nervous system

Concise Notes on the Nervous System (Unit 18)

Functions of the Nervous System:

1. Sensory Input: Gathers information from sensory receptors about internal and external changes.

2. Integration: Processes sensory information and determines the necessary response.

3. Motor Output: Sends signals to muscles or glands to initiate a response.

Organization of the Nervous System:

• Structural Classification:

  • Central Nervous System (CNS): Includes the brain and spinal cord, responsible for integration and command.

  • Peripheral Nervous System (PNS): Composed of cranial and spinal nerves, acting as communication lines between the CNS and the rest of the body.

• Functional Classification:

  • Sensory (Afferent) Division: Carries signals to the CNS from sensory receptors.

    • Somatic sensory fibers: From skin, muscles, and joints.

    • Visceral sensory fibers: From internal organs.

  • Motor (Efferent) Division: Transmits impulses from the CNS to effectors (muscles & glands).

    • Somatic Nervous System: Voluntary control of skeletal muscles.

    • Autonomic Nervous System: Involuntary control of cardiac/smooth muscles & glands.

      • Sympathetic Division: Activates "fight or flight" response.

      • Parasympathetic Division: Promotes "rest and digest" functions.

Nervous Tissue Components:

1. Neurons: Specialized cells that conduct nerve impulses.

   • Cell body: Contains nucleus, organelles, and metabolic structures.

   • Processes:

     • Dendrites: Receive signals.

     • Axons: Transmit signals away from the cell body.

   • Myelin Sheath: Insulating layer that speeds up signal transmission (formed by Schwann cells in PNS and oligodendrocytes in CNS).

2. Neuroglia (Glial Cells): Supportive cells that do not conduct impulses.

   • CNS Glial Cells:

     • Astrocytes: Provide structural support and regulate the brain's chemical environment.

     • Microglia: Act as immune cells, removing debris.

     • Ependymal Cells: Produce and circulate cerebrospinal fluid (CSF).

     • Oligodendrocytes: Produce myelin in the CNS.

   • PNS Glial Cells:

     • Schwann Cells: Produce myelin in the PNS.

     • Satellite Cells: Protect neuron cell bodies.

Neuron Communication & Action Potential:

• Resting Membrane Potential: Inside of the neuron is more negative (-70mV) than the outside.

  • More Na⁺ outside, more K⁺ inside.

• Depolarization: Na⁺ channels open → Na⁺ enters → Inside becomes more positive.

• Action Potential: Electrical impulse travels down the axon.

• Repolarization: K⁺ exits the neuron, restoring negative charge inside.

• Sodium-Potassium Pump: Restores resting state (3 Na⁺ out, 2 K⁺ in).

Transmission at Synapses:

1. Action potential reaches the axon terminal.

2. Calcium (Ca²⁺) enters, triggering neurotransmitter release.

3. Neurotransmitter crosses the synaptic cleft and binds to receptors on the next neuron.

4. If threshold is met, a new action potential begins.

Reflexes & Reflex Arc:

• Reflex: Rapid, automatic response to a stimulus.

• Reflex Arc Components:

  1. Sensory Receptor (detects stimulus).

  2. Sensory Neuron (transmits signal to CNS).

  3. Integration Center (processes and decides response).

  4. Motor Neuron (sends command to effector).

  5. Effector Organ (muscle or gland that responds).

• Examples:

  • Somatic Reflexes: Knee-jerk reflex (stimulates skeletal muscle).

  • Autonomic Reflexes: Regulates heart rate, digestion, blood pressure.

This is a concise summary of Unit 18 Nervous System. Let me know if you need more details! 😊Here are concise notes along with a Question & Answer (Q&A) section based on your provided material.

Concise Notes

Three-Neuron Reflex Arc

• Involves five elements: Receptor, Sensory Neuron, Interneuron, Motor Neuron, Effector.

• Example: Flexor (Withdrawal) Reflex – a response to pain, like pulling your hand away from a hot object.

Central Nervous System (CNS) & Brain Anatomy

1. Cerebral Hemispheres

   • Paired left and right halves, forming the superior part of the brain.

   • Contain gyri (ridges), sulci (shallow grooves), and fissures (deep grooves).

   • Divided into lobes based on cranial bones.

2. Cerebral Cortex

   • Primary Somatic Sensory Area (Parietal Lobe): Processes pain, temperature, touch.

   • Primary Motor Area (Frontal Lobe): Controls voluntary movement.

   • Special Sensory Areas:

     • Visual (Occipital Lobe), Auditory & Olfactory (Temporal Lobe).

   • Broca’s Area (Motor Speech) – Important for speech production.

3. White Matter & Basal Nuclei

   • Corpus Callosum connects hemispheres.

   • Basal Nuclei regulate motor control.

4. Diencephalon (Interbrain)

   • Thalamus: Sensory relay station.

   • Hypothalamus: Controls body temperature, metabolism, emotions, and the pituitary gland.

   • Epithalamus: Includes the pineal gland (secretes melatonin) and choroid plexus (produces cerebrospinal fluid).

5. Brain Stem

   • Midbrain: Visual and auditory reflex centers.

   • Pons: Helps regulate breathing.

   • Medulla Oblongata: Controls vital functions (heart rate, breathing, blood pressure).

   • Reticular Formation: Regulates consciousness and wakefulness.

6. Cerebellum

   • Controls balance, coordination, and precise timing of muscle activity.

7. Protection of CNS

   • Meninges: Dura mater, Arachnoid mater, Pia mater.

   • Cerebrospinal Fluid (CSF): Cushions the brain and spinal cord.

   • Blood-Brain Barrier: Restricts harmful substances from entering the brain.

8. Brain Dysfunctions

   • Traumatic Brain Injuries: Concussion, Contusion, Cerebral Edema.

   • Stroke (CVA): Caused by blocked blood flow, leading to paralysis or speech issues.

   • TIA (Mini-Stroke): Temporary ischemia.

9. Spinal Cord

   • Extends from the brainstem to L1/L2.

   • Contains gray matter (cell bodies) and white matter (nerve tracts).

   • 31 pairs of spinal nerves.

   • Dorsal root (sensory input), Ventral root (motor output).

10. Peripheral Nervous System (PNS)

    • Cranial Nerves (12 pairs): Serve the head/neck, except the vagus nerve (extends to thoracic/abdominal organs).

    • Spinal Nerves (31 pairs): Form plexuses that serve the limbs.

11. Autonomic Nervous System (ANS)

    • Controls involuntary functions (heart rate, digestion).

    • Two divisions:

      • Sympathetic ("Fight or Flight") – Prepares the body for stress.

      • Parasympathetic ("Rest and Digest") – Conserves energy and maintains daily body functions.

Q&A Section

1. Reflex Arc & CNS

Q: What are the five components of a three-neuron reflex arc?
A: Receptor, Sensory Neuron, Interneuron, Motor Neuron, Effector.

Q: What is an example of a reflex arc?
A: The flexor (withdrawal) reflex, such as pulling your hand away from a hot object.

2. Brain Structure & Function

Q: What are the main regions of the brain?
A: Cerebral Hemispheres, Diencephalon, Brain Stem, Cerebellum.

Q: What are the three main parts of the cerebral hemispheres?
A: Cortex (gray matter), White Matter, Basal Nuclei.

Q: What is the function of the primary motor cortex?
A: Controls voluntary skeletal muscle movements.

Q: What is the function of the primary somatic sensory area?
A: Processes pain, temperature, and touch sensations.

3. Diencephalon & Brain Stem

Q: What are the three parts of the diencephalon?
A: Thalamus, Hypothalamus, Epithalamus.

Q: What are the functions of the hypothalamus?
A: Controls temperature, metabolism, emotions, and pituitary gland.

Q: What are the functions of the brain stem?
A: Controls breathing, heart rate, and reflex actions.

4. Protection & Disorders of the CNS

Q: What are the protective layers of the brain?
A: Meninges (Dura Mater, Arachnoid Mater, Pia Mater).

Q: What is cerebrospinal fluid (CSF), and what is its function?
A: A fluid cushion that protects the brain and spinal cord.

Q: What is the function of the blood-brain barrier?
A: Protects the brain by preventing harmful substances from entering.

Q: What are the effects of a stroke (CVA)?
A: Can cause paralysis, loss of speech, or even death.

5. Spinal Cord & PNS

Q: What is the function of the spinal cord?
A: Acts as a two-way communication pathway between the brain and body.

Q: How many spinal nerves are there?
A: 31 pairs.

Q: What is the difference between sensory (afferent) and motor (efferent) nerves?
A:

• Sensory (afferent) nerves carry impulses toward the CNS.

• Motor (efferent) nerves carry impulses away from the CNS.

6. Autonomic Nervous System

Q: What are the two divisions of the autonomic nervous system?
A: Sympathetic (Fight or Flight) & Parasympathetic (Rest and Digest).

Q: What neurotransmitters are used in the autonomic nervous system?
A:

• Acetylcholine (ACh) – Used by parasympathetic fibers.

• Norepinephrine (NE) – Used by sympathetic fibers.

Q: What are some effects of the sympathetic nervous system?
A: Increases heart rate, breathing rate, and blood pressure during stress.

Spinal Nerve Plexuses

A nerve plexus is a network of interconnected spinal nerves that serve motor and sensory functions in specific regions of the body. The ventral rami of spinal nerves (except for thoracic nerves T1–T12) form these plexuses.

There are four major spinal nerve plexuses:

1. Cervical Plexus (C1–C5)

• Location: Neck (formed by C1–C5 spinal nerves)

• Function: Serves the muscles and skin of the neck, shoulder, and diaphragm

• Important Nerve:

  • Phrenic nerve – Controls the diaphragm (important for breathing)

Example of Dysfunction: Damage to the phrenic nerve can lead to respiratory failure.

2. Brachial Plexus (C5–T1)

• Location: Shoulder region (formed by C5–T1 spinal nerves)

• Function: Innervates the shoulders, arms, and hands

• Important Nerves:

  • Axillary nerve – Serves the shoulder

  • Radial nerve – Supplies the posterior arm & forearm (wrist extension)

  • Median nerve – Controls forearm flexors and fingers

  • Ulnar nerve – Supplies hand muscles ("funny bone" nerve)

  • Musculocutaneous nerve – Controls the biceps and forearm muscles

Example of Dysfunction:

• Radial nerve damage – Causes wrist drop (inability to extend the wrist).

• Median nerve compression – Leads to carpal tunnel syndrome.

3. Lumbar Plexus (L1–L4)

• Location: Lower back (formed by L1–L4 spinal nerves)

• Function: Controls the abdominal wall, anterior thigh, and hip flexors

• Important Nerves:

  • Femoral nerve – Innervates the anterior thigh muscles (quadriceps)

  • Obturator nerve – Serves the medial thigh (adductors)

Example of Dysfunction:

• Femoral nerve injury – Leads to weakness in hip flexion and knee extension.

4. Sacral Plexus (L4–S4)

• Location: Lower back & pelvis (formed by L4–S4 spinal nerves)

• Function: Serves the lower limbs, buttocks, and pelvic organs

• Important Nerves:

  • Sciatic nerve – The largest nerve in the body; supplies the posterior thigh, leg, and foot

  • Tibial nerve – Serves the posterior lower leg (calf) and foot

  • Common fibular nerve – Supplies the anterior and lateral leg muscles

Example of Dysfunction:

• Sciatic nerve compression (sciatica) – Causes severe pain and weakness in the leg.

• Common fibular nerve damage – Leads to foot drop (difficulty lifting the foot).

Summary Table

Key Takeaways

• The ventral rami of spinal nerves form plexuses to distribute nerve signals to limbs.

• The sciatic nerve (from the sacral plexus) is the largest nerve in the body.

• Damage to plexuses can lead to loss of sensation or movement in the affected areas.

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