Central Nervous System - Anatomy & Physiology

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The central nervous system consists of; the brain (Prefix = "encephalo") and the spinal cord (Prefix = "myelo").

Cerebrum, brain stem and cerebellum. Image courtesy of BioMed Archive


Sensory neurons from both the internal and external environment relay information to the CNS. The CNS processes sensory information and initiates motor outputs. Effector and motor neurons from the CNS relay the appropriate outputs to effector organs.

The Autonomic Nervous System

The autonomic nervous system relays sensory information from, and motor information to, the internal environment. It therefore plays an important role in the maintenance of homeostasis.

The Somatosensory Nervous System

The somatosensory nervous system relays sensory information from, and motor information to, the external environment.

White and Grey Matter

White Matter

White matter consists of accumulations of myelinated axons. Myelinated axons are wrapped in myelin. Myelin is composed of lipid and protein in an 80:20 ratio. It insulates axons to give efficient action potential conduction. Myelin is provided by oligodendrocytes in the CNS. They respond poorly in injury. Schwann cells in the PNS myelinate one axon only. A "funiculus" is a large region of white matter in the spinal cord.

White and grey matter. Image courtesy of BioMed Archive

Grey Matter

The outer portions of the cerebral cortex and the inner portions of the spinal cord are composed of grey matter. Grey matter is also found in columns and scattered in brainstem nuclei. It is composed of neuronal cell bodies, plus glial cells.

Upper and Lower Motor Neurons

Lower Motor Neuron (LMN)

LMNs are efferent neurons which connect the CNS to smooth or skeletal muscle. Autonomic LMNs connect to smooth muscle. Somatic LMNs connect to skeletal muscle. Those innervating the muscles of the axial and peripheral skeleton have their cells bodies in the ventral horn of the spinal cord. Injury causes LMN weakness. This is characterised by; Depressed reflexes, decreased tone and neurogenic muscle atrophy.

Upper Motor Neuron (UMN)

The upper motor neuron comprises the motor system of the CNS. This is responsible for; Initiating voluntary movement and maintenance of tone and posture. In man, direct connections exist between neurons in the motor cortex and LMNs in the spinal cord. This is known as the "pyramidal system". In animals, there are scattered groups of interconnected neurons in the cortex and brainstem, which ultimately synapse with LMNs in the brainstem and spinal cord. This is called the "extrapyramidal system". UMN injury results in increased extensor tone, giving; Stiffness, spasticity, delay in the onset of protraction, and a longer stride, disinhibition of the LMN relfex ability, causing increased reflexes, the inability to stimulate the LMN and UMN weakness results.