Central and Peripheral Nervous System

Key Ideas:

1. Structure of the Meninges:

   • The meninges are three layers of membranes that form a barrier between the central nervous system (CNS) and the peripheral nervous system (PNS).
   • The layers include the pia mater (closest to the CNS), the arachnoid mater, and the dura mater (farthest from the CNS), which is tough and protective.

2. Role of the Meninges:

   • The dura mater acts as a protective sack, preventing the brain from sustaining injuries like concussions by floating it in cerebrospinal fluid.
   • The meninges provide structural support and protect the CNS from physical damage.

3. Neuronal Distribution:

   • Nearly all neurons in the CNS are contained within the meninges, except for motor neurons that exit the CNS to innervate peripheral tissues.
   • Motor neurons in the PNS control voluntary skeletal muscle movement and autonomic functions such as gland secretion and cardiac muscle activity.

4. Peripheral Neurons:

   • Peripheral neurons include sensory neurons and autonomic neurons, which are located outside the CNS and transmit information to and from it.
   • Neurons are categorized as peripheral or central based on the location of their cell bodies rather than the location of their axons or dendrites.

5. Shared Vulnerabilities:

   • Peripheral neurons share vulnerabilities with each other due to their location outside the CNS, making them susceptible to certain disorders.
   • For example, congenital insensitivity to pain results from mutations affecting sensory neurons that respond to injury, as well as autonomic neurons responsible for sweating.

6. Understanding Diseases:

   • Diseases affecting either the CNS or the PNS often exhibit distinct symptoms based on the affected neuronal populations.
   • Exploring these diseases provides insights into the complexities of neural development and function in both the central and peripheral nervous systems.

---

Key Ideas:

1. Difference Between Central and Peripheral Nervous System Vulnerability:

   • Diseases affecting the nervous system typically target either the central nervous system (CNS) or the peripheral nervous system (PNS), rarely affecting both.
   • The meninges act as a barrier protecting the CNS from toxins, viruses, and damage, making the PNS more vulnerable to external threats.

2. Capacity for Repair:

   • The PNS has a greater capacity for repair compared to the CNS. Axonal injuries in the PNS can often repair themselves, while traumatic injuries to the CNS generally do not heal.

3. Vulnerabilities of the Peripheral Nervous System:

   • The PNS is vulnerable to toxins like botulinum toxin, which primarily affects peripheral neurons but cannot breach the meninges.
   • Viruses such as polio and herpes zoster exploit different mechanisms to infiltrate peripheral neurons, causing diseases like polio-induced paralysis and shingles.

4. Examples of Viral Infections:

   • Polio virus enters through the synapse between motor neurons and voluntary muscles, ultimately causing motor neuron death and muscle weakness.
   • Herpes zoster virus, responsible for shingles, infects sensory neurons, remaining dormant until reactivation, which can lead to a painful rash in the affected dermatome.

5. Unique Characteristics of Shingles:

   • Shingles typically manifests as a rash localized to one side of the body, following the sensory distribution of the affected neurons.
   • The distribution of shingles often corresponds to specific nerve pathways, such as the ophthalmic division of the trigeminal nerve.

6. Effects of Toxins:

   • Toxins like sarin primarily affect the PNS due to their inability to penetrate the CNS, causing peripheral nerve damage without directly impacting the brain or spinal cord.

7. Future Topic: Brain Tumors:

   • The next segment will explore brain tumors, shifting the focus to disorders affecting the CNS.

• 

CNS Regional Functions