Chapter 15

Chapter 15: Neurologic System Study Guide

The nervous system is a complex, integrated hierarchy divided structurally into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). It functions to detect environmental changes and initiate responses to maintain a dynamic steady state.

1. Organisation of the Nervous System

Structural Divisions

  • Central Nervous System (CNS):  Consists of the brain and spinal cord, protected by the cranium and vertebral column.

  • Peripheral Nervous System (PNS):  Comprises the cranial nerves (projecting from the brain) and spinal nerves (projecting from the spinal cord).

Functional Divisions

  • Somatic Nervous System:  Regulates voluntary motor control (e.g., skeletal muscles).

  • Autonomic Nervous System (ANS):  Regulates the internal environment through involuntary control of organ systems (viscera). It is subdivided into:

  • Sympathetic Division:  Mobilises energy stores (fight-or-flight).

  • Parasympathetic Division:  Conserves and restores energy (rest-and-digest).

Directional Pathways

  • Afferent (Ascending) Pathways:  Carry sensory impulses from peripheral receptors toward the CNS.

  • Efferent (Descending) Pathways:  Transmit motor impulses away from the CNS to effector organs or skeletal muscles.

2. Cells of the Nervous System

Nervous tissue consists of two primary cell types:  Neurons  (the primary conducting cells) and  Neuroglia  (supporting cells).

The Neuron

Neurons are electrically excitable cells composed of three main parts:

  1. Cell Body (Soma):  Located mainly in the CNS. Dense collections in the CNS are called  nuclei ; in the PNS, they are  ganglia .

  2. Dendrites:  Receptive extensions that carry impulses toward the cell body.

  3. Axons:  Long projections that carry impulses away from the cell body. The  axon hillock  is where action potentials begin.

Structural Classification of Neurons

Type,Characteristics,Example/Location

Unipolar,One process (axon) branching from the cell body.,Retina

Pseudounipolar,One axon and one dendrite fused near the cell body; the axon extends into the CNS.,Sensory neurons in cranial/spinal nerves

Bipolar,"Two distinct processes (one axon, one dendrite) arising from the cell body.",Connects rod and cone cells in the retina

Multipolar,Multiple processes and extensive branching; the most common type.,Motor neurons

Functional Classification of Neurons

  • Sensory (Afferent):  Carry impulses from peripheral receptors to the CNS. Mostly pseudounipolar.

  • Associational (Interneurons):  Transmit impulses from neuron to neuron within the CNS only. Multipolar.

  • Motor (Efferent):  Transmit impulses from the CNS to an effector organ (muscle or gland). Multipolar.

Neuroglia (Supporting Cells)

Neuroglia comprise approximately 50% of the total brain and spinal cord volume.| Cell Type | System | Primary Functions || ------ | ------ | ------ || Astrocytes | CNS | Form blood-brain barrier; provide nutrient transport; scar-forming cells. || Oligodendrocytes | CNS | Produce the myelin sheath for CNS axons. || Microglia | CNS | Phagocytic cells; responsible for clearing cellular debris and immune defence. || Ependymal Cells | CNS | Line ventricles and choroid plexuses; involved in cerebrospinal fluid (CSF) production. || Schwann Cells | PNS | Produce the myelin sheath for PNS axons; direct axonal regrowth. || Satellite Cells | PNS | Surround and protect neuron cell bodies in ganglia; facilitate communication. |

3. Central Nervous System Divisions

The brain is divided into three primary embryonic vesicles: the Forebrain, Midbrain, and Hindbrain.

I. Forebrain (Prosencephalon)

  • Telencephalon:

  • Cerebral Cortex:  The largest portion of the brain, composed of grey matter (cell bodies) arranged in folds (gyri) and grooves (sulci).

  • Basal Ganglia:  Subcortical nuclei (caudate nucleus, putamen, globus pallidus) involved in voluntary movement and cognitive function.

  • Diencephalon:

  • Thalamus:  Major integrating centre for afferent impulses to the cerebral cortex.

  • Hypothalamus:  Maintains internal environment (homeostasis); regulates ANS, temperature, endocrine function, and emotional expression.

  • Epithalamus:  Forms the roof of the third ventricle.

  • Subthalamus:  Motor activity centre.

II. Midbrain (Mesencephalon)

Serves as a relay centre for motor and sensory tracts and contains auditory and visual reflex centres.

  • Tectum:  Includes superior colliculi (visual motor) and inferior colliculi (auditory motor).

  • Tegmentum:  Contains the red nucleus and substantia nigra (dopamine synthesis).

  • Cerebral Peduncles:  Efferent fibres linking the cortex to the brainstem.

III. Hindbrain (Rhombencephalon)

  • Cerebellum:  Responsible for reflexive, involuntary fine-tuning of motor control, balance, and posture. It exerts  ipsilateral  (same side) control.

  • Pons:  A bridge transmitting information between the cerebellum and brainstem.

  • Medulla Oblongata (Myelencephalon):  The lowest portion of the brainstem. Controls vital reflexes (heart rate, respiration, blood pressure) and is the site where many motor tracts  decussate  (cross over).

4. Cerebral Lobes and Functional Areas

The cerebrum is divided into four main lobes:

  • Frontal Lobe:

  • Prefrontal Area:  Goal-oriented behaviour, short-term memory, and thought elaboration.

  • Primary Motor Area (Precentral Gyrus):  Voluntary motor control.

  • Broca Area:  Motor aspects of speech (usually in the left hemisphere).

  • Parietal Lobe:

  • Postcentral Gyrus:  Primary somatic sensory input.

  • Sensory Association Area:  Storage, analysis, and interpretation of stimuli.

  • Occipital Lobe:  Contains the primary visual cortex and visual association areas.

  • Temporal Lobe:

  • Primary Auditory Cortex:  Hearing and related association.

  • Wernicke Area:  Reception and interpretation of speech.

  • Additional functions:  Memory consolidation and smell.

5. The Spinal Cord and Reflex Arcs

Spinal Cord Structure

The spinal cord lies within the vertebral canal. A cross-section reveals:

  • Grey Matter (Inner Core):  Butterfly-shaped region.

  • Posterior (Dorsal) Horn:  Primarily interneurons and sensory neuron axons.

  • Lateral Horn:  Contains cell bodies involved with the ANS.

  • Anterior (Ventral) Horn:  Contains nerve cell bodies for efferent (motor) pathways.

  • White Matter (Outer Layer):  Contains ascending (sensory) and descending (motor) spinal tracts.

The Reflex Arc

The reflex arc is the basic unit of the nervous system that responds to stimuli. Components include:

  1. Receptor:  Detects the stimulus.

  2. Afferent (Sensory) Neuron:  Transmits the impulse to the CNS.

  3. Interneuron:  (Optional) Provides a link between sensory and motor neurons.

  4. Efferent (Motor) Neuron:  Transmits the impulse from the CNS to the periphery.

  5. Effector:  The muscle or gland that carries out the response.

6. Protective Structures and CSF Circulation

Protective Layers

  1. Cranium:  Eight bones protecting the brain.

  2. Meninges:  Three protective membranes:

  3. Dura Mater:  Outermost, tough, double-layered membrane.

  4. Arachnoid Mater:  Spongy, web-like middle layer.

  5. Pia Mater:  Delicate innermost layer adhering to the brain and spinal cord surface.

Cerebrospinal Fluid (CSF)

CSF is a clear, colorless fluid that cushions the CNS and prevents the brain from tugging on nerve roots.

  1. Production:  Produced by  ependymal cells  in the  choroid plexuses  of the ventricles.

  2. Circulation Path:

  3. Lateral ventricles

  4. Interventricular foramen (Foramen of Monro)

  5. Third ventricle

  6. Cerebral aqueduct (Aqueduct of Sylvius)

  7. Fourth ventricle

  8. Subarachnoid space (via lateral and median apertures)

  9. Reabsorption:  Reabsorbed into the venous circulation through  arachnoid villi .

7. Peripheral and Autonomic Nervous Systems

Spinal Nerves

There are 31 pairs of mixed (sensory and motor) spinal nerves:

  • 8 Cervical, 12 Thoracic, 5 Lumbar, 5 Sacral, 1 Coccygeal.

  • Dermatomes:  Specific areas of skin innervated by a single spinal cord segment.

Autonomic Nervous System (ANS) Comparison

The ANS uses a two-neuron system (preganglionic and postganglionic).| Feature | Sympathetic (Thoracolumbar) | Parasympathetic (Craniosacral) || ------ | ------ | ------ || Origin | T1 through L2 | Cranial nerve nuclei (III, VII, IX, X) and sacral cord || Function | Energy mobilisation; "Fight or Flight" | Energy conservation; "Rest and Digest" || Ganglia Location | Near the spinal cord (paravertebral) | Near or inside the effector organ || Preganglionic Neurotransmitter | Acetylcholine | Acetylcholine || Postganglionic Neurotransmitter | Norepinephrine (mostly) | Acetylcholine || Receptors | α-adrenergic, β-adrenergic | Cholinergic |