Introduction to Neuroanatomy

Original Editor - Joanne Garvey and Naomi O'Reilly

Top Contributors - Joanne Garvey, Lucinda hampton, Naomi O'Reilly, Laura Ritchie, Kim Jackson, Rachael Lowe, Kate Sampson, Admin, Tarina van der Stockt, Simisola Ajeyalemi, Adam Vallely Farrell, Rucha Gadgil, Rewan Elsayed Elkanafany, Jess Bell, WikiSysop, Mande Jooste, Scott Buxton and George Prudden  

Overview

The nervous system is made up of vast neural networks; signalling within these circuits enables thinking, language, feeling, learning, memory, and all function and sensation.  It is well-established that through plasticity of existing cells our nervous systems can adapt to situations not previously encountered, but it also has been shown that cells (NSCs) are plastic and involved in creating new connections in adaptation and response to injury.

The Nervous System has three specific functions:

1. Sensory Input - Sensory receptors present in the skin and organs respond to external & internal stimuli by generating nerve impulses that to the central nervous system
2. Integration - The brain and spinal cord of the Central Nervous System combine and sum up all the data received from the body and send out nerve impulses.
3. Motor Output  - The nerve impulses from the Central Nervous System go to the effectors (muscles and glands). Muscle contractions and gland secretions are responses to stimuli received by sensory receptors.

The Nervous System is divided into two main divisions. 

1. Central Nervous System (CNS)
2. Peripheral Nervous System (PNS)

Central Nervous System (CNS)

Spinal cord and brain sit in respective cavities

The CNS two parts: Brain; Spinal Cord

1. Brain

The Brain is divided into four main parts:

• Brain stem, consisting of the medulla, pons, and midbrain
• Cerebellum
• Diencephalon, with the thalamus and hypothalamus
• Cerebral hemispheres (comprised of the cerebral cortex, basal ganglia, white matter, hippocampi and amygdalae). The right and left hemisphere are connected by the corpus callosum which facilitates communication between both sides of the brain. The Hemispheres are then further divided into four lobes.

2. The Spinal Cord (the caudal extension of the CNS).

Neurological Conditions: many neurological conditions affect the CNS. They range dramatically in scope, impact, and nature of the effect. Some conditions lead to progressively impaired movement eg Parkinson disease. Huntington chorea. The demyelination in multiple sclerosis can cause acute attacks, and over time, chronic degradation of function. Others may impact cognition such as the various dementias. Epilepsy can cause uncontrolled excitation. Headaches often impair the daily function of patients. Traumatic injuries can cause plegia or paresis and may result a wide range of deficits depending on the location and extent of the lesion.

Neurons

Neurons are cells of the nervous system, located within the grey matter, and responsible for all neurological functions of the brain.

They are any of the impulse-conducting cells that constitute the brain, spinal column, and nerves in vertebrates, consisting of a nucleated cell body with one or more dendrites and a single axon. See Neurone link for more detailed information

Cerebrum

The cerebrum consists of two cerebral hemispheres, the right and left hemisphere are connected by the corpus callosum which facilitates communication between both sides of the brain, with each hemisphere in the main connection to the contralateral side of the body i.e. the left hemisphere of the cerebrum receives information from the right side of the body resulting in motor control of the right side of the body and vice versa.

The hemispheres are then further divided into four lobes;

1. Occipital
2. Parietal
3. Temporal (medial part of which are a series of structures including the Hippocampus)
4. Frontal

Cerebral Cortex

The outer layer of the cerebral hemisphere is termed the cerebral cortex. This is inter-connected via pathways that run sub-cortically. It is these connections as well as the connections from the cerebral cortex to the brainstem, spinal cord and nuclei deep within the cerebral hemisphere that form the white matter of the cerebral hemisphere. The deep nuclei include structures such as the basal ganglia and the thalamus.

Basal Ganglia

The “basal ganglia” refers to a group of subcortical nuclei within the brain responsible primarily for motor control, as well as other roles such as motor learning, executive functions, emotional behaviours, and play an important role in reward and reinforcement, addictive behaviours and habit formation.

The Hypothalamus

Hypothalamus

The hypothalamus is an organ central to many autonomous functions of the human body, notably the regulation of homeostasis. It has a significantly large efferent output to the ANS and has a highly significant role in the control of pituitary endocrine function.

The hypothalamus lies on either side of the 3rd ventricle, below the thalamus and between the optic chiasm and the midbrain. It receives a large input from limbic structures. See link for detailed description.

Meninges

The CNS is enclosed within the skull and vertebral column. These structures are separated by a series of membranes known as the Meninges. The Pia Mater is separated from the delicate arachnoid membrane by the subarachnoid space, which is then in turn separated from the Dura mater by the Sub-dural space.

Neuroglial Cells

The brain is made up of more than just neurones. Although there are about 86-100 billion neurons in the brain, there are about the same number of glial cells in the brain. Glial cells, or neuroglia, are cells that surround the neurones of the central nervous system embedded between them, providing both structural and physiological support.

There are four main classes of neuroglial cells (see link) within the CNS.

1. Astrocytes.
2. Oligodendrocytes
3. Ependymal cells
4. Microglial Cells

And in the PNS:

1. Schwann Cell: Found only in the PNS. Responsible for the myelination of the peripheral nerves by wrapping the cell around the axon. There are multiple layers of scwann cell membrane wrapped around the nerve. One schwann cell wraps around one axon and provides myelin for one internode. They are important for regeneration of damaged peripheral axons.

Limbic System

The limbic system refers to a number of areas within the brain lying mainly on the medial side of the temporal lobe. It includes a number of structures as seen in the diagram.

• The limbic system provides high level processing of sensory information. The main outflow of the limbic system is to the prefrontal cortex and the hypothalamus as well as to cortical areas. It appears to have a role in attaching behavioural significance and response to a given stimulus.
• Damage to this area has profound effects on emotional responses.
• Long term potentiation (LTP) is the increase in the strength of a synaptic transmission with repetitive use, it can be seen to be effected in the hippocampus (primarily is involved with memory) and is thought to be important for memory acquisition.

Brainstem

Brainstem red

The Brainstem lies at the base of the brain and the top of the spinal cord. It is the structure that connects the cerebrum of the brain to the spinal cord and cerebellum. It is composed of 3 sections in descending order: the midbrain, pons, and medulla oblongata. It is responsible for many vital functions of life, such as breathing, consciousness, blood pressure, heart rate, and sleep.  

See link

CNS Blood Supply

Circle of Willis

The CNS vasculature provides the nutrients necessary for the correct functioning of the central nervous system (CNS).

Brain: Arterial blood supply to the brain comes from four vessels;

• Right and Left Internal Carotid 
• Right and Left Vertebral Arteries

The internal carotid arteries branch to form two major cerebral arteries, the anterior and middle cerebral arteries. The right and left vertebral arteries come together at the level of the pons on the ventral surface of the brainstem to form the midline basilar artery.

Circle of Willis: The basilar artery joins the blood supply from the internal carotids in an arterial ring at the base of the brain (in the vicinity of the hypothalamus and cerebral peduncles) called the circle of Willis. The posterior cerebral arteries arise at this confluence, as do two small bridging arteries, the anterior and posterior communicating arteries. Conjoining the two major sources of cerebral vascular supply via the circle of Willis presumably improves the chances of any region of the brain continuing to receive blood if one of the major arteries becomes occluded.

Spinal Cord: The spinal cord is supplied by a single anterior spinal artery and paired posterior spinal arteries. Anterior spinal artery: arises from the vertebral arteries and extends from the level of the lower brainstem to the tip of the conus medullaris. It supplies the ventral medial surface of the medulla and anterior 2/3 of the spinal cord. The posterior spinal arteries supply the dorsal 1/3 of the cord. There are reinforcing branches from other arteries along the length of the cord.

If occlusion occurs, it is normally of the anterior spinal artery, producing loss of power and spinothalamic sensory deficit, but dorsal column sensory capabilities are maintained.

Venous Drainage

The cerebrum, cerebellum and brainstem are drained by numerous veins, which empty into the dural venous sinuses. The spinal cord is supplied by anterior and posterior spinal veins, which drain into the internal and external vertebral plexuses .

If occlusion of either of these venous systems then raised intracranial pressure can develop.

The Cerebellum

Cerebellum

The cerebellum is a vital component in the human brain as it plays a role in motor movement regulation and balance control. The cerebellum is neuron-rich, containing 80% of the brain’s neurones organized in a dense cellular layer, and it's surface area when unfolded is nearly 75% of the surface area of the cerebrum. See link

Spinal Cord

The spinal cord is part of the central nervous system and consists of a tightly packed column of nerve tissue that extends downwards from the brainstem through the central column of the spine. It is a relatively small bundle of tissue (weighing 35g and just about 1cm in diameter) but is crucial in facilitating our daily activities. See link.

The spinal cord carries nerve signals from the brain to other parts of the body (importantly the muscles we use to move) and receives sensory input from the body, partially processes it, and then transmits that information to the brain.

Peripheral Nervous System (PNS)

The peripheral nervous system includes the nerves and ganglia that are outside of the central nervous system.  The peripheral nervous system is made up of two divisions: the somatic nervous system and the autonomic system. Each part of this system plays a vital role in how information is communicated throughout the body.

Autonomic Nervous System (ANS)

The autonomic system is the part of the peripheral nervous system that's responsible for regulating involuntary body functions. Functions of the ANS include the regulation of “circulation, respiration, metabolism, secretion, body temperature, and reproduction.”

The ANS is divided into two Divisions:

1. Sympathetic: Preganglionic neurons found in lateral horn of spinal cord from upper thoracic to mid-lumbar cord (T1-L3). Postganglionic cell bodies found in vertebral and prevertebral ganglia. Uses Noradrenalin as postganglionic transmitter.
2. Parasympathetic: Preganglionic neurons have cell bodies in the brainstem and sacrum. Postganglionic cell bodies are found adjacent to or within the walls of the organ they supply. Uses acetylcholine (ACh) as postganglionic transmitter.

Somatic Nervous System

The somatic system is the part of the peripheral nervous system responsible for carrying sensory and motor information to and from the central nervous system. The somatic nervous system derives its name from the Greek word soma, which means "body."

Cranial and spinal nerves contribute to the somatic nervous system. Cranial nerves provide voluntary motor control and sensation to the head and face. Spinal nerves supply the trunk and limbs. The posterior rami travel backwards to supply the vertebral column, vertebral muscles and skin of the back whilst the anterior rami supply the limbs and anterior trunk. The majority of anterior rami combine to form nerve plexuses from which many major peripheral nerves stem. The exception to this is the anterior rami of the thoracic region which travel relatively independently from one another without forming plexuses, as the intercostal and subcostal nerves of the trunk.

Nervous plexuses are as follows:

C1-C4 form the cervical plexus

C5-T1 combine into the brachial plexus

T12-L4 form the lumbar plexus

L4 - S4 combine into the sacral plexus

Sensory Systems

The sensory system is where information is transmitted to the spinal cord and brain from peripheral sensory receptors. These are specialised neurons or nerve endings.

The sensory receptor, afferent axon and cell body are known as the primary afferent. The process by which the signals are transmitted through this system is known as sensory transduction. The signal produced is sent to the CNS via peripheral or cranial nerves via several synapses, eventually terminating in the cortex where it is analysed.

There are five main sensory systems in mammals.

1. touch/pressure
2. vision
3. hearing and balance
4. taste
5. smell/olfaction

Sensory receptors

A sensory receptor's adequate stimulus is the stimulus modality for which it possesses the adequate sensory transduction apparatus. Adequate stimulus can be used to classify sensory receptors:

• Baroreceptors respond to pressure in blood vessels
• Chemoreceptors respond to chemical stimuli
• Electromagnetic radiation receptors respond to electromagnetic radiation[1] Infrared receptors respond to infrared radiation
• Photoreceptors respond to visible light
• Ultraviolet receptors respond to ultraviolet radiation
• Electroreceptors respond to electric fields Ampullae of Lorenzini respond to electric fields, salinity, and to temperature, but function primarily as electroreceptors
• Hydroreceptors respond to changes in humidity
• Magnetoreceptors respond to magnetic fields
• Mechanoreceptors respond to mechanical stress or mechanical strain
• Nociceptors respond to damage, or threat of damage, to body tissues, leading (often but not always) to pain perception
• Osmoreceptors respond to the osmolarity of fluids (such as in the hypothalamus)
• Proprioceptors provide the sense of position
• Thermoreceptors respond to temperature, either heat, cold or both
  

Pain Systems

Pain is defined as an unpleasant sensory or emotional experience, associated with potential or actual tissue damage. Nociception defines the processing of information about damaging stimuli by the nervous system up to the level of the cortex.

Potentially damaging mechanical, thermal, and chemical stimuli are detected by nerve endings called nociceptors, which are found in the skin, on internal surfaces such as the periosteum, joint surfaces, and in some internal organs. The concentration of nociceptors varies throughout the body; they are found in greater numbers in the skin than in deep internal surfaces. The nociceptors are unspecialized free nerve endings that have their cell bodies outside the spinal column in the dorsal root ganglia. Nociceptors are categorized according to the axons which travel from the receptors to the spinal cord or brain.

Nociceptors have a certain threshold; that is, they require a minimum intensity of stimulation before they trigger a signal. Once this threshold is reached a signal is passed along the axon of the neuron into the spinal cord.

In some conditions, excitation of pain fibers becomes greater as the pain stimulus continues, leading to a condition called hyperalgesia.

There are two types of nociceptor:

1. A delta fibres: activated by high threshold mechanoreceptors. thinly myelinated.
2. Unmyelinated C-fibres: activated by polymodal nociceptors(PMN) and respond to intense mechanical stimulation, high temperatures and irritant chemicals.

There are three main pathways that transmit nociceptive signals to the brain:

1. Spinothalamic tract
2. Spino reticular tract.
3. Spino mesencephalic

Motor Systems

Motor systems are the areas of the nervous system responsible for controlling movement. The movement can either be guided by input from the sensory systems (closed-loop) or triggered by a sensory cue, or an internal desire to move (open-loop). Most movements involve both types of control. Closed loop movements tend to involve the axial muscles (posture and balance) while open-loop movements involve the peripheral muscles (locomotion and fine skilled movements).

1. Level 1 - Highest level - concerned with initiation, planning, and programming of the movement. Response to internal desire to move (limbic system and post. parietal cortex.
2. Level 2 - Cerebellum - responsible for balance and coordination of the movement.
3. Level 3 - Control of lower descending neurons via supraspinal descending motor pathways (corticospinal/pyramidal tract and tecto and rubrospinal, vestibulo spinal, extrapyramidal).
4. Level 4 - Low level motor organisation in the spinal cord. Descending motor paths, and interneurons. Mediation of spinal cord reflexes. This is where the central pattern generators are situated.
5. Level 5 - Lowest level. output neuron of the CNS to the muscle (motorneuron). Receives important input from sensory organs in the periphery - the muscle spindle and golgi tendon organ.