Overview of the Nervous System

Overview of the Nervous System

Structure of the Nervous System

  • The nervous system can generally be divided into two main parts:

    • Central Nervous System (CNS)

    • Comprises the brain and the spinal cord.

    • Peripheral Nervous System (PNS)

    • Consists of the somatic nervous system and autonomic nervous system.

Central Nervous System (CNS)

  • **Components: **

    • Brain

    • Spinal Cord

Peripheral Nervous System (PNS)

  • Components:

    • Somatic Nervous System

    • Connects to skeletal muscles, allowing voluntary movement.

    • Autonomic Nervous System

    • Regulates internal organs and bodily activities automatically.

Autonomic Nervous System Branches

  • Sympathetic Nervous System:

    • Increases arousal, preparing the body for survival-related actions.

    • Associated with the four F's: fighting, fleeing, feeding, mating.

    • Example: Increases heart rate and blood flow to muscles when faced with danger.

  • Parasympathetic Nervous System:

    • Counteracts the sympathetic system; calms the body and reduces arousal.

    • Returns body to resting state after danger has passed, e.g., slows heart rate, redirects blood to digestive organs.

    • Both systems operate in coordination rather than as completely separate systems.

Nerves and Nerve Structure

  • Nerves:

    • Defined as bundles of axons similar to electrical cables.

    • Held together by connective tissue and contain blood vessels providing nutrients and oxygen.

Somatic vs. Autonomic Nerves

  • Somatic Nerves:

    • Innervate muscles; consist of sensory neurons that send signals from muscles to spinal cord and motor neurons that send signals from the spinal cord to muscles.

  • Autonomic Nerves:

    • Automatically regulate internal body functions without voluntary control.

Terminology Related to the Neuraxis

  • Neuraxis:

    • Central axis of the nervous system comprising the brain and spinal cord.

    • Terminology based on the neuraxis includes:

    • Rostral (Anterior): Towards the front of the brain or top of the spinal cord.

    • Caudal (Posterior): Towards the back of the brain or bottom of the spinal cord.

    • Dorsal: Above the neuraxis (e.g., top of the brain); towards the back of the spinal cord.

    • Ventral: Below the neuraxis (e.g., bottom of the brain); towards the front of the spinal cord.

    • Lateral: Towards the side of the structure.

    • Medial: Towards the center.

Relationship of Terms in Humans vs. Other Animals

  • In humans:

    • The neuraxis is bent due to upright posture; terminology still applied but may refer to different orientations, e.g.,

    • Rostral: Front/head

    • Caudal: Back/feet

    • Dorsal: Top of the head

    • Ventral: Bottom of the head

Structure of the Spinal Cord

  • The spinal cord is housed within the spinal column, which consists of multiple vertebrae connected by flexible cartilage.

  • Vertebrae Composition:

    • Cervical Vertebrae (C1-C6): Located in the neck.

    • Thoracic Vertebrae: 12 vertebrae in the chest region.

    • Lumbar Vertebrae: 5 vertebrae in the waist area.

    • Sacral Vertebrae: 5 fused vertebrae forming a single bone.

Spinal Cord Structure in Cross-Section

  • The spinal cord contains a butterfly-shaped area of gray matter (cell bodies) surrounded by white matter (myelinated axons).

  • Dorsal Root: Enters the spinal cord; contains sensory neuron cell bodies located outside the spinal cord, known as a dorsal root ganglion.

  • Ventral Root: Contains motor neuron cell bodies inside the spinal cord, exits through the ventral horn to form the spinal nerve, which contains both sensory and motor neurons.

Clinical Application: Patient Symptoms and Spinal Nerve Damage

  • Patient 1: Loss of sensation in left arm but able to move it.

    • Possible damage to sensory pathways related to left arm sensation.

  • Patient 2: Unable to move right arm but can feel it.

    • Suggests motor pathway damage affecting right arm.

  • Patient 3: Cannot move or feel legs.

    • Indicates severe damage affecting lower spinal cord functioning.

Conclusion and Next Steps

  • Preparation to discuss the brain’s structure and functions in the following segments, further exploring the overall organization and specifics of the central nervous system.