Chapter 10 Nervous system

Nervous System Overview

Communication Systems:

• Two primary systems control the body:

  • Nervous System: Rapid communication through nerve impulses.

  • Endocrine System: Slower communication via hormones in the bloodstream.

Divisions of the Nervous System:

• Central Nervous System (CNS): Consists of the brain and spinal cord.

• Peripheral Nervous System (PNS): Includes all nerves outside the CNS; further divided into:

  • Autonomic Nervous System (ANS): Regulates automatic functions (e.g., heart rate, digestion).

  • Somatic Nervous System: Controls voluntary movements.

Neurons

• Major Features of Neurons:

  • Cell Body: Main part containing the nucleus.

  • Dendrites: Short branches conducting impulses to the cell body.

  • Axon: Long projection conducting impulses away from the cell body.

• Types of Neurons Based on Function:

  • Sensory Neurons: Carries impulses to the CNS (afferent).

  • Motor Neurons: Sends impulses away from CNS to effectors (efferent).

  • Interneurons: Connects sensory and motor neurons within the CNS.

Glial Cells

• Glial Cells (Neuroglia): Support and protect neurons.

• Functions:

  • Hold neurons together structurally and functionally.

  • Regulate neuron function.

• Types of Glial Cells:

  • Astrocytes: Provide structural support and regulate nutrients.

  • Oligodendrocytes: Form the myelin sheath in CNS.

  • Microglia: Act as immune defense in the CNS.

Nerve Structures

• Nerves: Bundles of axons in the PNS, wrapped in connective tissue:

  • Endoneurium: Surrounds each axon.

  • Perineurium: Surrounds groups of axons (fascicles).

  • Epineurium: Encloses the entire nerve.

• Myelin Sheath: Forms around axons, influencing conduction speed; myelinated axons conduct impulses faster than unmyelinated axons.

Reflex Arcs

• Definition: Simplest neuron pathway allowing for reflex actions (e.g., knee-jerk response).

• Components: Includes sensory neurons, interneurons, and motor neurons.

• Types of Reflex Arcs:

  • Two-Neuron Reflex Arc:

    • Involves a sensory neuron and a motor neuron.

    • Example: The patellar reflex (knee-jerk) where a tap on the knee causes an immediate contraction of the quadriceps muscle.

  • Three-Neuron Reflex Arc:

    • Involves sensory neurons, interneurons, and motor neurons; allows for more complex actions.

    • Example: Withdrawal reflex, such as when touching a hot surface. The sensory neuron activates an interneuron, which then activates a motor neuron to withdraw the hand quickly away from the stimulus.

Clinical Conditions of Nervous System

• Multiple Sclerosis: Autoimmune condition characterized by the loss of myelin in CNS, leading to impaired nerve conduction.

• Cerebral Palsy: Non-progressive brain injury causing movement abnormalities, often due to perinatal factors.

• Parkinson's Disease: Degeneration of dopamine-producing neurons; causes motor symptoms such as tremors.

• Acute Cerebrovascular Accident (Stroke): Disruption of blood flow to the brain leading to neuron death.

Cerebrospinal Fluid (CSF) Functions:

• Circulates around the brain and spinal cord, providing cushioning and protection; involved in nutrient transport and waste removal.

• Hydrocephalus: Condition caused by blockage of CSF flow, leading to increased pressure in the brain.

Autonomic Nervous System (ANS) Divisions:

• Sympathetic Nervous System: Prepares the body for stress-related activities (fight-or-flight response).

• Parasympathetic Nervous System: Restores the body to a state of calm (rest-and-repair).

• Neurotransmitters: Chemicals (like acetylcholine and norepinephrine) used for communication within the autonomic pathway.

Quick Checks

• Difference Between CNS and PNS: CNS is the central control system (brain and spinal cord), while PNS includes all peripheral nerves.

• Features of a Neuron: Cell body, dendrites, axon.

• Types of Neurons: Sensory, motor, interneurons.

• Glia vs. Neurons: Glia support neurons; three types include astrocytes, oligodendrocytes, and microglia.

Autonomic Nervous System (ANS) Divisions:

• Sympathetic Nervous System: Prepares the body for stress-related activities (fight-or-flight response).

• Parasympathetic Nervous System: Restores the body to a state of calm (rest-and-repair).

• Enteric Nervous System: Often referred to as the "second brain," it governs the function of the gastrointestinal system and operates independently of the CNS but can be modulated by both the sympathetic and parasympathetic systems.

• Neurotransmitters: Chemicals (like acetylcholine and norepinephrine) used for communication within the autonomic pathway.

Cranial Nerves Overview

• Definition: Cranial nerves are a set of twelve paired nerves that arise directly from the brain (including the brainstem) instead of the spinal cord. They are responsible for a variety of sensory and motor functions.

List of Cranial Nerves:

1. Olfactory Nerve (I): Responsible for the sense of smell.

2. Optic Nerve (II): Carries visual information from the retina to the brain.

3. Oculomotor Nerve (III): Controls most of the eye's movements, as well as the constriction of the pupil.

4. Trochlear Nerve (IV): Controls the superior oblique muscle of the eye, which is responsible for rotational movement.

5. Trigeminal Nerve (V): Responsible for sensation in the face and motor functions such as biting and chewing. It has three branches - ophthalmic, maxillary, and mandibular.

6. Abducens Nerve (VI): Controls lateral eye movement by innervating the lateral rectus muscle.

7. Facial Nerve (VII): Responsible for facial expressions, the sense of taste from the anterior two-thirds of the tongue, and some functions in the ear.

8. Vestibulocochlear Nerve (VIII): Responsible for hearing and balance (equilibrium).

9. Glossopharyngeal Nerve (IX): Involved in taste and other functions of the throat and tongue, including swallowing.

10. Vagus Nerve (X): Controls functions of the heart, lungs, and digestive tract; it also contributes to the gag reflex.

11. Accessory Nerve (XI): Controls certain shoulder and neck muscles, allowing head rotation and shoulder elevation.

12. Hypoglossal Nerve (XII): Controls tongue movements essential for speech and swallowing.

Functions and Clinical Relevance:

• Sensory Functions: Many cranial nerves are primarily sensory (e.g., I, II, VIII), conveying information from sensory organs to the brain.

• Motor Functions: Others are primarily motor (e.g., III, IV, VI, XI, XII), controlling muscles of the head and neck.

• Mixed Nerves: Some cranial nerves have both sensory and motor functions (e.g., V, VII, IX, X).

• Clinical Conditions: Damage to cranial nerves can lead to specific symptoms depending on the nerve affected, such as loss of smell (olfactory nerve), facial paralysis (facial nerve), or difficulty swallowing (vagus, glossopharyngeal nerves).

Understanding cranial nerves is crucial for diagnosing neurological conditions and understanding their impact on daily functions.

The brain is the central organ of the nervous system, responsible for processing sensory information, coordinating movement, and controlling cognition and behavior. It consists of multiple structures, including the cerebrum, cerebellum, and brainstem, each having specialized functions:

• Cerebrum: Involved in higher brain functions such as thought, emotion, learning, and voluntary muscle movements; divided into lobes (frontal, parietal, temporal, occipital).

• Cerebellum: Coordinates muscle movements and maintains posture and balance.

• Brainstem: Controls vital functions such as breathing, heart rate, and blood pressure; includes the midbrain, pons, and medulla oblongata. The brain is protected by the skull, meninges, and cerebrospinal fluid, which also provides cushioning and nutrient transport.