Anatomy

The Brainstem is the central stalk of neural tissue, ahead of the spinal cord, to which the cerebral hemispheres, the basal ganglia, thalamus and cerebellum are attached.

The brainstem is divided into three sections: above the cord is the Medulla, above that is the Pons, and the Midbrain is at the rostral (headwards) end of the brainstem. At the rostral end of the midbrain, the ventral surface divides into two cerebral peduncles which connect with each side of the thalamus and internal capsule. The aqueduct expands to become the third ventricle.

The brainstem consists of neurones of different types:

1. Axons passing between the brain and spinal cord in both directions
2. Nuclei and axons of cranial nerves
3. The Reticular formation, which contains specialised areas of neural tissue concerned with different functions, but without distinct anatomical boundaries. These include areas of neural tissue controlling respiration, cardiovascular and gastrointestinal functions, micturition, sensitivity to pain, sleep and waking, and other functions.
4. Nerve cells and axons connecting the cerebellum with the remainder of the brain, including structures such as the olive in the medulla, and the cerebellar peduncles.
5. Structures such as the colliculi and substantia nigra of the midbrain, which have specific connections and functions.

Top

12 Cranial nerves originate from the brainstem and base of the brain, and have a wide range of functions, including:

1. Special senses (I: Smell; II: Vision; VIII: Hearing)
2. Eye Movements (III, IV, VI)
3. Sensory and Motor functions in the head and neck: (V, VII, XI)
4. Balance (VIII)
5. Autonomic functions: (III, IX, X)
6. Contol of the tongue, larynx and speech (IX, X, XII)

Anatomy of the Forebrain : Thalamus and Cerebral Cortex   Top

The thalamus consists of two large groups of nuclei, one on each side of the third ventricle. The structure is approximately the size of a walnut and many of the axons of these nuclei project to the cerebral cortex.

There are many groups of nuclei concerned with different functions within the thalamus. Two separate groups of thalamic nuclei are concerned with innocuous sensations and noxious sensations; both project to the primary sensory recieving area of the cortex. Others are relay stations associated with the coordination of movement, and special senses such as vision and hearing.

Cerebral Cortex

The surface of the cerebral hemispheres is convoluted and folded, creating some constant deep fissures (sulci) which help to divide the cerebrum into lobes. The most prominent sulcus is the central sulcus; it is a deep groove that separates the frontal and parietal lobes, and is a constant feature of the human brain.

The gyri are separated by the sulci, are grey folds on the surface of the brain. Prominent gyri are seen on each side of the central sulcus - the pre- and post-central gyri. The precentral gyrus is concerned with fine control of movement, while the post-central gyrus is the primary sensory receiving area concerned with fine sensation.

Lobes of the Cerebral Hemispheres

The cerebral hemispheres are divided into the frontal, occipital, temporal, and parietal lobes. The occipital lobe is concerned with processing visual information, and the temporal lobe is concerned with hearing and speech. A deep part of the temporal lobe, the hippocampus, is concerned with memory and learning. The frontal lobe is concerned with thought, planning and executive functions.

Physiology   Top

The sense of Fine Touch is a highly sensitive discriminative sensation, most highly developed in the fingers, lips and genitalia.

The sensory receptors that mediate fine touch sensation all have myelinated axons and complex end organs, that allow them to distinguish moving or vibratory stimuli from maintained pressure or stretch of the skin. The former group are rapidly adapting receptors, whereas the receptors that maintain their discharge of action potentials throughout a period of mechanical stimulation are called slowly adapting receptors.

Sensory receptors have overlapping receptive fields (the area of skin served by a single axon and its branches) that are smallest in the fingers and lips, where they account for the ability to discriminate between mechanical stimuli that are close together. In skin areas where discrimination between two mechanical stimuli is poor (such as the skin of the back), the receptive fields are large.

The central pathways of receptors concerned with fine touch include the dorsal columns of the same side of the body (ipsilateral), the dorsal column nuclei (n. gracilus and n. cuneatus).

Information is relayed through the dorsal column nuclei and these axons pass through the brainstem and project to the contralateral thalamic nuclei. This relay station sends its axons to the primary somatosensory receiving area of the cerebral cortex (the post-central gyrus).

In each of these relay stations there is a process of contrast enhancement, which helps to provide more precise information concerning the location of touch stimuli.

Proprioception and Kinaesthesia requires input from Vibration receptors, Muscle Spindles, Golgi Tendon Organs and Joint receptors. The central pathays are similar to those for fine touch.

Pain and Nociception   Top

Pain is a subjective response to injurious or inflammatory (noxious) stimuli. The sensory mechanism depends on conduction of afferent impulses in unmyelinated nerve fibres with bare nerve endings. A network of neurones in the superficial layers of the dorsal horn process this information, along with other afferent inputs which can modulate the signal that passes into ascending pain pathways.

The classical anatomical description of the ascending pain pathway is the spinothalamic tract, which originates in the dorsal horn and ascends in the contralateral lateral columns to the thalamus. We now know that there are a number of other ascending pain pathways, including the spino-reticular tract and the post-synaptic dorsal column pathway. The classical anatomical view was that the dorsal columns consisted of axon collaterals of primary afferent neurones, and transmitted innocuous sensations. But modern evidence is that the dorsal columns do have an ascending post-synaptic system conerned with pain, and explains why tractotomies are not always successful in producing permanent pain relief.

The dorsal horn contains a netork of neurones that alter the strength of the nociceptive signal passed t ascencing pathays. The Gate Theory describes the importance of a balanced input between small and large fibre activity. Other factors that modulate the pain pathway within the dorsal horn are descending pathways from the braintem (medulla), midbrain (periaqueductal gray matter) and other areas of the central nervous system.

Testing the senses is a basic routine part of clinical assessment in neurological cases. In spinal injuries such as complete transection of the spinal cord as a result of trauma, there is complete absence of sensation of all types below the level of the lesion.

Chronic Pain is associated with neuroplastic changes in neurones and nociceptive pathways.

The origins of Phantom Limb Pain are discussed.