How the Brain Works The Facts Visually Explained by DK (z-lib.org)

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At the neuromuscular junction, the end

of the axon releases acetylcholine, a

neurotransmitter (see p.24). The acetylcholine

binds to receptors in the muscle cell

membrane. This triggers chemical

reactions that make the muscle

fiber contract.

The upper and lower motor neurons meet in the ventral horn of the spinal cord.

The outer part of the ventral horn carries

nerves that run to the hands and feet;

the central part carries nerves to

the upper arms and thighs.

BRAIN FUNCTIONS AND THE SENSES

Making a Move

From spine to muscleInside the spinal cord, the axons of the corticospinal tract, which are covered with a myelin sheath, form the white matter. The gray matter at the center of the spinal cord consists of the cell bodies of motor neurons. The ends of the corticospinal axons (known as upper motor neurons) synapse on to motor neurons (known as lower motor neurons) in the ventral horn of the gray matter. The axons of the lower neurons exit the spine through gaps in the vertebrae (see p.12) and extend to the muscle fibers. The point where the nerve endings activate the muscle fibers to complete the movement is called the neuromuscular junction.

Upper motor neurons

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Executing movementNerve signals make a muscle

contract and pull on the associated joint to move the part of the limb

just beyond it. Muscles used in fine movements have more nerve endings than those used for simple movements.

MUSCLE

HOW LONG DOES IT

TAKE FOR A SIGNAL

TO TRAVEL FROM

BRAIN TO MUSCLE?

Signals can travel

from the brain to our

muscles at a speed of up

to 395 ft (120 m)

per second.

RADIAL NERVE

WHITE MAT TER

GR AY

MAT TER

Acetylcholine

Muscle contracts and moves joint,

causing arm to

bend

AXON TERMINAL

2

3

MUSCLE FIBER

Receptor for acetylcholine

SYNAPTIC CLEFT

Lower motor neurons

RIGHT ARM

Lower motor neurons pass signals from spinal cord to muscles

VENTRAL

HORN

Direction

of signal

US_098_099_Making_a_move.indd 99 20/09/2019 12:35