Neuroanatomy: An Overview

Overview

The nervous system is divided into:

• The central nervous system (CNS), which consists of the:
  • Brain
  • Spinal cord
• The peripheral nervous system (PNS), which is divided into:
  • The somatic nervous system, which consists of:
    • Motor nerves
    • Sensory nerves
    • Spinal nerves
    • Cranial nerves
• The autonomic nervous system which is divided into:
  • The sympathetic nervous system (‘fight or flight’)
  • The parasympathetic nervous system (‘rest and digest’)
  • The enteric nervous system

The Somatic Nervous System

Overview

The somatic nervous system is the part of the peripheral nervous system associated with voluntary movement via skeletal muscles. The somatic nervous system consists of:

• Sensory neurones:
  • Relay sensation to the CNS
• Motor neurones:
  • Relay motor impulses from the CNS to muscles
• Spinal nerves:
  • Nerves that carry both sensory information into and motor impulses out of the spinal cord
• Cranial nerves:
  • Carry impulses into and out of the brain stem

Motor neurones

The cell bodies of motor neurones are found in the cerebral cortex, brain stem, or spinal cord. Motor neurones convey impulses from these areas to muscles and glands. Motor neurones are divided into:

• Upper motor neurones (UMNs):
  • Travel from the cerebral cortex through different tracts and synapse with lower motor neurones in the brainstem or spinal cord
• Lower motor neurones (LMNs):
  • Transmits signals from UMNs to the effector muscles to perform a movement

UMN and LMN signs play an important role in working out where a neurological lesion is. These are discussed in more detail in Upper Motor Neurones, Lower Motor Neurones, and Descending Tracts.

Sensory neurones

Sensory neurones are nerves that contain only sensory (afferent) nerve fibres. These convey impulses from sensory receptors to the central nervous system to the dorsal root ganglia of a spinal nerve.

Damage to sensory nerves can lead to problems with:

• Coordination and balance, due to a loss of proprioception sense
• Temperature sensation
• Pain sensation – such as in those with diabetes mellitus and infections of the feet

Spinal nerves

Spinal nerves are mixed nerves that carry both motor signals out of and sensory signals into the spinal cord. There are:

• 8 pairs of cervical nerves (C1-C8)
• 12 pairs of thoracic nerves (T1-T12)
• 5 pairs of lumbar nerves (L1-L5)
• 5 pairs of sacral nerves (S1-S5)
• 1 pair of coccygeal nerves (Co1)

Each spinal nerve is made from a combination of nerve fibres from its:

• Dorsal root – carries afferent sensory signals from receptors to the spinal cord
• Ventral root – carries efferent signals from the spinal cord to effectors (e.g. muscles)

Cranial nerves

Cranial nerves are pairs of nerves that emerge directly from the brain and brainstem. They are considered to be part of the peripheral nervous system, although technically speaking, the nerves originating from the brain are part of the CNS.

The cranial nerves include:

• Cranial nerves emerging from the cerebrum:
  • CN I – olfactory nerves
  • CN II – optic nerves
• Cranial nerves emerging from the brainstem:
  • Cranial nerves emerging from the midbrain:
    • CN III – oculomotor nerves
    • CN IV – trochlear nerves
  • Cranial nerves emerging from the pons:
    • CN V – trigeminal nerves
    • CN VI – abducens nerves
    • CN VII – facial nerves
    • CN VIII – vestibulocochlear nerves
• Cranial nerves emerging from the medulla:
  • CN IX – glossopharyngeal nerves
  • CN X – vagus nerves
  • CN XI – accessory nerves
  • CN XII – hypoglossal nerves

Cranial nerves and their clinical significance are discussed in more detail in Cranial Nerves.

The Autonomic Nervous System

Overview

The autonomic nervous system describes the division of the nervous system that is involved in the control of unconscious functions, including the control of smooth muscles, internal organs, and glands. Autonomic functions include the control of heart rate and respiration.

The autonomic nervous system uses a two-neurone efferent pathway, meaning a preganglionic neurone from the spinal cord must synapse onto a postganglionic neurone before innervating the target structure.

The sympathetic nervous system

Activation of the sympathetic nervous system results in a ‘fight or flight’ response. It is made up of cell bodies that run in the lateral aspect of the spinal cord from T1 to L2-3 which form the preganglionic neurones. These synapse with postganglionic neurones in differing locations.

The parasympathetic nervous system

Activation of the parasympathetic nervous system results in a ‘rest and digest’ response. It is made up of cell bodies found in the brainstem (cranial nerves III, VII, IX, or X) and the sacral spinal cord (S2-S4) which form preganglionic neurones. These synapse with postganglionic neurones in different locations.

The enteric nervous system

The enteric nervous system is another division of the autonomic nervous system involved in controlling functions of the gut, such as peristalsis and secretion of digestive juices. 

Neurotransmitters and receptors in the autonomic nervous system

The autonomic nervous system requires preganglionic neurones to synapse with postganglionic neurones which innervate target structures. Neurotransmitters and receptors can be targeted by drugs and have different effects.

Preganglionic neurones of both the sympathetic and parasympathetic divisions release acetylcholine (ACh) which binds to nicotinic receptors (nAChR) on postganglionic neurones.

The postganglionic neurones release either:

• ACh (which binds to muscarinic receptors on effector tissues) – all parasympathetic postganglionic neurones
• Noradrenaline (NA, adrenergic neurones) – nearly all sympathetic postganglionic neurones

A summary of some muscarinic receptors is as follows:

A summary of some adrenergic receptors is as follows:

The effects of the autonomic nervous system

A summary of the effects of the autonomic nervous system is as follows: