Spinal Cord and its Functions: Study Notes

For the complete syllabus, check out our PMDC MDCAT Syllabus 2025 article.

Spinal Cord:

• It is thick, whitish nerve cord that lies below the medulla and extends down through the neuronal canal of vertebrae up to the hips.
• It can be divided into four divisions: cervical, thoracic, lumbar, and sacral.
• Almost all of the structures below the neck receive sensory and motor innervation from the spinal cord.
• It has an inner butterfly shaped grey matter containing a central canal and outer portion of white matter.

PERIPHERAL NERVOUS SYSTEM

• The peripheral nervous system (PNS) is made up of nerve tissue and fibers outside the brain and spinal cord, such as the 12 pairs of cranial and 31 pairs of spinal nerves.
• The PNS thus connects the CNS to the rest of the body and can itself be subdivided into the somatic and autonomic nervous systems.

Parasympathetic and Sympathetic Nervous System

Central Nervous System (CNS):

• Structure:
  • Brain and spinal cord
• Function:
  • Integrative and control centers

Peripheral Nervous System (PNS):

• Structure:
  • Cranial nerves and spinal nerves
• Function:
  • Communication lines between the CNS and the rest of the body

Divisions of Peripheral Nervous System:

1. Sensory (Afferent) Division:

• Structure:
  • Somatic and visceral sensory nerve fibers
• Function:
  • Conducts impulses from receptors to the CNS

1. Motor (Efferent) Division:

• Structure:
  • Motor nerve fibers
• Function:
  • Conducts impulses from the CNS to effectors (muscles and glands)

Motor Division Subdivisions:

• Autonomic Nervous System (ANS) (Involuntary):
• Structure:
  • Visceral (internal organs) motor
• Function:
  • • Sends input to internal receptors and conducts impulses from CNS to cardiac muscles, smooth muscles, and glands
  • Affects heart rate, digestion, respiration rate, salivation, perspiration, pupil diameter, and sexual arousal
• Sympathetic Division:
• Function:
  • • • Mobilizes body systems during activity; quick response "fight or flight"
  • Neurotransmitter: Noradrenaline (Adrenergic)
• Parasympathetic Division:
• Function:
  • • • Conserves energy; non-immediate response "rest and digest"
      • Neurotransmitter: Acetylcholine (Cholinergic)
  • Promotes "housekeeping" functions during rest
• Somatic Nervous System (SNS) (Voluntary):
• Structure:
  • Somatic motor
• Function:
  • • Provides input from sense organs (touch, hearing, and sight)
  • Conducts impulses from CNS to skeletal muscles

RECEPTORS

• The modified neurons or epithelial cells which occur either singly or in groups with other cell types or receive stimuli from external or internal environments are called receptors.
• The receptors acts as a receptionist of nervous system.
• The stage of awareness of stimulus is called as “sensation”.
• The impulses are converted into perception.

TYPES OF RECEPTORS:

Depending upon the type of energy detected by receptors, they can be classified into following groups.

• Thermoreceptors
• Chemoreceptors
• Mechanoreceptors
• Photoreceptors
• Pain Receptors

WORKING OF SENSORY RECEPTORS IN SKIN

FREE NERVE ENDINGS:

• They are the simplest type of receptors having free ends without any protective capsule.
• These are found in epidermis or dermis layer of skin.
• All of these receptors adapt very slowly to stimulation.

ENCAPSULATED RECEPTORS:

• The nerve endings of these receptors are enclosed in capsule of connective tissues.
• They are also found in skin.

Types:

These are of following two types.

Meissner’s Corpuscles:

• Their capsules consist of thick collagen fibers with spiral and highly coiled nerve ending.
• Abundant in hairless areas of skin such as finger tips, lips, eye-lids, nipples, palms, soles etc. These are sensitive to touch.

Pacinian Corpuscles:

• In this corpuscle, the sensory nerve ending is surrounded in concentric onion like layers of membranes alternating with fluid filled spaces.
• These are located in dermis as well as in some internal organs and freely moveable joints.
• Pacinian corpuscles respond to rapid change in pressure associated with touch and vibration.

Carotid and aortic arch detect changes in arterial mean pressure and pulse pressure.

Carotid sinus and aortic arch have chemoreceptors which are sensitive to Carbon dioxide and H+ in blood.

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