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Understanding the Nervous System for MCAT

Jun 3, 2025

MCAT Biology Chapter 4: Nervous System

Overview

  • The nervous system allows organisms to sense environmental stimuli such as pain and temperature.
  • Coordinates sensory information and responds to stimuli.
  • Responsible for control of muscular movement, neuromuscular reflexes, glandular secretions, and higher-level mental functions.
  • Operates through electrical and chemical signals.

Main Objectives

  1. Cells of the Nervous System
    • Neurons and Glial cells.
  2. Transmission of Neural Impulses
    • Action potential, synapse.
  3. Organization of the Human Nervous System
    • Central and Peripheral nervous systems, autonomic nervous system, reflexes.

Cells of the Nervous System

Neurons

  • Transmit electrical impulses and translate them into chemical signals.
  • Each neuron has a nucleus located in the cell body (Soma), along with the endoplasmic reticulum and ribosomes.
  • Dendrites: Receive messages from other cells.
  • Axon Hillocks: Integrates signals and initiates action potentials if excitatory enough.
  • Axon: Long appendage that terminates in proximity to a target structure. Insulated by myelin.
  • Myelin Sheath: Increases speed of conduction, produced by oligodendrocytes (CNS) and Schwann cells (PNS).
  • Nodes of Ranvier: Gaps in the myelin sheath for rapid signal conduction.
  • Axon Terminal: Maximizes neurotransmission.

Glial Cells

  • Support neurons.
  • Astrocytes: Nourish neurons, form blood-brain barrier.
  • Ependymal Cells: Produce cerebrospinal fluid.
  • Microglia: Ingest waste products and pathogens.
  • Oligodendrocytes and Schwann Cells: Produce myelin.

Transmission of Neural Impulses

Action Potential

  • Resting Membrane Potential: -70 mV, maintained by sodium-potassium ATPase.
  • Depolarization: Sodium influx causes positive charge, leading to action potential.
  • Repolarization: Potassium efflux restores negative charge.
  • Hyperpolarization: Temporary excess negativity.
  • Restoration: Sodium-potassium pump restores resting potential.

Synapse

  • Chemical Transmission: Neurotransmitters relay messages across synaptic cleft.
  • Removal of Neurotransmitters:
    1. Enzymatic breakdown.
    2. Reuptake into presynaptic neuron.
    3. Diffusion out of the synaptic cleft.

Organization of the Human Nervous System

Central Nervous System (CNS)

  • Brain and Spinal Cord: White and gray matter; brain stem controls basic functions.
  • Spinal Cord: Extends from brainstem, protected by vertebral column.

Peripheral Nervous System (PNS)

  • Somatic Nervous System: Sensory and motor neurons.
  • Autonomic Nervous System: Regulates involuntary functions and has sympathetic and parasympathetic divisions.
    • Sympathetic (Fight or Flight): Increases heart rate, dilates pupils.
    • Parasympathetic (Rest and Digest): Decreases heart rate, increases digestion.

Reflexes

  • Reflex Arcs: Neural circuits that control reflexive behavior.
  • Monosynaptic Reflex Arc: Single synapse between sensory and motor neuron.
  • Polysynaptic Reflex Arc: At least one interneuron between sensory and motor neurons.

Note: Ensure to understand the functions of different neuron types and their roles in the transmission of signals. Remember the difference between sympathetic and parasympathetic systems, as well as glial cell functions for comprehensive MCAT biology preparation.

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