The Nervous System: Structure and Function

The Nervous System

Overview and Functions

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals to and from different parts of the body. It is essential for sensation, integration, and response to internal and external stimuli.

• Sensory Input: The process of gathering information from sensory receptors that monitor changes (stimuli) occurring inside and outside the body.

• Integration: The nervous system processes and interprets sensory input and decides if action is needed.

• Motor Output: The response or effect that activates muscles or glands (effectors).

• Example: Touching a hot surface (stimulus) is detected by sensory receptors, processed by the brain, and results in the withdrawal of the hand (motor output).

Classification of the Nervous System

Central Nervous System (CNS)

The CNS serves as the command center of the body, consisting of the brain and spinal cord. It interprets incoming sensory information and issues instructions based on past experience and current conditions.

• Brain: The main control center for processing information and directing responses.

• Spinal Cord: Conducts signals to and from the brain and controls reflex activities.

Peripheral Nervous System (PNS)

The PNS consists of nerves extending from the brain and spinal cord. It acts as communication lines linking all parts of the body to the CNS.

• Spinal Nerves: Carry impulses to and from the spinal cord.

• Cranial Nerves: Carry impulses to and from the brain.

• Function: Connects sensory organs, muscles, and glands to the CNS.

Functional Subdivisions of the PNS

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

• Motor (Efferent) Division: Transmits commands from the CNS to effectors (muscles and glands).

• Somatic Nervous System: Controls voluntary movements of skeletal muscles.

• Autonomic Nervous System: Regulates involuntary activities of smooth muscle, cardiac muscle, and glands. Subdivided into:

  • Sympathetic Division: Prepares the body for 'fight or flight' responses.

  • Parasympathetic Division: Promotes 'rest and digest' activities.

Nervous Tissue: Supporting Cells (Neuroglia)

Types of Neuroglia in the CNS

• Astrocytes: Most abundant; support neurons, maintain the blood-brain barrier, and regulate the chemical environment.

• Microglial Cells: Act as phagocytes, removing debris and defending against pathogens.

• Ependymal Cells: Line cerebrospinal fluid-filled cavities and help circulate cerebrospinal fluid.

• Oligodendrocytes: Produce myelin sheaths around CNS nerve fibers, increasing the speed of impulse transmission.

Types of Neuroglia in the PNS

• Satellite Cells: Surround neuron cell bodies in the PNS, providing support and regulating the environment.

• Schwann Cells: Form myelin sheaths around peripheral nerve fibers, aiding in rapid signal conduction.

Nervous Tissue: Neurons

Structure of Neurons

Neurons are specialized cells that transmit electrical impulses throughout the body. They have unique structures adapted for their function.

• Cell Body: Contains the nucleus and metabolic center of the neuron.

• Processes: Extensions from the cell body, including:

  • Dendrites: Conduct impulses toward the cell body; neurons may have many dendrites.

  • Axon: Conducts impulses away from the cell body; each neuron has a single axon that may branch at its end (axon terminals).

• Myelin Sheath: Fatty material covering axons, providing insulation and increasing the speed of impulse transmission.

• Nodes of Ranvier: Gaps in the myelin sheath that facilitate rapid conduction of nerve impulses.

Neuron Classification by Structure

• Multipolar Neurons: Many processes extend from the cell body; most common type; includes motor and interneurons.

• Bipolar Neurons: One axon and one dendrite; found in special sensory organs (e.g., retina).

• Unipolar Neurons: Single process that splits into two branches; primarily sensory neurons in the PNS.

Neuron Classification by Function

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

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

• Interneurons (Association Neurons): Connect sensory and motor neurons within the CNS.

Neural Communication: The Synapse

The synapse is the functional junction between two neurons or between a neuron and an effector cell. Neurotransmitters are released from axon terminals to transmit signals across the synaptic gap.

Neural Impulse Transmission

Resting Membrane Potential

• The resting membrane is polarized, with the external face slightly positive and the internal face slightly negative.

• The chief extracellular ion is sodium ($Na^+$), and the chief intracellular ion is potassium ($K^+$).

• The membrane is relatively impermeable to both ions at rest.

Action Potential Generation and Propagation

• Depolarization: A stimulus changes the membrane's permeability, allowing $Na^+$ to enter the cell, reversing the membrane polarity locally.

• Action Potential: If the stimulus is strong enough, depolarization spreads, and an action potential is initiated.

• Propagation: The action potential travels along the length of the neuron as adjacent areas of the membrane depolarize.

• Repolarization: $K^+$ ions diffuse out, restoring the negative charge inside the membrane and positive charge outside.

Equation for Resting Membrane Potential:

$V_m = V_{inside} - V_{outside}$

Where $V_m$ is the membrane potential, typically around -70 mV in neurons.

Receptors and Effectors

• Sensory Receptors: Specialized to detect specific types of stimuli (e.g., pain, temperature, touch, pressure, proprioception).

• Effectors: Muscles or glands that carry out the response to nervous system signals.

• Examples:

  • Free nerve endings: Pain and temperature receptors

  • Meissner’s corpuscle: Touch receptor

  • Lamellar corpuscle: Deep pressure receptor

  • Muscle spindle and tendon organ: Proprioceptors (sense body position and movement)

Summary Table: Major Divisions of the Nervous System

Division	Main Components	Function
Central Nervous System (CNS)	Brain, Spinal Cord	Integration, command center, interprets sensory input, issues instructions
Peripheral Nervous System (PNS)	Cranial and Spinal Nerves	Communication lines between CNS and rest of body
Sensory (Afferent) Division	Sensory receptors, afferent nerves	Transmits sensory information to CNS
Motor (Efferent) Division	Somatic and Autonomic systems	Transmits commands from CNS to effectors
Somatic Nervous System	Skeletal muscles	Voluntary control
Autonomic Nervous System	Smooth muscle, cardiac muscle, glands	Involuntary control (sympathetic and parasympathetic)