Chapter 9 The nervous system

Chapter 9: The Nervous System

Overview of the Nervous System

The nervous system is a vital network that maintains homeostasis in the body through electrical signals. It plays crucial roles in providing sensation, allowing for mental functioning, emotional responses, and activating muscles and glands. As the master controlling and communicating system of the body, it orchestrates a range of physiological processes crucial for survival and adaptation.

Divisions of the Nervous System

The nervous system is divided into two major parts:

1. Central Nervous System (CNS)

   • Comprises the brain and spinal cord.

2. Peripheral Nervous System (PNS)

   • Consists of nerves outside the CNS, which includes the cranial nerves and spinal nerves. This division can further be categorized into:

   • Autonomic Nervous System

   • Somatic Nervous System

   • Autonomic Nervous System contains:

     • Parasympathetic - responsible for relaxing the body.

     • Sympathetic - prepares the body for stressful situations.

Functional Divisions of the Peripheral Nervous System

Sympathetic Nervous System

• Engages the stress response, gearing the body up for fight, flight, or freeze. It is characterized by rapid physiological changes:

  • Increased heart rate and rapid, shallow breathing.

  • Dilation of pupils, which may heighten sensitivity to light.

  • Gut activity decreases, impacting digestion.

  • Blood flow shifts to skeletal muscles, diverting it from the brain, potentially clouding judgment.

  • Hormonal rush leading to feelings of anxiety.

Parasympathetic Nervous System

• Facilitates relaxation responses, which include:

  • A return to a slow, rhythmic heartbeat.

  • Deeper and slower breathing.

  • Constriction of pupils.

  • Restoration of gut activity to support digestion and nutrient absorption.

  • Enhanced blood flow to the gut, lungs, and brain, which helps uplift mood and foster relaxation.

Functional Classification

1. Sensory (AFFERENT) Neurons

   • Transmit impulses to the CNS from sensory receptors across the body, keeping the CNS apprised of internal and external stimuli.

2. Motor (EFFERENT) Neurons

   • Convey impulses from the CNS to effector organs, like muscles and glands, facilitating actions—like pulling away from a hot surface.

Structure of Neurons

Neurons are the functional cells of the nervous system, distinguished by long, threadlike fibers that extend from the cell body to transmit impulses. These fibers are classified as:

• Dendrites: Fibers that carry impulses toward the cell body (the term 'dendrite' derives from the Greek word for tree).

• Axons: Fibers that carry impulses away from the cell body towards muscles or glands.

Supporting Cells in the Nervous System

Neuroglia, or glial cells, found in both the CNS and PNS, provide support but do not transmit electrical signals.

Types of Glial Cells

• CNS:

  • Astrocytes: Aid in maintaining the blood-brain barrier.

  • Microglia: Act as the immune defense within the CNS.

  • Oligodendrocytes: Produce myelin necessary for insulatory functions.

  • Ependymal cells: Produce cerebrospinal fluid (CSF).

• PNS:

  • Schwann cells: Form myelin sheaths around nerve fibers.

  • Satellite cells: Provide cushioning and protective support.

Myelin Sheath

The myelin sheath, a whitish fatty substance, insulates and protects the axon, enhancing the speed of electrical signal transmission. Myelin is formed by Schwann cells in the PNS and oligodendrocytes in the CNS.

• Nodes of Ranvier: These gaps between myelinated portions of the axon facilitate rapid signal transmission by allowing electrical impulses to jump from one node to the next.

Importance of Myelination

Destruction of the myelin sheath can lead to demyelinating diseases, such as Multiple Sclerosis (MS) and Guillain-Barre Syndrome.

Structure of a Neuron

• Includes essential components such as the nucleus, nucleolus, axon, cell body, and dendrites, which work in sync to facilitate neuronal function.

Nerve Impulses: Basic Concepts

Nerve impulses are characterized by their all-or-nothing principle, similar to firing a gun; they are either conducted across the axon or not at all. The stages of a nerve impulse include:

• Polarized State: The resting state with a more negative charge inside the cell.

• Depolarization: The process where the membrane's charge becomes less negative, moving toward a positive charge.

• Repolarization: The return to the resting polarized state after depolarization occurs.

Synapse

At a synapse, nerve impulses are transmitted between neurons through neurotransmitters. The releasing process entails:

• Presynaptic cells releasing neurotransmitters in response to an action potential.

• Neurotransmitters crossing the synaptic cleft and binding to receptors on postsynaptic cells, which can either increase or decrease the likelihood of triggering an action potential.

Neurotransmitters

Common neurotransmitters include:

• Norepinephrine

• Serotonin

• Dopamine

• Acetylcholine

The Spinal Cord

It serves as a two-way conduction pathway between the brain and body, encased within the vertebral column, with 31 pairs of spinal nerves emerging from it.

Gray and White Matter

• Gray Matter: Comprises cell bodies and unmyelinated axons.

• White Matter: Comprises myelinated axons.

Spinal Nerves

Spinal nerves are attached to the spinal cord through two roots:

• Dorsal Root: Contains sensory neuron cell bodies.

• Ventral Root: Comprises motor fibers that supply muscles and glands. All spinal nerves are mixed nerves containing both sensory and motor fibers.

Reflex Arc Components

1. Receptor: Detects stimulus.

2. Sensory Neuron: Transmits nerve impulses toward the CNS.

3. Central Nervous System: Integrates the information and plans a response.

4. Motor Neuron: Transmits impulses from the CNS to the effector.

5. Effector: Receives impulses and carries out a response.

Types of Reflexes

• Stretch Reflex: Helps maintain muscle tone and posture (e.g., knee-jerk reflex).

• Withdrawal Reflex: A protective pathway that withdraws from harmful stimuli (e.g., pulling back from heat).

Autonomic Nervous System (ANS)

Regulates actions of glands, smooth muscles, and heart muscles, divided into the:

• Parasympathetic Division: Responsible for