Anatomy and Physiology

The nervous system acts as the body's communication network, coordinating actions, sensations, and responses by transmitting signals between the brain, spinal cord, and all other body parts. It is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). 

Anatomy of the nervous system

Cells of the nervous system

The nervous system is composed of two primary types of cells: 

  • Neurons: The basic functional units of the nervous system, responsible for sending and receiving nerve impulses. They have three main parts:

    • Cell body (soma): The core of the neuron, containing the nucleus.

    • Dendrites: Branch-like extensions that receive incoming signals from other neurons.

    • Axon: A long, tail-like projection that carries electrical signals away from the cell body to a synapse.

  • Glial cells (neuroglia): Supportive cells that do not transmit nerve impulses themselves. They provide structural support, insulate neurons with myelin, and maintain the internal environment. 

Central Nervous System (CNS)

The CNS is the body's integration and command center, comprising the brain and spinal cord. Both are protected by bones (skull and vertebrae), connective tissue membranes (meninges), and cerebrospinal fluid. The CNS is composed of: 

  • Gray matter: Consists of neuron cell bodies, dendrites, and glial cells. In the brain, it forms the outer cerebral cortex, while in the spinal cord, it forms a central, butterfly-shaped core.

  • White matter: Composed mostly of myelinated axons, which allow for rapid communication. In the brain, it is located beneath the cortex, and in the spinal cord, it forms the outer layer. 

Major CNS components include:

  • Brain:

    • Cerebrum: The largest part of the brain, divided into two hemispheres and four lobes responsible for higher functions like thought, language, and memory.

    • Cerebellum: The "little brain" located at the back of the skull that coordinates voluntary movements, posture, and balance.

    • Brainstem: Connects the cerebrum and cerebellum to the spinal cord. It controls vital involuntary functions such as breathing and heart rate.

  • Spinal Cord: A vital two-way pathway for signals traveling between the brain and the rest of the body. It also serves as a reflex center. 

Peripheral Nervous System (PNS)

The PNS consists of all the nerves extending outside the CNS, linking it to the body's organs, limbs, and skin. The PNS is subdivided into the somatic and autonomic nervous systems. 

  • Somatic Nervous System (SNS): Controls voluntary body movements by sending signals to skeletal muscles and relays sensory information from the skin and joints to the CNS.

  • Autonomic Nervous System (ANS): Regulates involuntary bodily functions like heart rate, breathing, and digestion. It is further divided into two opposing parts:

    • Sympathetic Nervous System: Initiates the "fight-or-flight" response, preparing the body for stressful situations by increasing heart rate and metabolism.

    • Parasympathetic Nervous System: Promotes "rest and digest" functions, calming the body and conserving energy. 

Physiology of the nervous system

The nervous system's function relies on neurons generating and transmitting electrical signals, known as action potentials, across junctions called synapses. 

The signaling process occurs in four main steps:

  1. Generation of a nerve impulse: Neurons at rest maintain a negative electrical charge inside their membrane. When a stimulus reaches a neuron, it triggers a rapid change in membrane potential called an action potential.

  2. Propagation along the axon: The action potential travels rapidly down the length of the axon. The fatty myelin sheath surrounding many axons greatly increases the speed of this electrical signal.

  3. Synaptic transmission: At the end of the axon, the signal reaches a synapse, the tiny gap between the axon terminal and the next neuron's dendrite. Here, the electrical signal triggers the release of chemical messengers called neurotransmitters.

  4. Signal reception and response: The neurotransmitters cross the synaptic cleft and bind to receptors on the postsynaptic neuron. This binding can either excite the next neuron, causing it to fire its own action potential, or inhibit it, preventing a signal from being sent. 

Overall functions of the nervous system:

  • Sensation and perception: Sensory neurons collect information about the internal and external environment, which is sent to the CNS for processing and interpretation.

  • Integration: The CNS processes and interprets the sensory information to determine the appropriate response.

  • Response: The CNS generates and sends out motor signals to muscles, glands, and organs to produce a coordinated and intentional action.

  • Homeostasis: The nervous system, particularly the autonomic division and the hypothalamus, maintains the body's stable internal environment by regulating functions like blood pressure, temperature, and fluid balance.