Detailed Notes -2 -Chapter-18-Neural Control And Coordination

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.