3. Nerve Cells

The Nervous System

• The communication and control centre of the body

• Involved in maintaining homeostasis

Central Nervous System (CNS):

• Brain

• Spinal Cord

Peripheral Nervous System (PNS):

• Nerves connecting the CNS with limbs and organs

• 12 pairs of cranial nerves, and 31 pairs of spinal nerves

Neuron

• A specialised cell that transmits signals along its length by generating electric currents

Parts of the Neuron

• Cell Body

• Dendrites

• Axon

• Schwann Cells

• Myelin Sheath

• Neurilemma

• Nodes of Ranvier

• Axon Terminal / Synapse

Cell Body

• The part of the neuron that contains the nucleus and controls the functioning of the rest of the cell

• Also contains common organelles (mitochondria, endoplasmic reticulum, ribosomes, Golgi apparatus)

Dendrites

• Short extensions from the cytoplasm / cell body that carry nerve impulses from surrounding neurons into the cell body

Axon

• A long extension of the cytoplasm that carries never impulses away from the cell body

Schwann Cells

• Special cells that wrap around the axon and form the myelin sheath

Myelin Sheath

• A white lipid layer that covers most axons

• Nerve fibres with myelin sheath are called myelinated fibres, those without a myelin sheath are called unmyelinated fibres

What are the purposes of the myelin sheath?

1. Acts as an insulator

2. Protects the axon from damage

3. Speeds up nerve impulse

Neurilemma

Outermost coil of the Schwann cells that help to repair damaged fibres

Nodes of Ranvier

Gaps in the myelin sheath at intervals along the axon

Axon Terminal / Synapse

The ending point of the cell that divides into many small branches

Type of Neurons (Function)

• Sensory Neurons

• Motor Neurons

• Interneurons

Sensory Neurons

• Carry messages from the sensory organs or skin to the CNS

Motor Neurons

• Carry messages from the CNS to the effectors (muscles and glands)

Interneurons

• Located in the CNS and are the link between sensory neurons and motor neurons

Types of Neurons (Structural)

• Multipolar Neurons

• Bipolar Neurons

• Unipolar Neurons

• Pseudounipolar Neurons

Multipolar Neurons

• One axon and multiple dendrites

• Includes most interneurons in brain and spinal cord, as well as motor neurons to carry messages to skeletal muscle

Bipolar Neurons

• One axon and one dendrite

• Includes sensory neurons and interneurons (eye, ear, nose)

Unipolar Neurons

• One extension, an axon

• Not found in humans (insects only)

Pseudounipolar Neurons

• One axon from cell body, separating into two extensions (cell body is to the side of the axon. One extension connects to dendrites and the other to the axon terminal.

• Most sensory neurons that carry messages to the spinal cord are pseudounipolar

Neuron, Nerve Fibre, and Nerve

• A neuron is a nerve cell

• A nerve fibre is any long extension of cytoplasm from a cell body (usually axon)

• A nerve is a bundle of nerve fibres held together by connective tissue

Neurotransmission

The transfer of a message from the axon of one neuron to the dendrites of another