Neural Control and Coordination

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 r==esting 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.==