Physiology of the Nervous System: Central Nervous System

General Design of the Nervous System

• The central nervous system is estimated to contain 80 to 100 billion neurons.
• A typical brain neuron receives incoming signals through synapses located mostly on dendrites, but also on the cell body.
• A neuron can have only a few hundred or as many as 200,000 synaptic connections from input fibers.
• The output signal travels by way of a single axon leaving the neuron, which may have many separate branches to other parts of the nervous system or peripheral body.

Sensory Part of CNS: Sensory Receptors

• Most activities of the nervous system are initiated by exciting sensory receptors, such as:
  • Visual receptors in the eyes
  • Auditory receptors in the ears
  • Tactile receptors on the surface of the body
• Sensory information enters the central nervous system through peripheral nerves and is conducted to multiple sensory areas, including:
  • Spinal cord at all levels
  • Reticular substance of the medulla, pons, and mesencephalon of the brain
  • Cerebellum
  • Thalamus
  • Areas of the cerebral cortex

Motor Part of CNS – Effector Organ

• The most important eventual role of the nervous system is to control various bodily activities.
• The motor part of the CNS is divided into two levels:
  • Lower brain or subcortical level: controls subconscious activities of the body, such as:
    • Control of arterial pressure and respiration
    • Control of equilibrium
    • Feeding reflexes
    • Emotional patterns
  • Higher brain or cortical level: responsible for higher functions, such as:
    • Reading, writing, and speaking
    • Calculations and composing music
    • Memory storage and retrieval

The Synapse

• A neuron is composed of three major parts: the soma, axon, and dendrites.
• The presynaptic terminals of the first neuron synapse with the dendrite or soma of the second neuron.
• Synapses can be either excitatory (secreting excitatory neurotransmitter) or inhibitory (secreting inhibitory neurotransmitter), depending on the type of postsynaptic receptor.

Transmission of Signals from the Motor Cortex to the Muscles

• Motor signals are transmitted from the cortex:
  • Directly to the spinal cord through the corticospinal tract (pyramidal tract)
  • Indirectly through multiple accessory pathways that involve the basal ganglia, cerebellum, and various nuclei of the brain stem (extrapyramidal system)
• The corticospinal tract originates from the primary motor cortex, premotor and supplementary motor cortex, and somatosensory cortex.

Brain Stem Control of Motor Functions

• The brain stem consists of the medulla, pons, and midbrain (mesencephalon).
• The brain stem contains motor and sensory nuclei that perform motor and sensory functions for the face and head regions.
• The brain stem is also involved in controlling respiration, the cardiovascular system, and many stereotyped movements of the body.

Somatosensory Cortex

• The somatosensory cortex is responsible for receiving and interpreting sensory information from the body.
• The somatosensory cortex is divided into two areas: somatosensory area I and somatosensory area II.
• The central fissure (central sulcus) separates the two areas, with the anterior half of the parietal lobe concerned with reception and interpretation of somatosensory signals.

Motor Pathways and Autonomic Nervous System

• The motor cortex controls motor activity and is divided into three subareas: primary motor cortex, premotor area, and supplementary motor area.
• The primary motor cortex controls discrete movements, while the premotor area and supplementary motor area control more complex movements and attitudinal movements of the body.