Detailed Notes Chapter-18-Neural Control And Coordination

Chapter-18-Neural  Control And Coordination

Introduction

1. Introduction to Coordination of Organ Systems:

   • Coordination is essential for organs and organ systems to maintain homeostasis, ensuring that the body’s internal environment remains stable.
   • Coordination involves the interaction and complementation of functions among two or more organs.

2. Example of Coordination during Physical Exercise:

   • During physical exercise, there is an increased demand for energy due to heightened muscular activity.
   • This increased activity requires more oxygen supply to the muscles.
   • Consequently, the body responds by increasing the rate of respiration, heart rate, and blood flow through the blood vessels to deliver more oxygen.
   • When physical exercise ceases, the activities of various organs gradually return to normal levels.

3. Organs Involved in Coordination during Physical Exercise:

   • Coordination during physical exercise involves various organs, including muscles, lungs, heart, blood vessels, and kidneys.

4. Role of Neural System and Endocrine System in Coordination:

   • The neural system and the endocrine system work together to coordinate and integrate the activities of organs.
   • The neural system provides a rapid coordination mechanism through an organized network of point-to-point connections.
   • The endocrine system provides chemical integration via the release of hormones into the bloodstream.

5. Specific Focus on the Neural System:

   • In this chapter, the focus will be on the neural system of humans.
   • Mechanisms of neural coordination, such as the transmission of nerve impulses and impulse conduction across synapses, will be explored.

18.1 NEURAL SYSTEM

1. Composition of Neurons:

   • The neural system consists of specialized cells called neurons.
   • Neurons are capable of detecting, receiving, and transmitting various types of stimuli, facilitating communication within the nervous system.

2. Neural Organization in Lower Invertebrates:

   • In lower invertebrates like Hydra, the neural organization is relatively simple.
   • It typically consists of a network of neurons, which enables basic sensory and motor functions.

3. Neural Organization in Insects:

   • In insects, the neural system exhibits a higher level of organization compared to lower invertebrates.
   • In addition to a network of neurons, insects possess a brain and multiple ganglia (clusters of nerve cell bodies).
   • This organization allows for more complex processing of sensory information and coordination of motor responses.

4. Neural System in Vertebrates:

   • Vertebrates, including humans, have a highly developed neural system.
   • This system is characterized by a central nervous system (CNS), which includes the brain and spinal cord, and a peripheral nervous system (PNS), consisting of nerves that extend throughout the body.
   • The CNS plays a crucial role in processing information, while the PNS relays sensory information to the CNS and transmits motor signals from the CNS to muscles and glands.
   • Vertebrates exhibit sophisticated sensory capabilities, complex motor coordination, and higher cognitive functions due to the advanced neural organization.

18.2 HUMAN NEURAL SYSTEM

1. Division of the Neural System:

   • The human neural system is divided into two main parts:
     • Central Nervous System (CNS)
     • Peripheral Nervous System (PNS)

2. Central Nervous System (CNS):

   • The CNS includes the brain and the spinal cord.
   • It serves as the primary site for information processing and control in the body.

3. Peripheral Nervous System (PNS):

   • The PNS comprises all the nerves in the body that are associated with the CNS (brain and spinal cord).
   • Nerve fibers of the PNS are of two types:
     • Afferent fibers: Transmit impulses from tissues/organs to the CNS.
     • Efferent fibers: Transmit regulatory impulses from the CNS to the relevant peripheral tissues/organs.

4. Subdivisions of the Peripheral Nervous System (PNS):

   • The PNS is further divided into two main divisions:
     • Somatic Nervous System: Relays impulses from the CNS to skeletal muscles, controlling voluntary movements.
     • Autonomic Nervous System: Transmits impulses from the CNS to involuntary organs and smooth muscles of the body, controlling involuntary functions.

5. Subdivisions of the Autonomic Nervous System (ANS):

   • The autonomic nervous system is further classified into:
     • Sympathetic Nervous System: Activates the body’s “fight or flight” response, preparing it for stressful situations.
     • Parasympathetic Nervous System: Promotes relaxation and regulates normal body functions during restful states.

6. Visceral Nervous System:

   • The visceral nervous system is a part of the peripheral nervous system.
   • It comprises a complex network of nerves, fibers, ganglia, and plexuses that facilitate the transmission of impulses between the central nervous system and the viscera (internal organs).

18.3 NEURON AS STRUCTURAL AND FUNCTIONAL UNIT OF NEURAL SYSTEM

1. Composition of Neurons:

   • Neurons are microscopic structures composed of three main parts: a. Cell body (soma): Contains cytoplasm with typical cell organelles and Nissl’s granules. b. Dendrites: Short fibers branching out from the cell body, containing Nissl’s granules. Dendrites transmit impulses towards the cell body. c. Axon: A long fiber extending from the cell body, with a branched distal end. Each branch terminates as a synaptic knob, which contains synaptic vesicles with neurotransmitters. Axons transmit nerve impulses away from the cell body, either to a synapse or to a neuro-muscular junction.

2. Classification of Neurons:

   • Neurons are classified based on the number of axons and dendrites they possess: a. Multipolar neurons: Have one axon and two or more dendrites. Found in the cerebral cortex. b. Bipolar neurons: Have one axon and one dendrite. Found in the retina of the eye. c. Unipolar neurons: Have one axon only. Commonly found in the embryonic stage.

3. Types of Axons:

   • Axons can be classified into two types: a. Myelinated axons: Enveloped by Schwann cells, forming a myelin sheath around the axon. Nodes of Ranvier are gaps between adjacent myelin sheaths. Myelinated nerve fibers are found in spinal and cranial nerves. b. Unmyelinated axons: Enclosed by Schwann cells that do not form a myelin sheath around the axon. Commonly found in both the autonomic and somatic nervous systems.

18.3.1 Generation and Conduction of Nerve Impulse

1. Polarization of Neuronal Membrane:

   • Neurons have polarized membranes, meaning there is an electrical potential difference across the membrane.
   • This polarization is maintained by selective permeability of ion channels, with potassium ions (K+) being more permeable while sodium ions (Na+) are nearly impermeable.
   • Inside the axon, there is a high concentration of K+ and negatively charged proteins, while outside the axon, there is a low concentration of K+ and a high concentration of Na+.

2. Resting Potential:

   • The maintenance of ionic gradients across the resting membrane is facilitated by the sodium-potassium pump, which actively transports 3 Na+ outwards for every 2 K+ into the cell.
   • This creates a positively charged outer surface and a negatively charged inner surface, resulting in a resting potential across the membrane.

3. Generation of Nerve Impulse (Action Potential):

   • When a stimulus is applied to a polarized membrane (e.g., at point A), the membrane becomes permeable to Na+.
   • This leads to a rapid influx of Na+ ions, causing depolarization and reversal of polarity at the site of stimulation.
   • The electrical potential difference across the membrane at the stimulated site is termed the action potential or nerve impulse.

4. Conduction of Nerve Impulse:

   • The depolarization at the stimulated site triggers a wave of depolarization along the axon.
   • Ahead of the depolarized site, the membrane has a positive charge on the outer surface and a negative charge on the inner surface.
   • This sets up a current flow from the depolarized site to adjacent regions, causing depolarization at those sites and generating successive action potentials.
   • This process repeats along the length of the axon, allowing the nerve impulse to be conducted.

5. Repolarization and Restoration of Resting Potential:

   • The rise in permeability to Na+ is short-lived and is quickly followed by a rise in permeability to K+.
   • K+ ions diffuse out of the membrane, restoring the resting potential.
   • This process allows the neuron to become responsive to further stimulation.

18.3.2 Transmission of Impulses

1. Types of Synapses:

   • Synapses are junctions between neurons where nerve impulses are transmitted.
   • There are two main types of synapses: electrical synapses and chemical synapses.

2. Electrical Synapses:

   • In electrical synapses, the membranes of pre- and post-synaptic neurons are very close together, allowing direct flow of electrical current between them.
   • Impulse transmission across electrical synapses is similar to impulse conduction along a single axon and is faster than transmission across chemical synapses.
   • However, electrical synapses are rare in the human nervous system.

3. Chemical Synapses:

   • Chemical synapses are more common and involve a synaptic cleft, a fluid-filled space separating the membranes of the pre- and post-synaptic neurons.
   • Transmission of impulses at chemical synapses involves neurotransmitters.

4. Role of Neurotransmitters:

   • Neurotransmitters are chemicals stored in vesicles within the axon terminals of the pre-synaptic neuron.
   • When an impulse (action potential) reaches the axon terminal, it triggers the movement of synaptic vesicles towards the membrane.
   • The vesicles then fuse with the membrane and release neurotransmitters into the synaptic cleft.

5. Activation of Post-synaptic Neuron:

   • Neurotransmitters released into the synaptic cleft bind to specific receptors on the post-synaptic membrane.
   • This binding opens ion channels in the post-synaptic membrane, allowing the entry of ions.
   • The entry of ions generates a new potential in the post-synaptic neuron, which may be either excitatory (promoting the generation of an action potential) or inhibitory (preventing the generation of an action potential).

18.4 CENTRAL NEURAL SYSTEM

1. Function of the Brain:

   • The brain serves as the central information processing organ of the body, often referred to as the “command and control system.”
   • It controls various functions, including:
     • Voluntary movements
     • Balance of the body
     • Functioning of vital involuntary organs (e.g., lungs, heart, kidneys)
     • Thermoregulation
     • Regulation of hunger and thirst
     • Circadian (24-hour) rhythms of the body
     • Activities of several endocrine glands
     • Human behavior
   • Additionally, the brain is responsible for processing sensory inputs and controlling various cognitive functions such as vision, hearing, speech, memory, intelligence, emotions, and thoughts.

2. Protection of the Brain:

   • The brain is well protected by the skull, providing a sturdy barrier against external injury.
   • Inside the skull, the brain is further shielded by cranial meninges, which consist of three layers:
     • Dura mater (outer layer)
     • Arachnoid (middle layer)
     • Pia mater (inner layer in contact with brain tissue)

3. Major Parts of the Brain:

   • The human brain can be divided into three major parts: a. Forebrain: Located at the front of the brain, it includes structures such as the cerebral cortex, thalamus, and hypothalamus. The forebrain is involved in higher cognitive functions and sensory processing. b. Midbrain: Positioned between the forebrain and hindbrain, the midbrain plays a role in coordinating sensory and motor responses, as well as controlling arousal and consciousness. c. Hindbrain: Located at the back of the brain, it comprises the cerebellum, pons, and medulla oblongata. The hindbrain is involved in regulating basic physiological functions such as breathing, heart rate, and coordination of movements.

18.4.1 Forebrain

1. Components of the Forebrain:

   • The forebrain consists of three main structures: a. Cerebrum b. Thalamus c. Hypothalamus

2. Cerebrum:

   • The cerebrum is the largest part of the human brain.
   • It is divided into two hemispheres, the left and right cerebral hemispheres, by a deep cleft.
   • These hemispheres are connected by a tract of nerve fibers called the corpus callosum.
   • The outer layer of the cerebral hemispheres is called the cerebral cortex, which is folded into prominent folds.
   • The cerebral cortex is composed of gray matter, consisting of neuron cell bodies, giving it a grayish appearance.
   • It contains motor areas, sensory areas, and association areas responsible for complex functions such as intersensory associations, memory, and communication.
   • Beneath the cerebral cortex lies white matter, composed of myelinated nerve fibers that give it an opaque white appearance.

3. Thalamus:

   • The thalamus is a major coordinating center for sensory and motor signaling.
   • It acts as a relay station, receiving sensory information from various sensory pathways and directing it to the appropriate regions of the cerebral cortex for processing.

4. Hypothalamus:

   • The hypothalamus is located at the base of the thalamus.
   • It plays a crucial role in maintaining homeostasis by controlling various physiological processes such as body temperature, hunger, thirst, and sleep-wake cycles.
   • The hypothalamus also contains neurosecretory cells that secrete hormones called hypothalamic hormones, which regulate the secretion of hormones from the pituitary gland.

5. Limbic System:

   • The inner parts of the cerebral hemispheres and associated deep structures such as the amygdala and hippocampus form the limbic lobe or limbic system.
   • Along with the hypothalamus, the limbic system is involved in regulating sexual behavior, emotional reactions (e.g., excitement, pleasure, rage, fear), and motivation.

18.4.2 Midbrain

1. Location and Position:

   • The midbrain is situated between the thalamus/hypothalamus of the forebrain and the pons of the hindbrain.
   • It forms the central portion of the brainstem.

2. Cerebral Aqueduct:

   • The midbrain contains a canal known as the cerebral aqueduct (also called the aqueduct of Sylvius).
   • This canal serves as a conduit for cerebrospinal fluid, allowing it to flow from the third ventricle in the diencephalon to the fourth ventricle in the hindbrain.

3. Corpora Quadrigemina:

   • The dorsal portion of the midbrain mainly consists of four round swellings known as the corpora quadrigemina (Latin for “quadruplet bodies”).
   • The corpora quadrigemina is further divided into two pairs of nuclei: a. Superior colliculi: These nuclei are involved in processing visual information and coordinating eye movements, particularly reflexive movements in response to visual stimuli. b. Inferior colliculi: These nuclei are involved in processing auditory information and coordinating auditory reflexes, such as turning the head towards a sudden noise.

18.4.3 Hindbrain

1. Components of the Hindbrain:

   • The hindbrain comprises three main structures: a. Pons b. Cerebellum c. Medulla (Medulla Oblongata)

2. Pons:

   • The pons is a region of the hindbrain containing fiber tracts that interconnect different regions of the brain.
   • It serves as a relay center, transmitting signals between the cerebrum and the cerebellum.
   • Additionally, the pons is involved in regulating certain functions such as sleep, respiration, and facial movements.

3. Cerebellum:

   • The cerebellum is a highly convoluted structure located at the back of the brain, beneath the cerebrum.
   • Its convoluted surface increases its surface area, providing space for a large number of neurons.
   • The cerebellum plays a crucial role in coordinating voluntary movements, maintaining balance, and fine-tuning motor control.
   • It receives sensory input from the spinal cord and various sensory pathways, allowing it to integrate and modulate motor signals.

4. Medulla (Medulla Oblongata):

   • The medulla oblongata is the lowermost part of the brainstem, connected to the spinal cord.
   • It contains vital centers that control essential physiological functions, including: a. Respiration: The medulla regulates breathing by monitoring levels of carbon dioxide and oxygen in the blood and adjusting breathing rate and depth accordingly. b. Cardiovascular reflexes: It controls heart rate, blood pressure, and other cardiovascular functions through reflex mechanisms. c. Gastric secretions: The medulla regulates digestive processes by controlling gastric secretions and gastrointestinal motility.

5. Brainstem:

   • The hindbrain, along with the midbrain, forms the brainstem, which serves as a critical link between the brain and the spinal cord.
   • The brainstem regulates basic physiological functions and plays a vital role in transmitting sensory and motor signals between the brain and the rest of the body.