The Nervous System

Overview

The nervous system is the primary controlling and communicating system of the human body. It is responsible for receiving sensory input, processing information, and coordinating responses throughout the body. The nervous system is essential for maintaining homeostasis, enabling movement, and supporting higher functions such as thought and emotion.

• Central Nervous System (CNS): Composed of the brain and spinal cord, the CNS acts as the main processing center for the entire nervous system.

• Peripheral Nervous System (PNS): Consists of nerves and ganglia outside the CNS, connecting the CNS to limbs and organs.

• The CNS and PNS work together as a single, coordinated unit.

Main Components of the Human Nervous System

• Brain: The control center for processing sensory information, generating thoughts, and directing responses.

• Spinal Cord: Transmits signals between the brain and the rest of the body; involved in reflex actions.

• Nerves: Bundles of axons that carry electrical impulses to and from the CNS.

Anatomical Divisions of the Nervous System

Central Nervous System (CNS)

The CNS includes the brain and spinal cord. It is responsible for integrating sensory information, generating thoughts, and directing motor responses.

• Brain: Contains billions of neurons and is the site of higher cognitive functions.

• Spinal Cord: Acts as a conduit for signals between the brain and peripheral nerves; also coordinates reflexes.

Peripheral Nervous System (PNS)

The PNS consists of nerves and ganglia that connect the CNS to the rest of the body. It is divided into sensory and motor divisions.

• Cranial Nerves: Emerge from the brain and innervate the head and neck.

• Spinal Nerves: Emerge from the spinal cord and innervate the trunk and limbs.

• Ganglia: Clusters of neuron cell bodies located outside the CNS.

Functional Organization of the Nervous System

Functions of the Nervous System

The nervous system performs three main functions:

1. Sensory Input: Receives stimuli from specialized receptors throughout the body.

2. Integration: Processes and interprets sensory input, deciding what action is needed.

3. Motor Output: Causes a response by activating effector organs (muscles or glands).

Functional Divisions of the PNS

• Sensory (Afferent) Division: Transmits sensory information from receptors to the CNS.

  • Somatic Afferents: Carry impulses from skin, skeletal muscles, tendons, and joints.

  • Visceral Afferents: Carry impulses from internal organs.

• Motor (Efferent) Division: Transmits motor commands from the CNS to effector organs.

  • Somatic Nervous System: Controls voluntary movements by activating skeletal muscles.

  • Autonomic Nervous System: Controls involuntary responses by regulating cardiac muscle, smooth muscle, and glands.

Autonomic Nervous System (ANS)

The ANS is further divided into:

• Sympathetic Division: Prepares the body for 'fight or flight' responses; increases heart rate, dilates pupils, inhibits digestion.

• Parasympathetic Division: Promotes 'rest and digest' activities; slows heart rate, stimulates digestion, constricts pupils.

These divisions often innervate the same organs but produce opposite effects.

Histology of Nervous Tissue

Cell Types in Nervous Tissue

• Neurons: Excitable cells that transmit electrical signals.

  • Extreme longevity; most neurons last a lifetime.

  • Amitotic; most do not divide after development.

  • High metabolic rate; require constant supply of oxygen and glucose

• Neuroglia (Glial Cells): Support, protect, and nourish neurons.

  • More numerous than neurons.

  • Include several types with specialized functions.

Structure of a Neuron

• Cell Body (Soma): Contains the nucleus and organelles; site of metabolic activity.

• Dendrites: Short, branched extensions that receive and transmit signals toward the cell body; not myelinated.

• Axon: Long, cylindrical projection that transmits signals away from the cell body; may be myelinated for faster transmission.

Neuron Classification

Structural Classification

• Anaxonic Neurons: Found in the brain and sensory organs; no obvious axon.

• Bipolar Neurons: Found in special sensory organs (e.g., retina, olfactory epithelium).

• Unipolar Neurons: Found in sensory part of the PNS.

• Multipolar Neurons: Common in the CNS; include all skeletal muscle motor neurons.

Functional Classification

• Sensory (Afferent) Neurons: Transmit impulses from sensory receptors to the CNS.

• Motor (Efferent) Neurons: Transmit impulses from the CNS to effectors (muscles or glands).

• Interneurons (Association Neurons): Connect sensory and motor neurons within the CNS; involved in higher functions such as memory and learning.

Synapses

Types of Synapses

• Neuro-neural Junction: Synapse between two neurons.

• Neuro-muscular Junction: Synapse between a neuron and a muscle cell.

• Neuro-glandular Junction: Synapse between a neuron and a gland cell.

Structure of a Synapse

• Presynaptic Cell: The neuron sending the signal.

• Postsynaptic Cell: The cell receiving the signal (neuron, muscle, or gland).

• Synaptic Cleft: The small gap between the presynaptic and postsynaptic membranes.

Neuroglia (Glial Cells)

Types of Neuroglia in the CNS

• Ependymal Cells: Line the central canal of the spinal cord and ventricles of the brain; secrete and circulate cerebrospinal fluid (CSF); contain stem cells for repair.

• Microglia: Act as phagocytes; clean up debris, waste, and pathogens.

• Astrocytes: Maintain the blood-brain barrier; provide structural support; repair damaged tissue; guide neuron development; control the interstitial environment.

• Oligodendrocytes: Form myelin sheaths around axons in the CNS.

Types of Neuroglia in the PNS

• Schwann Cells (Neurolemmocytes): Form myelin sheaths around axons in the PNS; gaps between Schwann cells are called nodes of Ranvier.

• Satellite Cells: Surround neuron cell bodies in ganglia; regulate the environment around neurons.

Myelination

Myelinated vs. Unmyelinated Axons

• Myelinated Axons: Have a myelin sheath that increases the speed of impulse transmission.

• Unmyelinated Axons: Lack a myelin sheath; transmit impulses more slowly.

Myelination Process

• Occurs during fetal development and the first year of life.

• Myelinating cells wrap around axons, forming multiple layers of membrane.

• The outermost layer in the PNS is called the neurolemma.

Gray Matter and White Matter

• Gray Matter: Contains neuron cell bodies and unmyelinated processes; found in the CNS.

• White Matter: Contains myelinated axons; organized into fiber tracts in the CNS.

Table: Comparison of CNS and PNS

Table: Types of Neurons

Key Equations

• Resting Membrane Potential: Where is the membrane potential.

• Nerve Impulse Transmission Speed: Where is speed, is distance, is time.

Example: Reflex Arc

A reflex arc is a simple neural pathway involving a sensory neuron, interneuron, and motor neuron. For example, touching a hot object activates sensory neurons, which send signals to the spinal cord. Interneurons process the information and immediately activate motor neurons, causing muscle contraction to withdraw the hand.

Additional info: The notes have been expanded to include definitions, examples, and tables for clarity and completeness.