The Brain, the Spinal Cord and Reflexes: Quick Revision

A brief note on The Brain, the Spinal Cord and Reflexes. Read it before a test. Try each question at the end before you open its answer.

The coverings and the regions

• Cranial meninges: dura mater (outer), arachnoid (thin, middle), pia mater (inner, on the brain).
• Forebrain: cerebrum, thalamus, hypothalamus. Midbrain. Hindbrain: pons, cerebellum, medulla.
• NCERT: the midbrain and the hindbrain form the brain stem.

The forebrain

• Cerebrum: two hemispheres joined by the corpus callosum. The cortex is the grey matter (motor, sensory and association areas). The inner part is the white matter.
• Thalamus: coordinating centre for sensory and motor signals.
• Hypothalamus: at the base of the thalamus. Centres for body temperature, eating and drinking. Neurosecretory cells.
• Limbic system: amygdala, hippocampus and the hypothalamus. Emotion and motivation.

The midbrain and the hindbrain

• Midbrain: cerebral aqueduct and four corpora quadrigemina on its back.
• Pons: fibre tracts. Cerebellum: very convoluted surface. Medulla: centres for respiration, cardiovascular reflexes and gastric secretions.

Reflex action

• Reflex: an involuntary response that needs a part of the CNS, with no conscious effort.
• Reflex arc: receptor, afferent neuron (dorsal root), efferent neuron, effector. The knee jerk is an example.
• The efferent path cut: no movement, but the stimulus is felt. The afferent path cut: no reflex and no feeling.

A worked example

An afferent fibre of 0.5 m and an efferent fibre of 0.5 m conduct at 50 m/s. There is one synapse of 0.5 ms. How long does the reflex take?

1. Each fibre: $0.5 \div 50 = 0.01$ s $= 10$ ms. Two fibres: $20$ ms.
2. Add the synapse: $20 + 0.5 = 20.5$ ms.

Practice with new numbers

A program to try

The program prints the parts of a reflex arc in order.

Mistakes to avoid

• Putting the arachnoid next to the brain. The pia mater touches the brain.
• Calling the cortex white matter. The cortex is the grey matter.
• Giving the centres of respiration to the pons. They are in the medulla.
• Saying that the afferent neuron leaves by the ventral root. It enters by the dorsal root.

Check yourself

1. Which part joins the two cerebral hemispheres?

2. Which part of the brain has centres for body temperature and for hunger and thirst?

3. What is the smallest reflex arc made of?

4. Which region of the brain holds the cerebral aqueduct?

Sources

• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 35, The Nervous System.
• NCERT (2023). Biology, Textbook for Class XI, chapter 18.