Introduction to the Human Nervous System

The human nervous system is a complex network that coordinates and controls bodily functions through the transmission of electrical signals. It is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS).

• Central Nervous System (CNS): Comprising the brain and spinal cord, the CNS is responsible for processing information and coordinating responses. It plays a critical role in higher functions like cognition, emotions, and memory.

• Peripheral Nervous System (PNS): This system connects the CNS to the limbs and organs. It includes sensory neurons that carry information to the CNS and motor neurons that relay commands from the CNS to muscles and glands.

Reflex Arc

The reflex arc is a neural pathway that mediates a reflex action. A flow chart for the pathway of a reflex arc can be structured as follows:

1. Stimulus

   • e.g., touching a hot surface↓

2. Sensory Receptor

   • Detects the stimulus↓

3. Sensory Neuron

   • Sends signal to the spinal cord↓

4. Interneuron (in spinal cord)

   • Processes information, relays signal↓

5. Motor Neuron

   • Sends signal to muscles↓

6. Effector (muscle)

   • Produces a response (e.g., pulling away)

Types of Receptors and Their Functions

1. Mechanoreceptors: Detect mechanical pressure or distortion (e.g., touch, sound).

2. Thermoreceptors: Sense changes in temperature (e.g., warm and cold sensations).

3. Photoreceptors: Respond to light; primarily involved in vision (e.g., rods and cones in the retina).

4. Chemoreceptors: Detect chemical changes (e.g., taste and smell).

5. Nociceptors: Respond to pain stimuli, indicating potential tissue damage.

Neuron

Neurons are specialized cells in the nervous system responsible for transmitting electrical signals. Their structure includes three main parts:

• Cell Body (Soma): Contains the nucleus and organelles; integrates signals received from other neurons.

• Dendrites: Branch-like extensions that receive signals from other neurons and transmit them to the cell body.

• Axon: A long projection that transmits electrical impulses away from the cell body to other neurons or muscles.

Neurons function by receiving information, processing it, and communicating with other neurons through synapses, which are junctions that facilitate signal transmission.

Types of Neuron

1. Sensory Neurons: Carry sensory information from receptors to the central nervous system (CNS).

2. Motor Neurons: Transmit commands from the CNS to muscles and glands.

3. Interneurons: Connect sensory and motor neurons within the CNS and facilitate communication between them.

Cells of the Nervous System

The human nervous system is composed of specialized cells that play crucial roles in its functioning:

• Neurons: Responsible for transmitting electrical signals.

• Supporting Cells (Glial Cells): Provide support, nourishment, and protection to neurons. They include:

  • Astrocytes: Maintain the blood-brain barrier and provide metabolic support.

  • Oligodendrocytes: Insulate axons in the central nervous system (forms myelin).

  • Microglia: Act as immune cells, responding to injury or disease.

  • Schwann Cells: Insulate axons in the peripheral nervous system.

Nerve Impulses

• Resting Potential: Typically around -70 mV (millivolts), indicating a negative charge inside the neuron compared to the outside.

• Threshold Potential: About -55 mV, the level that must be reached for an action potential to occur.

• Action Potential Peak: Usually around +30 mV, where the inside of the neuron becomes positively charged.

• Repolarization: The return to resting potential often occurs within milliseconds, typically lasting about 1-2 milliseconds per action potential.

Ion Movement During Action Potential

• Sodium (Na+) Entry: Rapid influx of Na+ ions occurs when voltage-gated sodium channels open, leading to depolarization.

• Potassium (K+) Exit: Following the peak of the action potential, K+ ions move out of the neuron, contributing to repolarization.

Synapse

A synapse is a junction between two neurons that allows for the transmission of signals. It is essential for communication within the nervous system. The process of signaling at a synapse involves the release of neurotransmitters from the presynaptic neuron, which then bind to receptors on the postsynaptic neuron, leading to either excitatory or inhibitory effects.

Types of Synapses

1. Chemical Synapse:

   • Most common type.

   • Involves the release of neurotransmitters from the axon terminal of the presynaptic neuron.

   • Signals are transmitted across the synaptic cleft to the postsynaptic neuron.

   • Can generate excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs).

2. Electrical Synapse:

   • Involves direct electrical communication between neurons.

   • Neurons are connected by gap junctions that allow for rapid transmission of ionic currents.

   • More common in invertebrates; found in some vertebrate tissues for synchronized activity (e.g., in cardiac muscles).

Neurotransmitter

Neurotransmitters are chemical messengers used by neurons to communicate with each other and with other types of cells. They play a crucial role in transmitting signals across the synapse from the presynaptic neuron to the postsynaptic neuron.

Types of Neurotransmitters:

1. Acetylcholine: Involved in muscle contraction and memory formation; it can be both excitatory and inhibitory.

2. Dopamine: Plays a key role in the brain's reward system and is involved in regulating mood, attention, and movement. It is generally excitatory.

3. Serotonin: Impacts mood, emotion, and sleep regulation; it has a calming effect and is considered inhibitory.

4. Norepinephrine: Involved in the body's fight-or-flight response, affecting attention and responding actions; it can be both excitatory and inhibitory.

5. Gamma-Aminobutyric Acid (GABA): The primary inhibitory neurotransmitter in the brain, it helps to reduce neuronal excitability.

6. Glutamate: The main excitatory neurotransmitter in the central nervous system, it plays crucial roles in learning and memory.

Central Nervous System (CNS)

Human Brain

The human brain can be further divided into several important subdivisions within its key regions:

1. Cerebrum: The largest part of the brain, involved in higher brain functions such as thought, action, and emotion.

   • Frontal Lobe:

     • Prefrontal Cortex: Involved in executive functions such as decision-making, problem-solving, and complex planning.

     • Motor Cortex: Controls voluntary movements of the body.

   • Parietal Lobe:

     • Somatosensory Cortex: Processes sensory information from the body, allowing us to perceive touch, pressure, and pain.

   • Temporal Lobe:

     • Auditory Cortex: Processes sound information.

     • Wernicke's Area: Important for language comprehension.

   • Occipital Lobe:

     • Visual Cortex: Responsible for processing and interpreting visual stimuli such as shapes, colors, and motion.

2. Cerebellum: Located under the cerebrum, responsible for

   • Coordination of Movement: Integrates sensory information to fine-tune motor activity.

   • Balance and Posture: Helps maintain equilibrium during movement.

   • Cognitive Functions: Recent research suggests it also plays a role in cognitive processes like attention and language.

3. Brainstem: Connects the brain to the spinal cord and controls essential functions.

   • Midbrain: Involved in vision, hearing, and motor control; contains pathways that relay information to higher centers.

   • Pons: Acts as a bridge between different parts of the brain; involved in regulating sleep and arousal.

   • Medulla Oblongata: Controls autonomic functions such as heart rate, breathing, and blood pressure.

4. Limbic System: Involved in emotions and memory.

   • Amygdala: Key role in processing emotions such as fear and pleasure.

   • Hippocampus: Essential for forming new memories and learning; involved in spatial navigation.

   • Thalamus: Acts as a relay station for sensory and motor signals to the cerebral cortex; involved in consciousness and alertness.

   • Hypothalamus: Regulates autonomic functions, controlling temperature, hunger, thirst, and circadian rhythms.

   • Cingulate Gyrus: Involved in processing emotions and regulating behavior.

Spinal Cord

The spinal cord is a cylindrical structure protected by the vertebral column, extending from the base of the brain to the lower back. It serves several critical functions:

• Conduction Pathway: The spinal cord acts as the main pathway for transmitting information between the brain and the rest of the body.

• Reflex Center: It mediates reflex actions, allowing for quick responses to stimuli without the need for direct involvement of the brain.

• Organization: It consists of segments that correspond to different regions of the body, known as spinal nerves, which branch off to communicate with various body parts.

Zones of the Spinal Cord:

The spinal cord is organized into segments, each corresponding to specific regions of the body. These segments branch off into spinal nerves for communication with various body parts. The main zones include:

• Cervical Zone: Controls upper limbs and neck.

• Thoracic Zone: Connects to the torso and upper limbs.

• Lumbar Zone: Related to lower limbs and pelvic organs.

• Sacral Zone: Governs pelvic and lower limb functions.

Peripheral Nervous System (PNS)

• The PNS connects the central nervous system (CNS) to the limbs and organs.

• It includes sensory neurons that carry information to the CNS and motor neurons that relay commands from the CNS to muscles and glands.

Divisions of the Peripheral Nervous System:

1. Somatic Nervous System:

   • Controls voluntary movements by innervating skeletal muscles.

   • Involves sensory receptors that send information about the external environment to the CNS.

2. Autonomic Nervous System (ANS):

   • Regulates involuntary bodily functions such as heart rate, digestion, and respiratory rate.

   • Divided into two branches:

     • Sympathetic Nervous System: Responsible for the body's fight-or-flight response, preparing the body for stressful situations.

     • Parasympathetic Nervous System: Responsible for maintaining normal body functions and conserving energy; it promotes the rest-and-digest response.