BIOL1019 Chap.1 Brain, Spinal Cord and Reflexes
Brain, Spinal Cord and Reflexes
The nervous system splits into a central part that processes and co-ordinates information and a peripheral part that carries it. This chapter starts in the spinal cord, where gray matter integrates and white matter carries tracts, and where dorsal roots bring sensory axons in while ventral roots send motor axons out. It then assigns one core job to each brain region, from the life-sustaining brainstem to the cerebrum.
The worked example follows a barefoot step onto a pin, in which a withdrawal reflex and a crossed extensor reflex run at the same time, and the practice questions test reflex classification and the effect of cutting a spinal root.
These topics open Week 1 and are tested in Test 1 as well as the final exam, and the Week 2 practical asks you to observe and time real reflexes, so the reflex arc is worth learning as a sequence you can apply to any stimulus and effector.
What this chapter covers
- 01
CNS and PNS: processing versus transmitting
- 02
Gray matter, white matter and tracts
- 03
Dorsal root ganglia, dorsal and ventral roots
- 04
Dermatomes as a clinical map
- 05
Monosynaptic and polysynaptic reflexes
- 06
Stretch, Golgi tendon, withdrawal and crossed extensor reflexes
- 07
Brainstem, cerebellum, diencephalon, limbic system and cerebrum
- 08
Cranial nerves and their four classes
Worked example · free
Classifying a tendon tap reflex
- +1Somatic, because the effector is skeletal muscle rather than an internal organ.
- +1Monosynaptic: the sensory neuron synapses directly with the motor neuron in the spinal cord.
- +1Arc: muscle spindle receptor, afferent sensory neuron, integrating centre in the cord, efferent motor neuron, thigh muscle as effector.
- +1It is fast because there is only one synapse and the brain is not involved in producing the response.
Key terms
- Gray matter
- Nervous tissue rich in neuron cell bodies that integrates information and initiates commands.
- White matter
- Nervous tissue made mainly of axons organised into ascending and descending tracts.
- Dermatome
- The strip of skin whose sensation is carried by one pair of spinal nerves.
- Crossed extensor reflex
- A reflex that extends the opposite limb while the stimulated limb withdraws, keeping the body supported.
- Medulla oblongata
- The lowest part of the brainstem, holding the cardiovascular and respiratory rhythmicity centres.
- Golgi tendon organ
- A tension receptor among tendon collagen fibres whose signals inhibit the same muscle through interneurons.
Brain, Spinal Cord and Reflexes FAQ
What is the difference between a tract and a nerve?
A tract is a bundle of axons inside the central nervous system that shares an origin and a destination, such as an ascending tract carrying sensation to the brain. A nerve is a bundle of axons in the peripheral nervous system. The optic nerve is the one exception the lectures point out, because it lies within the CNS.
Why is every spinal nerve called a mixed nerve?
Each spinal nerve forms where a dorsal root carrying sensory axons joins a ventral root carrying motor axons. The nerve beyond that junction therefore contains both sensory and motor fibres, which is why damage to a single spinal nerve can affect sensation and movement in the region it supplies.
How do I tell monosynaptic and polysynaptic reflexes apart?
Count the synapses between the sensory and motor neurons. A monosynaptic reflex has one, so it is very fast and simple, like the stretch reflex. A polysynaptic reflex passes through a pool of interneurons, can activate several muscle groups and can inhibit some muscles, as in withdrawal, crossed extensor and Golgi tendon reflexes.
What does the Week 2 practical ask me to do with reflexes?
The Nervous System Practical, worth 10%, is completed in groups during the Week 2 workshop. Groups use Lt and a tendon-hammer biosensor to test reflexes, time them and observe the Babinski reflex, then submit one group report before the workshop ends. Each student must commit the pre-practical activity beforehand to avoid a 50% deduction.
Exam move
Draw the spinal cord in cross-section from memory and label the dorsal root ganglion, dorsal root, ventral root, gray horns and white matter, then trace one reflex arc across it. Make a two-column table pairing each brain region with its single most examinable function. Finally, practise one classification question for every reflex named in the lectures, giving its type, number of synapses and receptor.
When you finish, explain each reflex aloud as a five-part arc and name where the integrating centre sits, because the lectorial's own checklist asks for the steps of a spinal reflex and for a specific example of each reflex type.
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