Brain Structure, Sensory Receptors & Motor Control

Overview

Lectures 22, 23, and 24 complete the neuroanatomy theme by covering the gross anatomy of the brain and its regions, the major ascending sensory pathways (dorsal column pathway and spinothalamic tract), the major descending motor pathway (corticospinal tract), sensory receptor types and transduction, and the neural circuits underlying motor control including voluntary movements and reflexes.

External Anatomy of the Brain — Lobes of the Cerebrum

The cerebral cortex (cerebrum) is the outermost layer of the brain, heavily folded into gyri (ridges) and sulci (valleys/furrows) to maximise surface area. It is divided into four lobes by major sulci:

| Lobe | Key Sulcus Boundaries | Primary Functions |

|---|---|---|

| Frontal lobe | Anterior to central sulcus; above lateral sulcus | Voluntary motor control (precentral gyrus = primary motor cortex), executive function, personality, language production (Broca's area) |

| Parietal lobe | Posterior to central sulcus; above lateral sulcus | Somatosensory processing (postcentral gyrus = primary somatosensory cortex), spatial awareness |

| Temporal lobe | Below lateral sulcus | Auditory processing, language comprehension (Wernicke's area), memory (hippocampus) |

| Occipital lobe | Posterior; behind parieto-occipital sulcus | Visual processing (primary visual cortex V1) |

Central sulcus: separates frontal and parietal lobes. The precentral gyrus (anterior to central sulcus) is the primary motor cortex. The postcentral gyrus (posterior to central sulcus) is the primary somatosensory cortex.

Internal Structures and White Matter

On coronal section, the brain shows:

• Grey matter (cerebral cortex) — outermost, contains cell bodies.
• White matter — beneath cortex, contains myelinated axon tracts.
• Deep nuclei (basal ganglia, thalamus) — clusters of grey matter deep within the white matter.

Three types of white matter tracts:

1. Commissural tracts: axons crossing from one hemisphere to the other. Primary example: corpus callosum — the large white matter band connecting the two cerebral hemispheres.
2. Projection tracts: axons extending between cortex and subcortical/spinal structures. Example: corticospinal tract.
3. Association tracts: axons within one hemisphere connecting different cortical areas (short or long distance).

Major Brain Regions: Medial View

| Region | Structures |

|---|---|

| Cerebrum | Cerebral cortex, corpus callosum, basal ganglia |

| Diencephalon | Thalamus (sensory relay), hypothalamus (autonomic & endocrine control) |

| Brainstem | Midbrain, pons, medulla oblongata |

| Cerebellum | Coordination, balance, fine motor control |

Sensory Pathways

Dorsal/Posterior Column Pathway (Fine Touch, Vibration, Proprioception)

Three-neuron chain from receptor to cortex:

1. Neuron #1: Unipolar neuron. Cell body in dorsal root ganglion. Peripheral fibre from skin receptor (touch, vibration, proprioception). Central fibre ascends ipsilaterally in dorsal columns → synapses on neuron #2 in the medulla oblongata.
2. Neuron #2: Cell body in medulla oblongata. Axon crosses (decussates) to the opposite side and ascends → synapses on neuron #3 in thalamus.
3. Neuron #3: Cell body in thalamus. Axon ascends to primary somatosensory cortex (postcentral gyrus) → conscious perception.

*Clinical point*: A lesion in the dorsal column (spinal cord) causes ipsilateral loss of fine touch/proprioception below the lesion. A cortical lesion causes contralateral loss.

Spinothalamic Tract (Pain, Temperature)

1. Neuron #1: Dorsal root ganglion. Synapse on neuron #2 in spinal cord (dorsal horn).
2. Neuron #2: Cell body in spinal cord ("tract cells"). Axon crosses immediately via anterior white commissure → ascends contralaterally → synapses on neuron #3 in thalamus.
3. Neuron #3: Thalamus → somatosensory cortex.

*Clinical point*: Pain/temperature decussates at the spinal cord level → lesion at spinal cord causes contralateral loss from that level down.

Motor Pathway — Corticospinal Tract

Upper Motor Neurons (UMN)

• Cell bodies in primary motor cortex (precentral gyrus).
• Axons descend through the internal capsule, brainstem, and decussate in the medulla oblongata (pyramidal decussation at the pyramids).
• Then descend in the lateral corticospinal tract of the spinal cord.
• Synapse onto lower motor neurons in the ventral horn of the spinal cord.

Lower Motor Neurons (LMN)

• Cell bodies in ventral horn of spinal cord.
• Axons exit via ventral root → spinal nerve → peripheral nerve → neuromuscular junction (NMJ) on skeletal muscle.

Primary Motor Cortex Organisation (the Motor Homunculus)

Specific regions of the precentral gyrus control specific body regions in a somatotopic map. Areas devoted to the face, hands, and tongue are disproportionately large (reflecting precision requirements).

Sensory Receptors (Lecture 23)

Sensory receptors convert (transduce) environmental stimuli into action potentials:

| Receptor Type | Stimulus | Ion channels | Adapts |

|---|---|---|---|

| Thermoreceptor | Temperature changes | Temperature-gated Na⁺ | Phasic (fast) |

| Chemoreceptor | Chemical concentration | Chemically-gated | Tonic or phasic |

| Mechanoreceptor — Tactile | Touch, pressure, vibration | Mechanically-gated Na⁺ | Mostly phasic |

| Mechanoreceptor — Proprioceptor | Muscle/tendon stretch | Mechanically-gated | Tonic (slow) |

| Mechanoreceptor — Baroreceptor | Vascular pressure/stretch | Mechanically-gated | Tonic |

| Nociceptor | Noxious/tissue-damaging stimuli | Temp/chem/mechanical-gated | Mostly tonic |

Tonic (slow-adapting) receptors: continually active; signal ongoing stimulus intensity. Example: proprioceptors.

Phasic (fast-adapting) receptors: silent at rest; fire with change and then stop. Detect changes. Example: temperature receptors.

Receptive fields: the area of skin from which a single sensory neuron collects signals. Small, densely packed receptive fields (fingers, lips) → high two-point discrimination and precise localisation. Large, sparse fields (trunk, limbs) → poor localisation.

Sensory intensity is encoded by the frequency of action potentials and the number of neurons activated.

Motor Control (Lecture 24)

Brain Regions for Voluntary Movement

1. Prefrontal cortex: plans the desired movement outcome (what to do).
2. Premotor cortex: organises the movement sequence to achieve the outcome (how to do it).
3. Primary motor cortex (precentral gyrus): directs execution of specific voluntary movements via UMNs.

Modulatory Structures

• Basal nuclei (basal ganglia): deep grey matter nuclei. Refine movements by selecting which to allow and which to inhibit. Damage causes hypokinetic disorders (Parkinson's disease — too little movement) or hyperkinetic disorders (Huntington's — too much movement).
• Cerebellum: stores motor programs; compares actual movement to planned movement (using proprioceptor and balance input); organises timing of muscle contractions. Damage causes ataxia — poor coordination, clumsy movements, intention tremor.

Neuromuscular Junction (NMJ)

• Specialised chemical synapse between an LMN axon terminal and a skeletal muscle fibre.
• Neurotransmitter: ACh (always excitatory).
• ACh binds chemically-gated Na⁺ channels on the sarcolemma → EPSP → action potential → muscle contraction.
• Unlike neuron-neuron synapses, each muscle fibre receives input from only ONE neuron at ONE site, and no summation is required — a single arriving AP very reliably causes contraction.

Motor Units

A motor unit = one lower motor neuron + all the skeletal muscle fibres it innervates.

• Small motor units (few fibres per neuron): fine, precise movements (fingers, tongue).
• Large motor units (many fibres per neuron): powerful, coarse movements (limb muscles).

Voluntary vs. Reflex Responses

| Feature | Voluntary | Reflex |

|---|---|---|

| Latency | 100+ ms | ~30–40 ms |

| Path | Receptors → somatosensory cortex → motor cortex → LMN | Receptors → spinal cord → LMN (no brain involvement) |

| Trainability | Improves with practice | Fixed / not trainable |

| Purpose | Complex interaction with environment | Rapid protection from injury |

The Stretch Reflex (e.g. Patellar/Knee-Jerk Reflex)

1. Tendon tap → sudden stretch of quadriceps.
2. Muscle spindle (proprioceptor) stretched → mechanically-gated Na⁺ channels open.
3. AP propagates along sensory axon to spinal cord.
4. Sensory axon synapses directly on motor neuron (no interneuron).
5. AP fires along motor axon → NMJ → quadriceps contracts → knee extends.
6. The same sensory neuron also activates an inhibitory interneuron that inhibits the hamstring motor neuron — preventing opposition.

The Withdrawal Reflex

• Response to painful stimuli: nociceptors activated → AP to spinal cord → interneurons excite flexor motor neurons (withdraw limb) and inhibit extensor motor neurons — a protective, automatic, spinal circuit.