Human Nervous System - CNS, Spinal cord

1. Anatomical Terminology Overview

• Directional Terms:
  • Anterior: Refers to the front side of the body.
  • Posterior: Refers to the back side of the body.
  • Superior: Positioned above or towards the head.
  • Inferior: Positioned below or towards the feet.
  • Medial: Closer to the midline of the body.
  • Lateral: Farther from the midline of the body.
  • Proximal: Closer to the trunk or the center of the body.
  • Distal: Farther away from the trunk or the center of the body.
• Relative Positioning Terms:
  • Ipsilateral: Refers to structures on the same side of the body (e.g., left arm and left leg).
  • Contralateral: Refers to structures on opposite sides of the body (e.g., left arm and right leg).
• Anatomical Planes:
  • Sagittal Plane: Divides the body into left and right parts.
  • Coronal (Frontal) Plane: Divides the body into anterior (front) and posterior (back) parts.
  • Transverse (Horizontal) Plane: Divides the body into superior (upper) and inferior (lower) parts.

2. Central Nervous System (CNS) vs. Peripheral Nervous System (PNS)

• Nervous System Overview:
  • The human nervous system is divided into two main subsystems:
    • Central Nervous System (CNS): Consists of the brain and spinal cord. Represented in gray in illustrations.
    • Peripheral Nervous System (PNS): Includes all nerves outside the CNS, represented in yellow.
• Differences between CNS and PNS:
  • CNS:
    • Located in the superior part of the body.
    • Encased and protected by bones (the skull for the brain and vertebrae for the spinal cord).
    • Responsible for processing and integrating information.
  • PNS:
    • Extends from the CNS to the limbs and other body parts.
    • Involves nerves that transmit sensory and motor signals between the CNS and the rest of the body.
    • Consists of two subsystems: Autonomic Nervous System (ANS) and Somatic Nervous System (SNS).

3. Peripheral Nervous System (PNS) Subsystems

• Autonomic Nervous System (ANS):
  • Involuntary system that regulates internal organs and glands without conscious control.
  • Divided into:
    • Sympathetic Nervous System: Activated in stress ("fight or flight" response).
    • Parasympathetic Nervous System: Activated in relaxation ("rest and digest" response).
• Somatic Nervous System (SNS):
  • Voluntary system that controls skeletal muscles.
  • Allows conscious control over movements (e.g., walking, picking up objects).

4. Structure and Function of the Spinal Cord

• Spinal Cord Anatomy:
  • Part of the Central Nervous System.
  • Located within the vertebral column.
  • Acts as a conduit for signals between the brain and the peripheral nerves.
  • Facilitates reflex actions independently of the brain.
• Functional Regions of the Spinal Cord:
  • Gray Matter: Contains neuron cell bodies, dendrites, and synapses.
    • Involved in reflexes and interneuron communication.
  • White Matter: Composed of myelinated axons.
    • Responsible for transmitting signals to and from the brain.

5. Role of the Nervous System in Body Function

• CNS (Central Nervous System):
  • Processes sensory information.
  • Coordinates voluntary and involuntary actions.
  • Integrates data from various parts of the body and generates appropriate responses.
• PNS (Peripheral Nervous System):
  • Transmits sensory information to the CNS.
  • Carries motor commands from the CNS to muscles and glands.
  • Divided into Somatic (voluntary control) and Autonomic (involuntary control) subsystems.

6. Key Points for Understanding the Nervous System

• Remember the anatomical terms for discussing body orientation and positioning.
• Understand the different planes used to view and analyze anatomical structures.
• Recognize the roles and differences between the CNS and PNS.
• Differentiate between the Autonomic Nervous System (involuntary) and the Somatic Nervous System (voluntary).
• Appreciate how the CNS (brain and spinal cord) is protected and functions to process and relay information throughout the body.

CNS Overview

• The Central Nervous System (CNS) is composed of the brain and the spinal cord.
• The CNS is protected by a three-part protection system: the meninges, the cerebrospinal fluid (CSF), and the bony structures (cranium and vertebrae).

Meninges

• The meninges are three protective layers that cover the brain and spinal cord:
  1. Dura Mater: The outermost, tough layer that provides primary protection.
  2. Arachnoid Mater: The middle layer, which has a spiderweb-like appearance. The space beneath it, called the subarachnoid space, contains CSF.
  3. Pia Mater: The innermost, delicate layer that closely adheres to the surface of the brain and spinal cord.
• These three layers together form the protective covering of the brain and spinal cord.

Cerebrospinal Fluid (CSF)

• Cerebrospinal Fluid (CSF) is a clear, colorless fluid that:
  • Provides physical protection by acting as a cushion for the brain and spinal cord.
  • Offers chemical protection by maintaining a stable environment, free of proteins and blood cells.
  • CSF is produced by the choroid plexus in the brain's ventricles at a rate of approximately 500 mL per day.
  • It circulates through the ventricles, the central canal of the spinal cord, and the subarachnoid space.
  • CSF is absorbed into the blood through structures called arachnoid villi.

 

Brain Ventricles

• The ventricles are interconnected cavities within the brain filled with CSF:
  • Two lateral ventricles (right and left)
  • Third ventricle
  • Fourth ventricle
• These ventricles are involved in the production and circulation of CSF, which provides cushioning and nutrients to the brain and spinal cord.

Spinal Cord Structure

• The spinal cord is an extension of the CNS and is protected by the same meninges (dura mater, arachnoid mater, and pia mater) as the brain.
• It consists of two types of matter:
  • Gray Matter: Located in the center, containing neuron cell bodies, dendrites, and unmyelinated axons. It forms an "H" or butterfly shape.
  • White Matter: Surrounds the gray matter, consisting of myelinated axons that form tracts to transmit signals to and from the brain.

Spinal Cord Function

• The spinal cord is responsible for:
  • Relaying sensory and motor information between the body and the brain.
  • Controlling reflexes that require quick responses, such as the withdrawal reflex (e.g., stepping on a sharp object).

Spinal Nerves and Roots

• Spinal nerves are 31 pairs of nerves that emerge from the spinal cord:
  • Dorsal Root: Contains sensory neurons that carry information from the body to the CNS.
  • Ventral Root: Contains motor neurons that carry commands from the CNS to muscles.
• Dorsal Root Ganglion: A cluster of sensory neuron cell bodies located in the dorsal root.

Reflex Arcs

• Reflex actions, such as the knee-jerk reflex, are processed at the level of the spinal cord and do not require input from the brain.
• Reflex arcs involve:
  • Sensory Neurons: Detect stimuli and transmit signals to the spinal cord.
  • Interneurons: Located in the spinal cord, connect sensory neurons to motor neurons.
  • Motor Neurons: Carry signals from the spinal cord to muscles to initiate movement.

CNS vs. Peripheral Nervous System (PNS)

• CNS: Comprises the brain and spinal cord.
• PNS: Consists of nerves and ganglia outside the CNS, including:
  • Somatic Nervous System: Controls voluntary movements and conveys sensory information.
  • Autonomic Nervous System (ANS): Regulates involuntary body functions and is divided into:
    • Sympathetic Nervous System: Activates 'fight or flight' response.
    • Parasympathetic Nervous System: Promotes 'rest and digest' state.

Cranial Nerves

• There are 12 pairs of cranial nerves that emerge directly from the brain:
  • Cranial Nerve I: Olfactory nerve - sense of smell.
  • Cranial Nerve II: Optic nerve - vision.
  • Cranial Nerves III to XII: Include various nerves controlling eye movements, facial muscles, hearing, balance, taste, and functions related to the head and neck.