonomous and somatic neural systems.

Generation and Conduction of Nerve Impulse:

• Neurons are excitable cells because their membranes are in a polarised state.

• Different types of ion channels are present on the neural membrane.

  • These ion channels are selectively permeable to different ions.

    • When a neuron is not conducting any impulse, i.e., resting, the axonal membrane is comparatively 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.

    • These ionic gradients across the resting membrane are maintained by the active transport of ions by the sodium-potassium 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 and therefore is polarised.

• The electrical potential difference across the resting plasma membrane is called the resting potential.

• When a stimulus is applied at a site on the polarised membrane, the membrane at site A 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.

  • The polarity of the membrane at site A is thus reversed and hence depolarised.

• The electrical potential difference across the plasma membrane at site A is called the action potential, which is in fact termed a nerve impulse.

• At sites immediately ahead, the axon (e.g., site B) 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 site A to site B.

• On the outer surface, current flows from site B to site A to complete the circuit of the current flow.

  • Hence, the polarity at the site is reversed, and an action potential is generated at site B.

  • Thus, the impulse (action potential) generated at site A arrives at site B.

• The sequence is repeated along the length of the axon and consequently, the impulse is conducted.

  • The rise in the stimulus-induced permeability to Na+ is extremely short-lived.

  • It is quickly followed by a rise in permeability to K+.

• Within a fraction of a second, K+ diffuses outside the membrane and restores the resting potential of the membrane at the site of excitation and the fiber becomes once more responsive to further stimulation.

Transmission of Impulses:

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

  • 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.

  • There are two types of synapses, namely, electrical synapses and chemical synapses.

    • At electrical synapses, 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.

      • The 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.

      • Electrical synapses are rare in our system.

    • At a 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.

      • The axon terminals contain vesicles filled with these neurotransmitters.

      • When an impulse (action potential) arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane where they fuse with the plasma membrane and release their neurotransmitters in the synaptic cleft.

      • The released neurotransmitters bind to their specific receptors, present on the post-synaptic membrane.

      • 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.

Central Nervous System:

• The brain is the central information processing organ of our body, and acts as the ‘command and control system’.

  • It controls:

    • Voluntary movements

    • Balance of the body

    • Functioning of vital involuntary organs (e.g., lungs, heart, kidneys, etc.),

    • Thermoregulation

    • Hunger and thirst

    • Circadian (24-hour) rhythms of our body

    • Activities of several endocrine glands

    • Human behavior.

  • It is also the site for processing vision, hearing, speech, memory, intelligence, emotions, and thoughts.

• The human brain is well protected by the skull.

  • Inside the skull, the brain is covered by cranial meninges consisting of:

    • an outer layer called dura mater.

    • a very thin middle layer called arachnoid.

    • an inner layer (which is in contact with the brain tissue) called the pia mater.

      

• The brain can be divided into three major parts:

  • Forebrain

  • Midbrain

  • Hindbrain

Forebrain:

• The forebrain consists of the cerebrum, thalamus, and hypothalamus.

  • The cerebrum forms the major part of the human brain.

  • A deep cleft divides the cerebrum longitudinally into two halves, which are termed the left and right cerebral hemispheres.

    • The hemispheres are connected by a tract of nerve fibers called the 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 grey matter due to its greyish appearance.

        • The neuron cell bodies are concentrated here giving the color.

      • The cerebral cortex contains motor areas, sensory areas, and large regions that are neither clearly sensory nor motor in function.

        • These regions called association areas are responsible for complex functions like intersensory associations, memory, and communication.

      • Fibers of the tracts are covered with the myelin sheath, which constitutes the inner part of the cerebral hemisphere.

      • They give an opaque white appearance to the layer and, hence, are called white matter.

  • The cerebrum wraps around a structure called the thalamus, which is a major coordinating center for sensory and motor signaling.

  • Another very important part of the brain called the hypothalamus lies at the base of the thalamus.

    • The hypothalamus contains a number of centers that control body temperature, the urge for eating and drink.

    • It also contains several groups of neurosecretory cells, which secrete hormones called hypothalamic hormones.

    • The inner parts of cerebral hemispheres and a group of associated deep structures like the amygdala, hippocampus, etc., form a complex structure called the limbic lobe or limbic system.

  • Along with the hypothalamus, it is involved in:

    • the regulation of sexual behavior

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

    • Motivation.

Midbrain:

• The midbrain is 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 of the midbrain consists mainly of four round swellings (lobes) called corpora quadrigemina.

Hindbrain:

• The hindbrain comprises pons, cerebellum, and medulla (also called the medulla oblongata).

  • Pons consists of fiber tracts that interconnect different regions of the brain.

  • Cerebellum has a very convoluted surface in order to provide additional space for many more neurons.

  • The medulla of the brain is connected to the spinal cord.

    • The medulla contains centers that control

      • Respiration

      • Cardiovascular reflexes

      • Gastric secretions.

• Three major regions make up the brain stem; mid-brain, pons, and medulla oblongata.

• The brain stem forms the connections between the brain and spinal cord.

Reflex Action and Reflex Arc:

• The entire process of response to peripheral nerve stimulation, that occurs involuntarily, i.e., without conscious effort or thought, and requires the involvement of a part of the central nervous system is called a reflex action.

• The reflex pathway comprises at least one afferent neuron (receptor) and one efferent (effector or excitator) neuron appropriately arranged in a series.

  • The afferent neuron receives a signal from a sensory organ and transmits the impulse via a dorsal nerve root into the CNS (at the level of the spinal cord).

  • The efferent neuron then carries signals from CNS to the effector.

• The stimulus and response thus form a reflex arc as shown below in the knee jerk reflex.