Human Nervous System: CNS and PNS Functions Explained

The Human Nervous System

Peripheral Nervous System (PNS)

Consists of cranial nerves and spinal nerves.

Central Nervous System (CNS)

Consists of the brain and spinal cord.

The Brain

• Cerebrum: Features left and right hemispheres with surface convolutions. It interprets sensory information and controls voluntary actions, emotions, and intellectual processes like thinking and speech.
• Cerebellum: Controls posture and balance, coordinating complex voluntary movements like walking.
• Brain Stem: Connects the brain and spinal cord, controlling involuntary actions such as heartbeat, breathing, body temperature, and sleep.

Spinal Cord

A column of nerve tissue protected by the spinal column. It connects the brain to the PNS, transmits nerve impulses, and controls many reflex actions.

Grey and White Matter

• Composition:
  • White matter: Composed of myelin-covered axons.
  • Grey matter: Composed of cell bodies, axons without myelin, and dendrites.
• Distribution: A cross-section shows grey matter located in the outer part of the brain and white matter in the inner part; in the spinal cord, this arrangement is reversed.

Peripheral Nervous System (PNS) Details

• The PNS connects the CNS to the limbs and organs, serving as a relay between the brain/spinal cord and the rest of the body.
• The PNS is not protected by the skull, vertebrae, or the blood-brain barrier, leaving it exposed to toxins and mechanical injury.
• Cranial nerves: Emerge from the brain.
• Ganglia: Groups of nerve cell bodies located at the sides of the spinal cord.
• Spinal nerves: Emerge from the spinal cord.

Peripheral Nerves

Composed of axons grouped together in bundles.

Types of PNS Nerves

• Sensory nerves: Transmit information from receptors to the CNS.
• Motor nerves: Transmit information from the CNS to effectors (glands, muscles).

A ganglion is a nerve cell cluster or group of nerve cell bodies located in the PNS and sensory system.

Divisions of the Nervous System

Somatic Nervous System

Involved in voluntary muscle movement and involuntary reflex arcs.

• Voluntary: Controlled by the brain (e.g., opening a door or speaking).
• Involuntary: Rapid automatic reflex actions originating in the spinal cord (e.g., pulling your hand away from a hot object).
• Three types of neurons are involved: sensory, relay, and motor neurons.

Autonomic Nervous System

Controls unconscious, automatic actions regulated by the CNS.

• Sympathetic: Increases heart rate and controls the fight-or-flight response during physical or emotional stress.
• Parasympathetic: Decreases heart rate and controls activities for rest and digestion.