Chapter 6: Neural Control and Coordination

Introduction

Coordination and Homeostasis

• Coordination is interaction and complementing functions of organs.
• Maintains homeostasis by synchronizing organ functions.
• Example: Physical exercise increases energy demand, oxygen supply, respiration rate, heartbeat, and blood flow.
• Post-exercise, organs like nerves, lungs, heart, kidneys return to normal.
• Neural system and endocrine system jointly coordinate body activities.
• Neural system: fast, point-to-point connections.
• Endocrine system: chemical coordination via hormones.

Memory Aids

• Coordination = organs working together for balance.
• Neural = fast signals; Endocrine = slow hormones.
• Exercise example: increased demand → increased supply.

Quick Checkpoints

• What is coordination in biology?
• How do neural and endocrine systems differ in coordination?
• Why does oxygen supply increase during exercise?

Neural System

Structure and Function

• Composed of specialized cells called neurons.
• Neurons detect, receive, and transmit stimuli.
• Lower invertebrates (e.g., Hydra) have simple neural networks.
• Insects have brains, ganglia, and neural tissues for better organization.
• Vertebrates possess highly developed neural systems.

Memory Aids

• Neurons = nerve cells for signal transmission.
• Hydra = simple nerve net.
• Insects = brain + ganglia; Vertebrates = advanced brain.

Quick Checkpoints

• What are neurons and their role?
• Describe neural organization in Hydra.
• How does neural complexity vary among animals?

Human Neural System

Structure and Divisions

• The human neural system has two parts: central neural system (CNS) and peripheral neural system (PNS).
• CNS includes the brain and spinal cord; it processes information and controls the body.
• PNS consists of all nerves outside the brain and spinal cord connected to CNS.
• PNS nerve fibers are afferent (to CNS) and efferent (from CNS).
• PNS divides into somatic neural system (to skeletal muscles) and autonomic neural system (to involuntary organs and smooth muscles).
• Autonomic system further splits into sympathetic and parasympathetic systems.
• Visceral nervous system is part of PNS controlling impulses between CNS and internal organs.

Memory Aids

• CNS = Brain + Spinal Cord (Control center)
• PNS = Nerves outside CNS (Communication lines)
• Afferent = Arrives at CNS; Efferent = Exits CNS
• Somatic = Skeletal muscles; Autonomic = Involuntary organs
• Sympathetic = Stress response; Parasympathetic = Rest and digest

Quick Checkpoints

• What are the two main parts of the human neural system?
• Which organs form the CNS?
• What types of fibers are in the PNS?
• How is the autonomic system subdivided?
• What is the role of the visceral nervous system?

Peripheral Neural System

Structure and Classification

• Peripheral neural system includes nerves outside brain and spinal cord.
• Two types of nerves: cranial nerves (12 pairs from brain) and spinal nerves (31 pairs from spinal cord).
• Cranial nerves include sensory, motor, and mixed types (e.g., olfactory, optic, facial, vagus).
• Spinal nerves classified into cervical (8 pairs), thoracic (12 pairs), lumbar (5 pairs), sacral (5 pairs), and coccygeal (1 pair).
• PNS subdivides into autonomic neural system and somatic neural system.
• Autonomic system controls involuntary organs; divided into sympathetic (stress response) and parasympathetic (rest) systems.
• Somatic system controls skeletal muscles.

Memory Aids

• Cranial nerves = 12 pairs from brain; Spinal nerves = 31 pairs from spinal cord.
• Autonomic = Involuntary control; Somatic = Voluntary control.
• Sympathetic = Thoracolumbar outflow; Parasympathetic = Craniosacral outflow.
• Sympathetic activates stress responses; Parasympathetic opposes it.

Quick Checkpoints

• What are the two types of peripheral nerves?
• How many pairs of cranial and spinal nerves exist?
• Which nerves are myelinated in PNS?
• What are the functions of sympathetic and parasympathetic systems?
• What does the somatic neural system control?

Neuron as Structural and Functional Unit of Neural System

Structure and Function

• A neuron has three parts: cell body, dendrites, and axon.
• Cell body contains cytoplasm, organelles, and Nissl's granules (rough ER with ribosomes) for protein synthesis.
• Dendrites are short, branched fibers transmitting impulses toward the cell body; they also contain Nissl's granules.
• Axon is a long fiber with branched distal ends ending in synaptic knobs containing synaptic vesicles with neurotransmitters.
• Axons transmit impulses away from the cell body to synapses or neuromuscular junctions.

Memory Aids

• Neuron parts: Cell body, Dendrites (toward), Axon (away).
• Nissl's granules = protein synthesis sites.
• Synaptic knob = neurotransmitter release point.

Quick Checkpoints

• What are the three main parts of a neuron?
• Where are Nissl's granules found and their function?
• Direction of impulse transmission in dendrites and axons?

Types of Neurons

Classification

• Based on axon and dendrites:
  • Unipolar: One axon, found in embryo.
  • Bipolar: One axon, one dendrite, found in retina.
  • Multipolar: One axon, multiple dendrites, found in cerebral cortex.
• Pseudounipolar: One axon divides into two, e.g., dorsal root ganglion.
• Non-polar: All processes equal, impulse conduction in any direction, e.g., coelenterates.
• Based on function:
  • Afferent (sensory): Carry impulses from receptors to CNS; dendrites form receptors.
  • Efferent (motor): Carry impulses from CNS to effectors; axon terminals form neuromuscular junction.

Memory Aids

• Unipolar = embryo, Bipolar = retina, Multipolar = brain.
• Afferent = arrive CNS, Efferent = exit CNS.

Quick Checkpoints

• How are neurons classified by structure?
• What is the function of afferent and efferent neurons?
• Examples of pseudounipolar and non-polar neurons?

Types of Axons

Classification and Features

• Two types: myelinated and non-myelinated.
• Myelinated axons are wrapped by Schwann cells forming myelin sheath with nodes of Ranvier; found in spinal and cranial nerves.
• Non-myelinated axons are enclosed by Schwann cells without myelin sheath; found in autonomic and somatic systems.

Memory Aids

• Myelinated = fast, nodes of Ranvier aid impulse speed.
• Non-myelinated = slow, no myelin sheath.

Quick Checkpoints

• What is the role of Schwann cells?
• Where are myelinated and non-myelinated axons found?
• What are nodes of Ranvier?

Generation and Conduction of Nerve Impulse

Resting State and Ionic Gradients

• Neurons are excitable due to polarized membranes with selective ion channels.
• At rest, membrane is more permeable to K⁺, less to Na⁺; inside axon has high K⁺ and negative proteins, outside has high Na⁺.
• Sodium-potassium pump maintains gradients by pumping 3 Na⁺ out and 2 K⁺ in.
• Resting potential is about -70 mV (inside negative, outside positive).

Action Potential and Impulse Conduction

• Stimulus opens Na⁺ channels, Na⁺ influx reverses polarity (depolarization) creating action potential (+30 to +45 mV).
• Current flows to adjacent membrane, triggering action potential there; impulse moves along axon.
• After Na⁺ influx, K⁺ channels open, K⁺ efflux restores resting potential (repolarization).
• Conduction is unidirectional; refractory period prevents backward impulse.
• In myelinated axons, impulse jumps node to node (saltatory conduction), faster than continuous conduction in non-myelinated axons.

Memory Aids

• Resting potential = -70 mV, action potential = +30 to +45 mV.
• Na⁺ in = depolarization, K⁺ out = repolarization.
• Saltatory conduction = jump nodes, faster impulse.

Quick Checkpoints

• What ions are involved in resting and action potentials?
• How does the sodium-potassium pump maintain resting potential?
• What is saltatory conduction?

Transmission of Impulses

Synapses and Types

• Impulse transmission occurs at synapses between neurons.
• Synapse = junction of pre-synaptic and post-synaptic neurons, may have synaptic cleft.
• Two types: electrical (direct current flow, rare) and chemical (neurotransmitter-mediated).

Chemical Synapse Mechanism

• Axon terminals contain vesicles with neurotransmitters (excitatory: acetylcholine; inhibitory: GABA).
• Impulse arrival causes vesicles to fuse with membrane, releasing neurotransmitters into synaptic cleft.
• Neurotransmitters bind receptors on post-synaptic membrane, opening ion channels to generate new potential.
• New potential can be excitatory or inhibitory.
• Impulse transmission is slower than electrical synapses.

Memory Aids

• Synapse = signal transfer point.
• Neurotransmitters: acetylcholine (excitatory), GABA (inhibitory).
• Chemical synapse slower than electrical.

Quick Checkpoints

• What are the two types of synapses?
• How do neurotransmitters transmit impulses?
• Difference between electrical and chemical synapses?

Central Neural System

Overview

• The brain is the central processing organ, controlling voluntary movements, balance, vital involuntary organs, thermoregulation, hunger, thirst, circadian rhythms, endocrine activities, and behavior.
• Processes vision, hearing, speech, memory, intelligence, emotions, and thoughts.
• Protected by the skull and covered by cranial meninges: dura mater (outer), arachnoid (middle), pia mater (inner).
• Divided into forebrain, midbrain, and hindbrain.
• Cerebrospinal fluid (CSF) cushions the brain, present in ventricles and subarachnoid space.

Memory Aids

• Brain parts: Forebrain (thinking), Midbrain (relay), Hindbrain (balance & vital functions).
• Meninx layers: Dura (durable), Arachnoid (spider-like), Pia (delicate).

Quick Checkpoints

• What are the three main parts of the brain?
• Which meninges layer is the thinnest?
• What is the role of cerebrospinal fluid?

Brain

Structure and Functions

• Forebrain includes olfactory lobes, cerebrum, and diencephalon.
• Cerebrum: largest part, two hemispheres connected by corpus callosum, cerebral cortex with gyri and sulci.
• Diencephalon: epithalamus (melatonin secretion), thalamus (sensory relay except smell), hypothalamus (controls hunger, thirst, hormones).
• Limbic system (amygdala, hippocampus) regulates emotions, sexual behavior, motivation.
• Midbrain: between forebrain and hindbrain, contains cerebral aqueduct and corpora quadrigemina (superior colliculi for sight, inferior colliculi for hearing).
• Hindbrain: pons (fiber tracts, breathing regulation), cerebellum (movement coordination, balance), medulla oblongata (controls respiration, cardiovascular reflexes).
• Brain stem (midbrain, pons, medulla) connects brain to spinal cord, entry/exit for 10 cranial nerves.
• Spinal cord: extends from medulla to second lumbar vertebra, conducts impulses, controls reflexes, has grey (inner) and white (outer) matter, ends as conus medullaris with filum terminale.

Memory Aids

• Forebrain = Olfactory + Cerebrum + Diencephalon.
• Corpora quadrigemina: Superior = Sight, Inferior = Hearing.
• Brain stem = Midbrain + Pons + Medulla.
• Limbic system controls emotions and motivation.

Quick Checkpoints

• What are the main parts of the forebrain?
• Which part of the brain controls balance and coordination?
• What is the function of the thalamus?
• Where is the cerebral aqueduct located?
• What does the medulla oblongata regulate?
• What connects the two cerebral hemispheres?
• What is the role of the spinal cord?