Vertebrate Nervous System: Structure, Function, and Reflexes

Vertebrate Nervous System: Structure and Components

Vertebrates possess a tubular nervous system, consisting of the brain and spinal cord.

Central Nervous System (CNS)

The Central Nervous System (CNS) consists of the brain and spinal cord. It is protected by two main types of envelopes:

• Bony Envelopes: The skull and vertebrae.
• Membranous Envelopes (Meninges): These consist of three layers: the dura mater, arachnoid mater, and pia mater. The cerebrospinal fluid (CSF) is located between the last two layers, functioning to cushion impacts.

Nerve fibers are grouped, forming the white matter. In the brain, the gray matter is located in the periphery (cortex), while the white matter is internal.

The Brain

The brain is composed of several major regions:

• Telencephalon (Cerebrum)

  It is divided into two cerebral hemispheres and features numerous ridges called gyri.

• Diencephalon

  Includes the hypothalamus, which controls basic sensations like thirst and sleep, and the pituitary gland, which controls and regulates all other endocrine glands.

• Mesencephalon (Midbrain)

  Controls many visual and auditory reflexes, and helps maintain muscle tone.

• Cerebellum

  Divided into two cerebellar hemispheres; its primary function is to control and coordinate movements and balance.

• Medulla Oblongata

  Controls vital automatic functions such as heartbeat and respiration.

Spinal Cord

The spinal cord is a tubular structure extending from the base of the brain to the second lumbar vertebra, responsible for motor functions and mediating reflexes.

Structure of the Spinal Cord

The gray matter includes four areas called horns (two anterior/ventral and two posterior/dorsal). It contains the central canal, lined by ependyma, through which cerebrospinal fluid flows.

Peripheral Nervous System (PNS)

The Peripheral Nervous System (PNS) consists of ganglia and nerves.

• Cranial Nerves: 12 pairs originating in the brain, innervating the head, sense organs, and some facial muscles.
• Spinal Nerves: 31 pairs, each having two branches (a motor branch and a sensory branch). They innervate the muscles of the arms, legs, and trunk.

Autonomic Nervous System (ANS)

The Autonomic Nervous System (ANS) controls visceral activities involuntarily. It is divided into two functional divisions:

• Sympathetic Division: Prepares the body for activity or stress situations (fight or flight).
• Parasympathetic Division: Prepares the body for rest and remedial situations (rest and digest).

Nervous System Actions and Reflexes

Involuntary Actions (Reflexes)

Involuntary actions are mediated by the Reflex Arc, which consists of the following elements:

1. Receptor
2. Sensory Neuron
3. Association Neuron
4. Motor Neuron
5. Effector Organ

• Unconditioned Reflexes: These are congenital (inborn).
• Conditioned Reflexes: These involve the cerebral cortex and are acquired through a process of learning.

Voluntary Actions

Voluntary actions involve the spinal cord, the brain, and specifically the cerebral cortex.

Receptors and Stimuli Classification

Receptors are specialized nerve endings or cells that detect stimuli. They can be in contact with neurons or grouped together to form sense organs.

Receptor Classification

Receptors are classified based on the origin of the stimuli they capture:

• Exteroceptors: Receive stimuli from the external environment.
• Interoceptors: Capture information from within the body. These are further classified as:
  • Proprioceptors: Located in muscles, tendons, and joints (reporting body position).
  • Visceroceptors: Report visceral activity.