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FUNCTIONS OF THE NERVOUS SYSTEM

  • The Nervous System is the body's CONTROL & COMMUNICATION center.

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ORGANIZATION OF THE NERVOUS SYSTEM:

  • Sensory Receptors: Detect stimuli from our surroundings.

  • Central Nervous System: Interpret stimuli and initiate a response.

  • Effectors: Muscles and Glands that carry out a response to the stimuli

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  • The Nervous System is divided into two parts:

    • The Central Nervous System (CNS) made up of the brain and spinal cord

    • The Peripheral Nervous System (PNS) made up of all other efferent neurons which are neurons that leave the CNS

Parts of A Nerve Cell

  • The structural and functional unit of the Nervous System is the NEURON or NERVE CELL

  • A neuron has a CELL BODY which contains a nucleus, organelles and fibre-like extensions on either side called AXONS & DENDRITES

  • A neuron also has supporting cells called NEUROGLIA 

    • In the CNS neurons have Astrocytes, Ependymal, Microglia and Oligodendrocytes

    • In the PNS neurons have Schwann Cells or Satellite Cells

NERVES:

  • Nerves and Neurons are NOT the same thing 

  • NERVES are BUNDLES of neuron axons in the PNS

    • They are classified as AFFERENT NERVES (going to the CNS), EFFERENT NERVES (going away from the CNS) or MIXED NEURONS (have both Afferent and Efferent Axons)​

  • A GANGLION is a tissue mass that forms due to a mass of nerve cell bodies​

NEURONS: 

  • Neurons are nerve cells that are made up of ONE DENDRITE, AXON and CELL BODY not multiple like Nerves

PARTS OF A NEURON

  • Axons are long extensions that convey messages from the neuron to other cells 

  • Axons can be over one meter long in length!

  • Axons are long extensions that convey messages from the neuron to other cells 

  • Axons can be over one meter long in length!

DENDRITES

  • Dendrites are short, highly branched extensions that receive incoming messages from other cells and carry this electrical message to its cell body.

  • Axons are long extensions that convey messages from the neuron to other cells 

  • Axons can be over one meter long in length!

AXONS

CELL BODY

  • The cell body contains the nucleus, mitochondria and other common organelles found in eukaryotic cells.

MYELIN & NON-MYELINATED NEURONS

MYELIN NEURONS:

  • Many axons are enclosed in a protein-rich insulating layer called the MYELIN SHEATH

    • The myelin sheath is made when SCHWANN CELLS wrap themselves around the axon​

    • This membrane that is created around the axon is  a lipid so it is a poor conductor of electricity which prevents electrical signals traveling through the axon from being lost

    • It provides insulation and protection to the axon (like plastic on a copper wire)

  • Also, the myelin sheath increases the speed of conduction of nerve impulses

  • At regular intervals a NODE OF RANVIER appears which is where the myelin sheath doesn't cover the axon

  • These nodes allow impulses to travel faster along the axon as the jump from one node the another

NON-MYELINATED  NEURONS:

  • These neurons are common in the CNS where the distance the signal travels within the neuron is shorter than the PNS

  • Signals travel slower since they have to move along the entire axon and  cannot jump from node to none

  • Axons have specialized endings called SYNAPTIC TERMINALS where signals are transferred from one neuron to another by releasing chemical messengers called neurotransmitters

  • The area between two neurons is called the SYNAPSE or SYNAPTIC CLEFT

  • The target of the signal may be another neuron or effector (muscle or gland) 

Three Types of Nerve Cells

SENSORY NEURONS

  • Transfer information or signals from the external environment (stimuli) to neurons in the CNS (brain or spinal cord)

  • Have cell bodies that are closer the AXON

***NEED TO KNOW THE DIAGRAM OF A SENSORY NEURON FOR THE TEST***

SIMPLE NERVE CIRCUIT

  1. THE REFLEX ARC

  • Automatic response to a stimuli, which mediates a reflex action by the CNS (brain or spinal cord)

  • Signal is sent from the sensory neuron (where receptor feel the stimulus in the skin or muscles) to an interneuron in the CNS (where this signal and its information is analyzed by the brain) and then into a motor neuron (where the response initiated by the CNS is able to be carried out by effectors which are muscles or glands)

  • This is a POLYSYNAPTIC REFLEX that requires two or more CNS synapses

    • eg. Pain withdrawal reflex or Pupillary light reflex (used to test for brain death or damage)​

2. REFLEX

  • We cannot control our reflexes at all

  • A reflex is carried out when information is analyzed in the CNS by the brain and this message is sent to a motor neuron in order for an action or reflex to be carried out 

  • A motor neuron carries this message to an effector cell (muscle or gland) and causes it to carry out a specific response to decrease the effect or stress of the stimulus initially felt by the body or sensory neuron

  • Is a MONOSYNAPTIC REFLEX that requires one synapse​

    • eg. A knee jerk reflex​

Polysynaptic Reflex

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