The Nervous System

Basics of the Nervous System

  • The nervous system allows organisms to respond quickly to changes in their environment.
  • It’s made up of central and peripheral components.
  • The central nervous system comprises the brain and spinal cord.
  • The peripheral nervous system consists of all the nerves that branch off from the brain and spinal cord.
  • Neurons, or nerve cells, are the primary components of the nervous system. They transmit electrical signals called nerve impulses throughout the body.

Structure of Neurons

  • Neurons have three major parts: a cell body, dendrites, and an axon.
  • The cell body maintains the health of the neuron and contains the nucleus.
  • Dendrites are branched structures that collect electrical signals from other cells and forward them to the cell body.
  • The axon carries electrical signals away from the cell body towards other neurons or effectors such as muscles or glands.

Types of Neurons

  • Sensory neurons carry signals from sensory organs (e.g., the eyes or skin) to the central nervous system.
  • Motor neurons transmit signals from the central nervous system to effectors.
  • Interneurons (relay neurons) communicate within the central nervous system and link sensory and motor neurons.

Synapses

  • Synapses are gaps between neurons where signals are transmitted from one neuron to the next.
  • Signals are passed across synapses using chemicals called neurotransmitters.
  • Neurotransmitters move across the synapse, bind to receptors on the second neuron and stimulate it to transmit the electrical signal.

Nerve Impulses

  • Nerve impulses, or action potentials, are temporary reversals in a neuron’s membrane potential caused by ions moving across the membrane.
  • This reversal propagates down the axon as an electrical signal.
  • Nerve impulses are all-or-nothing events, meaning they are always of the same strength when they’re triggered - they cannot be weaker or stronger.

Reflex Actions

  • Reflex actions bypass the brain to provide a quicker response to stimuli.
  • Reflex arcs explain the neural pathway involved in a reflex action.
  • An example is the knee-jerk reflex: when the tendon below the kneecap is hit, sensory neurons send signals to spinal cord interneurons that then signal motor neurons to contract the thigh muscle.

The Brain

  • The brain is divided into the cerebrum, the cerebellum, and the medulla oblongata.
  • The cerebrum controls voluntary actions and is the site of intelligence, learning, and memory.
  • The cerebellum regulates balance and coordination of movement.
  • The medulla oblongata controls involuntary actions like heart rate and breathing.