Human Physiology - Neurons & the Nervous System II

BIO 301
Human Physiology

Neurons & the Nervous System - Part 2

The Human Nervous System consists of the Central Nervous System & the Peripheral Nervous System.

Central Nervous System:

Brain

2 - Spinal cord

Peripheral Nervous System:

Cranial nerves

2 - Spinal nerves (31 pair) & their branches

Source: http://mail.med.upenn.edu/~hessd/Lesson3.htm

Divisions of Peripheral Nervous System -

1 - Somatic - supplies & receives fibers (neurons) to & from the skin, skeletal muscles, joints, & tendons

Used with permission of John Kimball

Parasympathetic division - important for control of 'normal' body functions, e.g., normal operation of digestive system
Sympathetic division - also called the 'fight or flight' division; important in helping us cope with stress

Source: http://faculty.washington.edu/chudler/nsdivide.html

Divisions of the Human Brain:

1 - Myelencephalon, which includes the medulla

2 - Metencephalon, which includes the pons and cerebellum

3 - Mesencephalon, which includes the midbrain (tectum and tegmentum)

4 - Diencephalon, which includes the thalamus and hypothalamus

5 - Telencephalon, which includes the cerebrum (cerebral cortex, basal ganglia, & medullary body)

Used with permission of John W. Kimball

Structures of the Brain:

Medulla (also called medulla oblongata) -

2 - contains ascending & descending tracts that communicate between the spinal cord & various parts of the brain

3 - contains 3 vital centers:

cardioinhibitory center, which regulates heart rate
respiratory center, which regulates the basic rhythm of breathing
vasomoter center, which regulates the diameter of blood vessels

Pons -

2 - Origin of four cranial nerves (V or trigeminal, VI or abducens, VII or facial, & VIII or vestibulocochlear)

3 - contains pneumotaxic center (a respiratory center)

Midbrain -

Corpora quadrigemina

Two pairs of rounded knobs on the upper surface of the midbrain mark the location of four nuclei, which are called collectively the "corpora quadrigemina." These masses contain the centers for certain visual reflexes, such as those responsible for moving the eyes to view something as the head is turned. They also contain the hearing reflex centers that operate when it is necessary to move the head so that sounds can be heard better.

2 - Cerebral peduncles - ascending & descending fiber tracts

3 - Origin of two cranial nerves (III or oculomotor & IV or trochlear)

Thalamus -

1 - Relay station for nearly all sensory impulses (except olfaction)

Source: http://songweaver.com/brain/index.html

Hypothalamus -

2 - Reception of sensory impulses from viscera

3 - Intermediary between nervous system & endocrine system

4 - Control of body temperature

5 - Regulation of food intake

6 - Thirst center

7 - Part of limbic system (emotions such as rage and aggression)

8 - Part of reticular formation

Reticular formation -

2 - needed for arousal from sleep & to maintain consciousness

Cerebellum -

1 - functions in coordination, maintenance of posture, & balance

Cerebrum -

2 - consists of 2 hemispheres divided by a fissure

Source: http://faculty.washington.edu/chudler/split.html

3 - includes cerebral cortex, medullary body, & basal ganglia:

Cortex:

outer 2 - 4 mm of the cerebrum
consists of gray matter (cell bodies & synapses; no myelin)

'folded', with upfolded areas called gyri & depressions or grooves called sulci
consists of four primary lobes

Source: http://faculty.washington.edu/chudler/lobe.html

functional areas include motor areas (initiate impulses that will cause contraction of skeletal muscles) (see A Map of the Motor Cortex), sensory areas (receive sensory impulses from throughout the body), and association areas (for analysis)

Source: http://faculty.washington.edu/chudler/functional.html

Medullary body:

the 'white matter' of the cerebrum; consists of myelinated axons
types of axons include:

commissural fibers - conduct impulses between cerebral hemispheres (and form the corpus callosum)

Source: http://faculty.washington.edu/chudler/sagittal.html

projection fibers - conduct impulses in & out of the cerebral hemispheres
association fibers - conduct impulses within hemispheres

Basal ganglia:

masses of gray matter in each cerebral hemisphere
important in control of voluntary muscle movements

Limbic System -

2 - located in part in cerebral cortex, thalamus, & hypothalamus

3 - Functions:

aggression
fear
feeding
sex (regulation of sexual drive & sexual behavior)

Spinal cord

Used with permission of John W. Kimball

The spinal cord extends from the skull (foramen magnum) to the first lumbar vertebra. Like the brain, the spinal cord consists of gray matter and white matter. The gray matter (cell bodies & synapses) of the cord is located centrally & is surrounded by white matter (myelinated axons). The white matter of the spinal cord consists of ascending and descending fiber tracts, with the ascending tracts transmitting sensory information (from receptors in the skin, skeletal muscles, tendons, joints, & various visceral receptors) and the descending tracts transmitting motor information (to skeletal muscles, smooth muscle, cardiac muscle, & glands). The spinal cord is also responsible for spinal reflexes.

Reflex- rapid (and unconscious) response to changes in the internal or external environment needed to maintain homeostasis

Reflex arc - the neural pathway over which impulses travel during a reflex. The components of a reflex arc include:

Spinal Nerves:

Source: http://nanonline.org/nandistance/nanneuro/modules/cranial/cranial.html

There are 31 pair of spinal nerves & each has a dorsal root and a ventral root. The dorsal root is sensory (all neurons conduct impulses into the spinal cord) while the ventral root is motor (all neurons conduct impulses out of the spinal cord). The dorsal root has a ganglion that contains the cell bodies of the sensory neurons that pass through the dorsal root. Each spinal nerve includes numerous sensory, or afferent, & motor, or efferent, neurons. Some of these neurons are classified as somatic, and these neurons conduct impulses to or from 'somatic' structures (skin, skeletal muscles, tendons, & joints). Other neurons are 'visceral', and these conduct impulses to or from 'visceral' structures (smooth muscle, cardiac muscle, and glands). Thus, all neurons in spinal nerves (& the peripheral nervous system) can be placed in one of four categories:

Somatic afferent
Somatic efferent
Visceral afferent
Visceral efferent

Somatic afferent neurons are sensory neurons that conduct impulses initiated in receptors in the skin, skeletal muscles, tendons, & joints. Receptors in the skin are responsible for sensing such things as touch, temperature, pressure, & pain and are called exteroceptors. Receptors in the skeletal muscles, tendons, & joints provide information about body position & movement and are called proprioceptors. Somatic afferent neurons are unipolar neurons that enter the spinal cord through the dorsal root & their cell bodies are located in the dorsal root ganglia.

Somatic efferent neurons are motor neurons that conduct impulses from the spinal cord to skeletal muscles. These neurons are multipolar neurons, with cell bodies located in the gray matter of the spinal cord. Somatic efferent neurons leave the spinal cord through the ventral root of spinal nerves.

Visceral afferent neurons are sensory neurons that conduct impulses initiated in receptors in smooth muscle & cardiac muscle. These neurons are collectively referred to as enteroceptors or visceroceptors. Visceral afferent neurons are unipolar neurons that enter the spinal cord through the dorsal root & their cell bodies are located in the dorsal root ganglia.

Visceral efferent neurons are motor neurons that conduct impulses to smooth muscle, cardiac muscle, & glands. These neurons make up the Autonomic Nervous System. Some visceral efferent neurons begin in the brain; others in the spinal cord. Because we're focusing on spinal nerves right now, we'll focus on those that begin in the spinal cord. It always takes two visceral efferent neurons to conduct an impulse from the spinal cord (or brain, in some cases) to a muscle or gland:

Visceral efferent 1 (also called the preganglionic neuron) is a multipolar neuron that begins in the gray matter of the spinal cord, which is where its cell body is located. This neuron leaves the cord through the ventral root of a spinal nerve, leaves the spinal nerve via a structure called the white ramus, then ends in an autonomic ganglion (either sympathetic or parasympathetic). In the ganglion, the visceral efferent 1 neuron synapses with a visceral efferent 2 neuron.
Visceral efferent 2 (also called the postganglionic neuron) is also a multipolar neuron and it begins in the sympathetic ganglion (which is where its cell body is located). Visceral efferent 2 neurons may exit the ganglion through the gray ramus, then proceed to some visceral structure (smooth muscle, cardiac muscle, or gland).

Source: http://www.mmi.mcgill.ca/Unit2/Mandl/lect20autonomicnervoussystem.htm

The 4 types of peripheral neurons: somatic afferent (top right), somatic efferent (bottom right),
visceral afferent (top left), and visceral efferent (bottom left).

Autonomic Nervous System:

2 - has two divisions: sympathetic & parasympathetic

sympathetic neurons leave the central nervous system through spinal nerves in the thoracic & lumbar regions of the spinal cord
parasympathetic neurons leave the central nervous system through cranial nerves plus spinal nerves in the sacral region of the spinal cord

Used with permission of John W. Kimball

sympathetic division - preganglionic neurons are relatively short & postganglionic neurons are relatively long
parasypathetic division - preganglionic neurons are relatively long & postganglionic neurons are very short

Chemical transmitters

cholinergic neurons - use acetylcholine as a neurotransmitter

includes all preganglionic neurons (both sympathetic & parasympathetic divisions), all parasympathetic postganglionic neurons, plus the sympathetic postganglionic neurons that supply the sweat glands

adrenergic neurons - used norepinephrine (also called noradrenalin) as a neurotransmitter

includes all postganglionic sympathetic neurons (except those that go to the sweat glands)

5 - Functions of the Autonomic Nervous System:

sympathetic division - prepares the body for strenuous physical activity in stressful situations. This response is often referred to as the 'fight-or-flight' response because the sympathetic division prepares the body to fight against or flee from a threat.
parasympathetic division - regulates important body functions such as digestion & 'slows down' the body after a 'flight-or-flight' response ('rest & digest')

6 - Control of Autonomic Nervous System - primary control center is the hypothalamus

Source: http://www.mmi.mcgill.ca/Unit2/Mandl/lect20autonomicnervoussystem.htm

Back to Neurons & the Nervous System I